**1. Introduction**

Hydrothermal resources at relatively shallow depth used today for geothermal power produc‐ tion are just pinpoints on a map of global scale. By far the biggest resource of geothermal energy is the crystalline basement in regions with normal to slightly above normal temperature gradients.Althoughthecrystallinebasementisnot completelyimpermeableduetothepresence of open fissures, fractures, or faults its overall permeability is generally far too low to achieve and maintain production flow rates sufficient for geothermal power production. The majority ofthe crystallinebasementmaythereforebe includedamongthe "petrothermalresources".The basicconceptofHDR-(Hot-Dry-Rock)orEGS-technology(EnhancedGeothermalSystems)thus consists of creating or enhancing large fracture surfaces in the crystalline basement in order to hydraulically connect two or several boreholes. During operation cold water injected in one of the boreholes heats up to rock temperature while circulating through the fracture system and is produced in the second well. To prevent boiling an overpressure is maintained in the geother‐ mal loop. Steam for power generation is produced in a secondary loop.

Depending on drilling depth (usually > 3 km) and temperature (usually > 150 °C) a doublet system of commercial size will operate at flow rates between 50 and 100 L/s and produce an electric power of 3 - 10 MWe. To ensure a service life of at least 25 years a separation distance of at least 0.5 to 2 km between the boreholes at depth and a total fracture surface area of 5 to 10 km2 is required. The volume of rock to be accessed by the fracture system has to be in the order of 0.1 – 0.3 km³. Due to the high flow velocities in the fractures especially near the injection and the production borehole the flow impedance of the fracture system (difference between inlet and outlet pressure devided by the outlet flow rate) is critical for the performance of the system. For energetic and economic reasons should not exceed 0.1 MPa s/L.

Basically two concepts had been designed and tested during the 40 years of HDR-research. In the beginning the crystalline basement at great depth had been regarded as an intact almost impermeable rock mass. It seemed therefore necessary to create artificial flow paths by means of hydraulic fracturing. The original concept (HDR-concept) [1-4] proposed by a group of the Los Alamos National Laboratory in the early 1970th consists of a doublet of deviated boreholes. The boreholes are drilled parallel to the azimuth of the least compressive principal stress and are connected by a set of large parallel fractures created during hydraulic fracturing tests in insolated borehole sections. These tensile fractures are oriented perpendicular to the least compressive principal stress.

The second concept (EGS-concept) promoted mainly by the Camborne School of Mines [5] and the University of Paris [6] is based on the observation of numerous natural fractures (joints and faults) even at great depth. The crystalline basement was therefore regarded as a broken material (discontinuum) and the idea was to shear and widen the natural fracture network by massive water injection in long uncased borehole sections. This process was named hydraulic stimulation. The second borehole is then directionally drilled into the region of enhanced permeability. Since the stimulated region is elongated in the direction of the maximum horizontal stress, the boreholes are aligned in this direction which is 90 ° off the direction of the HDR-concept.

HDR-Concept EGS-Concept

#### **Figure 1.** Basic concepts

demandinginvarious aspects it seems almost certainthatgeothermalmulti-fracture-systemsof

Hydrothermal resources at relatively shallow depth used today for geothermal power produc‐ tion are just pinpoints on a map of global scale. By far the biggest resource of geothermal energy is the crystalline basement in regions with normal to slightly above normal temperature gradients.Althoughthecrystallinebasementisnot completelyimpermeableduetothepresence of open fissures, fractures, or faults its overall permeability is generally far too low to achieve and maintain production flow rates sufficient for geothermal power production. The majority ofthe crystallinebasementmaythereforebe includedamongthe "petrothermalresources".The basicconceptofHDR-(Hot-Dry-Rock)orEGS-technology(EnhancedGeothermalSystems)thus consists of creating or enhancing large fracture surfaces in the crystalline basement in order to hydraulically connect two or several boreholes. During operation cold water injected in one of the boreholes heats up to rock temperature while circulating through the fracture system and is produced in the second well. To prevent boiling an overpressure is maintained in the geother‐

Depending on drilling depth (usually > 3 km) and temperature (usually > 150 °C) a doublet system of commercial size will operate at flow rates between 50 and 100 L/s and produce an electric power of 3 - 10 MWe. To ensure a service life of at least 25 years a separation distance of at least 0.5 to 2 km between the boreholes at depth and a total fracture surface area of 5 to 10 km2 is required. The volume of rock to be accessed by the fracture system has to be in the order of 0.1 – 0.3 km³. Due to the high flow velocities in the fractures especially near the injection and the production borehole the flow impedance of the fracture system (difference between inlet and outlet pressure devided by the outlet flow rate) is critical for the performance of the

Basically two concepts had been designed and tested during the 40 years of HDR-research. In the beginning the crystalline basement at great depth had been regarded as an intact almost impermeable rock mass. It seemed therefore necessary to create artificial flow paths by means of hydraulic fracturing. The original concept (HDR-concept) [1-4] proposed by a group of the Los Alamos National Laboratory in the early 1970th consists of a doublet of deviated boreholes. The boreholes are drilled parallel to the azimuth of the least compressive principal stress and are connected by a set of large parallel fractures created during hydraulic fracturing tests in insolated borehole sections. These tensile fractures are oriented perpendicular to the least

The second concept (EGS-concept) promoted mainly by the Camborne School of Mines [5] and the University of Paris [6] is based on the observation of numerous natural fractures (joints and faults) even at great depth. The crystalline basement was therefore regarded as a broken material (discontinuum) and the idea was to shear and widen the natural fracture network by massive water injection in long uncased borehole sections. This process was named hydraulic

mal loop. Steam for power generation is produced in a secondary loop.

system. For energetic and economic reasons should not exceed 0.1 MPa s/L.

this kind can be realized in the near future.

96 Effective and Sustainable Hydraulic Fracturing

**1. Introduction**

compressive principal stress.

Due to the enormous size of the created or enhanced fracture systems mainly water or brine without proppants were considered as frac-fluids since it seemed too costly or technically impossible to place proppant material over such large areas. All tests in the crystalline basement were accompanied by intense induced seismicity. Localizing and mapping the sources of induced seismicity thus became the most important tool for investigating the evolution of the fracture systems during water injection and most of the projects used this method to define the target for the second or third well. On the other hand induced seismicity has become a major obstacle for further development of the HDR- or EGS-technology since on some locations the population was shocked by events with magnitudes bigger than 3 [7].

The HDR-concept was followed only during the first years of development. Warned by the inability to create vertical fractures in the pioneering Los Alamos project and convinced by the arguments of the EGS-proponents that shearing of natural fractures is the predominant failure mechanism this concept was abandoned [8] and all projects after the 1980th followed the new EGS-concept. The rapid adoption of this concept was to a big part due to its technical simplicity. In particular, it required no high-temperature open hole packers, which created enormous technical problems in the Los-Alamos-Project. The change in the leading concept had severe consequences:

**•** 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.

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.

EGS — Goodbye or Back to the Future http://dx.doi.org/10.5772/56458 99

**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.

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

Re-production from [9].

**2.2. Camborne**


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 wing-cracks.
