Impact of fluid injection on fracture reactivation at The Geysers geothermal field

Abstract

We analyze the spatiotemporal distribution of fault geometries from seismicity induced by fluid injection at The Geysers geothermal field. The consistency of these faults with the local stress field is investigated using (1) the fault instability coefficient I comparing the orientation of a fault with the optimal orientation for failure in the assumed stress field and (2) the misfit angle β between slip vectors observed from focal mechanisms and predicted from stress tensor. A statistical approach is applied to calculate the most likely fault instabilities considering the uncertainties from focal mechanisms and stress inversion. We find that faults activated by fluid injection may display a broad range in orientations. About 72% of the analyzed seismicity occurs on faults with favorable orientation for failure with respect to the stress field. However, a number of events are observed either to occur on severely misoriented faults or to slip in a different orientation than predicted from stress field. These events mostly occur during periods of high injection rates and are located in proximity to the injection wells. From the stress inversion, the friction coefficient providing the largest overall instability is μ = 0.5. About 91% of the events are activated with an estimated excess pore pressure <10 MPa, in agreement with previous models considering the combined effect of thermal and poroelastic stress changes from fluid injection. Furthermore, high seismic activity and largest magnitudes occur on favorably oriented faults with large instability coefficients and low slip misfit angles.

Deciphering the Stress State of Seismogenic Faults in Oklahoma and Southern Kansas Based on an Improved Stress Map

“…The required pore pressure increases are consistent with the estimated pore pressure using similar geomechanical analysis for multiple induced clusters in Texas (Quinones et al, ; Snee & Zoback, ). The observed pore pressure range is also consistent with the findings in the Geyser geothermal field in Martínez‐Garzón et al (), where faults with a broad range of orientations are activated by fluid injection and the misoriented faults are mostly activated during high injection rates in proximity to the injection wells by an estimated pore pressure increase of ∼10 MPa.…”

Insights Into Complex Subdecimeter Fracturing Processes Occurring During a Water Injection Experiment at Depth in Äspö Hard Rock Laboratory, Sweden

“…We assume that the strength of the rock mass surrounding the injection borehole is limited by ubiquitous fractures and joints with a friction coefficient of 0.85 taken from laboratory experiments and that pore pressure is approximately hydrostatic, which at the depth of 400 m corresponds to 4 MPa. We calculated the fault instability coefficient for 21 stable fault plane solutions and selected the respective nodal planes favorably oriented for slip in the local stress field (for details see Martínez‐Garzón et al, ; Vavryčuk, ). Interestingly, the estimated stress states for favorably oriented nodal planes of most events projected on the Mohr circle diagram suggest that they are critically stressed (i.e., located close to the failure envelope), despite of apparent heterogeneity in the kinematics represented by the mechanisms.…”

“…The analysis of full seismic moment tensors reveals the amount of volumetric and shear strain during faulting and allows to infer the respective contributions from fracture opening and closing and shear displacements (Norris et al, ) possibly related to changes in pore fluid pressure (e.g., Fischer & Guest, ; Rutledge et al, ; Staněk & Eisner, ; Zhao et al, ) and permeability (Baig et al, , ; Martínez‐Garzón et al, ; Norris et al, ; Zhao et al, ). Full moment tensors also provide information on fault plane orientations that can be used to calculate the local stress field through stress tensor inversion (e.g., Martínez‐Garzón et al, , ). Focal mechanisms may also be used to detect the potential of earthquake propagation along preexisting and potentially highly stressed faults leading to large seismic events (e.g., Martínez‐Garzón et al, ; Walsh & Zoback, ; Yeck et al, ).…”

Temporal static stress drop variations due to injection activity at The Geysers geothermal field, California

“…The following analysis was performed using a high‐quality IS data set from the NW part of The Geysers geothermal field. The cluster of seismicity located in the vicinity of Prati‐9 and Prati‐29 injection wells consisted of 1254 relocated events [ Kwiatek et al ., ; Martínez‐Garzón et al ., , , ] recorded between 10 of December 2007 and 23 of August 2014 by a local seismic network composed of 31 three‐component short‐period surface geophones operated by Lawrence‐Berkeley National Laboratory. For a representative group of 354 events high‐quality stress drops were calculated using mesh spectral ratio technique [ Kwiatek et al ., ].…”