Mechanics of Induced Seismicity
We are investigating the physics of injection-induced seismicity including full poroelastic coupling of stress and pore pressure, and time-dependent earthquake nucleation. In one study we model stress and pore pressure due to specified injection rate in a homogeneous, poroelastic medium. During injection poroelastic coupling may increase or decrease the seismicity rate, depending on the orientation of the faults relative to the injector. If injection-induced stresses inhibit slip, abrupt shut-in can lead to locally sharp increases in seismicity rate; tapering the flux mitigates this effect. This is shown in the first figure along the x-axis, and also in the time snapshots in Figure 2.
The maximum magnitude event has been observed to occur postinjection. We suggest the seismicity rate at a given magnitude depends on the nucleation rate, the size distribution of fault segments, and if the background shear stress is low, the time-varying volume of perturbed crust. This leads to a rollover in frequency-magnitude distribution for larger events, with a “corner” that increases with time. Larger events are absent at short times, but approach the background frequency with time; larger events occurring post shut-in are thus not unexpected as seen in the simulation of Figure 3.
Current work is now exploring more realistic geometries, with injection into a sandstone layer overlying basement containing normal faults. The faults may be either low or high permeability and connected or isolated from the reservoir (Figure 4).