Wastewater Injection and Slip Triggering: Results from a 3d Coupled Reservoir/Rate-and-State Model

This paper present results from a combined model that brings together injection physics, reservoir dynamics, and fault physics to better explain the primary controls on induced seismicity. We created a 3D fluid flow simulator with embedded discrete fracture technique, coupled with a 3D displacement discontinuity geomechanics model that uses rate and state friction to model stable or unstable rupture events. The model incorporates reservoir properties including vertical and horizontal extent; stratification including top-seal, reservoir, and basement; multiple permeability and porosity. Injection parameters include rate and pressure. Fault properties include size, 2D permeability, and frictional properties. Several suites of simulations were run to evaluate the relative importance of each of the factors from all three parameter groups. We find that the injection parameters interact with the reservoir parameters in the context of the fault physics. For a given reservoir and fault properties, injection rate increases magnitude and frequency of earthquakes, and volume is unimportant. For a different reservoir, these relations may change, leading to the need to specify/examine the injection parameters only within the context of a particular faulted reservoir. Both injection and reservoir properties can interact with the fault properties to trigger or impede slip, so that the magnitudes of induced earthquakes depend on all three groups of parameters. For example, the fault permeability structure is a key factor in inducing earthquakes in basement in many reservoir scenarios. In some cases, the main component in inducing seismicity include the pressure on the fault and its rate of change, which affect how big of a fault area is being affected, and therefore initial earthquake size. By implication, selecting reservoirs for wastewater disposal may involve prioritizing those reservoirs with higher permeability as it takes longer for fluid pressure to exceed critical pressure and trigger large unstable rupture events on nearby faults.