Final Report Induced Seismicity

Office Of Basic Energy Sciences U.S. Department Of Energy

DE-FGO3-90ER14152

Paul Segall Department of Geophysics, Stanford University

Stanford, California 94305-22 15 Tel: (41 5) 725-7241 E-Mail: segall@kilauea.stanford.edu

September 18, 1997 P~ 2

1 INTRODUCTION The objective of this project has been to develop a fundamental understanding of seismicity associated with energy production. Earthquakes are known to be associated with oil, gas, and geothermal energy production. The intent is to develop physical models that predict when seismicity is likely to occur, and to determine to what extent these earthquakes can be used to infer conditions within energy reservoirs. Early work focused on earthquakes induced by oil and gas extraction. Just completed research has addressed earthquakes within geothermal fields. such as The Geysers in northern California, as well as the interactions of dilatancy, friction. and shear heating. on the generation of earthquakes. The former has involved modeling t hermo- and poro-elastic effects of geothermal production and water injection. Global Positioning System (GPS) receivers are used to measure deformation associated with geothermal activity, and these measurements along with seismic data are used to test and constrain thermo-mechanical models.

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DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employets, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, process, or service by trade name, trademark, manufac- turer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

DISCLAIMER

Portions of this document may be illegible electronic image products. Images are produced from the best available original document.

2 THE GEYSERS GEOTHERMAL FIELD The Geysers geothermal field in northern California coast rasrges is the largest producing geothermal field in the world. Peak capacity in the mid-198O’s reached 2,000 MW. Since that time power production has declined due to falling steam pressure within the reservoir. The principal strategy for mitigating the effects of pressure loss has been water injection. Roughly 28% of the steam condensate is reinjected. Barker et al., [1992] found that water injection has good potential for allowing additional energy recovery, however in some cases liquid water has breached producing wells making them unsuitable for steam production. Tritium tracers and stable isotopic indicators show that injectate is produced as steam as much as 2 to 3 km away, at sites at lower vapor phase pressure than the injection point. This is interpreted to indicate that the injectate flows considerable distances in the liquid phase before flashing to steam [Barker et al., 19921 .

Significant microseismicity occurs at The Geysers, and essentially all researchers agree that the majority of the earthquakes there are artificially induced. There is little unanimity. however. about the mechanism of the induced seismicity. Empirically, seismicity correlates well with both steam production and liquid injection (Figure 1). There are those that have suggested that water injection is the dominant cause of the microseismicity, whereas others point to effects of steam production. Within these categories, workers have argued that the cause is decrease in temperature. changes in pore-pressure, or to changes in deformation mechanism. Stark [1992] finds a good correlation between the location of injection wells and epicenters of events with depths greater than 4,000 ft. Shallower events apparently do not correlate with injection. Stark [1992] also reports a temporal correlation between the onset of injection in a particular area and an increase in seismicity rate. He notes however, that not all the seismicity at The Geysers is induced by injection, and estimates [Stark, personal coniniunication 19921 that roughly half of the events are production related.

The mechanics of the induced seismicity are not been well understood. Microearthquakes correlated with water injection are interpreted to mark the path of liquid injectate within the reservoir. If this interpretation is correct, induced seismicity could be a valuable indicator of fluid flow within the reservoir. However, it is not clear whether the injection induced seismicity results from increased fluid pressure or whether it is dominantly a thermoelastic effect. Furthermore, it is not always easy to separate “injection induced” earthquakes from those induced by background cooling or pore-pressure declines, and the reservoir scale effects of cooling and pore-pressure decline have never been quantified. Without a clear physical understanding of the mechanism, it is possible that the distribution of microseismicity may be misinterpreted.

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3 Results A. Stress changes within hydrocarbon and geothermal reservoirs.

Earthquakes have been induced by oil and gas production, where pore pressures have decreased, in some cases by several tens of MPa. It has previously been suggested that such earthquakes are caused by poroelastic stressing of crust surrounding the reservoir [ Segall, 19891. Induced earthquakes are also common in geothermal fields, such as The Geysers, where strong correlations between both steam production and condensate injection, and earthquake activity have been observed over the last several decades.

Stress measurements within hydrocarbon reservoirs show that the least horizontal stress decreases with declining reservoir pressure, as predicted by poroe1asticit.y. Approximating the reservoir as a homogeneous ellipsoidal inclusion in an elastic full SFQ c p undergoing spa- tially uniform changes in pore-pressure and temperature, predicts stress changes within the reservoir given by

XAT E,, (1 - 2u) 1

where Q’ is the Biot pore-pressure coefficient, p is shear modulus, u the drained Poisson’s ratio. and X is the coefficient of thermal expansion. The E,, depend on the shape of the reservoir through

where S,,,, are t,he Eshelby shape factors. Fcx circular disk shaped reservoirs, isothermal reduction in pore pressure induces a relative horizontal tension within the reservoir given by

where a3 and a l are the reservoir semi-major axes in the vertical and horizontal directions, respectively. The horizontal stress change immediately above and below the reservoir is

with production inducing increased compression. The simple poro-elastic model predicts stress changes constitant with in situ stress observations. For example, at Ekofisk, we ap- proximate uS/u1 = 150 m / 4 km = 0.04 (/Van den Bark and Thomas, 1980). For Poisson’s ratio in the range of 0.15 to 0.20,the model together with the observed value of Aa,/Ap - 0.8

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yield Q - 1, which is in good agreement with laboratory data reported by Teafel et al.,

Production induced stressing may promote frictional sliding on pre-existing faults. Within the reservoir itself, normal faulting is promoted if the regional stress is extensional and the Biot coefficient is sufficiently large, CY > 0.85 for reasonable coefficients of friction. On the other hand, dilatant fracturing and normal faulting are always promoted, in extensional en- vironments, near the edge of the reservoir or in regions of high pore-pressure gradient. It is suggested such fracturing could enhance fracture permeability in tight rocks adjacent to portions of the reservoir that experience large reductions in pore-pressure due to production. In regional compressional environments, production modestly favors reverse faulting above and below the reservoir.

(1991).

B. Reservoir scale deformation and subsidence. In order to characterize reservoir scale deformation at The Geysers we have resurveyed

leveling networks last occupied in the late 1970's by the U.S.G.S. Water Resources Divi- sion [5lossop and Segall, 1997al. During the 1990's the elevations of a number of survey monuments, across The Geysers geothermal field, were remeasured using GPS receivers. Lofgren et al.; 119811 had determined that the region was subsiding, with a maximum rate of 0.048 f 0.0055 m/yr between 1973 and 1977. To render the leveling and GPS surveys comparable. we transformed them to the same reference frame using the GEOID 96 geoid model. For the period 1977-1996 a maximum subsidence rate of 0.047 rt 0.002 m/yr is de- termined. The subsidence is clearly associated with the actively produced geothermal field (Figure 2) .

Subsidence indicates volume contraction within the reservoir, consistent with both the the poroelastic and thermoelastic deformation mechanisms suggested by Lofgren [ 19811 and Denlinger et al. [1981]. For the case of thermoelastic strain we can relate ~ k k to a reservoir temperature change. AT, via the volumetric coefficient of thermal expansion, a,, as follows

Ekk = @,AT. (5) Values of a, M 3 x have been measured for the reservoir greywackes for tem- peratures of 250 "C. Hence for the previously cited minimum volume strain of 5 x a temperature change of some 17 "C, between 1977 and 1996, would be required. Reservoir wide energy balance considerations limit the average temperature change within the reser- voir to be less than 5.3 "C [Segall and Fitzgerald, 19971. This is insufficient to explain the observed subsidence.

"C

Pore pressure changes AP induce strains of

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where K is the quasi-static bulk moduli of the reservoir and surroundings and K, is the bulk moduli of the mineral grains that make up the rock. The steam pressure reduction within the reservoir fractures between 1977 and 1996 is at most about 2 x lo6 Pa [Barker et al., 19921. For the observed subsidence to be caused by pore-pressure changes requires a quasi- static bulk moduli, K < 3.6 x lo9 Pa, considerably less than moduli inferred from seismic wave speeds. This apparent discrepancy is due to the fractured nature of the reservoir. The observed fracture porosity and estimated fracture stiffness leads to estimates of Kefjectzve of 1.6 - 3.5 x 1Q6 Pa, in good agreement with that inferred from the subsidence.

We have analyzed the subsidence using a very simple model consisting of dist+ilted point centers of contraction, estimating the position and strength of contraction from the measured subsidence. The inferred position of the contraction sources shows a close correlation with the location of steam pressure lows that had developed in The Geysers by 1987 (Figure 3).

C. Thermal and Mechanical Effects of Liquid Injection. Injection induced seismicity has been observed in several geothermal fields around the

world, including The Geysers. A common explanation for such seismicity is the reduction in effective normal stress across faults within the reservoir, due to increase in pore pressure. lye have investigated the role of advective cooling and consequent thermoelastic stressing in triggering earthquakes [Mossop and Segall, 1997bl. We consider a simp'? model of injection of cold water into a hot permeable fault zone. Heat flow within the fault is assumed to be solely by advection and outside the fault to be purely by conduction. Coupling between the temperature and pore pressure fields is neglected. As the wall rock around the fault cools the compressisle stress acting across the fault is reduced. For parameters appropriate to The Geysers vie are able to quantitatively compare the reduction in effective normal stress induced by pore pressure and thermoelasticity. The results are that the thermoelastic reduction in effective normal stress can be greater than that due to pore pressure. The thermoelastic stresses are on the order of 10 MPa for mass flo-ur rates of 10 kg s-', water rock temperature differences of 200°C. fault permeability thickness products. kh, of 10-11m3 and times. t , of a year or so (Figure 4). These results are relatively unaffected by the additional complications introduced by including thermal expansion of the injected fluid or flashing of the injectate to steam.

These results strongly suggest that thermal stressing is a significant factor in triggering Seismicity at The Geysers. This, in turn, implies that microearthquakes can provide a reasonable tracer of subsurface fluid flow. Our results also have importance for any operation

where the reduction in effective stress is a concern, such as well stimulation and stress characterization. For cases where the temperature difference between rock and injectate is small or where the hydraulic conductivity is low then thermoelasticity can be ignored. For larger temperature differences and moderate fracture conductivities thermoelasticity has increasing importance. It provides a mechanism for increasing permeability of already moderately permeable fractures and contributes to any stress test where mass flow rate is high.

D. Fluid rock coupling in the nucleation of earthquakes.

The previous work emphasizes the coupling of fluids and rock at the scales of the reservoir and injector wells. There is also important coupling that occurs on a more fundamental level and is central to the .,ucleation of earthquakes, both natural and induced. vlt: analyze the conditions for unstable slip of a fluid infiltrated fault using a rate and state dependent friction model including the effects of dilatancy and pore compaction [Segall and Rice, 19951. We postulate the existence of a steady state drained porosity of the fault gouge which depends on slip velocity as $ss = $0 + ~log(v/vo) over the range considered, where ZI is sliding velocity and E and vo are constants. Porosity evolves toward steady state over the same distance scaIe. d,, as “state.” This constitutive model predicts changes in porosity upon step changes in sliding velocity that are consistent with the drained experiments of Marone et al. (1990). For undrained loading, the effect of dilatancy is to increase (strengthen) d-r,,/d log ’u by p S s & / ( o - p ) P , where pss is steady state friction, CT and p are fault normal stress and pore pressure. and ,8 is a combination of fluid and pore compressibilities. Assuming E - 1.7 x from fitting the Rlarone et al. data, we find the “dilatancy strengthening” effect to be reasonably consistent with undrained tests conducted by Lockner and Byerlee (1994). Linearized perturbation analysis of a single degree of freedom model in steady sliding shows that unstable slip occurs if the spring stiffness is less than a critical value given by

where a and b are coefficients in the friction law and F(c*) is a function of the model hydraulic diffusivity c* (diffusivity/diffusion length2). In the limit c* -+ 00 F(c*) -+ 0, recovering the drained result of Ruina (1983). In the undrained limit, c* ---f 0, F(c*) ---f 1, so that for sufficiently large E slip is always stable to small perturbations. Under undrained conditions (0 - p ) must exceed E , L L ~ ~ / / ? ( ~ - u) for instabilities to nucleate, even for arbitrarily reduced stiffness. This places constraints on how high the fault zone pore pressure can be, to rationalize the absence of a heat flow anomaly on the San Andreas fault, and still allow earthquakes to nucleate without concommitant fluid transport. For the dilatancy constitutive

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laws examined here, numerical simulations do not exhibit large interseismic increases in fault zone pore pressure. The simulations do, however, exhibit a wide range of interesting behavior including: sustained finite amplitude oscillations near steady state and repeating stick slip events in which the stress drop decreases with decreasing diffusivity, a result of dilatancy strengthening. For some parameter values we observe “aftershock” like events that follow the principal stick-slip event. These aftershocks are noteworthy in that they involve rerupture of the surface due to the interaction of the dilatancy and slip weakening effects rather than to interaction with neighboring portions of the fault. This mechanism may explain aftershocks that appear to be located within zones of high mainshock slip, although poor resolution in mainshock slip distributions can not be ruled out.

4 Reports 1-esulting from BES funding Journal Articles

Segall, P.. J.R. Grasso, and A. Mossop, Poroelastic stressing and induced seismicity near the Lacq gas field: southwestern France, J . Geophys. Res., vol. 99, pp. 15,423-15,438, 1994.

Segall. P. and J.R. Rice, Dilatancy, compaction, and slip instability of a fluid infiltrated fault. J . Geophys. Res., ‘u. 100; p- 22, 155-22,171, 1995.

hlossop. A. and P. Segall, Subsidence at The Geysers geothermal field, N. California from a comparison of GPS and leveling surveys, Geophys. Res. Lett. Vol. 24 , No. 14, p. 1839., 1997.

Segall, P. and S. Fitzgerald, A note on induced stress changes in hydrocarbon and geothermal reservoirs. Tectonophysics in press, 1997.

Yfossop. A. and P. Segall, Induced seismicity in geothermal fields: Thermoelastic Inejction model. J . Geophys. Res., in final preparation.

Mossop. A. and P. Segall, Pressure changes within The Geysers geothermal field, N. Califor- nia inferred from GPS and leveling surveys, J . Geophys. Res., in preparation.

Mossop, A. and P. Segall. Microseismicity, steam production and water injection at The Geysers geothermal field. Geology, in preparation.

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Abstracts and Conference Proceedings

Segall, P. and J.R. Rice, Dilatancy, compaction, and slip instability of a fluid-infiltrated fault, EOS. Transactions, American Geophysical Union, Vol. 75, No. 44, p. 425,1994.

MOSSOP, Antony, and P. Segall. Induced seismicity at The Geysers, Northern California (EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION ; Vol. 75, No. 44, Suppl., p. 444, 1994).

MOSSOP, Antony. and P. Segall. Stresses induced by injection of cold fluids into hot fractured rock. (EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION ; Vol. 76, No. 46. Supp l:, p. 354, 1995).

Segall, P. and J.R. Rice, Dilatancy, compaction, and slip instability of a fluid infiltrated fault. J. Gtb,hys. Res., v. 100, p. 22. 155-22,171, 1995.

Segall. P. and J.R. Rice. Stability of slip on a fluid-saturated fault with shear heating, dilatancy. and rate and state dependent friction, Transactions, American Geophysical Union, Vol. 76, No. 17. p. 282, 1995.

Segall. P. and J.R. Rice, Slip instabilities: Unstable friction or shear heating? (abstract), IUGG X X I General Assembly Abstracts. Boulder, Colo., A357, 1995.

Slossop. Antony. and P. Segall. Subsidence at The Geysers geothermal field, N. Calif0 rnia (EOS. TRANSACTIONS, AMERICAN GEOPHYSICAL UNION ; Vol. 77, No. 46. Suppl . p. 146, 1996).

llossop. Antony. 11. Murray, S. Owen and P. Segall. Subsidence at The Geysers geothermal field: results and simple models. (PROCEEDINGS 22nd STANFORD GEOTHERMAL IYORKSHOP : p. 377-382. 1997).

Mossop, ,4ntony. Closed form estimates of injection induced temperature changes. (19th NEM' ZEALAND GEOTHERMAL WORKSHOP, In Press, 1997).

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5 References Cited

Barker, B.J., M.S. Gulati, M.A. Bryan, and K.L. Reidel. Geysers reservoir performance. In Monograph o n the Geysers geothermal field, Special report no. 17, pages 167-178. Geothermal Resources Council, 1992.

Denlinger, R.P., W.P. Isherwood, and R.L. Kovach. Geodetic analysis of reservoir depletion Journal of Geophysical Research, at The Geysers steam field in northern California.

86:6091-6096, 1981.

Lofgren, B.E. , Monitoring crustal deformation in the geyser-clsar lake region. In Research in The Geysers- Clear Lake geothermal area, northern California. Geological survey pro- fessional paper 1141, United States Government printing office, 1981.

of simulated fault gouge, J. Geophys. Res., 95, 7007-7025, 1990. Marone, C., C.B. Raleigh, and C.H. Scholz, Frictional behavior and constitutive modeling

h'fossop, A., and P. Segall, Subsidence at The Geysers geothermal filed, N. California from a comparison of GPS and leveling surveys, Geophysical Research Letters, in press, 1997a.

MOSSOP, A., and P. Segall, Induced Seismicity in Geothermal Fields I: A Thermoelastic Injection hlodel, in preparation for Journal of Geophysical Research, 1997b.

Ruina: A.: Slip instability and state variable friction laws, J . Geophys. Res., 88, 10,359- 10.370, 1983.

Segall, P., 1989. Earthquakes triggered by fluid extraction, Geolog, 17942-946.

Segall. P. and S.D. Fitzgerald. A note on induced stress changes in hydrocarbon and geother- mal reservoirs. Tectonophysics (in press).

Stark. h1..4.. Mcro earthquakes - A tool to track injected water in The Geysers geothermal reservoir". In Monograph o n the Geysers geothermal f ield, Special report no. 17, pages 111-117. Geothermal Resources Council, 1992.

Teufel. L.I.zi., D.W. Rhett: and H.E. Farrell, 1991, Effect of reservoir depletion and pore pressure drawdown on in situ stress and deformation in the Ekofisk field, North Sea: In Rogiers, J.-C., (Editor), Rock Mechanics as a Multidisciplznary Science, Balkema, Rotterdam. p. 63-72.

Van den Bark, E. and O.D. Thomas, Ekofisk, 1980, First of the giant oil fields in west,ern

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Europe, in M.T. Hablbouty, ed. Giant Oil and Gus Fields of the Decade 1968-1978, A.A.P.G. Memoir 30, Oklahoma, pp. 195-224.

Williamson, K.H., Development of a Reservoir Model for The Geysers geothermal field. In Monograph o n the Geysers geothermal field, Special report no. 17, pages 179-187. Geothermal Resources Council, 1992.

6 Figure Captions

Figure 1: Correlation between seismicity, steam production, and water injection at The Geysers.

Figure 2: Subsidence between the 1977 leveling and 1996 GPS surveys, holding V626 fixed.

Figure 3: The best fitting point contraction sources with, superimposed, measured pressure lows as of 1987 [Williamson, 19921.

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Figure 4: Thermal stress acting as a function of radial distance from the injection borehole. Stress is the component normal to the fault zone and parameters are taken to be appropriate for a typical injector in The Geysers geothermal reservoir.

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( o w w c D w c D w o 0 0 0 0 0 0 0 0 + + + + + + + + w w w w w w w w 0 0 0 0 0 0 0 0

b W l n d c 9 C u T L O 9 4 9 9 9 4 9 9

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I - - I - - - \.- a

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0 km 5 I I

237' 10' 237' 15' 237' 20'

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8 ) 198f OBSERVED PRESSURES

Isobar ipda) rt mean 80 level, based on measured re servofr pressures

Miles

\ 0 km 5 - 237' 10' 237' 15' 237' 20'

-0 50 150 200 250 R adiai Distance (m)

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