Inferring In Situ Hydraulic Pressure From Induced Seismicity Observations: An Application to the Cooper Basin (Australia) Geothermal Reservoir

Abstract

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.

Inferring Hydraulic Pressure Changes from Induced Seismicity Observations: Three Showcases from Geothermal Reservoirs

“…Our first dataset stems from a geothermal reservoir at St. Gallen (Switzerland) where fluid injection activities have caused a series of a few hundred earthquakes occurring on a single fault zone as indicated by the distribution of relative hypocentre locations [7]. Compared to the dataset used by Baisch [5], the number of earthquakes in this dataset is two orders of magnitude smaller and overlap of the slip area of neighbouring earthquakes is less pronounced. With this dataset, we aim to explore the performance of SHPM for small datasets with only few fracture patches slipping repeatedly.…”

“…With our third dataset, we aim to investigate the performance of SHPM in a scenario where initial stress conditions are strongly heterogeneous. The dataset stems from the same geothermal reservoir in the Cooper Basin that has already been studied by Baisch [5]. Our analysis, however, is based on data from a restimulation of the Habanero#1 well in 2005, where the initial stimulation has resulted in heterogeneous stress conditions prior to restimulation.…”

“…In a recent study, Baisch [5] has proposed a new approach for inferring in situ fluid pressure changes from induced seismicity observations, which could be an important step towards calibrating numerical models. The approach, hereafter referred to as seismohydraulic pressure mapping (SHPM), is based on relative stress and pressure changes occurring between repeated slippage of the same patch of a fracture.…”

“…The approach, hereafter referred to as seismohydraulic pressure mapping (SHPM), is based on relative stress and pressure changes occurring between repeated slippage of the same patch of a fracture. Baisch [5] demonstrates the performance of the SHPM approach using induced seismicity data from an EGS reservoir (Enhanced Geothermal System) in the Cooper Basin, Australia. For this, he has made several assumptions which are closely linked to certain characteristics of the induced seismicity in the Cooper Basin, namely, that all earthquakes have occurred on the same, large-scale (planar) fault and share a common source mechanism, and the slip area of neighbouring events is overlapping.…”

“…For this, he has made several assumptions which are closely linked to certain characteristics of the induced seismicity in the Cooper Basin, namely, that all earthquakes have occurred on the same, large-scale (planar) fault and share a common source mechanism, and the slip area of neighbouring events is overlapping. Although these conditions may seem to be very specific, Baisch [5] speculates that similar conditions may actually prevail at other locations, thus allowing application of the same method.…”