PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering

Stanford University, Stanford, California, February 11-13, 2013

SGP-TR-198

RELATIVE IMPORTANCE OF PROCESSES LEADING TO STRESS CHANGES IN THE

GEYSERS GEOTHERMAL AREA

Johannes B. Altmann1, Oliver Heidbach

1, Roland Gritto

2

1Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences,

Section 2.6 Seismic Hazard and Stress Field, Telegrafenberg, 14473 Potsdam, Germany 2Array Information Technology, USA

e-mail: jaltmann@gfz-potsdam.de

ABSTRACT

Quantification of the different processes that

contribute to the spatio-temporal stress changes at

geothermal sites is critical for the understanding of

induced seismicity. Poroelastic coupling and thermal-

mechanical processes during hot water extraction and

cold water injection are probably the key processes

that contribute to the stress changes in The Geysers

geothermal field. In particular, with the onset of

massive injection of wastewater into the reservoir the

number of induced events with magnitudes M > 4

increased significantly. This raised the question

which stress changing process is the key control of

the induced seismicity rate. In addition to man-made

stress changes the tectonic loading from the Pacific

plate motion relative to the North America plate also

alters the stress state as well as the co-seismic stress

change of the induced events itself.

In order to assess the relative importance of the

various stress changing processes, we built a 3D

thermo-hydro-geomechanical numerical model of

The Geysers geothermal field. The model accounts

for the far-field tectonic loading as well as for the co-

seismic stress changes of the M > 4 events. To solve

the resulting fully coupled partial differential

equations we use the commercial finite element code

Abaqus. After the implementation of an initial stress

state that is calibrated against data of the orientations

of maximum horizontal stress and stress regime from

earthquake focal mechanism solutions, we apply

kinematic far-field boundary conditions derived from

continuous GPS stations. First, preliminary results

reveal that tectonic loading contributes within the

reservoir little compared to the pore-pressure

diffusion on time scales of several years.

Furthermore, the relative importance of stress

changes due to pore pressure diffusion strongly

depends on the spatial and temporal scales that are

analyzed as well as on the uncertainties of the rock

properties in particular the permeability of the rock

matrix.

INTRODUCTION

Fluid injection under high pressure to enhance

permeability of geothermal reservoirs and massive

injection of waste water to maintain reservoir is

common practice (Tester et al., 2007). These

processes are beside the hot water production, i.e.

heat extraction from the underground the key man-

made processes that alter the subsurface stress state.

However, in the past decade the accompanying

induced seismicity became an increasing problem

not only at geothermal reservoirs (Evans et al., 2012;

Majer et al., 2012, 2007) but also related to other

types of underground mining activities such as

hydrocarbon exploitation, as well as potash and coal

mining (Suckale, 2009, 2010).

Whether injection-related seismicity or production-

related seismicity is considered, both types of

seismicity are induced by a stress change. These are

caused by the injected or produced fluid. There are

two components of the fluid, leading to stress

changes. Fluid has a pore pressure component as well

as a temperature component. Fluid injection increases

the pore pressure increase and thus reduces the

normal stress along nearby faults, and thus can cause

seismicity (Byerlee, 1978). Fluid extraction and thus

pore pressure decrease results in subsidence of the

surface around the production well. The heat

extraction and injection of cold water also changes

the stress field by changing the thermal stresses.

Furthermore, depending on the predominant tectonic

regime, in extensional regimes normal faults may be

developed at the flanks of the reservoir, in

compressional regimes thrust faults may be

developed above and below the reservoir (Segall and

Fitzgerald, 1998). Heating up the cold injected fluid

cools down the hot rock and leads to volumetric

contraction. This contraction reduces the friction on

nearby faults (Eberhart-Philips and Oppenheimer,

1984) or even can create new cracks within the rock

volume, both mechanisms resulting in potential

seismicity.

Besides the man-made induced stress changes natural

processes such as plate tectonics and co-seismic

stress transfer due to major seismic events can alter

the stress state substantially. In The Geysers

geothermal field these processes can potentially

contribute significantly to the overall stress state.

To quantify the relative importance of these man-

made and natural processes we built a 3D thermo-

hydro-geomechanical numerical model of The

Geysers geothermal field that describes the stress

changes caused by fluid flow and by temperature

changes as well as the ones from natural tectonic

processes. Our model is a regional approach of the

whole reservoir in contrast to other work that focus

on single fluid injection experiment or 2D effects of

poroelastic effects (Bruel, 2007; Altmann et al.,

2010; Kohl and Megel, 2007; Rutqvist et al., 2006;

2007; Ghassemi and Zhou, 2011; Fakcharoenphol,

2012;)

As we set-up a regional model of The Geysers area,

we also consider fluid injection and production at

different locations. This enables us to model injection

and production scenarios within the entire geothermal

field. As the seismic activity clearly increased with

the start of SEGEP and SRGRP, our main interest of

investigation is on the period starting on 1997 with

SEGEP till now. We also implemented the major

fault system of The Geysers as well as the reservoir

bounding faults in order to assess at a later stage the

reactivation potential of these long fault segments

and the stress changes on these due to reservoir

production and long-term waste water injection in

comparison with tectonic loading.

In the following sections we describe briefly the

overall tectonic setting and the monitored induced

seismicity. We then present the technical and

geometrical setup of our model followed with a

description how we introduce and calibrate the initial

stress field and the loading from plate tectonics.

TECTONIC SETTING AND GEOLOGY

The Geysers geothermal field is located in northern

California, in an active tectonic region. It is

approximately 150 km north of San Francisco, about

60 km east of the San Andreas fault and 20 km south

of the Clear Lake basin.

The Geysers is located east of the Maacama fault and

is bounded to the east by the Collayomi fault.

Maacama as well as Collayomi faults are part of the

broad right-lateral San Andreas transform fault

system, where the Pacific plate adjoins the North

American plate (McLaughlin, 1981; Allis, 1982;

Donnelly et al., 1993; Boyle et al., 2011). Figure 1

shows a map of northern California, in which the

location of The Geysers is marked as solid red line.

The arrows show observed GPS velocity

measurements with respect to a fixed North

American plate. GPS-measurement were taken from

the USGS SFBay area GSP network.

Numerous authors (Donnelly et al., 1993; Bufe et al.,

1981; Oppenheimer 1986; Eberhart-Philips, 1988;

Allis and Shook, 1999) point out that seismic

measurements indicate that the Clear Lake area,

where The Geysers geothermal field is located, is

under extension. The Clear Lake basin is assumed to

be a pull-apart basin (Crowell, 1974; Hearn et al.,

1988; Donnelly et al., 1993) within the broad San

Andreas fault system. The reason for the described

complex tectonic and geological setting in the Clear

Lake area is the subduction of the Farallon plate

beneath the North American plate, which ended

about 3 Ma ago at the latitude of the Clear Lake area

(Donnelly et al., 1993; Boyle et al., 2011). After the

Farallon plate was subducted beneath the North

American plate 3 Ma ago, volcanism commenced in

the Clear Lake area ca. 2.1 Ma ago (Donnelly et al.,

1993). There is evidence that there is a large silicic

magma body beneath the Clear Lake area,

responsible for the heat anomaly of this area (Stimac

et al., 1992; Donnelly et al., 1993; Boyle et al, 2011).

An intrusion of felsic rock into the Meta-Greywacke

about 1.2 Ma ago led to the felsite layer, part of The

Geysers geothermal reservoir (Brikowski 2001). The

felsite layer varies in depth from 0.7 km depth in the

southeast Geysers up to 2.5 km depth in the

northwest Geysers (Brikowski 2001; Boyle et al.,

2011).

Figure 1: Map of The Geysers geothermal field (red

solid line) within the broad San Andreas

fault (SAF) system. Grey solid lines show

the traces of the major faults. Velocities

from continuous GPS sites show horizontal

displacement with respect to a fixed North

American plate in the interseismic phase

where the SAF is locked.

The enormous heat in the underground leads to a

natural geothermal activity in The Geysers area,

Maacama fault

Collayomi fault

SAF

where hot spring and fumaroles appear at the surface.

Due to this obvious thermal activity, already in the

1920’s The Geysers were discovered as potential

energy source and eight wells were drilled during this

time. Commercial energy production started in 1955

and was expanded since then to recently eighteen

active power plants (Lipman et al., 1978; Sanyal and

Enedy, 2011).

OBSERVATION OF INDUCED SEISMICITY

The massive development of the geothermal field as

well as a lack of both natural and artificial recharge,

the reservoir pressure began to decline in the late

1980’s. As a consequence, productivity began to

decrease (Stark et al., 2005; Goyal and Conant, 2010;

Sanyal and Enedy, 2011). In order to prevent the

reservoir pressure to further decline, two injection

projects were set up in order to pump secondary-

treated and tertiary-treated wastewater from nearby

communities into the underground (Goyal and Box,

2004; Stark et al, 2005; Goyal and Conant, 2010;

Sanyal and Enedy, 2011). The Southeast Geysers

Effluent Project (SEGEP) started in 1997, the Santa

Rosa – Geysers Recharge Project (SRGRP) started in

2003. Since then, the reservoir pressure was

stabilized.

Despite of a decreasing production rate since the end

1980’s, seismicity with events of magnitude M > 1.5

increased (Figure 2). A comparison of injection rate,

production rate and observed seismicity (Figure 3)

shows a clear correlation between injection rate and

observed seismic events of M > 1.5. Especially after

the start of the two wastewater injection projects in

1997 and 2003, seismicity increased.

Figure 2: Comparison of injection, production and

observed seismicity in The Geysers. Figure

from Hartline, 2012.

After the start of SRGRP in 2003, also the occurrence

of M > 4 earthquakes increased. But this increase

could temporally not correlated to the wastewater

injection (Majer and Peterson, 2007; Boyle et al.,

2011). Rutqvist and Oldenburg (2008) suggest

injection-related seismicity to occur due to cooling-

induced shear slip along fractures, whereas Denlinger

and Bufe (1982) suggest production-related

seismicity to occur due to temperature and pore

pressure decrease (Boyle et al., 2011).

Figure 3: Correlation between water injection and

observed seismicity (M > 1.5). Figure from

Hartline, 2012.

MODEL GEOMETRY AND DISCRETIZATION

For our geomechanical-numerical model of The

Geysers region we have chosen a rectangular area of

approximately 60 km by 40 km that is oriented in the

direction of the horizontal velocities derived from the

continuous GPS station P195 (Figure 4).

Figure 4: Rectangle shows the location and size of

the geomechanical-numerical model. The

orientation of the model is such that it

aligned along the horizontal GPS velocity

of station P195. Red arrows and triangles

show the kinematic boundary conditions

that will be applied to the model to realize

tectonic loading; triangles denote that the

boundary is fixed.

The size for the geomechanical-numerical model is

chosen at this regional scale in order to model the far

field effect of tectonic loading on the state of stress

within The Geysers and on the reservoir bounding

faults, and that the distance between the model

boundaries and the area of interest (reservoir) is large

enough to avoid any boundary effects on the results.

However, the model is small enough to have a high

resolution in the center of the model to implement

also injection and production points (Figure 5).

Figure 5: Finite element model of The Geysers area.

The reservoir (blue layer) is located in the

model center. Seven major faults included

are: Maacama fault (MF), two branches

of the Geyser Peak fault (GPF), two

branches of the Wight Way fault (WWF),

Cobb Mountain fault (CMF), Collayomi

fault (CF).

The model geometry consists of three horizontal

stratigraphic layers, representing the Greywacke

layer from the surface to a depth of approximately

2.5 km, the Felsite layer down to a depth of 5 km and

the upper crust from 5 km to a depth of 15 km

(Figure 5 and 6).

Figure 6: Geothermal reservoir of The Geysers on

top of the Felsite layer, realized in the

geomechanical-numerical model.

The model geometry also contains seven major faults

of this area, which are implemented as vertical

contact surfaces, intersecting the entire model from

the surface to the bottom in 15 km depth. The

geothermal reservoir itself is located in the center of

the geomechanical-numerical model on top of the

Felsite layer.

In order to solve the fully coupled differential

equations of poroelasticity and thermal diffusion we

discretize the model geometry into 1.2 million

tetrahedral poroelastic elements, to which pore

pressure and temperature can be addressed to. The

resolution increases from 25 m around the injection

and production points to 200 m at the reservoir

boundary up to 2.5 km at the model boundaries.

In order to simulate the effect of fluid injection and

withdrawal on the pore pressure field and on the state

of stress, we included 65 injection and production

points in total. Injection points are located at SEGEP

and SRGRP injection wells, production points around

the power plants. Figure 7 shows a part of the

injection and production points within the reservoir.

Figure 7: Injection and production sites are indicated

as yellow dots, rupture planes of the

induced events with magnitude four and

larger are shown as inclined purple

planes. Faults are represented as large

vertical, colored planes.

Additionally, we included ten rupture planes of

magnitude 4 earthquakes that occurred since the start

of SEGEP in end of 1997 as planar contact surfaces.

For the orientations of the rupture planes we used

strike and dip from earthquake moment tensor

solutions. This gives us the possibility to model the

effect of co-seismic slips on the state of stress.

NUMERICAL MODELING OF STRESS AND

STRESS CHANGES

The model process consists of several consecutive

steps with increasing model complexity. First we

start with the definition of an appropriate initial stress

state of the model area. Then we apply the lateral

kinematic boundary conditions to simulate the

stressing of past tectonic loading. These two initial

phases will deliver the starting stress field before

fluid injection and production are considered.

15 k

m

MF

GPF

WWF

CMF

CF

Initial State Of Stress

The k-ratio as the ratio of the mean horizontal stress

to vertical stress can be used as parameter to describe

the reference stress state of the Earth’s crust. The

compilation of k-ratios presented in Figure 8 shows

that it varies in the first two kilometers significantly.

Measurements from the KTB site (Brudy et al., 1997)

and the SAFOD pilot hole (Hickman and Zoback,

2004) show that the k-ratio is decreasing with depth

and tends to converge to lithostatic (k=1) at greater

depth. As SAFOD is in an tectonic very active region

the k-ratio is much clearly higher due to higher

horizontal stresses compared to the KTB site that is

in central Europe in a tectonically inactive region.

The simplest way to define an initial state of stress is

obtained by applying gravity to a geomechanical-

numerical model with the model bottom and the

model sides fixed in normal directions. However, this

procedure results in an uniaxial stress state that is not

justfied by comparing the stress state with field data

(Figure 8).

In order to define a reference stress state Sheorey

(1994) presents a one-dimensional analytical elasto-

static thermal earth model. He parameterizes the

structure of his earth model in terms of thermal

gradient, thermal expansion coefficient varying with

depth and temperature, gravity and elastic constant

varying with depth. He takes into account the effects

of thermal expansion and gravitational compaction,

and derives an expression for the k-ratio for the

uppermost kilometers of the earth’s crust

(

) (1)

where E is the Young’s modulus in GPa and H depth

in m. This k-ratio is valid for areas with no lateral

density changes and no active tectonics. For

E=90 GPa, the value for the KTB site (Brudy et al.,

1997), this k-ratio fits very well the measured data

from the KTB site (Figure 8). This implies that the

simple uniaxial boundary conditions would not result

in an appropriate initial stress state. Thus, in our

model we thus use the k-ratios that result from the

Sheory model (1994) as it fits the observed data best.

In practical terms of modeling, we build a box around

The Geysers geomechanical-numerical model with

model boundaries inclined to the center of the earth.

In the following, this box is called Sheorey-box. The

inclined model boundaries increases the horizontal

stresses compared to a model with vertical model

boundaries, when the model compacts due to the

applied gravity forces.

The Sheorey-box consists of two layers, an upper

load frame with the same depth like The Geysers

model and a lower layer called compaction layer.

With Equation (1) Sheorey (1994) gives an estimate

of the k-ratio in the upper crust. This formula is valid

for an area without tectonic stresses and with no

lateral density variations. Thus, Equation (1) is used

to calculate an initial state of stress. E is known for

the different layers of The Geysers model, so that we

are able to calculate a k-ratio dependent on H. This

gives us theoretical curves of the k-ratio, one for each

Young’s modulus of the model.

Figure 8: Worldwide measurements of k-ratios

compared to different model of reference

stress states (for details see Hergert &

Heidbach, 2011 and references given

therein).

Figure 9: Geomechanical-numerical model of The

Geysers with round Sheorey-box.

Now, gravity is applied, and from the resulting stress

field k-ratios along vertical paths are calculated,

which are in areas of no lateral density variations.

We use the finite element solver ABAQUS to solve

the resulting poroelastic partial differential equations

in order to perform our modeling studies. We fit this

modeling results to the theoretical curves calculated

after Equation (1). The modeled k-ratios can be

improved by variation of the Poisson’s ratios of The

Geysers model and of the load frame, and by

variation of the Young’s modulus of the compaction

layer.

Figure 10 shows the best fit of modeled and

theoretical k-ratios along two vertical profiles. There

load

frame

compaction

layer

dep

th [

km

]

are three different curves, because of three different

layers in the model, which have different Young's

moduli.

Figure 10: Comparison of k-ratio curves after

Sheorey (1994) (solid lines) with modeled

k-ratios along two vertical paths (location

1, location 2). The difference between the

modeled k-ratios in depths between 2 km

and 3 km is due to different thicknesses of

Greywacke and Felsite layers.

This result is used as the initial stress state of the

model area, the basis which in the following tectonic

loading and fluid injection and production is applied

on. The initial stress state does not consider any

stress changes due to tectonics. As The Geysers is

located in an active tectonic region, tectonic loading

will change the initial stress state. In the following,

we apply boundary conditions to the model,

representing tectonic loading in this area.

Stress Change Due To Tectonic loading

The Pacific plate moves north-northwest with 51.5

mm/a with respect to a fixed North American plate.

In The Geysers area, this movement decreases from

31 mm/a at the Maacama fault to 21 mm/a east of the

Collayomi fault. This means a difference of 10 mm/a

within our modeling area (Figure 4). Therefore, to

model this effect, the southwestern model boundary

is aligned along the measured GPS-vector, so that we

can apply a shear boundary condition of 10 mm/a to

this model boundary. The northeastern model

boundary is fixed, and along the other model

boundaries we apply a linearly decreasing velocity

from 10 mm/a in the west to zero in the east. These

boundary conditions are marked as red arrows and

triangles in Figure 4.

For our modeling approach, we use poroelastic

material properties, given in Table 1.

Table 1: Poroelastic material properties with void

ratio n, hydraulic conductivity kf, Poisson's

ratio, bulk modulus of the solid grains Kg,

bulk modulus of the wetting liquid Kf and

drained bulk modulus Kd. Layer Grey-

wacke

Reservoir Felsite Upper

Crust

n (%) 1.5 1.5 1.5 1.5

kf (m/s) 1.0E-10 5.0E-8 5.0E-10 1.0E-10

0.32 0.35 0.32 0.30

Kg (GPa) 40 50 75 100

Kf (GPa) 2.1 2.1 2.1 2.1

Kd (GPa) 18 26 37 50

After applying these boundary conditions to the

model, the initial state of stress has changed. We use

the regime stress ratio (RSR) after Simpson (1997)

and the orientation of the maximum horizontal stress

to compare the modeling results with observed data.

The RSR has values between 0 and 3 and is a

continuous scale for the stress regime, covering radial

extension (RSR=0), extension or normal faulting

(RSR=0.5), transtension (RSR=1.0), strike slip

(RSR=1.5), transpression (RSR=2.0), compression or

thrust faulting (RSR=2.5) and constriction

(RSR=3.0).

Figure 11 show a RSR contour plot in a depth of 2

km. Values range from 0.65 to 1.3 within The

Geysers. This means, that there is a transtensional

regime with some normal faulting and strike slip

components. This result, we compare to focal

mechanism solutions of earthquakes occurred in The

Geysers (Figure 12). These data show normal

faulting and strike slip regimes. The modeling results,

which also show regime between normal faulting and

strike slip, fit to these observations.

Figure 13 shows a contour plot of the orientation of

the maximum horizontal stress SH in 2 km depth. The

SH orientation varies in the model results between 8

and 18 degrees within The Geysers. These values are

in average about 10 degrees smaller than the

observations (Figure 12), but are within the standard

deviation of mean SH orientation with 27° ± 20°.

To summarize, after applying tectonic boundary

conditions to the geomechanical-numerical model,

the results can explain observed data of tectonic

regimes and SH orientations. In the next step, fluid

0 0.5 1 1.5 2 2.5 3

k-ratio (mean horizontal stress / vertical stress)

14

12

10

8

6

4

2

0

De

pth

in k

m

E = 20 GPa

E = 40 GPa

E = 60 GPa

Model, Location 1

Model, Location 2

Greywacke

Felsite

Upper Crust

injection and production is applied to the model and

stress changes due to these processes are modeled.

Figure 11: RSR (Regime stress ratio) contour plot in

2 km depth. Values within The Geysers

range between 0.65 and 1.3 indicating

transtension to strike slip regime. Note

that The Geysers is in the center of the

model (see Figure 4).

Figure 12: Stress map of The Geysers area. Lines

show the orientation of the maximum

horizontal stress SH derived from

earthquake focal mechanisms solutions

following the World Stress Map quality

ranking scheme (Heidbach et al., 2010);

color indicate the tectonic regime with

NF=normal faulting (red), green strike-

slip (green) and TF=Thrust Faulting

(blue). Rose diagram shows the mean SH

orientation with 27° ± 20°.

Figure 13: Orientation of the maximum horizontal

stress SH in 2 km depth.

Fluid Injection And Production Induced Stress

Changes

Here, we show first results of modeling the pore

pressure and stress change due to fluid injection and

production at The Geysers. For this purpose, we

modeled a scenario of fluid injection and production

starting in end of 1997 with the start of SEGEP,

added in 2003 the injection due to SRGRP and ended

in 2013 with our modeling. Fluid injection due to

SEGEP is represented by fluid injection at 10

injection points, due to SRGRP at further 13 injection

points and due to production at 42 production points.

Injected and produced fluid amounts correspond to

real injected and produced amounts. We have

averaged the injected and produced amounts over the

entire model run time and over all injection and

production points. This means, 28 l/s at each SEGEP

injection point, 30 l/s at each SRGRP injection point

and 48 l/s at each production point.

Figure 14 shows the pore pressure change within The

Geysers. The reservoir pressure is lowered by

approximately 20 to 30 MPa, with higher values in

the vicinity of the production points and with lower

values towards the reservoir boundary. There are

local increases of pore pressure in the vicinity of the

injection points. The overall picture is a decrease of

pore pressure in the reservoir. A decrease in pore

pressure increases the stress components. Figure 15

shows the change in maximum principal stress in the

reservoir. Fluid production and pore pressure

decrease increases the maximum principal stress by

10 to 30 MPa with higher values in the vicinity of the

production points.

Figure 14: Pore pressure change inside The Geysers

reservoir due to injection and production

of fluid.

Figure 15: Change in maximum principal stress

inside The Geysers reservoir due to

injection and production of fluid.

DISCUSSION

As no stress magnitude data are available from The

Geysers, we used the reference state of Sheorey

(1994) that provides k-ratios as well as information

on the SH orientation and the tectonic regime to

calibrate the initial stress state of the model.

The data from the KTB-site can be well explained by

the Sheorey-concept (Figure 8), but not the data from

the SAFOD pilot hole. In principle the Sheorey

reference stress state is only valid for regions with no

active tectonics and no large lateral density

variations. This holds on for the KTB-site, but not for

the SAFOD pilot hole.

Also The Geysers is in an active tectonic region.

Calculating the k-ratio without applying tectonic

boundary conditions, k-ratios and thus RSR values

are too low compared to measured data. Figure 16

shows the RSR in a depth of 2 km before applying

tectonic boundary conditions. The values within The

Geysers range between 0 and 0.3; thus the regime is

highly extensional, and the measured tectonic

regimes could not explained by these results. The

tectonic loading increases the horizontal stresses and

thus the k-ratio and the RSR. Compare the results of

Figure 16 to those of Figure 11, where RSR is plotted

after tectonic loading is applied.

Figure 16: RSR contour plot in 2 km depth, only due

to gravity field (no tectonic loading).

Not only RSR values change by adding tectonic

loading to the modeling process. There is also a

change in the SH orientation. Before applying tectonic

loading to the model, the SH orientation covers the

entire range from 0 to 180 degrees (Figure 17).

The more or less stable measured orientation (Figure

12) cannot be explained by these results. Therefore, it

is essential to add tectonic loading to the modeling

process.

Injection and production of fluid have a clear impact

on the pore pressure field and on the state of stress.

However, the results showing changes in the order of

a few tens of MPa are overestimated; these are

preliminary results that depend significantly on the

assumed permeability of the rock. There are

estimates that the reservoir pressure dropped by a few

MPa (Mossop and Segall, 1997).

Figure 17: Orientations of the maximum horizontal

stress in 2 km depth, only due to gravity

field (no tectonic loading).

CONCLUSION

In order to quantify the relative importance of the

man-made and natural processes that change the

stress state in The Geysers geothermal field a

regional model is needed. Furthermore, we showed

that the implementation of an appropriate initial

stress field of The Geysers is of key importance.

Tectonic regimes and orientations of the maximum

horizontal stress, both derived from focal mechanism

solutions, can be explained by the model results with

the initial stress field and tectonic loading due to the

locked San Andreas fault system.

First results, preliminary results, of long-term

injection and long-term production of fluid show a

decreasing pore pressure and increasing effective

stresses. However, the absolute values seem to be

overestimated given the pore pressure observations

that indicate a smaller stress drop.

For the future, we have developed a tool to make the

permeability of the reservoir dependent on the pore

pressure. When pore pressure exceeds a certain pore

pressure value, permeability increases. This will

increase the fluid flow and lower the pore pressure

changes. Additionally, we intend to investigate stress

changes due to co-seismic slip and in particular the

stress loading history at the reservoir bounding faults.

ACKNOWLEDGEMENT

The work presented was mainly funded by the

Department of Energy Geothermal Technologies

Program under Award Number DE-EE0002756-002

and co-funded by the European Commission within

the FP7 project GEISER, grant agreement no. 24132.

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