**2. Characteristics of key projects**

Nine research projects and 3 commercial HDR-projects have been performed since the beginning of Hot Dry Rock research at around 1970 [9]. The major projects are described briefly in the following paragraphs.

#### **2.1. Los Alamos**

This first HDR-project was located at Fenton Hill at the rim of a large caldera. HDR-Systems were established at two levels in a biotite-granodiorite body at around 2800 m depth (Fenton Hill I) and in a heterogeneous metamorphic complex below 3500 m (Fenton Hill II) [10]. Rock temperature was 190 °C at the upper level and above 230 °C at the lower level.

The main elements of the shallow system are two vertical fractures created by water-frac tests. The design of the deeper system was according to the HDR-concept as shown in Fig. 1. Since the Los Alamos team felt certain about the stress directions both boreholes were drilled and completed before stimulation. In the target zone the wells were directionally drilled with a dip of 60° and parallel to the azimuth of the minimum horizontal stress. The length of the uncased sections was 1000 m, their vertical distance 300 m. The deeper borehole reached a depth of 4400 m [3]. About a dozen hydraulic fracturing operations were conducted in various intervals of the deviated section of the deeper well (EE2). High temperature open hole packers, casing packers, and PBR (Polished Bore Receptacles) in combination with sanding-off the bottom part of the hole were used to insolate borehole sections of 20 to 150 m length within the 1000 m long open hole section [11,12]. Most of the tests showed a high frac-pressure of about 40 MPa (wellhead pressure) indicating a high value of the normal stress acting on the fracture (around 0.8 of the vertical stress). The most extensive hydraulic fracturing operation was conducted in the uppermost 20 m of the open hole at 3500 m depth by injecting 21,500 m³ of water at flow rates up to 130 L/s. From the spatial distribution of induced seismicity as shown in Fig. 2 it was concluded, that a volumetric structure of roughly 800 x 800 m with a thickness of 200 m was stimulated. The strike of this structure was perpendicular to the direction of the borehole azimuth as expected, but instead of being vertical it was dipping toward the East parallel to the borehole axis. A satisfactory connection between the boreholes could be achieved after sidetracking the upper well into the region of induced seismicity and after stimulating this new well section. A circulation test revealed a thermal power output of 10 MW at a production flow rate of 12 – 14 L/s. Fluid losses and flow impedance were 20–30 % and 2.1 MPa s/l respectively. The Fenton Hill test site was abandoned due to declining financial support.

**Figure 2.** Hypocenters of seismic signals induced during the massive water frac-test in borehole EE1 at Fenton Hill (view is along the strike direction of the stimulated structure). EE-3A is the sidetrack of borehole EE-3, that was lend through the stimulated region. Note that the side track is not in the same plane as EE-2 but about 150 m in front of it. Re-production from [9].

#### **2.2. Camborne**

**•** Boreholes were no longer directional drilled parallel to the azimuth of the minimum horizontal stress but more or less parallel to the azimuth of the maximum horizontal stress. **•** Very long almost vertical borehole sections containing hundreds of natural fractures were

**•** The development of high temperature packers was no longer important and was disre‐

**•** Heat exchanging area as a measure for the service life of a HDR-system was replaced by

**•** Geometrical simple fracture mechanical models were replaced by geometrical complex

It will be shown that mainly this change of concept is responsible for the poor progress of HDRtechnology during the last 3 decades. The following chapters will critically review the results and observations of the major EGS-projects and proof that the basic mechanism controlling the stimulation process is not the shearing of the joint network but the formation of single large

Nine research projects and 3 commercial HDR-projects have been performed since the beginning of Hot Dry Rock research at around 1970 [9]. The major projects are described briefly

This first HDR-project was located at Fenton Hill at the rim of a large caldera. HDR-Systems were established at two levels in a biotite-granodiorite body at around 2800 m depth (Fenton Hill I) and in a heterogeneous metamorphic complex below 3500 m (Fenton Hill II) [10]. Rock

The main elements of the shallow system are two vertical fractures created by water-frac tests. The design of the deeper system was according to the HDR-concept as shown in Fig. 1. Since the Los Alamos team felt certain about the stress directions both boreholes were drilled and completed before stimulation. In the target zone the wells were directionally drilled with a dip of 60° and parallel to the azimuth of the minimum horizontal stress. The length of the uncased sections was 1000 m, their vertical distance 300 m. The deeper borehole reached a depth of 4400 m [3]. About a dozen hydraulic fracturing operations were conducted in various intervals of the deviated section of the deeper well (EE2). High temperature open hole packers, casing packers, and PBR (Polished Bore Receptacles) in combination with sanding-off the bottom part of the hole were used to insolate borehole sections of 20 to 150 m length within the 1000 m long open hole section [11,12]. Most of the tests showed a high frac-pressure of about 40 MPa (wellhead pressure) indicating a high value of the normal stress acting on the fracture (around

temperature was 190 °C at the upper level and above 230 °C at the lower level.

stimulated by injecting very large quantities of water.

fracture network models lacking fracture mechanical mechanisms.

garded.

wing-cracks.

accessible rock volume.

98 Effective and Sustainable Hydraulic Fracturing

**2. Characteristics of key projects**

in the following paragraphs.

**2.1. Los Alamos**

This first major project following the EGS-concept started in 1977 [5] and was operated by the Camborne School of Mines. The test site (Rosemanowes Quarry) is located near the centre of the Permian Carnmenellis granite pluton which is outcropping at the surface. Two orthogonal vertical joint sets were encountered at depth striking NNW-SSE and WSW-ENE. The stress conditions were strike slip with the maximum horizontal stress oriented NW-SE [13]. Two wells were drilled to 2000 m depth. Their arrangement is similar to the deep system in Fenton Hill but they deviate parallel to the direction of the maximum horizontal stress. Their vertical distance is about 300 m in the deviated part. Both boreholes had long open hole sections of 700 m and 360 m respectively. They were both drilled prior to stimulation. A hydraulic connection was achieved after a massive water injection in the lower well (26,000 m³) and a less massive stimulation in the upper well. The hydraulic connection however was poor and the fracture system extended during a long term circulation test with injection flow rates obviously too high for the system. A better connection was achieved after drilling a third well perpendicular to the strike of the fracture system and intersecting it some 300 m below the other two wells. Seismic activity recorded during the frac-tests (figure 3) showed that the activated fracture system had grown predominantly downward and was elongated in the direction parallel to the maximum horizontal stress [14]. The width of the seismic cloud (spatial distribution of the seismic sources) was less than 200 m. It showed an internal clustering of events along long vertical channels. A substantial thermal drawdown was observed during the first year of a circulation test indicating that the structure was much less "volumetric" than expected.

through down to the maximum depth of the boreholes (5 km). Typical for a rift setting is the high density of almost rift-parallel faults. Temperature anomalies at the site and in the region around Soultz are indications that some of the faults are permeable, transporting water from great depth into the Permian and Triassic cap rock. The joint systems are clustered with a high density of joints in fracture zones and a much lower density in competent rock [16]. Fracture zones and joints are mainly sub-vertical and striking 160°. The stress field is characterized by a low minimum horizontal stress (σ<sup>h</sup> ≅ 0.54 σV) and a maximum horizontal stress almost equal to the vertical stress [17, 18]. It was supposed that there was a transition from normal faulting conditions in the top part to strike slip conditions below 3000 m. The direction of the maximum horizontal stress as determined from the orientation of drilling induced fractures and borehole break-outs is 170 °. Temperature reached 201 °C at 5000 m depth. Large scale in-situ permea‐ bility of the granite was determined to less than 35 μD. But some of the faults intersected by the boreholes had transmissibilities between 0.1 and 50 d m (darcy meter; 1 d m = 10-12 m³) demonstrating that much more than 90 % of the water in the granite is carried by a few highly

EGS — Goodbye or Back to the Future http://dx.doi.org/10.5772/56458 101

Two HDR-Systems were established in the depths levels 2800 – 3600 m [20] and 4400 m – 5000 m [21] respectively (figure 4). The design of both systems was according to the EGS-concept, but instead of drilling production and injection wells first and connecting them afterwards the first borehole was massively stimulated after completion and the next borehole directionally drilled into the target zone defined by the spatial distribution of induced seismicity. In this way a doublet system was established at the upper and a triplet system at the lower level. All boreholes drilled at Soultz had open hole sections of 500 – 750 m in the bottom part. These sections were stimulated by injecting large volumes of water (between 10,000 and 35,000 m³). Flow rates were comparatively low (35 – 55 L/s) and in some cases the tests were started at flow rates as low as 1 L/s in order to allow the pressure to spread out in the joint network thus stimulating as many joints as possible. Both in the upper and the lower system it was necessary to stimulate the second and (in case of the triplet) also the third well before a satisfactory connection was achieved. Borehole separation was 450 m in the doublet system and 600 m

between the central injection hole and the two production holes in the triplet system.

Self propping of fractures was quite efficient and sustainable. Transmissibility of single fractures exceeded 1 d·m and was only slightly pressure dependent. Both fracture systems (upper and lower system) had open boundaries. Active pumping in the production wells was therefore introduced for the first time in an EGS-project in order to avoid fluid losses. Pro‐ duction flow rates reached 25 L/s in the production well of the upper system and more than 30 L/s for the two production boreholes (cumulated) of the lower system. Reinjection of the production flow in the central injection well of the lower system became a problem since induced seismicity started at an injection flow rate of about 20 L/s. In many aspects: depth, size, borehole distance, flow-impedance, circulation flow rates and fluid losses the two systems at Soultz mark the frontier of present HDR-technology. Part of this success however may be due to the favorable tectonic conditions in a rift setting and the results obtained so far are still

permeable faults and not by the joint network [19, 20].

insufficient for a commercial system.

**Figure 3.** Front view of the seismic cloud of the EGS-system at Rosemanowes. Note the channel-like structures inside the seismic cloud. Re-production from [9]

#### **2.3. Soultz-sous-Forêts**

This project started in 1988 [15]. The site of Soultz is located in the central part of the Upper Rhine Valley 6 km east of the Western main fault. The top of the Granite is at 1400 m and holds through down to the maximum depth of the boreholes (5 km). Typical for a rift setting is the high density of almost rift-parallel faults. Temperature anomalies at the site and in the region around Soultz are indications that some of the faults are permeable, transporting water from great depth into the Permian and Triassic cap rock. The joint systems are clustered with a high density of joints in fracture zones and a much lower density in competent rock [16]. Fracture zones and joints are mainly sub-vertical and striking 160°. The stress field is characterized by a low minimum horizontal stress (σ<sup>h</sup> ≅ 0.54 σV) and a maximum horizontal stress almost equal to the vertical stress [17, 18]. It was supposed that there was a transition from normal faulting conditions in the top part to strike slip conditions below 3000 m. The direction of the maximum horizontal stress as determined from the orientation of drilling induced fractures and borehole break-outs is 170 °. Temperature reached 201 °C at 5000 m depth. Large scale in-situ permea‐ bility of the granite was determined to less than 35 μD. But some of the faults intersected by the boreholes had transmissibilities between 0.1 and 50 d m (darcy meter; 1 d m = 10-12 m³) demonstrating that much more than 90 % of the water in the granite is carried by a few highly permeable faults and not by the joint network [19, 20].

m and 360 m respectively. They were both drilled prior to stimulation. A hydraulic connection was achieved after a massive water injection in the lower well (26,000 m³) and a less massive stimulation in the upper well. The hydraulic connection however was poor and the fracture system extended during a long term circulation test with injection flow rates obviously too high for the system. A better connection was achieved after drilling a third well perpendicular to the strike of the fracture system and intersecting it some 300 m below the other two wells. Seismic activity recorded during the frac-tests (figure 3) showed that the activated fracture system had grown predominantly downward and was elongated in the direction parallel to the maximum horizontal stress [14]. The width of the seismic cloud (spatial distribution of the seismic sources) was less than 200 m. It showed an internal clustering of events along long vertical channels. A substantial thermal drawdown was observed during the first year of a circulation test indicating that the structure was much less "volumetric" than expected.

**Figure 3.** Front view of the seismic cloud of the EGS-system at Rosemanowes. Note the channel-like structures inside

This project started in 1988 [15]. The site of Soultz is located in the central part of the Upper Rhine Valley 6 km east of the Western main fault. The top of the Granite is at 1400 m and holds

the seismic cloud. Re-production from [9]

100 Effective and Sustainable Hydraulic Fracturing

**2.3. Soultz-sous-Forêts**

Two HDR-Systems were established in the depths levels 2800 – 3600 m [20] and 4400 m – 5000 m [21] respectively (figure 4). The design of both systems was according to the EGS-concept, but instead of drilling production and injection wells first and connecting them afterwards the first borehole was massively stimulated after completion and the next borehole directionally drilled into the target zone defined by the spatial distribution of induced seismicity. In this way a doublet system was established at the upper and a triplet system at the lower level. All boreholes drilled at Soultz had open hole sections of 500 – 750 m in the bottom part. These sections were stimulated by injecting large volumes of water (between 10,000 and 35,000 m³). Flow rates were comparatively low (35 – 55 L/s) and in some cases the tests were started at flow rates as low as 1 L/s in order to allow the pressure to spread out in the joint network thus stimulating as many joints as possible. Both in the upper and the lower system it was necessary to stimulate the second and (in case of the triplet) also the third well before a satisfactory connection was achieved. Borehole separation was 450 m in the doublet system and 600 m between the central injection hole and the two production holes in the triplet system.

Self propping of fractures was quite efficient and sustainable. Transmissibility of single fractures exceeded 1 d·m and was only slightly pressure dependent. Both fracture systems (upper and lower system) had open boundaries. Active pumping in the production wells was therefore introduced for the first time in an EGS-project in order to avoid fluid losses. Pro‐ duction flow rates reached 25 L/s in the production well of the upper system and more than 30 L/s for the two production boreholes (cumulated) of the lower system. Reinjection of the production flow in the central injection well of the lower system became a problem since induced seismicity started at an injection flow rate of about 20 L/s. In many aspects: depth, size, borehole distance, flow-impedance, circulation flow rates and fluid losses the two systems at Soultz mark the frontier of present HDR-technology. Part of this success however may be due to the favorable tectonic conditions in a rift setting and the results obtained so far are still insufficient for a commercial system.

**Project Stress st. Well Frac-int. Well Traj. VIN QIN Pwc A Cloud-Dip Ref. [km] [m³] [L/s] [MPa] [km²]**

EGS — Goodbye or Back to the Future http://dx.doi.org/10.5772/56458 103

Falkenberg normal HB4a 0.25 vertical 25 3.5 2.2 0.014 60 [23] Fenton H. I normal 2.8 sub-vert. 587 0.15 [24] Fenton H. I normal 2.8 sub-vert. 761 0.16 [24] Fenton H. I normal 2.8 sub-vert. 5018 0.53 [24] Fenton H. II normal 3.7 55° II Sh 150 0.027 [24] Fenton H. II normal 3.7 55° II Sh 890 0.085 [24] Fenton H. II normal 3.7 55° II Sh 3183 0.27 [24] Fenton H. II normal 3.7 55° II Sh 3183 1 [24] Fenton H. II normal 3.7 55° II Sh 4702 1.1 [24] Fenton H. II normal EE2 3.45-3.47 55° II Sh 22000 108 38 0.7 65 [8,12] Camb. II strike s. RH12 1.74-2.12 60° II SH 18500 20-90 14 0.6 sub-vert. [25] Camb. II strike s. RH15 2.1-2.25 60° II SH 5700\* 200 15 0.04 sub-vert. [13] Hijiori normal SKG-2 1.79-1.80 vertical 2000 17-100 15 0.15 60 [2,26,27] Hijiori normal HDR-1 2.03-2.21 vertical 2100 17-67 26 0.25 60 [2,26] Ogachi (rever.) OGC-1 1.00-1.01 vertical 10140 11 19 0.5 30 [2,28] Ogachi (rever.) OGC-1 0.71-0.72 vertical 5440 8 22 0.3 sub-hor. [2,28] Soultz I strike s. GPK1 2.85-3.40 vertical 25300 0.2-36 9 1 sub-vert. [20,29] Soultz I strike s. GPK2 3.21-3.88 sub-vert. 28000 12-50 12 0.8 sub-vert. [20] Soultz II strike s. GPK2 4.40-5.00 sub-vert. 23400 30-50 14.5 3 sub-vert. [22] Cooper B. reverse Hab. 1 4.14-4.42 vertical 20000 14-26 60 3 sub-hor. [9] Basel strike-s. Basel 1 4.63-5.00 vertical 11650 0.2-55 30 0.9 sub-vert. [30]

**Table 1.** Stimulation parameters and of major stimulation tests in HDR-projects, VIN: injected volume, QIN: injection

General observations can be summarized as follows: All tests except a gel-test in the Camborne II system were accompanied by intense seismic activity. In all cases the seismic clouds were approximately 2-dimensional with a thickness in the range of the spatial resolution of the localization method. Some seismic clouds were twisted or bended or showed long tubular internal structures and straight boundaries. Width to length ratio measured along their main axis ranged generally from 0.3 to 3. The sparse seismic sources of the gel-frac were arranged tubularly. Some seismic clouds (Soultz II GPK2., Basel 1, and Fenton Hill II EE1) showed an alignment with well trajectories. Seismic clouds in strike-slip regions were vertical or subvertical with a general trend slightly off the direction of the maximum horizontal stress. For normal stress conditions (Hijiori, HDR-1 and Fenton Hill, EE2) the seismic clouds dipped 60° or 65° respectively toward the direction of the minimum horizontal stress. For reverse stress conditions (Cooper Basin and most likely Ogachi) the seismic clouds were horizontal or subhorizontal. Seismic activity started generally far below the jacking pressure (pressure equal to the normal stress on the fracture) but the pressure always approached the at the end of the tests. In relation to depth the maximum injection pressure pwc (measured at the well head) was comparatively low for vertical or sub-vertical fractures, higher for steeply dipping fractures

flow rate, pwc: maximum well head pressure, A: area of the "seismic cloud"

and much higher for sub-horizontal fractures.

**3.1. General observations**

**Figure 4.** Seismic clouds of all stimulation tests of the upper EGS-system (Soultz I) and of the first stimulation test of the lower EGS-system (Soultz II) at Soultz [22]. General strike direction of the seismic clouds is NNW-SSE in both cases. The deeper well is GPK2, the other GPK1. The deep system was later intersected by two additional boreholes (GPK3 and GPK4) and enlarged during stimulation tests in these wells. Note the low seismic source density in the southern wing of the seismic cloud.

#### **3. Observations and results of stimulation and circulation tests**

Though the number of stimulation tests in EGS-projects is quite limited as compared to the millions of frac-tests in oil and gas reservoirs an attempt was made to find some general relationships between test-parameters and test-results. This was done with little hope since reliable data was sparse and test conditions very variable. The only constants for almost all tests were rock type (granite and granodiorite) and frac-fluid (water or brine with one exception) and the fact that all tests were done in uncased borehole sections. All other test parameters and conditions were quite variable (Tab. 1): Stress condition ranged from normalto reverse faulting, length of frac-interval from 3 to 750 m, injected volume from 20 m³ to 35.000 m³, flow rates from 6 L/s to 200 L/s. Furthermore some tests were performed with constant flow rate, others with stepwise increased flow rates. Well trajectories were predominantly vertical to sub-vertical but some tests (Fenton Hill II and Camborne II) were performed in inclined borehole sections.


**Table 1.** Stimulation parameters and of major stimulation tests in HDR-projects, VIN: injected volume, QIN: injection flow rate, pwc: maximum well head pressure, A: area of the "seismic cloud"

#### **3.1. General observations**

**Figure 4.** Seismic clouds of all stimulation tests of the upper EGS-system (Soultz I) and of the first stimulation test of the lower EGS-system (Soultz II) at Soultz [22]. General strike direction of the seismic clouds is NNW-SSE in both cases. The deeper well is GPK2, the other GPK1. The deep system was later intersected by two additional boreholes (GPK3 and GPK4) and enlarged during stimulation tests in these wells. Note the low seismic source density in the southern

Though the number of stimulation tests in EGS-projects is quite limited as compared to the millions of frac-tests in oil and gas reservoirs an attempt was made to find some general relationships between test-parameters and test-results. This was done with little hope since reliable data was sparse and test conditions very variable. The only constants for almost all tests were rock type (granite and granodiorite) and frac-fluid (water or brine with one exception) and the fact that all tests were done in uncased borehole sections. All other test parameters and conditions were quite variable (Tab. 1): Stress condition ranged from normalto reverse faulting, length of frac-interval from 3 to 750 m, injected volume from 20 m³ to 35.000 m³, flow rates from 6 L/s to 200 L/s. Furthermore some tests were performed with constant flow rate, others with stepwise increased flow rates. Well trajectories were predominantly vertical to sub-vertical but some tests (Fenton Hill II and Camborne II) were performed in

**3. Observations and results of stimulation and circulation tests**

wing of the seismic cloud.

102 Effective and Sustainable Hydraulic Fracturing

inclined borehole sections.

General observations can be summarized as follows: All tests except a gel-test in the Camborne II system were accompanied by intense seismic activity. In all cases the seismic clouds were approximately 2-dimensional with a thickness in the range of the spatial resolution of the localization method. Some seismic clouds were twisted or bended or showed long tubular internal structures and straight boundaries. Width to length ratio measured along their main axis ranged generally from 0.3 to 3. The sparse seismic sources of the gel-frac were arranged tubularly. Some seismic clouds (Soultz II GPK2., Basel 1, and Fenton Hill II EE1) showed an alignment with well trajectories. Seismic clouds in strike-slip regions were vertical or subvertical with a general trend slightly off the direction of the maximum horizontal stress. For normal stress conditions (Hijiori, HDR-1 and Fenton Hill, EE2) the seismic clouds dipped 60° or 65° respectively toward the direction of the minimum horizontal stress. For reverse stress conditions (Cooper Basin and most likely Ogachi) the seismic clouds were horizontal or subhorizontal. Seismic activity started generally far below the jacking pressure (pressure equal to the normal stress on the fracture) but the pressure always approached the at the end of the tests. In relation to depth the maximum injection pressure pwc (measured at the well head) was comparatively low for vertical or sub-vertical fractures, higher for steeply dipping fractures and much higher for sub-horizontal fractures.

#### **3.2. Size of the stimulated region**

In contrary to the common praxis in EGS-literature of the last 2 decades but in accordance with an earlier study [24] not the volume but the area of the seismic cloud was taken as the measure for the size of the stimulated region. This was done because of the 2-dimensional nature of the seismic clouds and the strong influence of the location error on their thickness. This area, called seismic area in the following, was grossly determined by drawing an envelope around the projection of the seismic clouds on a plane parallel to its main orientation. Despite of the big variation in test and test-site conditions a clear correlation was found between seismic area and injected volume (figure 5a). 75% of the data points can well be fitted by a power law with exponent n = 0.6. Accordingly the ratio of injected volume and seismic area is fitted by a power law with exponent (1 – n) = 0.4 (figure 5b). Seismic area and the ratio of injected volume and seismic area did not correlate with flow rate or length of the frac-interval. These findings and the high coefficient of correlation of both parameters with injected volume allow establishing the following working hypotheses:

the first well (GPK1) and propagated sub-vertically downward (figure 6). At the end of stimulation it reached a length of about 700 m. Seismicity was spreading predominantly to one side of this channel during migration. In the final test period several other channels developed starting from almost the same region as the first but with different dip (figure 6). Views along some of these channels indicate that the main structure created by the stimulation process was

EGS — Goodbye or Back to the Future http://dx.doi.org/10.5772/56458 105

a large wing crack with a central shear zone and the typical bended wings (figure 7).

**Figure 6.** Evolution of the seismic cloud during the first stimulation test in the Soultz I system (front view) [31].

**Figure 7.** Seismic cloud of all located events of the first stimulation test in the Soultz I system (well GPK1). Left: front

view, middle and right: views along indicated directions [32].

View 1 View 2


**3.** Static fracture models should apply; friction pressure losses in the fractures are negligible.

**Figure 5.** Results of the stimulation tests: a) Area of the seismic clouds vs. injected volume, b) ratio of seismic area and injected volume vs. injected volume. Fitting lines and coefficients of determination are for the solid data points.

#### **3.3. Characteristics and internal structure of the stimulated region**

Some of the seismic clouds showed long channel-like internal features. In the Soultz I system a first channel started to grow from the main outlet in the top part of the open hole section of the first well (GPK1) and propagated sub-vertically downward (figure 6). At the end of stimulation it reached a length of about 700 m. Seismicity was spreading predominantly to one side of this channel during migration. In the final test period several other channels developed starting from almost the same region as the first but with different dip (figure 6). Views along some of these channels indicate that the main structure created by the stimulation process was a large wing crack with a central shear zone and the typical bended wings (figure 7).

**3.2. Size of the stimulated region**

104 Effective and Sustainable Hydraulic Fracturing

the following working hypotheses:

of the length of the frac-interval.

y = 2,180.41x0.59 R² = 0.93

1.E+01 1.E+02 1.E+03 1.E+04 1.E+05

**VIN [m³]**

a) b) **VIN [m³]**

**3.3. Characteristics and internal structure of the stimulated region**

**Figure 5.** Results of the stimulation tests: a) Area of the seismic clouds vs. injected volume, b) ratio of seismic area and injected volume vs. injected volume. Fitting lines and coefficients of determination are for the solid data points.

Some of the seismic clouds showed long channel-like internal features. In the Soultz I system a first channel started to grow from the main outlet in the top part of the open hole section of

1.E+03

1.E+04

1.E+05

**A [m²]**

1.E+06

1.E+07

In contrary to the common praxis in EGS-literature of the last 2 decades but in accordance with an earlier study [24] not the volume but the area of the seismic cloud was taken as the measure for the size of the stimulated region. This was done because of the 2-dimensional nature of the seismic clouds and the strong influence of the location error on their thickness. This area, called seismic area in the following, was grossly determined by drawing an envelope around the projection of the seismic clouds on a plane parallel to its main orientation. Despite of the big variation in test and test-site conditions a clear correlation was found between seismic area and injected volume (figure 5a). 75% of the data points can well be fitted by a power law with exponent n = 0.6. Accordingly the ratio of injected volume and seismic area is fitted by a power law with exponent (1 – n) = 0.4 (figure 5b). Seismic area and the ratio of injected volume and seismic area did not correlate with flow rate or length of the frac-interval. These findings and the high coefficient of correlation of both parameters with injected volume allow establishing

**1.** The stimulation process is mainly volume-controlled. This means, the majority of the injected volume is creating new fracture volume. Hydraulic diffusion (including fluid losses into the rock matrix) is not essential. Fluid efficiency η (ratio of created fracture

**2.** The number of fractures created or stimulated in the frac-interval is close to "1" regardless

**3.** Static fracture models should apply; friction pressure losses in the fractures are negligible.

y = 4.59E-04x4.08E-01 R² = 8.61E-01

1.E+01 1.E+02 1.E+03 1.E+04 1.E+05

1.E-04

1.E-03

1.E-02

**b [m]**

1.E-01

1.E+00

volume and injected volume) is high (at least higher than 0.5).

**Figure 6.** Evolution of the seismic cloud during the first stimulation test in the Soultz I system (front view) [31].

**Figure 7.** Seismic cloud of all located events of the first stimulation test in the Soultz I system (well GPK1). Left: front view, middle and right: views along indicated directions [32].

#### **3.4. Number of stimulated fractures**

Direct information on the number of conductive fractures was obtained by flow and temper‐ ature logging during stimulation and post-stimulation injection or production tests. In no case was the number of hydraulically significant fractures bigger than 4 and often they were in close vicinity to each other. An example from a well in the Soultz I system as shown in figure 8 demonstrates that even these small numbers should only be regarded as an upper limit [32]. The 750 m long uncased section of this well contained one significant fault with a transmissi‐ bility of 0.1 d m at 3500 m depth prior to stimulation. This fault consumed more than 90% of the injected fluid during pre-stimulation hydraulic tests. The contribution of the several hundreds of joints, a number of fracture zones and additional faults, as well as the numerous drilling induced fractures encountered by ultrasonic borehole-televiewer measurements was insignificant. During stimulation a group of drilling induced en-echelon fractures in the uppermost open hole section opened in the early test-phase and remained the dominant hydraulic feature throughout the test absorbing about 2/3 of the injected flow rate. The remaining third was absorbed by the fault at 3500 m and by 3 other fractures. A redistribution of the flow fraction of these three fractures between stimulation and production proofed that they were merely low impedance connections to the main fracture originating in the upper‐ most part of the open hole section (figure 8).

**3.5. Aperture of stimulated fractures**

**EGI data (Fluorimetry on site)**

**injection of 150 kg of SN fluorescein dissolved in 950 l of fresh water** 

evaluation. The tracer break-through volume for this model is given by:

**SN fluorescein concentration (µg/l)**

Information on the aperture of the stimulated fractures can be obtained from tracer tests performed during circulation. The tracer response curves of EGS-systems found in reports and publications were of surprisingly uniform shape (figure 9). All had a steeply rising tracer concentration after tracer break-through, a single maximum and a monotonously declining tracer concentration afterwards. Some showed minor inflections in the tail that were inter‐ preted by some authors as tracer arrivals from multiple flow paths but as an effect of tracer reinjection by others. None of the curves showed clear indications of multiple fracture flow.

> **07/07/05 27/07/05 16/08/05 05/09/05 25/09/05 15/10/05 04/11/05 24/11/05 14/12/05 03/01/06 Time (days)**

**Figure 9.** Tracer response curve recorded during a long-term circulation test in the Soultz II system. Source: [33].

*Vb* <sup>=</sup> *<sup>π</sup>bT* · *aG* <sup>2</sup>

For these reasons and in order get comparable results it seemed reasonable to use the same simple proxy model, namely that of a doublet in an infinite fracture of uniform aperture for

With *bT:* aperture of the fracture and *aG*: geometrical inlet to outlet distance. The values of *bT* determined with this equation are listed in Table 2 and plotted in figure 10. For comparison the values of *b* given by the ratio of seismic area and injected volume and the corresponding fit-line are included. In 3 cases (Fenton Hill I & II and Camborne II) is the aperture determined from the tracer break-through volume by a factor of 5 to 10 smaller than the aperture given by the ratio of injected volume and seismic area. In other 3 cases (Soultz I, Hijiori and Ogachi) both aperture values agree quite well. For Soultz II the aperture from the tracer break-through volume is consistent with the data from Soultz I, Hijiori and Ogachi though the ratio of injected volume and seismic area is comparatively low. The majority of the aperture values are in the

**BRGM data (HPLC)**

EGS — Goodbye or Back to the Future http://dx.doi.org/10.5772/56458 107

<sup>3</sup> (1)

**Figure 8.** Right: Spinner flow logs recorded during the main stimulation test and during a post-stimulation production test in well GPK1 (Soultz I system). Left: model illustrating that fractures Z1, Z2, and Z3 are low impedance flow paths connection the well to the main fracture Z1 [31].

#### **3.5. Aperture of stimulated fractures**

**3.4. Number of stimulated fractures**

106 Effective and Sustainable Hydraulic Fracturing

most part of the open hole section (figure 8).

connection the well to the main fracture Z1 [31].

Direct information on the number of conductive fractures was obtained by flow and temper‐ ature logging during stimulation and post-stimulation injection or production tests. In no case was the number of hydraulically significant fractures bigger than 4 and often they were in close vicinity to each other. An example from a well in the Soultz I system as shown in figure 8 demonstrates that even these small numbers should only be regarded as an upper limit [32]. The 750 m long uncased section of this well contained one significant fault with a transmissi‐ bility of 0.1 d m at 3500 m depth prior to stimulation. This fault consumed more than 90% of the injected fluid during pre-stimulation hydraulic tests. The contribution of the several hundreds of joints, a number of fracture zones and additional faults, as well as the numerous drilling induced fractures encountered by ultrasonic borehole-televiewer measurements was insignificant. During stimulation a group of drilling induced en-echelon fractures in the uppermost open hole section opened in the early test-phase and remained the dominant hydraulic feature throughout the test absorbing about 2/3 of the injected flow rate. The remaining third was absorbed by the fault at 3500 m and by 3 other fractures. A redistribution of the flow fraction of these three fractures between stimulation and production proofed that they were merely low impedance connections to the main fracture originating in the upper‐

**Figure 8.** Right: Spinner flow logs recorded during the main stimulation test and during a post-stimulation production test in well GPK1 (Soultz I system). Left: model illustrating that fractures Z1, Z2, and Z3 are low impedance flow paths Information on the aperture of the stimulated fractures can be obtained from tracer tests performed during circulation. The tracer response curves of EGS-systems found in reports and publications were of surprisingly uniform shape (figure 9). All had a steeply rising tracer concentration after tracer break-through, a single maximum and a monotonously declining tracer concentration afterwards. Some showed minor inflections in the tail that were inter‐ preted by some authors as tracer arrivals from multiple flow paths but as an effect of tracer reinjection by others. None of the curves showed clear indications of multiple fracture flow.

**Figure 9.** Tracer response curve recorded during a long-term circulation test in the Soultz II system. Source: [33].

For these reasons and in order get comparable results it seemed reasonable to use the same simple proxy model, namely that of a doublet in an infinite fracture of uniform aperture for evaluation. The tracer break-through volume for this model is given by:

$$\mathcal{V}\_b = \frac{\pi \, b\_{\mathcal{V}} \cdot a\_{\mathcal{G}}\,^2}{3} \tag{1}$$

With *bT:* aperture of the fracture and *aG*: geometrical inlet to outlet distance. The values of *bT* determined with this equation are listed in Table 2 and plotted in figure 10. For comparison the values of *b* given by the ratio of seismic area and injected volume and the corresponding fit-line are included. In 3 cases (Fenton Hill I & II and Camborne II) is the aperture determined from the tracer break-through volume by a factor of 5 to 10 smaller than the aperture given by the ratio of injected volume and seismic area. In other 3 cases (Soultz I, Hijiori and Ogachi) both aperture values agree quite well. For Soultz II the aperture from the tracer break-through volume is consistent with the data from Soultz I, Hijiori and Ogachi though the ratio of injected volume and seismic area is comparatively low. The majority of the aperture values are in the range of centimeters. For comparison: (hypothetical) tensile fractures with a fracture area of 1 km² would have an average aperture of about 1 mm. (2009). AP 3000 p1-62. 

24 6 reverse faulting conditions reverse faulting stress conditions

27 27 Niitsuma, H, & Asanuma, H. Niitsuma, H, Asanuma, H & Jones, R.

S. (ed.) Deep Heat Mining Basel – Seismic Risk Analysis. Basel, Amt für Umwelt und Energie;

> through time is by at least a factor of 10 lower. This means a layer compatible with the thermal draw-down of Camborne II could have a maximum thickness of 1 m. The long term thermal response of such a layer is indistinguishable from that of a discrete fracture. Summarizing one can conclude that the observed thermal draw down curves give no reason to introduce more than one fracture or a porous layer (volumetric fracture system) instead of a discrete fracture

> > 0 0.5 1 1.5 2 2.5 3

0.01 m 10 m Camborne

EGS — Goodbye or Back to the Future http://dx.doi.org/10.5772/56458 109

*<sup>I</sup> <sup>m</sup>*<sup>3</sup> (2)

*bH* = 12*T <sup>f</sup>* <sup>3</sup> (3)

**time [a]**

**Figure 11.** Evolution of the temperature draw-down in the production well of the Camborne II system (red dots from [34]) and fit-curve calculated for a doublet in an infinite fracture (solid blue line) and for a doublet in a porous layer with thickness 10 m (dashed line) for well distance: aH = 263 m, flow rate: Q = 15 L/s, injection temperature: Tin = 20 °C,

The inter-well transmissibility of the fractures was determined from the flow impedance "I" by using the model of a doublet in an infinite fracture of homogeneous and isotropic trans‐ missibility imbedded in an impermeable matrix. For this model the transmissibility is approx‐

The values calculated with Eq. 2 are listed in Tab. 2. The transmissibility of the fractures of the Camborne II and the Soultz I & II systems exceeds 1 d·m, a value which is rarely achieved with conventional propped fractures. The fractures of the other systems are in the range of 0.1 d·m or lower. The "hydraulic aperture" bH corresponding to these transmissibility values can be

*<sup>T</sup> <sup>f</sup>* <sup>=</sup> <sup>2</sup>*<sup>μ</sup>*

as the main flow path between injection and production well.

70

imately given by the following formula:

T0 = 80.5 °C.

calculated by:

**3.7. Hydraulic properties**

72

74

76

**production temperature [°C]**

78

80

82

**Figure 10.** Fracture apertures determined from tracer break-through volumes (open symbols) and given by the ratio of injected volume and seismic area of the stimulation tests (filled symbols). Fitting line is identical with the fitting line of figure 5b.

#### **3.6. Heat exchanging area**

The observation of the thermal draw down is probably the most sensitive method to distin‐ guish single fracture flow from multi-fracture or volumetric flow. Already two (thermally independent) fractures instead of one extent the time scale for the thermal draw-down by the square-root of two. Volumetric flow is indicated when the production flow is constant over a prolonged time period. Unfortunately data for only two cases, Camborne II and Hijiori was available. Both thermal draw down curves could well be fitted by using an analytical model and reasonable thermo-physical values for the fluid and rock. The model calculates the evolution of the production temperature for a doublet in an infinite fracture of uniform transmissibility with transient conductive heat flow from the rock matrix toward the fracture (figure 11). The inlet-outlet distance of 260 m determined by using this model agrees quite well with the geometrical inlet-outlet distance of the Camborne II system (Table 2). For two fractures this distance would reduce to 190 m, which is hardly compatible with the geometrical configuration. For Hijiori the inlet-outlet distance obtained by the model is already for one fracture smaller than the geometrical distance. Two fractures would make this discrepancy even bigger. The thermal draw down curve of a porous layer with a thickness of 10 m instead of a discrete fracture would have a thermal break through-time (end time of constant produc‐ tion temperature) of about 1 year for Camborne (figure 11). The observed thermal break-

6

through time is by at least a factor of 10 lower. This means a layer compatible with the thermal draw-down of Camborne II could have a maximum thickness of 1 m. The long term thermal response of such a layer is indistinguishable from that of a discrete fracture. Summarizing one can conclude that the observed thermal draw down curves give no reason to introduce more than one fracture or a porous layer (volumetric fracture system) instead of a discrete fracture as the main flow path between injection and production well.

**Figure 11.** Evolution of the temperature draw-down in the production well of the Camborne II system (red dots from [34]) and fit-curve calculated for a doublet in an infinite fracture (solid blue line) and for a doublet in a porous layer with thickness 10 m (dashed line) for well distance: aH = 263 m, flow rate: Q = 15 L/s, injection temperature: Tin = 20 °C, T0 = 80.5 °C.

#### **3.7. Hydraulic properties**

6

range of centimeters. For comparison: (hypothetical) tensile fractures with a fracture area of 1

1.E+01 1.E+02 1.E+03 1.E+04 1.E+05

VIN [m³]

**Figure 10.** Fracture apertures determined from tracer break-through volumes (open symbols) and given by the ratio of injected volume and seismic area of the stimulation tests (filled symbols). Fitting line is identical with the fitting line

The observation of the thermal draw down is probably the most sensitive method to distin‐ guish single fracture flow from multi-fracture or volumetric flow. Already two (thermally independent) fractures instead of one extent the time scale for the thermal draw-down by the square-root of two. Volumetric flow is indicated when the production flow is constant over a prolonged time period. Unfortunately data for only two cases, Camborne II and Hijiori was available. Both thermal draw down curves could well be fitted by using an analytical model and reasonable thermo-physical values for the fluid and rock. The model calculates the evolution of the production temperature for a doublet in an infinite fracture of uniform transmissibility with transient conductive heat flow from the rock matrix toward the fracture (figure 11). The inlet-outlet distance of 260 m determined by using this model agrees quite well with the geometrical inlet-outlet distance of the Camborne II system (Table 2). For two fractures this distance would reduce to 190 m, which is hardly compatible with the geometrical configuration. For Hijiori the inlet-outlet distance obtained by the model is already for one fracture smaller than the geometrical distance. Two fractures would make this discrepancy even bigger. The thermal draw down curve of a porous layer with a thickness of 10 m instead of a discrete fracture would have a thermal break through-time (end time of constant produc‐ tion temperature) of about 1 year for Camborne (figure 11). The observed thermal break-

24 6 reverse faulting conditions reverse faulting stress conditions

27 27 Niitsuma, H, & Asanuma, H. Niitsuma, H, Asanuma, H & Jones, R.

28 29 . Personal Communication. . Induced seismicity, AP 3000 report. In: Baisch,

S. (ed.) Deep Heat Mining Basel – Seismic Risk Analysis. Basel, Amt für Umwelt und Energie;

> Fenton Hill I Fenton Hill II Camborne II Hijiori Ogachi Soultz I Soultz II

(2009). AP 3000 p1-62.

km² would have an average aperture of about 1 mm.

 

108 Effective and Sustainable Hydraulic Fracturing

1.E-04

1.E-03

1.E-02

b [m]

of figure 5b.

**3.6. Heat exchanging area**

1.E-01

1.E+00

The inter-well transmissibility of the fractures was determined from the flow impedance "I" by using the model of a doublet in an infinite fracture of homogeneous and isotropic trans‐ missibility imbedded in an impermeable matrix. For this model the transmissibility is approx‐ imately given by the following formula:

$$T\_f = \frac{2\mu}{l} \quad \left[m^3\right] \tag{2}$$

The values calculated with Eq. 2 are listed in Tab. 2. The transmissibility of the fractures of the Camborne II and the Soultz I & II systems exceeds 1 d·m, a value which is rarely achieved with conventional propped fractures. The fractures of the other systems are in the range of 0.1 d·m or lower. The "hydraulic aperture" bH corresponding to these transmissibility values can be calculated by:

$$b\_H = \sqrt[3]{12T\_f} \tag{3}$$

This formula is for smooth fracture surfaces and was experimentally confirmed for fractures in rock but with coefficients slightly higher than 12. The hydraulic apertures calculated with this formula are by about a factor of 10 to 100 lower than the apertures derived from the tracer break-through volume (Tab. 2). This in turn means that one should expect fracture transmis‐ sibilities between 500 d·m and 7 Mio. d·m from the apertures of the tracer tests. This stupendous discrepancy can not be explained by turbulence or viscosity effects. It is also unlikely that asperities or particles are plugging the fractures to such a degree that only a small fraction of the fractures is open for the flow. In this case the tracer break-through volume would also be reduced to a high degree. The most plausible explanation is that the fractures consist of a series of wide open and very narrow fracture elements. In this case the average aperture is mainly determined by the wide fracture elements whereas the transmissibility is mainly determined by the narrow fracture elements. This kind of arrangement can neither be explained by tensile fracture propagation nor by the shearing of existing fractures or faults.

**4. Interpretation and discussion**

of faults or fracture zones [41, 42].

It is obvious that the actual EGS-concept is inconsistent with almost all observations and results described in the last chapter. The main reason for this inconsistency is most likely the wrong model for the granite underlying this concept. It´s basic assumption is that the granite due to the presence of joints has to be considered as a discontinuum (figure 12) and can therefore be regarded a as coulomb-material. Accordingly the coulomb friction failure criterion is the obvious choice for the stimulation process. A more realistic model (figure 12) however considers the granite as a continuum on the scale of joints and as a discontinuum on the scale

EGS — Goodbye or Back to the Future http://dx.doi.org/10.5772/56458 111

**Figure 12.** Conceptual models for the granite. Left: granite as a discontinuum on the scale of joints [13], right: granite

The Coulomb failure criterion is therefore applicable only on the scale of faults or fracture zones whereas on the scale of joints hydraulic stimulation needs a failure mechanism that includes the formation of new fracture surface like the classical hydraulic fracturing models. Tensile fracture models are however unable to explain the onset of the failure process at a pressure far below the minimum principal stress. Furthermore they are hardly consistent with the intense seismicity and the source mechanism of the seismic events. For these reasons a new model is required that combines tensile fracture propagation with the shearing of natural

as a discontinuum on the scale of faults or fracture zones and as a continuum on the scale of joints [40, 41].

discontinuities. The most obvious choice is the wing-crack model (figure 13).


**Table 2.** Operation parameters and results of circulation tests in major HDR-systems, aG: geometrical distance between inlet and outlet, T0: rock temperature, QEX: production flow rate, I: impedance (ratio of pressure difference between inlet and outlet and production flow rate), μ: viscosity of produced water; T: transmissibility, bH: hydraulic aperture, Vb: tracer break-through volume, bT: aperture calculated from tracer break-through volume, aH: inlet to outlet distance calculated from observed thermal draw-down.

Well test analysis of post-stimulation injection or production tests point into the same direction. All post-stimulation hydraulic tests in Soultz and in Basel showed very long fracture linear or bilinear flow periods often persisting over 10 hours or more. Such long periods need a very long start line for the flow. In cases where several hundred Meter long axial fractures are present (Soultz II well GPK4 and probably Basel) these long starting lines may be identical with the trace of these fractures along the borehole wall. In other cases they are most likely identical with the long channel-like features of the seismic clouds. Theses channels play most likely a dominant role for the flow distribution in the stimulated fractures. Channels may also exist on the scale of joints and may result in a highly anisotropic transmissibility of the large scale fracture.

#### **4. Interpretation and discussion**

This formula is for smooth fracture surfaces and was experimentally confirmed for fractures in rock but with coefficients slightly higher than 12. The hydraulic apertures calculated with this formula are by about a factor of 10 to 100 lower than the apertures derived from the tracer break-through volume (Tab. 2). This in turn means that one should expect fracture transmis‐ sibilities between 500 d·m and 7 Mio. d·m from the apertures of the tracer tests. This stupendous discrepancy can not be explained by turbulence or viscosity effects. It is also unlikely that asperities or particles are plugging the fractures to such a degree that only a small fraction of the fractures is open for the flow. In this case the tracer break-through volume would also be reduced to a high degree. The most plausible explanation is that the fractures consist of a series of wide open and very narrow fracture elements. In this case the average aperture is mainly determined by the wide fracture elements whereas the transmissibility is mainly determined by the narrow fracture elements. This kind of arrangement can neither be explained by tensile

**Project aG T0 QEX I μ T bf Vb bT aH References [m] [°C] [L/s] [MPa·s/L] [Pa·s] [d·m] [m] [m³] [m] [m]** Fenton Hill I 230 190 6 1.6 1.5E-04 .19 1.3E-04 100 1.8E-03 - [9] Fenton Hill II 150 230 6 2.1 1.3E-04 0.12 1.2E-04 100 4.2E-03 - [8] [3] Camborne II 250 80 15 0.6 3.6E-04 1.20 2.5E-04 240 3.6E-03 263 [13] Hijiori 90 250 4 0.6 1.2E-05 0.04 7.9E-05 90 1.1E-02 65 [35-38] Ogachi 80 240 1.7 8 1.2E-04 0.03 7.2E-05 289 4.3E-02 - [39] Soultz I 450 170 25 0.23 1.6E-04 1.39 2.6E-04 6000 2.9E-02 - [20] Soultz II 600 200 12 0.25 1.4E-04 1.1 2.4E-04 11500 3.1E-02 - [34]

**Table 2.** Operation parameters and results of circulation tests in major HDR-systems, aG: geometrical distance between inlet and outlet, T0: rock temperature, QEX: production flow rate, I: impedance (ratio of pressure difference between inlet and outlet and production flow rate), μ: viscosity of produced water; T: transmissibility, bH: hydraulic aperture, Vb: tracer break-through volume, bT: aperture calculated from tracer break-through volume, aH: inlet to

Well test analysis of post-stimulation injection or production tests point into the same direction. All post-stimulation hydraulic tests in Soultz and in Basel showed very long fracture linear or bilinear flow periods often persisting over 10 hours or more. Such long periods need a very long start line for the flow. In cases where several hundred Meter long axial fractures are present (Soultz II well GPK4 and probably Basel) these long starting lines may be identical with the trace of these fractures along the borehole wall. In other cases they are most likely identical with the long channel-like features of the seismic clouds. Theses channels play most likely a dominant role for the flow distribution in the stimulated fractures. Channels may also exist on the scale of joints and may result in a highly anisotropic transmissibility of the large

outlet distance calculated from observed thermal draw-down.

110 Effective and Sustainable Hydraulic Fracturing

scale fracture.

fracture propagation nor by the shearing of existing fractures or faults.

It is obvious that the actual EGS-concept is inconsistent with almost all observations and results described in the last chapter. The main reason for this inconsistency is most likely the wrong model for the granite underlying this concept. It´s basic assumption is that the granite due to the presence of joints has to be considered as a discontinuum (figure 12) and can therefore be regarded a as coulomb-material. Accordingly the coulomb friction failure criterion is the obvious choice for the stimulation process. A more realistic model (figure 12) however considers the granite as a continuum on the scale of joints and as a discontinuum on the scale of faults or fracture zones [41, 42].

**Figure 12.** Conceptual models for the granite. Left: granite as a discontinuum on the scale of joints [13], right: granite as a discontinuum on the scale of faults or fracture zones and as a continuum on the scale of joints [40, 41].

The Coulomb failure criterion is therefore applicable only on the scale of faults or fracture zones whereas on the scale of joints hydraulic stimulation needs a failure mechanism that includes the formation of new fracture surface like the classical hydraulic fracturing models. Tensile fracture models are however unable to explain the onset of the failure process at a pressure far below the minimum principal stress. Furthermore they are hardly consistent with the intense seismicity and the source mechanism of the seismic events. For these reasons a new model is required that combines tensile fracture propagation with the shearing of natural discontinuities. The most obvious choice is the wing-crack model (figure 13).

**Figure 13.** Wing-crack model, left: onset of shearing, middle: wing initiation, right: wing propagation.

The formation of wing-cracks is one of the micro-mechanisms discussed in material science to explain the inelastic behavior and failure of brittle material under compression. The basic observation is that fractures of finite length failing in shear will not propagate along their own plane but will form tensile wing-fractures (figure 13). Referring to results of [42] Lehner & Kachanow [43] stated that the wings start to grow at an angle of 70° to the plane of the initial shear fracture and gradually turn into the direction of the maximum principal stress. Intro‐ ducing the parameter τex which is the part of the shear stress exceeding the Coulomb friction failure line (figure 13) the criterion for the initiation of the wings can be written as [42]:

$$
\pi\_{ex} = \frac{\sqrt{3}\kappa\_{ic}}{2\sqrt{\pi L\_{\text{max}}}} \tag{4}
$$

With L: length of the wings, σ1, σ3: maximum and minimum principal stress respectively, p: fluid pressure in the wing-crack, Ф: angle between the normal of the joint and the maximum principal stress. For wing-cracks of the scale of joints or bigger the terms containing fracture

This formula shows that wing propagation is stable as long as the fluid pressure is smaller than the minimum principal stress and that the wing length can become long in comparison to L0 only when the fluid pressure approaches σ3. This means, in competent granite (with a low density of joints) large scale wings (in relation to the size of the joints) can only develop at a pressure close to the frac-extension pressure of conventional tensile fractures. In fracture zones where the joint density is high the situation is different (figure 14). Here the wings of the joint being sheared first may connect to the next pair of joints soon after their initiation. The fluid-pressure required for this is presumably not much higher than the pressure for wing initiation. When this pressure is maintained a through-going series of joints and wing-cracks can develop. This series is acting as one large scale shear fracture. Correspondingly much larger wings can emerge from the end of this series than from the ends of a single joint when

**Figure 14.** Wing-crack mechanism in a fracture zone, left: wings of the first sheared joint connect to the next pair of joints, middle: chain of wing-cracks reach the boundaries of the fracture zone, right: large scale wings emerge from

*<sup>π</sup>*(*σ*<sup>3</sup> - *<sup>p</sup>*) (6)

EGS — Goodbye or Back to the Future http://dx.doi.org/10.5772/56458 113

toughness KIC can be neglected and the equation reduces to:

exposed to the same pressure.

the boundary of the fracture zone.

*L*

*<sup>L</sup>* <sup>0</sup> <sup>=</sup> <sup>2</sup>*τexcos<sup>ϕ</sup>*

With τex = τ-μ(σn-p), τ: shear stress on the fracture, σn: normal stress on the wings, p: fluid pressure in the wings, KIC: fracture toughness of the rock, L0: half length of the initial fracture (figure 13). Inserting typical values for the fracture toughness of granite KIC = 1.5 MPa m1/2 and for the joint half length L0 = 5 m one gets τex = 0.33 MPa. This low value indicates that joints being sheared will inevitably develop wings.

In the simplest approximation of long straight wings parallel to the axis of the maximum principal stress propagation of the wings of a stress driven fracture is governed by the following equation [24]:

$$\sqrt{\frac{L}{L\_{\
u}}} = \frac{K\_{\Lambda^{\circ}}}{2(\sigma\_{3} \cdot p)\sqrt{\pi L\_{\
u}}\_{\
u}} + \sqrt{\left(\frac{K\_{\Lambda^{\circ}}}{2(\sigma\_{3} \cdot p)\sqrt{\pi L\_{\
u}}\_{\
u}}\right)^{2} + \frac{2\tau\_{ex}\cos\phi}{\pi\{\sigma\_{3} \cdot p\}}}\tag{5}$$

With L: length of the wings, σ1, σ3: maximum and minimum principal stress respectively, p: fluid pressure in the wing-crack, Ф: angle between the normal of the joint and the maximum principal stress. For wing-cracks of the scale of joints or bigger the terms containing fracture toughness KIC can be neglected and the equation reduces to:

$$\frac{L}{\frac{L}{L\_{\text{-}0}}} = \frac{2\pi\_{\text{ex}}\cos\phi}{\pi\{\sigma\_3 \cdot p\}}\tag{6}$$

This formula shows that wing propagation is stable as long as the fluid pressure is smaller than the minimum principal stress and that the wing length can become long in comparison to L0 only when the fluid pressure approaches σ3. This means, in competent granite (with a low density of joints) large scale wings (in relation to the size of the joints) can only develop at a pressure close to the frac-extension pressure of conventional tensile fractures. In fracture zones where the joint density is high the situation is different (figure 14). Here the wings of the joint being sheared first may connect to the next pair of joints soon after their initiation. The fluid-pressure required for this is presumably not much higher than the pressure for wing initiation. When this pressure is maintained a through-going series of joints and wing-cracks can develop. This series is acting as one large scale shear fracture. Correspondingly much larger wings can emerge from the end of this series than from the ends of a single joint when exposed to the same pressure.

**Figure 13.** Wing-crack model, left: onset of shearing, middle: wing initiation, right: wing propagation.

*<sup>τ</sup>ex* <sup>=</sup> <sup>3</sup>*<sup>K</sup> IC* 2 *πL* <sup>0</sup>

being sheared will inevitably develop wings.

*L*

*<sup>L</sup>* <sup>0</sup> <sup>=</sup> *KIC*

2(*σ*<sup>3</sup> - *p*) *πL* <sup>0</sup>

following equation [24]:

112 Effective and Sustainable Hydraulic Fracturing

With τex = τ-μ(σn-p), τ: shear stress on the fracture, σn: normal stress on the wings, p: fluid pressure in the wings, KIC: fracture toughness of the rock, L0: half length of the initial fracture (figure 13). Inserting typical values for the fracture toughness of granite KIC = 1.5 MPa m1/2 and for the joint half length L0 = 5 m one gets τex = 0.33 MPa. This low value indicates that joints

In the simplest approximation of long straight wings parallel to the axis of the maximum principal stress propagation of the wings of a stress driven fracture is governed by the

<sup>+</sup> ( *KIC*

2(*σ*<sup>3</sup> - *p*) *πL* <sup>0</sup>

) 2 + 2*τexcosϕ*

*<sup>π</sup>*(*σ*<sup>3</sup> - *<sup>p</sup>*) (5)

(4)

The formation of wing-cracks is one of the micro-mechanisms discussed in material science to explain the inelastic behavior and failure of brittle material under compression. The basic observation is that fractures of finite length failing in shear will not propagate along their own plane but will form tensile wing-fractures (figure 13). Referring to results of [42] Lehner & Kachanow [43] stated that the wings start to grow at an angle of 70° to the plane of the initial shear fracture and gradually turn into the direction of the maximum principal stress. Intro‐ ducing the parameter τex which is the part of the shear stress exceeding the Coulomb friction failure line (figure 13) the criterion for the initiation of the wings can be written as [42]:

**Figure 14.** Wing-crack mechanism in a fracture zone, left: wings of the first sheared joint connect to the next pair of joints, middle: chain of wing-cracks reach the boundaries of the fracture zone, right: large scale wings emerge from the boundary of the fracture zone.

Before the wings become very long they will probably grow in height, i.e. in the direction perpendicular to the 2-D wing-crack model of figure 14. This can happen at any fluid-pressure higher than the pressure given by equation 4. This is a plausible explanation for the evolution of the channel-like features in the seismic clouds preceding their lateral propagation. It seems that stimulating at low flow rates as in Ogachi and in the starting periods of Basel and Soultz accentuate the formation of these channels but channeling was indicated also on other sites. For normal and reverse stress conditions these channels were predominantly horizontal or sub-horizontal (Fenton Hill, Hijiori, Ogachi, and Cooper Basin). For strike slip conditions they were pre-dominantly vertical or sub-vertical (Soultz, Basel, Camborne). In the Camborne system extremely long vertical channels developed during a long term circulation test. This demonstrates that the wing-crack mechanism may lead to uncontrolled large scale fracture growth during the operation of the EGS-Systems at a fluid pressure significantly lower than the minimum principal stress.

Neglecting fracture toughness the normalized shear displacement at the root of the wings is approximately given by:

$$\frac{\mathbf{U}}{\mathbf{E}\_{\text{o}}} = \frac{\mathbf{4} \cdot \mathbf{r}\_{ex}}{\mathbf{E}} \tag{7}$$

not surprising. Since they are tensile fractures they may only show up where they intersect

EGS — Goodbye or Back to the Future http://dx.doi.org/10.5772/56458 115

EGS-Project Basel

1000 m

**Figure 15.** Processed (collapsed) seismic cloud of the stimulation test in Basel, view about 10° from vertical toward W, blue arrows: direction of the maximum horizontal stress, collapsing performed by Q-Con GmbH [44], stress direction

Generally the large scale wing-crack model delivers a plausible explanations for almost all observations described in the previous chapters in particular for: the onset of fracture propa‐ gation at a fluid pressure much lower than the minimum principal stress, the high intensity and mechanism of induced seismicity, the occurrence of channel-like features in the seismic clouds, the long lasting fracture linear or bilinear flow periods during post-stimulation well tests, the occurrence of high magnitude after-shocks, the large fracture apertures derived from tracer break-through volumes and from the ratio of fracture area and injected volume. It also explains the striking discrepancy between the only moderate fracture transmissibility and the large apertures. It is clear that a more rigorous study requires 3-D-wing-crack models since the 2-D-model neglects the vertical stress gradients and it may be due to these stress gradients

prominent natural discontinuities and cause them to shear.

N

that some of the seismic clouds showed twisted wings.

[30].

With E': Young´s modulus for plane strain conditions. This equation is a good approximation of a formula given in [43] for the straight wing model. The normalized aperture of the wings at their root is given by:

$$\frac{\frac{\partial}{\partial t}}{\frac{\partial}{\partial t}} = \frac{4\pi\varepsilon\_m \cos\phi}{E^\*} \tag{8}$$

These formulas were applied to the stimulated fracture in Basel, whose seismic cloud showed a remarkably clear wing-crack shape (figure 15) after processing source location data with the so called "collapsing method" [44].

The results of the wing-crack model agree quite well with the observations when an angle of Ф = 80° is assumed. This direction is within the uncertainty-limits of the stress data [30]. The calculated ratio of L/L0 ≈ 2 agrees quite well with the observed data (figures 15, 16). Similarly yields the wing-crack model the same large aperture values as derived from the tracer tests and from the ratio of injected volume and seismic area. For comparison: Static tensile fracture models like the 2-D Griffith fracture would yield average apertures of about 1 mm. One of the most striking results is the very high displacement of 100 mm at the root of the wings (for Ф = 80°). This easily explains the high number of seismic events and the occurrence of high magnitudes in the central part of the seismic cloud of Basel. Interestingly the wings showed up only during the shut-in and flow-back period though they had presumably been formed much earlier. This and the fault plane solutions of the post-fracturing events [30] indicate that the seismic signals of the wings were induced not by forward-sliding but by back-sliding. This behavior can hardly be explained by shearing of a natural fault but is easily explained with the wing-crack model. The low density of seismic sources of the wings as observed in Basel is not surprising. Since they are tensile fractures they may only show up where they intersect prominent natural discontinuities and cause them to shear.

Before the wings become very long they will probably grow in height, i.e. in the direction perpendicular to the 2-D wing-crack model of figure 14. This can happen at any fluid-pressure higher than the pressure given by equation 4. This is a plausible explanation for the evolution of the channel-like features in the seismic clouds preceding their lateral propagation. It seems that stimulating at low flow rates as in Ogachi and in the starting periods of Basel and Soultz accentuate the formation of these channels but channeling was indicated also on other sites. For normal and reverse stress conditions these channels were predominantly horizontal or sub-horizontal (Fenton Hill, Hijiori, Ogachi, and Cooper Basin). For strike slip conditions they were pre-dominantly vertical or sub-vertical (Soultz, Basel, Camborne). In the Camborne system extremely long vertical channels developed during a long term circulation test. This demonstrates that the wing-crack mechanism may lead to uncontrolled large scale fracture growth during the operation of the EGS-Systems at a fluid pressure significantly lower than

Neglecting fracture toughness the normalized shear displacement at the root of the wings is

With E': Young´s modulus for plane strain conditions. This equation is a good approximation of a formula given in [43] for the straight wing model. The normalized aperture of the wings

These formulas were applied to the stimulated fracture in Basel, whose seismic cloud showed a remarkably clear wing-crack shape (figure 15) after processing source location data with the

The results of the wing-crack model agree quite well with the observations when an angle of Ф = 80° is assumed. This direction is within the uncertainty-limits of the stress data [30]. The calculated ratio of L/L0 ≈ 2 agrees quite well with the observed data (figures 15, 16). Similarly yields the wing-crack model the same large aperture values as derived from the tracer tests and from the ratio of injected volume and seismic area. For comparison: Static tensile fracture models like the 2-D Griffith fracture would yield average apertures of about 1 mm. One of the most striking results is the very high displacement of 100 mm at the root of the wings (for Ф = 80°). This easily explains the high number of seismic events and the occurrence of high magnitudes in the central part of the seismic cloud of Basel. Interestingly the wings showed up only during the shut-in and flow-back period though they had presumably been formed much earlier. This and the fault plane solutions of the post-fracturing events [30] indicate that the seismic signals of the wings were induced not by forward-sliding but by back-sliding. This behavior can hardly be explained by shearing of a natural fault but is easily explained with the wing-crack model. The low density of seismic sources of the wings as observed in Basel is

E' (7)

E' (8)

*U <sup>L</sup>* <sup>0</sup> <sup>=</sup> <sup>4</sup> · *<sup>τ</sup>ex*

*b <sup>L</sup>* <sup>0</sup> <sup>=</sup> <sup>4</sup>*τexcos<sup>ϕ</sup>*

the minimum principal stress.

114 Effective and Sustainable Hydraulic Fracturing

approximately given by:

at their root is given by:

so called "collapsing method" [44].

**Figure 15.** Processed (collapsed) seismic cloud of the stimulation test in Basel, view about 10° from vertical toward W, blue arrows: direction of the maximum horizontal stress, collapsing performed by Q-Con GmbH [44], stress direction [30].

Generally the large scale wing-crack model delivers a plausible explanations for almost all observations described in the previous chapters in particular for: the onset of fracture propa‐ gation at a fluid pressure much lower than the minimum principal stress, the high intensity and mechanism of induced seismicity, the occurrence of channel-like features in the seismic clouds, the long lasting fracture linear or bilinear flow periods during post-stimulation well tests, the occurrence of high magnitude after-shocks, the large fracture apertures derived from tracer break-through volumes and from the ratio of fracture area and injected volume. It also explains the striking discrepancy between the only moderate fracture transmissibility and the large apertures. It is clear that a more rigorous study requires 3-D-wing-crack models since the 2-D-model neglects the vertical stress gradients and it may be due to these stress gradients that some of the seismic clouds showed twisted wings.

widening of the joint network. The data rather suggest that generally only one large fracture is formed during massive stimulation tests regardless of the length of the test interval. The formation of these single fractures can well be explained by the wing-crack model. Wing-cracks have a significantly smaller area to volume ratio than tensile fractures of equal size and need therefore larger fluid volumes for an envisaged fracture area. The large shear displacement at the wing roots enables high magnitude seismic events during the propagation period and strong seismic after-shocks by back-sliding. The magnitudes seem to increase with the seismic area and may finally set a limit for the dimensioning of the individual wing-cracks. The poststimulation transmissibility of wing cracks is presumably very heterogeneous and highly anisotropic. Wide open channels may persist at the roots of the main wings and at the roots of smaller wings within the central shear fracture. These channels are presumably oriented perpendicular to the slip direction and are of uttermost importance for the positioning of the second well to avoid thermal short-circuiting. The transmissibility of the fracture areas in between theses channels and of the large wings is much lower but is most likely in the range of 0.1 - 1 d·m thus enabling flow rates in the order of 1 to more than 10 L/s per wing-crack.

EGS — Goodbye or Back to the Future http://dx.doi.org/10.5772/56458 117

Theses findings suggest that the present EGS-concept will never lead to EGS-systems of industrial size and performance. It has to be abandoned and be replaced by a multi-fracture scheme as foreseen in the original Hot-Dry-Rock concept with the main difference that the tensile fractures of this concept have to be replaced by wing-cracks. This requires a more sophisticated design and planning in particular for the positioning, completion and treatment of the second well. Industrial systems of this type require wells being drilled parallel to the axis of the minimum principal stress, i.e. horizontal wells for normal and strike slip stress conditions and vertical wells for reverse faulting stress conditions. An industrial system may consist of about 30 to 40 equidistant fractures connecting two 1km long parallel well sections with a well separation of about 500 m. Systems of these dimensions should operate for at least 25 years at flow rates of 100 L/s, an electric power output between 5 and 10 MW and a pumping power of less than 1 MW. Directional drilling and packer technology have improved signifi‐ cantly during the last three decades and multi-fracture concepts are applied with great success in unconventional gas reservoirs. Though the conditions and requirements in geothermal applications are more demanding in various aspects it seems almost certain that geothermal

multi-fracture-systems of this type can be realized in the near future.

Address all correspondence to: jung.geotherm@googlemail.com

Jung-Geotherm, Isernhagen, Germany

**Author details**

Reinhard Jung\*

**Figure 16.** Calculated diagrams for the start of wing-crack propagation (left) and for the end of stimulation (right) in Basel, top: Mohr-diagram (σ1 = 130 MPa, σ3 = 69.6 MPa at 4600 m depth), middle: normalized wing length L/L0, bot‐ tom: shear displacement U and aperture b at the root of the wings, calculations with equations (6-8), stress data with minor modifications from [30], Φ = 80°.

#### **5. Summary and way forward**

Observations and results of all major EGS-projects leave no doubt, that hydraulic stimulation can not be regarded as merely a pressure diffusion process accompanied by shearing and widening of the joint network. The data rather suggest that generally only one large fracture is formed during massive stimulation tests regardless of the length of the test interval. The formation of these single fractures can well be explained by the wing-crack model. Wing-cracks have a significantly smaller area to volume ratio than tensile fractures of equal size and need therefore larger fluid volumes for an envisaged fracture area. The large shear displacement at the wing roots enables high magnitude seismic events during the propagation period and strong seismic after-shocks by back-sliding. The magnitudes seem to increase with the seismic area and may finally set a limit for the dimensioning of the individual wing-cracks. The poststimulation transmissibility of wing cracks is presumably very heterogeneous and highly anisotropic. Wide open channels may persist at the roots of the main wings and at the roots of smaller wings within the central shear fracture. These channels are presumably oriented perpendicular to the slip direction and are of uttermost importance for the positioning of the second well to avoid thermal short-circuiting. The transmissibility of the fracture areas in between theses channels and of the large wings is much lower but is most likely in the range of 0.1 - 1 d·m thus enabling flow rates in the order of 1 to more than 10 L/s per wing-crack.

Theses findings suggest that the present EGS-concept will never lead to EGS-systems of industrial size and performance. It has to be abandoned and be replaced by a multi-fracture scheme as foreseen in the original Hot-Dry-Rock concept with the main difference that the tensile fractures of this concept have to be replaced by wing-cracks. This requires a more sophisticated design and planning in particular for the positioning, completion and treatment of the second well. Industrial systems of this type require wells being drilled parallel to the axis of the minimum principal stress, i.e. horizontal wells for normal and strike slip stress conditions and vertical wells for reverse faulting stress conditions. An industrial system may consist of about 30 to 40 equidistant fractures connecting two 1km long parallel well sections with a well separation of about 500 m. Systems of these dimensions should operate for at least 25 years at flow rates of 100 L/s, an electric power output between 5 and 10 MW and a pumping power of less than 1 MW. Directional drilling and packer technology have improved signifi‐ cantly during the last three decades and multi-fracture concepts are applied with great success in unconventional gas reservoirs. Though the conditions and requirements in geothermal applications are more demanding in various aspects it seems almost certain that geothermal multi-fracture-systems of this type can be realized in the near future.

#### **Author details**

Reinhard Jung\*

**Figure 16.** Calculated diagrams for the start of wing-crack propagation (left) and for the end of stimulation (right) in Basel, top: Mohr-diagram (σ1 = 130 MPa, σ3 = 69.6 MPa at 4600 m depth), middle: normalized wing length L/L0, bot‐ tom: shear displacement U and aperture b at the root of the wings, calculations with equations (6-8), stress data with

Observations and results of all major EGS-projects leave no doubt, that hydraulic stimulation can not be regarded as merely a pressure diffusion process accompanied by shearing and

minor modifications from [30], Φ = 80°.

116 Effective and Sustainable Hydraulic Fracturing

**5. Summary and way forward**

Address all correspondence to: jung.geotherm@googlemail.com

Jung-Geotherm, Isernhagen, Germany

## **References**

[1] Smith, M. C, Aamodt, R. L, Potter, R. M, & Brown, D. W. Manmade geothermal res‐ ervoirs: Proc. 2nd US Symposium on Geothermal Energy, (1975). San Francisco Cali‐ fornia, , 1781-1787.

[13] Parker, R. H. Overview. In: Parker R.H. ed., Hot Dry Rock geothermal energy, Phase

EGS — Goodbye or Back to the Future http://dx.doi.org/10.5772/56458 119

[14] Baria, R, Green, A. S. P, & Hearn, K. C. Microseismic results. In: Parker R.H. ed., Hot Dry Rock geothermal energy, Phase 2B final report of the Camborne School of Mines

[15] Kappelmeyer, O, Gerard, A, Schloemer, W, Ferrandes, R, Rummel, F, & Benderitter, Y. European HDR Project at Soultz-sous-Forêts: General presentation. In: Geothermal

[16] Sausse, J, & Genter, A. Types of permeable fractures in granite.- Geological Soc., Lon‐ don, Special Publications (2005). doi:10.1144/GLS.SP.2005.240.01.01., 240, 1-14.

[17] Cornet, F. H, Bérard, F. H, & Bourouis, S. How close to failure is a granite rock mass at 5 km depth?. International Journal Rock Mechanics Mining Sciences (2007). , 47-66.

[18] Klee, G, & Rummel, F. Hydrofrac stress data for the European HDR research project test site Soultz-sous-Forêts. International Journal of Rock Mechanics, Mining Science

[19] Jung, R. Hydraulic fracturing and hydraulic testing in the granitic section of borehole

[20] Jung, R. HDR-Projekt Soultz- Erschließung permeabler Risszonen für die Gewinnung geothermischer Energie aus heißen Tiefengesteinen. Schlussbericht zum Forschungs‐ vorhaben 0326690A, Archiv Nr. 118977, Bundesanstalt für Geowissenschaften und

[21] Baria, R, Jung, R, Tischner, T, Teza, D, Baumgärtner, J, Dyer, B, Hettkamp, T, Nich‐ olls, J, Michelet, S, Sanjuan, B, Soma, N, Asanuma, H, & Garnish, J. Creation of a HDR/EGS reservoir at 5000 m depth at the European HDR project.- Proc. 31st Stan‐

[23] Jung, R. Hydraulic in situ investigations of an artificial fracture in the Falkenberg Granite. Int. J. Rock Mech. Sci. & Geomech. Abstr.; (1989). , 26(3), 301-308.

[24] Murphy, H, Keppler, H, & Dash, Z. Does hydraulic fracturing theory work in jointed rock masses?- Geothermal Resources Council, Transactions Oct. (1983). , 7, 461-466.

[25] Camborne School of MinesGeothermal Energy Project. Internal report (1985). , 2-42.

[26] Nedo, F. Y. Summary of Hot Dry Rock Geothermal Power Project. Geothermal Ener‐ gy Technology Dep., New Energy and Ind. Tech. Dev. Org., Tsukuba, Japan; (1997).

[27] Tezuka, K, & Niitsuma, H. Stress estimated using microseismic clusters and its rela‐ tionship to the fracture system of the Hijiori hot dry rock reservoir, Eng. Geol. 56;

[22] Weidler, R. Personal Communication, Geothermeon, Landau, Germany, (2001).

GPK1, Soultz Sous Forêts. Geotherm. Sci. & Tech. (1991). , 3, 149-198.

Energy in Europe, J.C. Bresee ed., Gordon and Breach Science Publ., (1992).

2B final report of the Camborne School of Mines Project, (1989). , 1, 3-38.

Project., (1989). , 2, 682-740.

Rohstoffe, Hannover, (1999).

(2000). , 47-62.

and Geomechanics Abstracts (1993). , 973-976.

ford Geothermal Workshop (2006). Stanford, Cal., US.


[13] Parker, R. H. Overview. In: Parker R.H. ed., Hot Dry Rock geothermal energy, Phase 2B final report of the Camborne School of Mines Project, (1989). , 1, 3-38.

**References**

fornia, , 1781-1787.

118 Effective and Sustainable Hydraulic Fracturing

(LA-11514), 11514.

Bulletin, March (1990).

doi:j.geothermics.(2011). in Press.

LA-UR-85-3334, (1985).

geothermal.inel.gov.(2006).

egon US; 1983., 24-27.

na, Hawaii, Aug. (1985). , 26-30.

[1] Smith, M. C, Aamodt, R. L, Potter, R. M, & Brown, D. W. Manmade geothermal res‐ ervoirs: Proc. 2nd US Symposium on Geothermal Energy, (1975). San Francisco Cali‐

[2] Tester, J. W, Brown, D. W, & Potter, R. M. Hot Dry Rock Geothermal Energy- A new Energy Agenda for the 21st Century. Los Alamos National Lab. Report MS (1989).

[3] Duchane, D, & Brown, D. Hot Dry Rock (HDR) geothermal energy research and de‐

[4] Duchane, D. Hot Dry Rock: A realistic energy option. Geothermal Resources Council

[5] Batchelor, A. S. The creation of Hot Dry Rock systems by combined explosive and hydraulic fracturing. In: proceedings of the International Conference on Geothermal

[6] Cornet, F. H. Experimental investigations of forced fluid flow through a granite rock mass. In: Proceedings of 4th Int. Seminar on the results of EC Geothermal Energy

[7] Evans, K. F, Zappone, A, Kraft, T, Deichmann, N, & Moia, F. A survey of the induced seismic responses to fluid injection in geothermal and CO2 reservoirs in Europe,

[8] Murphy, H. Hot Dry Rock phase II reservoir engineering. Los Alamos Nat. Lab. Rep.

[9] MIT The Future of Geothermal Energy- Impact of Enhanced Geothermal systems (EGS) on the United States in the 21st CenturyIdaho Nat. Lab., Idaho US, http://

[10] Laney, R, Laughlin, A. W, & Aldrich, M. J. Geology and geochemistry of samples from the Los Alamos National Laboratory HDR Well EE-2, Fenton Hill, New Mexico.

[11] Rowley, J. C, Pettitt, R. A, Matsunaga, I, Dreesen, D. S, Nicholson, R. W, & Sinclair, A. R. Hot-Dry-Rock Geothermal reservoir fracturing initial field operations- (1982). Proceedings Geothermal Resources Council 1983 Annual Meeting, Oct. Portland, Or‐

[12] Dreesen, D. S, & Nicholson, R. W. Well completion and operations for the MHF of Fenton Hill HDR Well EE-2. Proceedings Geothermal Resources Council, Kailua-Ko‐

Energy, May 1982. Florence, Italy. BHRA Fluid Eng. Bedford; (1982). , 321-342.

velopment at Fenton Hill, New Mexico. GHC Bulletin (2002). , 12-19.

Demonstration, Florence, Italy, April 27-30, (1989). , 189-204.

Los Alamos Scientific Lab., Los Alamos NM, USA; (1981).


[28] Kaieda, H, Hisatoshi, I, Kenzo, K, Koichi, S, Hiroshi, S, & Koichi, S. Review of Ogachi HDR Project in Japan. Proc. IGA World Geotherm. Congress 2005, Antalya Turkey, April 2005; (2005). , 24-29.

[39] Kenzo, K. Technology of reservoir estimation for Hot Dry Rock geothermal power. Volumetric Estimation of the Ogachi Reservoir by Tracer Test. Denryoku Chuo Ken‐

EGS — Goodbye or Back to the Future http://dx.doi.org/10.5772/56458 121

[40] Genter, A, Dezayes, C, & Gentier, S. Lede´sert B., Sausse´ J. Conceptual fracture mod‐ el at Soultz based on geological data. In: Bundesanstalt für Geowissenschaften und Rohstoffe und den staatlichen Geologischen Diensten in der Bundesrepublik Deutschland (eds.) International Conference 4th HDR Forum, 29-30 Sep. 1998, Stras‐ bourg, France. Geologisches Jahrbuch, Sonderhefte, Heft SE1, Reihe E, Geophysik,

[41] Valley, B. C. The relation between natural fracturing and stress heterogeneities in deep-seated crystalline rocks at Soultz-sous-Forêts (France), PhD thesis. ETH Zürich,

[43] Lehner, F, & Kachanov, M. On modelling of "winged" cracks forming under com‐

[44] Baisch, S, & Vörös, R. Personal Communication. Q-Con, Bad Bergzabern, Germany;

[42] Cotterell, B, & Rice, J. R. International Journal of Fracture 16; (1980). , 155-169.

pression. International Journal of Fracture 77; (1996). RR75., 69.

kyujo Abiko Kenkyujo Hokoku, (2000). p.(U99018)

(2002). , 93-102.

(2007). (17385)

(2007).


[39] Kenzo, K. Technology of reservoir estimation for Hot Dry Rock geothermal power. Volumetric Estimation of the Ogachi Reservoir by Tracer Test. Denryoku Chuo Ken‐ kyujo Abiko Kenkyujo Hokoku, (2000). p.(U99018)

[28] Kaieda, H, Hisatoshi, I, Kenzo, K, Koichi, S, Hiroshi, S, & Koichi, S. Review of Ogachi HDR Project in Japan. Proc. IGA World Geotherm. Congress 2005, Antalya Turkey,

[29] Jones, R, Beauce, A, Fabriol, H, & Dyers, B. Imaging induced microseismicity during the 1993 injection test at Soultz-sous-Forêts France, Proc. IGA World Geothermal

[30] Häring, M. O, Schanz, U, Ladner, F, & Dyer, B. C. Characterization of the Basel1 En‐ hanced Geothermal System. Geothermics; (2008). doi:10.1016/j.geothermics.

[31] Evans, K. F, Moriya, H, Niitsuma, H, Jones, R. H, Phillips, W. S, Genter, A, Sausse, J, Jung, R, & Baria, R. Microseismicity and permeability enhancement of hydro-geolog‐ ic structures during massive fluid injections into granite at 3 km depth at the Soultz

[32] Niitsuma, H, Asanuma, H & Jones, R. Induced seismicity, AP 3000 report. In: Baisch, S. (ed.) Deep Heat Mining Basel – Seismic Risk Analysis. Basel, Amt für Umwelt und

[33] Tischner, T, Pfender, M, & Teza, D. Hot Dry Rock Projekt Soultz: Erste Phase der Er‐ stellung einer wissenschaftlichen Pilotanlage. Abschlussbericht zum Vorhaben 0327097, Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit,

[34] Ledingham, P. Circulation results 1983-1986, 7:2 Thermal Model. In: Parker R.H. ed., Hot Dry Rock geothermal energy, Phase 2B final report of the Camborne School of

[35] Matsunaga, I, Yanagisawa, N, Sugita, H, & Tao, H. Reservoir monitoring by tracer testing during a long term circulation test at the Hijiori HDR Site. Proc. 27th Work‐ shop on Geothermal Reservoir Eng., Stanford Univ. Stanford, Jan. (2002). , 28-30. [36] Tenma, N, Yamaguchi, T, Tezuka, K, Oikawa, Y, & Zyvolovski, G. Comparision of heat extraction from production wells in the shallow and deep reservoirs at the Hi‐ jiori test site using FEHM code. Proc. 26. Workshop on Geothermal Reservoir Engi‐

neering, Stanford Uni., January February 1, 2001, SGP-TR-162; (2001). , 29.

Geothermal Reservoir Eng., Stanford Univ. Stanford, Jan. (2001). , 29-31.

voir Eng., Stanford Univ. Stanford, Jan. (2001). , 29-31.

[37] Matsunaga, I, Sugita, H, & Tao, H. Tracer monitoring by a fibre optic fluorometer during a long-term circulation test at the Hijiori HDR Site. Proc. 26. Workshop on

[38] Oikawa, Y, Tenma, N, Yamaguchi, T, Karasawa, H, Egawa, Y, & Yamauchi, T. Heat extraction experiment at Hijiori Test Site. Proc. 26. Workshop on Geothermal Reser‐

HDR site. Geophys. J. Int., (2005). , 2005(160), 388-412.

April 2005; (2005). , 24-29.

120 Effective and Sustainable Hydraulic Fracturing

2008.06.002.

Congress, Florence, Italy; (1995).

Energie; (2009). AP 3000 p1-62.

1.4.Berlin, Germany; (2006). , 2001-31.

Mines Project., (1989). , 1, 390-408.


**Chapter 6**

**Understanding Hydraulic Fracture Growth,**

Additional information is available at the end of the chapter

**Monitoring**

Norm R. Warpinski

**1. Introduction**

tal surety.

reservoir [21].

http://dx.doi.org/10.5772/55974

**Effectiveness, and Safety Through Microseismic**

Microseismic monitoring has become a valuable tool for optimizing stimulations, comple‐ tions, and overall field development, particularly in unconventional reservoirs. This technology was initially rooted in geothermal energy [1,2], but subsequently was used for many years in research projects to understand fracturing in unconventional reservoirs, such as in the Multiwell Experiment [3,4], the M-Site fracture diagnostics laboratory [5-8], the Carthage Cotton Valley fracturing test [9,10], and for other processes, such as drill cuttings injection [11]. It finally reached a level of sophistication and reliability to function as a service technology in the early 21st century [12,13], and many thousands of hydraulic fractures have been monitored since that time. In addition to providing a "window" into the subsurface for fracture optimization and control, the large amount of microseismic data that has been gathered provides a significant database that can be used for environmen‐

Microseismicity occurs because of geomechanical changes to the reservoir as a result of the fracturing process [14,15], and detection and location of these "events" provides a methodol‐ ogy to monitor fracture growth patterns and overall dimensions. One of the curious features of microseismic technology is that no one has ever seen the slippage plane of a microseism that was induced by a hydraulic fracture. As a result, the understanding of microseismicity has been through a down-scaling of earthquake seismology [16], examination of fracture behav‐ iour in minebacks [17,18], comparisons with rock bursts and laboratory acoustic emissions [19,20], and geomechanics considerations of the way in which hydraulic fractures perturb a

> © 2013 Warpinski; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

> © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.
