LAUSR

An approach is proposed for estimating in situ reservoir pressure changes based on induced earthquakes with overlapping slip patches. The approach is independent of absolute (tectonic) stresses and can also be applied in a scenario where the (unknown) tectonic stresses are strongly heterogeneous. Based on Coulomb friction, the approach makes use of the fact that stress‐strength conditions are in equilibrium at the time of seismic activation. Coseismic stress changes inferred from seismogram recordings provide the stress‐state relative to the failure stress immediately after seismic activation. If the same fracture patch is seismically activated again, relative changes of fluid pressure, that is, the fluid pressure change at which stress‐strength conditions reach equilibrium again, can be inferred from the stress‐deficit when accounting for stress re‐load from neighboring patches. The performance of the technique is demonstrated using a data set of 20,736 earthquakes induced during a hydraulic injection experiment in a geothermal reservoir in the Cooper Basin, Australia. The spatial hypocenter distribution indicates that the seismicity mainly occurred on a single‐layer structure with slip direction controlled by the regional stress field as evidenced by fault mechanisms. Due to the high earthquake density, interearthquake stress interaction is pronounced. The spatial stress impact associated with each earthquake is simulated using a slider‐block type propagation pattern based on Okada's analytical solutions. Using the scalar seismic moment determined from S‐wave spectra, the spatio‐temporal evolution of hydraulic pressure changes is inferred on a reservoir scale. Results indicate that hydraulic flow is most pronounced into NW‐SE direction.