Abstract
Over recent decades, it has become clear that the extraction of fluids from underground reservoirs can be linked to seismicity and aseismic deformation around producing fields. Using a simple model with uniform fluid extraction from a reservoir, Segall (1989, “Earthquakes Triggered by Fluid Extraction,” Geology, 17(10), pp. 942–946) illustrated how poroelastic stresses resulting from fluid withdrawal may be consistent with earthquake focal mechanisms surrounding some producing fields. Since these stress fields depend on the spatial gradient of the change in pore fluid content within the reservoir, both quantitative and qualitative predictions of the stress changes surrounding a reservoir may be considerably affected by assumptions in the geometry and hydraulic properties of the producing zone. Here, we expand upon the work of Segall (1989, “Earthquakes Triggered by Fluid Extraction,” Geology, 17, pp. 942–946 and 1985, “Stress and Subsidence Resulting From Subsurface Fluid Withdrawal in the Epicentral Region of the 1983 Coalinga Earthquake,” J. Geophys. Res. Solid Earth, 90, pp. 6801–6816) to provide a quantitative analysis of the surrounding stresses resulting from fluid extraction and diffusion in a horizontal reservoir. In particular, when considering the diffusion of fluids, the spatial pattern and magnitude of imposed stresses is controlled by the ratio between the volumetric rate of fluid extraction and the reservoir diffusivity. Moreover, the effective reservoir length expands over time along with the diffusion front, predicting a time-dependent rotation of the induced principal stresses from relative tension to compression along the ends of the producing zone. This reversal in perturbed principal stress directions may manifest as a rotation in earthquake focal mechanisms or varied sensitivity to poroelastic triggering, depending upon the criticality of the pre-existing stress state and fault orientations, which may explain inferred rotations in principal stress directions associated with some induced seismicity.
Earthquake focal mechanisms of the induced seismicity in 2006 and 2007 below Basel (Switzerland)
