high injection rates counteract formation of far reaching fluid migration pathways at the geysers geothermal field deep underground water injections induce seismicity when the seismic fractures coalesce into far reaching pathways for fluid migration the migrating fluid may reach pre existing faults and by de

discrete element modeling of stress and strain evolution within and outside a depleting reservoir stress changes within and around a depleting petroleum reservoir can lead to reservoir compaction and surface subsidence affect drilling and productivity of oil wells and influence seismic waves used for monitoring of reservoir performance currently modeling efforts are split into more or less coupled geomechanical normally linearly elastic fluid flow and geophysical simulations there is evidence from e g induced seismicity that faults may be triggered or generated as a result of reservoir depletion the numerical technique that most adequately incorporates fracture formation is the dem discrete element method this paper demonstrates the feasibility of the dem here pfc particle flow code to handle this problem using an element size of 20 m 2 d and 3 d simulations have been performed of stress and strain evolution within and around a depleting reservoir within limits of elasticity the simulations largely reproduce analytical predictions the accuracy is however limited by the element size when the elastic limit is exceeded faulting is predicted particularly near the edge of the reservoir simulations have also been performed to study the activation of a pre existing fault near a depleting reservoir

source mechanisms of induced earthquakes at the geysers geothermal reservoir at the geysers geothermal reservoir in northern california evidence strongly suggests that activities associated with production of electric power cause an increase in the number of small earthquakes first degree dynamic moment tensors are used to investigate the relationship between induced earthquakes and injection of water into a well as part of a controlled experiment in the northwest geysers the estimation of dynamic moment tensors in the complex shallow crust at the geysers is challenging so the method is described in detail with particular attention given to the uncertainty in the results for seismic events in the moment magnitude range of 0 9 2 8 spectral moduli of dynamic moment tensors are reliably recovered in the frequency range of 1 100 hz but uncertainty in the associated spectral phases limits their use to a few simple results a number of different static moment tensors are investigated with the preferred one obtained from parameters of a model fitted to the spectral modulus of the dynamic moment tensor moment tensors estimated for a group of 20 earthquakes exhibit a range of source mechanisms with over half having significant isotropic parts of either positive or negative sign corner frequencies of the isotropic part of the moment tensor are about 40 larger than the average of the deviatoric moment tensor some spatial patterns are present in source mechanisms with earthquakes closely related in space tending to have similar mechanisms but at the same time some nearby earthquakes have very different mechanisms tensional axes of displacement in the source regions are primarily horizontal while the pressure axes range from near horizontal to vertical injection of water into the well in the center of the study area clearly causes an increase in the number of earthquakes per day but an effect upon source mechanisms is not evident

a note on induced stress changes in hydrocarbon and geothermal reservoirs abstract 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 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 poroelasticity for circular disk shaped reservoirs isothermal reduction in pore pressure induces a relative horizontal tension within the reservoir 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 0 85 for reasonable coefficients of friction on the other hand dilatant fracturing and normal faulting are always promoted in extensional environments near the edge of the reservoir or in regions of high pore pressure gradient it is suggested that 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 the ratio of thermal to poroelastic stress can be quite large in geothermal reservoirs such as the geysers reservoir wide energy balance considerations suggest that the average temperature has declined at the geysers by 6 c during the past 20 years reservoir average stress changes are thus on the order of 2 mpa and are certainly much larger near injection wells and steam producing fractures

a 3 d study of the effects of thermomechanical loads on fracture slip in enhanced geothermal reservoirs heat extraction from enhanced geothermal systems egs can greatly affect the behavior of joints and other discontinuities in the reservoir fracture permeability can change in response to fluid injection extraction rock cooling variations of stress field and mineral dissolution precipitation the reduction in effective stress caused by pore pressure increase can cause the slippage of discontinuities thus inducing seismicity studies have shown that thermal stresses generated by cold water injection have a similar effect in order to assess the influence of thermal stresses on fracture opening and slippage a 3 d coupled heat extraction thermal stress elastic displacement discontinuity model is used in this study the effects of each mechanism on fracture slip is estimated with particular reference to the coso geothermal field the results indicate that under typical field conditions a substantial increase in fracture slip is observed when thermal stresses are taken into account the temporal evolution of the thermal stresses suggests that the rock mass deformation will not cease upon stoppage of water injection which can be a cause of delayed seismic activity

geomechanical response to seasonal gas storage in depleted reservoirs a case study in the po river basin italy 1 underground gas storage ugs in depleted hydrocarbon reservoirs is a strategic practice to cope with the growing energy demand and occurs in many places in europe and north america in response to summer gas injection and winter gas withdrawal the reservoir expands and contracts essentially elastically as a major consequence of the fluid gas and water pore pressure fluctuations depending on a number of factors including the reservoir burial depth the difference between the largest and the smallest gas pore pressure and the geomechanical properties of the injected formation and the overburden the porous medium overlying the reservoir is subject to three dimensional deformation with the related cyclic motion of the land surface being both vertical and horizontal we present a methodology to evaluate the environmental impact of underground gas storage and sequestration from the geomechanical perspective particularly in relation to the ground surface displacements long term records of injected and removed gas volume and fluid pore pressure in the lombardia gas field northern italy are available together with multiyear detection of vertical and horizontal west east displacement of the land surface above the reservoir by an advanced permanent scatterer interferometric synthetic aperture radar psinsar analysis these data have been used to calibrate a 3 d fluid dynamic model and develop a 3 d transversally isotropic geomechanical model the latter has been successfully implemented and used to reproduce the vertical and horizontal cyclic displacements on the range of 8 10 mm and 6 8 mm respectively measured between 2003 and 2007 above the reservoir where a ugs program has been underway by stogit eni s p a since 1986 following a 5 year field production life because of the great economical interest to increase the working gas volume as much as possible the model addresses two ugs scenarios where the gas pore overpressure is pushed from the current 103 pi where pi is the gas pore pressure prior to the field development to 107 pi and 120 pi results of both scenarios show that there is a negligible impact on the ground surface with deformation gradients that remain well below the most restrictive admissible limits for the civil structures and infrastructures

geomechanical simulation of the stress tensor rotation caused by injection of cold water in a deep geothermal reservoir we present a three dimensional thermohydromechanical numerical study of the evolution and distribution of the stress tensor within the northwest part of the geysers geothermal reservoir in california including a detailed study of the region around one injection well from 2003 to 2012 initially after imposing a normal faulting stress regime we calculated local changes in the stress regime around injection wells our results were compared with previously published studies in which the stress state was inferred from inverting the focal plane mechanism of seismic events our main finding is that changes in stress tensor orientation are caused by injection induced progressive cooling of the reservoir as well as by the seasonal variations in injection rate because of the gravity flow and cooling around a liquid zone formed by the injection the vertical stress reduction is larger and propagates far below the injection well at the same time the horizontal stress increases mostly because of stress redistribution below and above the cooling area these two phenomena cause the rotation of the stress tensor and the appearance of a strike slip regime above inside and below the cooling area the cooling and the associated rotation of the stress regime can play a significant role in the observed long term deepening of the microseismicity below active injection wells

the world stress map database release 2016 crustal stress pattern across scales abstract knowledge of the present day crustal in situ stress field is a key for the understanding of geodynamic processes such as global plate tectonics and earthquakes it is also essential for the management of geo reservoirs and underground storage sites for energy and waste since 1986 the world stress map wsm project has systematically compiled the orientation of maximum horizontal stress shmax for the 30th anniversary of the project the wsm database has been updated significantly with 42 870 data records which is double the amount of data in comparison to the database release in 2008 the update focuses on areas with previously sparse data coverage to resolve the stress pattern on different spatial scales in this paper we present details of the new wsm database release 2016 and an analysis of global and regional stress pattern with the higher data density we can now resolve stress pattern heterogeneities from plate wide to local scales in particular we show two examples of 40 60 shmax rotations within 70 km these rotations can be used as proxies to better understand the relative importance of plate boundary forces that control the long wave length pattern in comparison to regional and local controls of the crustal stress state in the new wsm project phase iv that started in 2017 we will continue to further refine the information on the shmax orientation and the stress regime however we will also focus on the compilation of stress magnitude data as this information is essential for the calibration of geomechanical numerical models this enables us to derive a 3 d continuous description of the stress tensor from point wise and incomplete stress tensor information provided with the wsm database such forward models are required for safety aspects of anthropogenic activities in the underground and for a better understanding of tectonic processes such as the earthquake cycle

reservoir conditions related to induced seismicity at the geysers steam reservoir northern california abstract seismicity at the geysers appears to be induced by steam production in which water boils to steam within a pervasively fractured reservoir subject to high levels of tectonic shear stresses and strains the steam reservoir is composed of fractured franciscan graywacke with a very heterogeneous multiple permeability and porosity distribution permeability is controlled by the larger fracture systems of the reservoir which act as a very permeable conduit to less permeable rock containing liquid water the reservoir fluid remains close to the liquidus conditions during production implying both a temperature and pressure decline in the rock matrix bounding the fracture systems as the reservoir is depleted reservoir earthquake focal mechanisms are nearly aligned with the regional tectonic strain field and reservoir earthquakes are indistinguishable from tectonic earthquakes elsewhere in the region within the reservoir the volume defined by the earthquakes has not changed within the resolution of the seismic network from 1975 to 1979 the log moment per earthquake also has not changed suggesting a steady state process of stress buildup and release added to the zero correlation of the fluid injection history with the earthquake activity the above observations suggest that elevated pore pressures and fluid injection are not likely causes for the geysers induced seismicity instead all available evidence points to the association of induced seismicity with pore pressure and temperature declines the two most likely mechanisms are either an increase in local shear stress levels with fissure deflation or a transition from stable to unstable sliding stick slip no other documented mechanism for induced seismicity are consistent with in situ reservoir conditions at the geysers

effects of long term fluid injection on induced seismicity parameters and maximum magnitude in northwestern part of the geysers geothermal field the long term temporal and spatial changes in statistical source and stress characteristics of one cluster of induced seismicity recorded at the geysers geothermal field u s are analyzed in relation to the field operations fluid migration and constraints on the maximum likely magnitude two injection wells prati 9 and prati 29 located in the northwestern part of the field and their associated seismicity composed of 1776 events recorded throughout a 7 year period were analyzed the seismicity catalog was relocated and the source characteristics including focal mechanisms and static source parameters were refined using first motion polarity spectral fitting and mesh spectral ratio analysis techniques the source characteristics together with statistical parameters b value and cluster dynamics were used to investigate and understand the details of fluid migration scheme in the vicinity of injection wells the observed temporal spatial and source characteristics were clearly attributed to fluid injection and fluid migration toward greater depths involving increasing pore pressure in the reservoir the seasonal changes of injection rates were found to directly impact the shape and spatial extent of the seismic cloud a tendency of larger seismic events to occur closer to injection wells and a correlation between the spatial extent of the seismic cloud and source sizes of the largest events was observed suggesting geometrical constraints on the maximum likely magnitude and its correlation to the average injection rate and volume of fluids present in the reservoir

mechanism of induced seismicity at the geysers geothermal reservoir california the geysers reservoir is situated in a 60 80 km wide right lateral shear zone associated with the north american pacific plate boundary a small fraction of the shear may be occurring as permanent aseismic creep which is being converted to stick slip movement in the geysers reservoir due to steam production two mechanisms could be responsible for the induced seismicity a large 100 bars increase in effective rock pressure and an increase in the coefficient of friction the first mechanism requires that the initial state of the reservoir be predominantly liquid dominated the second mechanism is a consequence of dehydration of the reservoir causing both the hardening of clay and fault gouge and the precipitation of silica

near surface microearthquakes at the geysers geothermal field california a 150 m length 6 level three component vertical geophone array was cemented into the 67 to 219 m depth interval 220 to 720 ft of unocal s well gdcf 63 29 during a plug and abandonment operation on april 7 1998 casing deformation has been observed in wells of the study area including the gdcf 63 29 well an objective of the study was to determine if shallow deformation at the geysers is manifested seismically near surface microearthquake activity was monitored for a period of one year during the latter four months monitoring was supplemented with four surface stations to help constrain locations of shallow seismicity event locations occurring within about 750 m of the array bottom have been determined for the 10 week period january 6 to march 16 1999 these events are distinct from surface monitored seismicity at the geysers in that they occur predominantly above the producing reservoir at depths ranging from about 220 to 1000 m 600 to c180 m elevation the shallow events tend to be episodic with relatively quiescent periods of up to three weeks occurring between swarms event locations show a northeast striking trend similar to seismicity trends mapped deeper in the reservoir and parallel to the strike of a major surface lineaments observed over the productive field however clear fault or fracture planes are not resolved from the hypocenters composite fault plane solutions suggest oblique reverse faulting in the overburden the shallowest seismicity terminates near the base of a serpentine unit a contact which is the locus of most of the well casing deformations logged in the area suggesting that reservoir contraction is accommodated along numerous discrete faults below the serpentine but as continuous plastic deformation in the serpentine it is hypothesized that the resulting strain discontinuity at the base of the serpentine explains the prevalence of wellbore deformation there the shallow above reservoir microseismicity is strongly correlated in time with the injection and the deeper injection induced seismic activity occurring in the reservoir immediately below this suggests that deep injection induced events trigger shallower events by a remote triggering mechanism which has been observed at a larger scale at the geysers and elsewhere

moment tensors of induced microearthquakes in the geysers geothermal reservoir from broadband seismic recordings implications for faulting regime stress tensor and fluid pressure

ten years of seismic monitoring over a gas field abstract ten years of seismic recording allows us to locate more than 800 tremors with magnitudes ranging from 1 0 to 4 2 in a small 10 by 10 by 5 km volume near a major gas field lacq france the seismic activity began 10 yr after gas extraction began nearly all of the epicenters are located within the lateral extent of the gas field the only perturbation of the stress or strain fields which has the same spatial and temporal scale as the seismicity is the 500 bar drop in gas pressure due to 20 yr of gas exploitation demonstrating that these events are induced the network established to monitor this activity consists of eight telemetered stations including a well bottom geophone located at 3200 m depth using three dimensional relocation techniques we computed new hypocenter positions for more than 280 events these better located hypocenters form a diffuse zone that mimics the local dome structure taking the regional tectonic stresses into account we suggest that these ruptures may be the result of unfolding of the dome structure

extensional tectonics at the geysers geothermal area california comparisons between the yearly volume of steam withdrawn from the geysers california geothermal reservoir and the number of earthquakes indicate that most of the seismicity is induced to establish the inducing mechanism an estimate of the stress field orientation was calculated from inversion of 210 fault plane solutions below 1 km depth the geysers is undergoing uniaxial extension with the least principal stress 3 oriented near horizontal at approximately 105 azimuth the consistency between the local and regional stress fields indicates that the regional tectonic stress is much larger than the stresses induced locally through geothermal activities the stress data are equivocal as to whether earthquakes are induced due to conversion of aseismic to seismic slip or by increased shear stresses arising from reservoir contraction the observed tendency for downward growth of seismicity may arise from a preferential fracture orientation that is near vertical in conjunction with a stress state where the gradient of 1 exceeds that of 3 evidence of extension at the geysers and in the surrounding clear lake region indicates a direct relationship to the presence of the quaternary clear lake volcanics

loading rate impact on fracturing pattern lessons from hydrocarbon recovery lacq gas field france using scaling parameters of induced seismicity in size and space domains seismic b value and spatial correlation dimension d respectively we analyze the fracturing pattern of the gas reservoir of lacq in correlation with the production rate on the basis of previous observations and a previous model segall et al 1994 we assume that the cause of induced seismicity is a poroelastic stressing such that the production rate can be viewed as a stress rate for the rocks adjacent to the reservoir during the 20 year study period the production rate has decreased dramatically leading to a change of poroelastic stress rate from 4 5 10 3 to 1 5 10 3 mpa yr accordingly we observe a significant decrease in the size of activated fractures and a reduction of the spatial clustering of fracturing both arguing for a stress rate impact on the fracturing pattern in the gas field we interpret these changes in terms of a change in the deformation mode of stressed rocks from a more brittle to a more ductile behavior as the stress rate was decreased this hypothesis is supported by independent evidence from a number of experimental numerical and field studies the loading rate thus appears as a possible candidate to drive changes of b and d in time and space the apparent sensitivity of the earth s crust fracturing pattern to loading rate variations of much lower frequency than the tidal variations suggests that the earth s crust deformation mode itself be frequency dependent also our observations may have strong implications in contexts where the loading rate is induced by human intervention e g hydrocarbon recovery mines suggesting the possibility of adjusting the loading rate so as to shape the fracturing pattern locally according to human needs

structural analysis of high pressure shear zones bacariza formation cabo ortegal nw spain abstract high pressure granulites of the bacariza formation cabo ortegal complex nw spain exhibit spectacular examples of ductile shear zones developed at different scales in rocks containing pre existing foliations a detailed structural analysis was carried out on these shear zones in order to unravel and compare the role of various parameters controlling the deformation process i e heterogeneous simple shear components of homogeneous deformation heterogeneous volume change and degree of non coaxiality although heterogeneous simple shear largely dominated negligible deviations from the ideal simple shear model were detected involving shortening along the structural directions perpendicular to the stretching axis within the foliation plane of the finite strain ellipsoid the relationship between displacement parallel to a half shear zone and the normal distance from its boundary provided the basis for the estimation of the stress exponent in the power law constitutive flow equation associated with each shear zone which is interpreted as a rheological indicator these geometric and rheological results and the thermobaric conditions of high pressure shear zone deformation indicate that these shear zones accommodated dominant plastic rock flow coeval with high pressure and high temperature deformations under moderate stress levels concomitant with elevated strain rates

poroelastic stressing and induced seismicity near the lacq gas field southwestern france o hundreds of shallow small to moderate earthquakes have occurred near the lacq deep gas field in southwestern france since 1969 these earthquakes are clearly separated from tectonic seismicity occurring in the pyrenees 25 km to the southwest the induced seismicity began when the reservoir pressure had declined by 30 mpa repeated levehng over the field shows localized subsidence reaching a maximum of 60 mm in 1989 segall 1989 suggested that poroelastic stressing associated with volumetric contraction of the reservoir rocks is responsible for induced seismicity associated with fluid extraction to test this model we compare the observed subsidence and hypocentral distributions with the predicted displacement and stress fields we find that the relationship between average reservoir pressure drop and subsidence is remarkably linear lending support to the linear poroelastic model displacements and stresses are computed based on a priori knowledge of the reservoir geometry materim properties and reservoir pressure changes the computed vertical displacements are found to be in excellent agreement with the subsidence observed from leveling stress perturbations accompanying gas extraction computed using the same parameters are found to be 0 2 mpa or less changes in coulomb failure stress are computed assuming that shp occurs on optimally oriented planes the predicted failure zones correlate very well with the spatial distribution of earthquakes if the perturbing stresses are small in comparison to the ambient regional deviatoric stresses and if the minimum regional compresslye stress axis is vertical accurate determination of focal mechanisms of the induced events would allow a more rigorous test of the poroelastic model and could lead to important inferences about the crustal stress state

hydro thermo mechanical analysis during injection of cold fluid into a geologic formation abstract we conducted in depth investigations of pore pressure buildup temperature and stress changes under isothermal or nonisothermal injection scenarios using numerical simulations the numerical simulation method combines fluid flow poroelasticity thermal diffusion and thermal stress and is based on the single phase fluid flow condition we also examined temporal evolutions of stress states and mobilized friction angles across base injection zone and caprock layers for two different stress regimes normal faulting and reverse faulting results show that injecting cold fluid shifts an area in which the mobilized friction angle becomes maximum which is where induced seismic events are likely to be triggered first for both stress regimes we also tested a hypothetical stepwise injection of cold fluid results show that this injection helps to improve the stability of the injection zone to some extent finally we suggest using dimensionless parameters to determine a prevalence of the thermal stress effect in the injection zone and discuss the impact of the initial stress regime and implications of using the single phase fluid flow condition

physics based forecasting of induced seismicity at groningen gas field the netherlands earthquakes induced by natural gas extraction from the groningen reservoir the netherlands put local communities at risk responsible operation of a reservoir whose gas reserves are of strategic importance to the country requires understanding of the link between extraction and earthquakes we synthesize observations and a model for groningen seismicity to produce forecasts for felt seismicity m 2 5 in the period february 2017 to 2024 our model accounts for poroelastic earthquake triggering and rupture on the 325 largest reservoir faults using an ensemble approach to model unknown heterogeneity and replicate earthquake statistics we calculate probability distributions for key model parameters using a bayesian method that incorporates the earthquake observations with a nonhomogeneous poisson process our analysis indicates that the groningen reservoir was not critically stressed prior to the start of production epistemic uncertainty and aleatoric uncertainty are incorporated into forecasts for three different future extraction scenarios the largest expected earthquake was similar for all scenarios with a 5 likelihood of exceeding m 4 0

induced stresses due to fluid extraction from axisymmetric reservoirs earthquakes can be induced by fluid extraction as well as by fluid injection segall 1989 proposed that poroelastic stresses are responsible for inducing earthquakes associated with fluid extraction here i present methods for computing poroelastic stress changes due to fluid extraction for general axisymmetric reservoir geometries the results ofgeertsma 1973 for a thin disk reservoir with uniform pressure drop are recovered as a special case predicted surface subsidence agrees very well with measured leveling changes over the deep lacq gas field in southwestern france the induced stresses are finite if the reservoir pressure changes are continuous computed stress changes are on the order of several bars suggesting that the preexisting stress states in regions of extraction induced seismicity are very close to frictional instability prior to production

the small effect of poroelastic pressure transients on triggering of production induced earthquakes in the groningen natural gas field over the past decade a steep increase in the number of seismic events has been observed in the groningen gas field the netherlands it is generally accepted that these are induced by compaction of the reservoir rock due to extensive depletion causing a buildup of strain energy in faults that may be released seismically we address the possible triggering of fault slip by the transient pressure field surrounding a well that has undergone a sudden rate change assuming a unilateral decoupling between displacement and pressure numerical experiments are conducted using a sequential finite volume finite element solution strategy that fully incorporates second order terms in the radial flow equation we investigate an idealized groningen like geometry to discover whether the hypothesized possibility of triggering induced seismicity exists explore how some controllable and uncontrollable variables influence its severity and possibly provide clues on how production strategy might be able to avoid its occurrence the results demonstrate that sudden production changes can indeed trigger near well seismic events but that the effect is very small compared to other potential causes changing from sudden to gradual changes in well rates is therefore not expected to lead to a significant reduction in the number or magnitude of production induced earthquakes in the groningen field

the evolution of seismic barriers and asperities caused by the depressuring of fault planes in oil and gas fields of south texas fluid pressures within some oil and gas fields of south texas have dropped to less than 20 per cent of their original values producing earthquakes with magnitudes up to 3 9 in recent years differential compaction of the depressurized region may be sufficient to result in the number and size of earthquakes generated or the faults may have been creeping prior to depressurization in either model the depressuring of fluids strengthens a fault and at first produces a barrier to slip as strain accumulates due to compaction or the continued aseismic slip of nearby portions of the fault stress builds up along the locked portions eventually forming high stress regions or asperities the asperities ultimately fail and earthquakes occur the process is repeated as long as the faults are active as the fluid pressures continue to decrease the barriers and subsequent asperities may increase in size and strength resulting in increasingly large and frequent earthquakes

on the physics based processes behind production induced seismicity in natural gas fields author s zbinden d rinaldi ap urpi l wiemer s abstract 2017 american geophysical union all rights reserved induced seismicity due to natural gas production is observed at different sites worldwide common understanding states that the pressure drop caused by gas production leads to compaction which affects the stress field in the reservoir and the surrounding rock formations and hence reactivates preexisting faults and induces earthquakes in this study we show that the multiphase fluid flow involved in natural gas extraction activities should be included we use a fully coupled fluid flow and geomechanics simulator which accounts for stress dependent permeability and linear poroelasticity to better determine the conditions leading to fault reactivation in our model setup gas is produced from a porous reservoir divided into two compartments that are offset by a normal fault results show that fluid flow plays a major role in pore pressure and stress evolution within the fault fault strength is significantly reduced due to fluid flow into the fault zone from the neighboring reservoir compartment and other formations we also analyze scenarios for minimizing seismicity after a period of production such as i well shut in and ii gas reinjection in the case of well shut in a highly stressed fault zone can still be reactivated several decades after production has ceased although on average the shut in results in a reduction in seismicity in the case of gas reinjection fault reactivation can be avoided if gas is injected directly into the compartment under depletion however gas reinjection into a neighboring compartment does not stop the fault from being reactivated

the stress state of the northwest geysers california geothermal field and implications for fault controlled fluid flow a dataset comprised of well constrained focal mechanisms for 6147 earthquakes recorded in the northwest geysers geothermal field during the period of 2005 2012 was utilized to conduct a detailed stress study within and below the geothermal reservoir the high quality focal mechanisms were organized into grid blocks of varying size using a 3d octree gridding algorithm in which discretization was governed by data density this method allows for separate inversions of contiguous blocks of seismicity at a relatively fine scale we obtained the three principal stress orientations for every grid block containing at least 25 events by inverting for the best fit stress tensor within each grid block the principal stress orientations were used to determine which of the two nodal planes for each focal mechanism had the highest ratio of resolved shear to normal stresses and was thus more likely to be the fault plane we found a normal strike slip faulting regime s hmax s v s hmin both within and below the reservoir consistent with the extensional and strike slip tectonics in the region surrounding the geysers in addition an average s hmax orientation of n26 e was obtained for the studied crustal volume these observations suggest that injection and production activities over the past 50 years do not appear to have significantly affected the local stress field the presumed fault planes are steeply dipping with northeast southwest to east west strike directions suggesting that these are the principal flow directions both within the low matrix permeability graywacke reservoir and in the wholly concealed granitic pluton locally referred to as the felsite basement below

change of stress regime during geothermal reservoir stimulation earthquakes are induced by man made changes of the stress field by injection or withdrawal of fluids in hydrocarbon production geothermal exploitation and wastewater disposal however the actual perturbation of the stress field and stress release by injection induced seismicity remains largely unknown we provide evidence for currently not understood hydromechanical processes after shut in of the well we invert earthquake focal mechanisms from a massive stimulation to invert for stress resolved in time and depth to obtain changes of the stress orientation and magnitude prior information about fracture orientations from well logs is taken into account comparison with independent stress measures reveals that stresses obtained from inversion of fluid induced seismicity are highly perturbed and not representative of the initial stress field the horizontal stresses change by tens of megapascals turning the stress regime from transitional normal faulting strike slip faulting to pure normal faulting the observed stress changes are attributed to large scale aseismic deformation

nucleation and arrest of dynamic rupture induced by reservoir depletion seismic events induced by the depletion of hydrocarbon reservoirs can cause damage to housing and cause societal and economic unrest however the factors controlling the nucleation and size of production induced seismic events are not well understood here we used geomechanical modeling of production induced stresses and dynamic rupture modeling to assess the conditions controlling down dip rupture size a generic model of offset depleting reservoir compartments separated by a fault was modeled in 2 d using the finite element package diana fea linear slip weakening was used to control fault friction behavior fault reactivation was computed in a quasi static analysis simulating stresses during reservoir depletion followed by a fully dynamic analysis simulating seismic rupture the sensitivity of reactivation and rupture size to in situ stress dynamic friction critical slip distance and reservoir offset was evaluated after reactivation a critical fault length was required to slip before seismic instability could occur in a subsequent fully dynamic analysis the propagation and arrest of dynamic rupture was simulated rupture remained mostly confined to the reservoir interval but could also propagate into the overburden and underburden or sometimes transition into a run away rupture propagation outside the reservoir interval was promoted by a critical in situ stress a large stress drop a small fracture energy and no or little reservoir offset with increasing offset up to the reservoir thickness reactivation was promoted but dynamic rupture size decreased 2019 the authors

relating 4d seismics to reservoir geomechanical changes using a discrete element approach a modified discrete element method is briefly introduced and used for modelling reservoir geomechanical response during fluid injection and depletion the modified approach works as a continuum method until some local failure is initiated after which it behaves like a discrete element method on a polygonal lattice the method is advantageous for modelling fracture developments in rocks it is applied here to synthetic models of two reservoirs taken from the north sea gullfaks and elginfranklin for gullfaks two cases of water injection were modelled one with low horizontal effective stress and the other with low vertical effective stress vertical fractures are developed in the first case whereas horizontal fractures are developed in the second case this would not have been seen using traditional methods based on 4d seismics data for the gullfaks field one may envision that horizontal fractures could have been formed the elgin franklin synthetic model is used to study various scenarios of changing stress field around the depleting reservoir based on 4d seismics data from this field one may see changes that could be interpreted in terms of possible fault reactivation

seal integrity and feasibility of co 2 sequestration in the teapot dome eor pilot geomechanical site characterization this paper reports a preliminary investigation of co2 sequestration and seal integrity at teapot dome oil field wyoming usa with the objective of predicting the potential risk of co2 leakage along reservoir bounding faults co2 injection into reservoirs creates anomalously high pore pressure at the top of the reservoir that could potentially hydraulically fracture the caprock or trigger slip on reservoir bounding faults the tensleep formation a pennsylvanian age eolian sandstone is evaluated as the target horizon for a pilot co2 eor carbon storage experiment in a three way closure trap against a bounding fault termed the s1 fault a preliminary geomechanical model of the tensleep formation has been developed to evaluate the potential for co2 injection inducing slip on the s1 fault and thus threatening seal integrity uncertainties in the stress tensor and fault geometry have been incorporated into the analysis using monte carlo simulation the authors find that even the most pessimistic risk scenario would require 10 mpa of excess pressure to cause the s1 fault to reactivate and provide a potential leakage pathway this would correspond to a co2 column height of 1 500 m whereas the structural closure of the tensleep formation in the pilot injection area does not exceed 100 m it is therefore apparent that co2 injection is not likely to compromise the s1 fault stability better constraint of the least principal stress is needed to establish a more reliable estimate of the maximum reservoir pressure required to hydrofracture the caprock

deformation characteristics of a clayey interbed during fluid injection surface deformation due to fluid withdrawal has long been observed at the surface above aquifers and oil reservoirs uplift associated with fluid injection has also been observed although the processes of subsidence and uplift are reversible in a poroelastic setting the presence of clayey interbeds can result in ground deformation behavior non reversible because of their low permeability and nonlinear behavior in this investigation a cam clay model is used in conjunction with a poroelastic model to simulate the presence of a laterally extensive lens shaped clayey interbed in a sandstone aquifer during fluid injection spatial and temporal changes associated with this interbed during aquifer pressurization are captured and can be clearly differentiated from the deformation of the surrounding poroelastic aquifer formation results for pore pressures and ground surface deformation patterns can be categorized into three distinct time intervals an early time interval where the aquifer is pressurized but not the interbed leading to a lower region above the interbed an intermediate time interval in which the interbed pressures slowly increase approaching the pressure of the adjacent aquifer leading to a potentially large surface uplift above the interbed and a late time interval in which pressure equilibration is achieved between the interbed and aquifer and a highly non symmetrical irregular surface deformation pattern results which may be higher or lower than the background depending on the interbed characteristics reservoir configuration is found to be an important factor influencing ground deformation behavior with more obvious deformation always occurring in a laterally confined aquifer as opposed to a laterally infinite aquifer

hydraulic stimulation of natural fractures as revealed by induced microearthquakes carthage cotton valley gas field east texas we produced a high resolution microseismic image of a hydraulic fracture stimulation in the carthage cotton valley gas field of east texas we improved the precision of microseismic event locations four fold over initial locations by manually repicking the traveltimes in a spatial sequence allowing us to visually correlate waveforms of adjacent sources the new locations show vertical containment within individual targeted sands suggesting little or no hydraulic communication between the discrete perforation intervals simultaneously treated within an 80 m section treatment i e fracture zone lengths inferred from event locations are about 200 m greater at the shallow perforation intervals than at the deeper intervals the highest quality locations indicate fracture zone widths as narrow as 6 m similarity of adjacent source waveforms along with systematic changes of phase amplitude ratios and polarities indicate fairly uniform source mechanisms fracture plane orientation and sense of slip over

earthquakes near rocky mountain house alberta and their relationship to gas production facilities a 1980 field survey of earthquake activity near rocky mountain house alberta using one digital and six conventional seismographs recorded 146 microearthquakes m 3 4 in 23 days and located 67 of them the located events and probably the unlocated events as well all occurred in a small zone approximately 4 km north south by 4 km east west by 1 km thick centred near 52 12 5 n 115 15 w at a depth of 4 0 km with an uncertainty of 2 km on each of the location coordinates the activity occurred below and or in the strachan d 3a sour gas reservoir which is a devonian aged limestone reef complex in a section of nearly flat lying unfaulted sediments the earthquakes had a composite thrust faulting mechanism with generally north trending intermediate dipping planes in the presence of a regional deviatoric stress field that was horizontally compressive and oriented approximately east west the earthquake activity may be related to the extraction of the natural gas and related fluids from the reservo

reservoir characterization using oil production induced microseismicity clinton county kentucky abstract microseismic monitoring tests were conducted from 1993 to 1995 in the seventy six oil field clinton county kentucky oil is produced from low porosity fractured carbonate rocks at 3 no injection operations were conducted gross changes in production rate correlate with event rate event rate lags changes in production rate by 2 to 3 weeks hypocenters and first motion data have revealed previously undetected low angle thrust faults above and below the currently drained depth intervals production history well logs and drill tests indicate that the seismically active faults or fractures are previously drained intervals that have subsequently recovered to hydrostatic pressure via brine invasion storage capacity computed for one of these drained fractures implies that total oil production represents about 20 of total pore volume correlation of older production intervals and well log porosity anomalies with the seismically active faults indicate that the oil reservoir in the study area is primarily a set of compartmentalized low angle thrust faults although low angle fracture sets have not previously been considered in the exploration and development of the area the mapped thrust faults are consistent with other investigators interpretations of oil associated with secondary fracture sets occurring along deeper seated wrench fault structures stress determined from composite focal mechanisms indicates a near surface

influence of poroelastic behavior on the magnitude of minimum horizontal stress sh in overpressured parts of sedimentary basins in many sedimentary basins of the world the minimum hori zontal stress s h is greater in overpressured zones than in normally pressured zones at equivalent depths a common explanation is that the frictional slip on listric normal faults keeps the difference between vertical stress s v and s h within certain bounds and the difference is smaller under lower effective stress i e higher pore pressure p p however in the overpressured parts of the central north sea graben united kingdom and the sable subbasin of the scotian shelf canada conventional friction envelopes underestimate the magnitude of s h these data instead indicate that s h increases at a rate proportional to but less than the rate of increase of p p a condition consistent with a p p induced deformation of the rock called poroelastic behavior this paper argues that whereas friction may govern s h in normally pressured basins poroelastic behavior is responsible for the unusually high s h in the overpres sured parts of these same basins data on the p p and s h gradients from these basins suggest that s h p p 0 7

the 2004 mw 4 4 rotenburg northern germany earthquake and its possible relationship with gas recovery we study the 20 october 2004 m w 4 4 rotenburg wumme neuenkirchen earthquake located in a previously aseismic region in the northern german sedimentary basin we constrain the source parameter by using different techniques a possible relationship between this event the regional tectonic setting and local gas recovery is investigated different waveform inversion and modeling approaches constrain the depth of the mainshock between 5 and 7 km the source mechanism was oblique normal faulting on planes striking roughly north south an inversion for kinematic rupture parameters indicates a unilateral rupture propagation toward the north consistent with the higher macroseismic intensities found toward the north in the region of hamburg compared with those at a similar distance toward the south in the region of hannover relocations of the mainshock and three of the largest aftershocks indicate that these events occurred within a few kilometers of three major gas fields and at depth close to gas production intervals comparison with seismicity triggered in the northern netherlands by depletion of similar gas reservoirs in a similar tectonic environment suggests that the m w 4 4 rotenburg event may be related to gas recovery online material focal mechanism and waveform fit

a comparison of long term changes in seismicity at the geysers salton sea and coso geothermal fields geothermal energy is an important source of renewable energy yet its production is known to induce seismicity here we analyze seismicity at the three largest geothermal fields in california the geysers salton sea and coso we focus on resolving the temporal evolution of seismicity rates which provides important observational constraints on how geothermal fields respond to natural and anthropogenic loading we develop an iterative regularized inversion procedure to partition the observed seismicity rate into two components 1 the interaction rate due to earthquake earthquake triggering and 2 the smoothly varying background rate controlled by other time dependent stresses including anthropogenic forcing we apply our methodology to compare long term changes in seismicity to monthly records of fluid injection and withdrawal at the geysers we find that the background seismicity rate is highly correlated with fluid injection with the mean rate increasing by approximately 50 and exhibiting strong seasonal fluctuations following construction of the santa rosa pipeline in 2003 in contrast at both salton sea and coso the background seismicity rate has remained relatively stable since 1990 though both experience short term rate fluctuations that are not obviously modulated by geothermal plant operation we also observe significant temporal variations in gutenberg richter b value earthquake magnitude distribution and earthquake depth distribution providing further evidence for the dynamic evolution of stresses within these fields the differing field wide responses to fluid injection and withdrawal may reflect differences in in situ reservoir conditions and local tectonics suggesting that a complex interplay of natural and anthropogenic stressing controls seismicity within california s geothermal fields

abrupt climatic change related to unstable ice sheet dynamics toward a new paradigm ice streams are fast currents of ice that drain some 90 of present day ice sheets and probably drained a similar percent of ice from former ice sheets ice streams may have life cycles akin to but much longer than surge cycles of certain mountain glaciers the floating tongues of marine ice streams coalesce to form floating ice shelves that buttress marine ice sheets and the grounded lobes of terrestrial ice streams stabilize terrestrial ice sheets ice shelves and ice lobes form during the life cycles of ice streams and disintegrate rapidly to initiate new life cycles successive life cycles continue until the ice sheet collapses a new paradigm is postulated in which abrupt climatic change is driven by iceberg outbursts associated with life cycles of marine ice streams and dust storms associated with life cycles of terrestrial ice streams for pleistocene northern hemisphere ice sheets these mechanisms regulated north atlantic surface temperatures and salinity and perhaps planetary albedo and atmospheric co2 and thereby drove climatic change which tended to be abrupt because ice stream life cycles began and ended abruptly ice streams proliferated along the margins of northern hemisphere ice sheets after they advanced beyond the crystalline shields of north america and eurasia and spread over softer sedimentary rocks overlain by thick ice cemented sediments that progressively thawed especially along river valleys and marine troughs an increasing fraction of these ice sheets was then subjected to the unstable life cycles of ice streams as seen in the pronounced spikiness of the late glacial oxygen isotope stratigraphy in the greenland ice core at dye 3 life cycles of greenland ice streams during the holocene seem to be linked to episodes of regional climatic cooling most recently the little ice age when superimposed on the present milankovitch hemicycle of reduced insolation over the north atlantic this could abruptly initiate a new worldwide glaciation cycle in the aftermath of present day co2 greenhouse warming proliferation of antarctic ice stream life cycles during the period of greenhouse warming may also trigger global climatic change by forcing a reorganization of circumpolar thermohaline circulation in the southern ocean

desorption induced shear failure of coal bed seams during gas depletion the recovery of natural gas from coal bed seams is usually accompanied by a significant increase of permeability induced by coal matrix shrinkage and stress relaxation upon gas desorption this advantageous increase in permeability may be impaired sometimes by mechanical failure of the reservoir rock and ensuing production of coal fines near wellbore stress concentration and reduction of lateral stresses are known to promote shear failure during depletion in oil and gas reservoir formations yet conventional analyses have shown limited success in predicting coal failure since other chemo physical mechanisms may be responsible in enhancing the conditions towards mechanical failure in the coal bed reservoir rock we show a set of triaxial experiments involving gas desorption from coal cores under zero lateral strain condition radial stress measured and controlled and constant total vertical stress meant to simulate the stress path during production far from the wellbore co2 is used as surrogate fluid for ch4 the experimental data indicates that desorption can significantly help reduce lateral stress and increasing deviatoric stress until shear failure occurs the results suggest that depletion induced shear failure is much more likely to occur in coal seams than in conventional non sorbing reservoir rocks the adsorptive mechanical coupling turns out to be a key phenomenon in the process numerical simulations at the representative elementary volume scale adopting a double porosity poromechanical model support the experimental findings and permit calculating a critical gas pressure for shear failure to happen this emergent phenomenon is comparable to the outcome of other situations such as mineral dissolution or thermal contraction where shrinkage relaxes lateral stress and acts as an intensifying driver for promoting shear failure within the reservoir rock coupled numerical simulation is needed to include near wellbore effects and validate our findings with actual field observations a thorough understanding of the coupled response of coal seams is necessary to enhance reservoir management and mitigate the effects of coal failure on fines production

subsidence at the geysers geothermal field n california from a comparison of gps and leveling surveys between 1994 and 1996 three gps surveys were conducted in the geysers region of northern california our aim was to constrain models of the stresses and strains induced by geothermal power production in that region each survey spanned the geysers geothermal field and consisted of typically 40 monuments these monuments had been previously employed in a series of first order leveling surveys during the 1970 s this earlier study had determined that the geysers region was subsiding with a maximum rate of 0 048 0 0055 m yr between 1973 and 1977 in order to be able to directly compare the leveling and gps surveys we transform them to the same reference frame using the geoid 96 geoid model for the period 1977 1996 we determine a maximum susbsidence rate of 0 047 0 002 m yr we then model this subsidence using a series of point sources of contraction and find their optimal configuration by applying the random cost method and the f test the minimum volume strain that we find consistent with this subsidence is approximately 5 10 4 such a strain cannot be explained by a thermoelastic mechanism but does seem to be consistent with poroelastic deformation and a quasi static reservoir bulk modulus k of 3 6 109 pa