3. Case study: the Geysers

Hutchings et al. [3] analyzed <sup>a</sup> ˜5.5 km<sup>3</sup> volume at the Northwest Geysers area (Figure 1a). Seven injection wells were in operation in this volume during the

period of the study (Figure 1b). The known locations of induced events, the timing, rate, and location of water injection, and the location of geologic alterations at the bottom of injection wells provide a test of tomography and a rock physics approach to reservoir property analysis. The study is divided into two time periods, approximately 42 days before and 31 days after injection rates changed significantly at several of the wells. Figure 1 shows microearthquake locations, recording station locations, and wells utilized in the study.

The study area is within a portion of The Geysers geothermal field that has a high temperature reservoir (HTR, with temperatures up to 400<sup>o</sup> C) that underlies a normal temperature reservoir (NTR, with temperatures about 260<sup>o</sup> C) shown in Figure 2 [44]. The HTR extends downward from about �2 km below mean sea level (bmsl) in the southwest to �3 km bmsl in the northeast. The NTR reservoir extends between �1.5 and �2 km bmsl [44]. The rocks are composed of relatively permeable greywacke in the NTR and of low-permeability, thermally altered greywacke in the HTR.

We separated data into two recording time periods and performed tomography, the first from 1 September to 8 October 2011 and the second from 9 October to 11 November 2011. We refer to these as period 1 and period 2 in the text. Tomography was performed with 23 recording stations, about one station per 1.3 km<sup>2</sup> , and with 378 and 369 earthquakes for the two time periods. The first time period is for 42 days before and the second is for 31 days after injection rates changed significantly at several of the wells.

Figures 3 and 4 show Vs and Vp tomography results for a cross section that passes through the bottom of wells WH34 and Prati-32 along with a cross section that passes through the bottom of wells Prati-9 and Prati-54. These are shown as AA<sup>0</sup> and BB<sup>0</sup> in Figure 1. Also shown is the resolution value for diagonal of the resolution matrix for the tomography. Figure 3a shows the thickening of the Greywacke seen in Figure 2. There is very little change in the Vp tomography

#### Figure 1.

(a) Regional setting and the location of the study area (white diamond) and locations of major faults. (b) Map view of study area—recording stations in blue squares, earthquakes from first period in magenta dots and those during the second period in black dots, wells in red (red square is top of well), and the cross section locations AA<sup>0</sup> , BB<sup>0</sup> , CC<sup>0</sup> , and DD<sup>0</sup> discussed in text. The study area is rotated 39<sup>o</sup> clockwise, so CC<sup>0</sup> is aligned almost due north.

Rock Physics Interpretation of Tomographic Solutions for Geothermal Reservoir Properties DOI: http://dx.doi.org/10.5772/intechopen.81226

#### Figure 3.

(a) Vp tomography results for period 1 along cross sections AA<sup>0</sup> and BB<sup>0</sup> shown in Figure 1b; (b) changes in Vp results for period 2, i.e., ΔVp; (c) resolution for period 1; (d) resolution for period 2.

results for the second time period. The red dots at the bottom of Prati-32 are location of the first events that occurred after injection started (located with the tomography results from the first time period). The black squares are the steam entry points. That the first few events occur near the wells and near the

Figure 4.

(a) Vs tomography results for period 1 along cross sections AA<sup>0</sup> and BB<sup>0</sup> shown in Figure 1b; (b) changes in Vs results for period 2, i.e., ΔVs; (c) resolution for period 1; (d) resolution for period 2.

#### Figure 5.

(a) Q p tomography results for period 1 along cross sections AA' and BB' shown in Figure 1b; (b) results for ΔQ p for period 2, Q p scale on left; (c) resolution for period 1; (d) resolution for period 2. Resolution scale is for diagonal of resolution matrix, where red is greater than 0.1 and blue is less than 0.01.

Rock Physics Interpretation of Tomographic Solutions for Geothermal Reservoir Properties DOI: http://dx.doi.org/10.5772/intechopen.81226

steam entry points (and presumably the water release points) and validates the accuracy of the earthquake locations, and subsequently the tomography results.

From Figure 4a, there are relatively high Vs anomalies below WH34, Prati-9 and possibly Prati-54 (extreme lower right) with no anomaly below Prati-32. These anomalies indicate about a 20% increase in Vs. Figure 4b shows the change in tomography results for Vs during period 2. The anomaly below Prati-9 shows a further increase in Vs and a new anomaly appeared below Prati-32. The anomalies below WH34 and Prati-54 have remained unchanged. The high velocity anomalies below WH34, Prati-32 and Prati-9 extend from the bottom of the wells and are not randomly occurring; we conclude they are not artifacts. Seismicity is distributed around the wells and throughout the deeper portions of the volume (>˜1.0 km). The resolution in the anomalous areas below the wells is in the range we consider acceptable for our purposes. The new anomaly below Prati-32 during period 2 demonstrates that temporal changes in reservoir properties can be observed over at least a month.

Figures 5 and 6 show Q p and Qs tomography results for the same cross sections as for Figures 3 and 4. The value for the diagonal of the resolution matrix is also shown. Figures 5b and 6b show the changes in Q p and Qs for the second time period, respectively. It is apparent from comparing the background Q values obtained in the inversion for the two time periods that the tomography has only significantly changed around the well bottoms. We conclude that the anomalies at the bottom of wells WH34 and Prati-9 are significant. Q p increased and Qs diminished considerably. The Q anomalies envelop the base of the wells and are not

#### Figure 6.

(a) Qs tomography results for period 1 along cross sections AA' and BB' shown in Figure 1b; (b) results for ΔQs for period 2, Qs scale on left; (c) resolution for period 1; (d) resolution for period 2. Resolution scale is for diagonal of resolution matrix, where red is greater than 0.1 and blue is less than 0.01.

located exactly where the Vp and Vs anomalies are observed. No other anomalies are apparent in the results. The anomaly at the bottom of Prati-9 increased in size and the anomaly below WH34 remained unchanged. The background values remained unchanged for the two time periods, so we are fairly confident the anomalies are real and the change below Prati-9 is real.

### 3.1 Interpretation

Our primary interpretation is that the anomalies we observe are the result of cold water injected into hot material. The wells below which no alteration was observed, and no earthquakes, were Prati-29, Prati-54, OF87A, OF51A and Prati-54. These are located at shallower depths and at cooler temperatures than wells WH34, Prati-32, and Prati-9, where alterations were observed and which are within the hot deep zone. Possible alterations below Prati-54 and OF51A exist at depths far enough below the wells to be located in the hot zone. These observations support our hypothesis that the alterations we observe are a result of cold water injected into very hot geology, which also causes earthquakes. Further, injected water has percolated down to very hot geology. Thus, water responding to gravity reaches as deep as 1.5 km below the wells (and possibly 4.5 km below Prati-54), causing the anomalies.

We further hypothesize that injection cools rock near the well bottoms, which stays cool and saturated as injection continues. Cooling near the well bottoms generates tensile cracks and subsequently micro-seismicity. As water turns to steam, a pressure front triggers more earthquakes away from the wells. However, there is not a concentration of seismicity in the high Vs anomalies, but throughout the deep zone, suggesting that the hot deep geology is not ductile enough to be aseismic.

' In addition to shear modulus, lambda also decreases (not shown), offsetting the increase in shear modulus so the change in Vp is not pronounced. Density does not increase because the fluid turns to steam. Poisson s ratio gets extremely low (not ' shown), mostly due to the significant increase in Vs with little change in Vp. Young s modulus is high and bulk modulus is low (not shown), which we interpret ' as pores filled with steam, but in other portions of the surrounding deep zone, Young s modulus is high and bulk modulus is normal, which we attribute to normal ' rock properties. In Eq. (6), lambda decreases in value by ˜Vs<sup>2</sup> , so a low value of lambda is not surprising. In Eq. (9), bulk modulus is similarly reduced by a factor <sup>s</sup> modulus is proportional to <sup>+</sup>Vs<sup>2</sup> proportional to ˜Vs<sup>2</sup> . In Eqs. (7) and (8), Young , so its increase is also not surprising.

There are high Q p and low Qs anomalies at the bottom of wells WH34, Prati-32 and Prati-9. The cold water in hot material causes fracturing and a significant increase in seismicity in addition to saturation (initially, before the water turns to steam). If a region is partially saturated, one would expect both intrinsic and extrinsic Q to be low, which could account for the low Qs anomaly. But the high Q p anomaly poses several issues. If material is fully saturated, intrinsic Q would be high, as there would be no movement between pores. However, Qs would still reflect the fractures because Vs is relatively unchanged by the presence of water. Q p might be high due to high intrinsic Q and high bulk modulus, so perhaps fractures are not reflected in Q p, meaning extrinsic Q p would also be high. Furthermore, the particle motion of shear waves is orthogonal to the compressional motion of the P-waves, alignment of fractures may also have an effect. Fractures parallel to P-wave particle motion would not alter Q and fractures perpendicular to shear-wave particle motion perpendicular to fractures would alter Q p. Since our

Rock Physics Interpretation of Tomographic Solutions for Geothermal Reservoir Properties DOI: http://dx.doi.org/10.5772/intechopen.81226

tomography does not differentiate between the geometry of attenuation types, we cannot say for sure which is occurring. There are some studies that support the difference between Q p and Qs under saturation conditions.

Tokzoz et al. [45] and Johnston and Tokzoz [46] both show that under dry conditions Q p � Qs, but under full saturation Qs < Q p, agreeing with our observations. This occurs under pressure but at ultrasonic frequencies. DeVilbiss-Munoz [47] shows that Q p and Qs increases significantly as water turns to steam and Mavkov and Nur [48] show Qs increases relative to Q p as saturation increases. Neither of these are consistent with our interpretation.

## 3.2 Reservoir analysis

We examine observations of normal temperature reservoir depths in the greywacke, from �1 to 2.5 km bmsl. The top of the steam/liquid boundary is at about 1 km bmsl and deepens to the southeast (Figure 2). Lowenstern and Janik [49] point out that the northwest portion of The Geysers has little condensed liquid and contains primarily steam-filled pores. Wells Prati-29 and OF87A-11 are shallower than the other wells in this study and were drilled into the greywacke. OF51A-11 and Prati-54 are deeper, but are located in the eastern portion of the field where the greywacke deepens, so their temperature is comparable to the shallower wells and is included in this discussion. Interestingly, no anomalies in our eight attributes are observed at the bottom of these wells. Temperatures are near 240°C and are apparently too low to induce the effects seen at the other wells drilled into the thermally altered greywacke with temperatures near 400°C or higher.

To identify anomalies at reservoir depths, we used the tomography results for the second time period, which used the results from the first time period as a starting model. Results in the depth range of interest here did not change significantly for the second time period (Figures 3–6). We examined values of the eight parameters for voxels that are 1 � 1 km laterally and 0.75 km in depth throughout the study volume. Each voxel is identified by whether the of majority of the volume visually shows high, low or no anomaly. Values are relative to what would be expected for normal geology at comparable depth and resolution. These observations are put into rows of Table 1, where one row is for each voxel; there are fifty rows for our approximately 5 � 5 km study area.

Figure 1 shows the surface projection of the location of the volumes, two volumes for each ID number. The ID numbers are listed in the first row of Table 1. The second column identifies the center of the voxel in kilometers relative to the origin for X and Y. Regarding the eight attributes, "0" indicates no anomaly, "+" a positive and "–" a negative anomaly. Generally, an anomaly is identified as being at least plus or minus 20% of average values at the same depth and geology. The final column provides a rock physics interpretation based on the eight conditions described in the introduction, which are derived from the basic principles outlined in Section 2. Zero change across the row indicates standard reservoir conditions, which is likely greywacke with steam-filled pores. It is assumed that there is not strict compliance with the eight descriptions in Section 2.8—for example, a voxel with a "0" value in the table may still show a slight change.

The anomalies identified in Table 1 can be seen in some of the cross sections. Figure 1 cross section CC<sup>0</sup> is aligned almost north-south, so cross section AA<sup>0</sup> represents the western portion and BB<sup>0</sup> represents the eastern portion of the study volume; the upper part of CC<sup>0</sup> represents the northern part and the bottom part of CC<sup>0</sup> represents the southern part through the middle of the study area.

### Applied Geophysics with Case Studies on Environmental, Exploration and Engineering Geophysics



Rock Physics Interpretation of Tomographic Solutions for Geothermal Reservoir Properties DOI: http://dx.doi.org/10.5772/intechopen.81226

#### Table 1.

Location of quadrants where tomography attributes are examined.

## 4. Discussion and conclusions

We analyze eight attributes tomographic images obtained from tomographic images: isotropic velocity (Vp and Vs), attenuation (Q p and Qs), and derived elastic moduli (lambda, bulk and Young's) and Poisson's ratio, in addition to earthquake locations. The known locations of induced events, the timing, rate, and location of water injection, and the location of geologic alterations at the bottom of injection wells provide a test of this rock physics approach to reservoir property analysis. We outline rock physics principles that can be used to interpret reservoir properties from these observations. We demonstrate that using a relatively high density of stations and examining anomalies, we obtain results in a shorter time period, with higher accuracy, and with fewer earthquakes than is typical for reservoir studies. We also apply a systematic rock physics evaluation of 50 1 km<sup>3</sup> volumes at reservoir depths and demonstrate the ability to identify reservoir properties. In the deeper portion of the volume (near the well bases, below the existing reservoir), seven of the eight attributes show significant effects of cold water injected into hot material and variations over a two-month time span. The results also suggest water is penetrating as deep as 1.5 km and possibly 4.5 km below the wells, even though temperatures reach at least 400°C in the country rock. This causes an increase in shear

modulus due to cooling, however, due to the temperature, the water quickly turns to steam.

We consider explanations for the relatively high Vs estimates in the hot deep zone exhibited in Figure 3. Noted earlier, these anomalous regions occur at depths comparable to or deeper than the well termination depths within the HTR (Figure 2). This is where the natural reservoir temperatures increase from a relatively homogenous 240°C in the NTR (˜900–1800 m bmsl) to at least 400°C, measured at the base of Prati-32 (2672 m bmsl), the deepest wellbore [44]. The pockets of high Vs observed in period 1 appear to spatially correspond with wells where water was actively being injected. Furthermore, during period 2, these anomalies intensified only below well bores in which the injection rate increased substantially relative to period 1 (Prati-32, Prati-9, Figure 3), including the unambiguous appearance of a new anomaly below Prati-32 associated with the injection of water into a previously undisturbed region of the HTR. The enhanced geothermal system (EGS) demonstration at Prati-32 is therefore an exemplary scenario for considering likely mechanisms to account for the observed evolution of higher Vs zones in the HTR, requiring changes in the physical properties of the reservoir material in the range of 30% decrease in bulk density or a 45% increase in shear modulus.
