Nucleation and Arrest of Dynamic Rupture Induced by Reservoir Depletion

Seismic events induced by the depletion of hydrocarbon reservoirs can cause damage to housing and cause societal and economic unrest. However, the factors controlling the nucleation and size of production-induced seismic events are not well understood. Here we used geomechanical modeling of production-induced stresses and dynamic rupture modeling to assess the conditions controlling down-dip rupture size. A generic model of (offset) depleting reservoir compartments separated by a fault was modeled in 2-D using the Finite Element package DIANA FEA. Linear slip-weakening was used to control fault friction behavior. Fault reactivation was computed in a quasi-static analysis simulating stresses during reservoir depletion, followed by a fully dynamic analysis simulating seismic rupture. The sensitivity of reactivation and rupture size to in situ stress, dynamic friction, critical slip distance, and reservoir offset was evaluated. After reactivation, a critical fault length was required to slip before seismic instability could occur. In a subsequent fully dynamic analysis the propagation and arrest of dynamic rupture was simulated. Rupture remained mostly confined to the reservoir interval but could also propagate into the overburden and underburden or sometimes transition into a run-away rupture. Propagation outside the reservoir interval was promoted by a critical in situ stress, a large stress drop, a small fracture energy, and no or little reservoir offset. With increasing offset (up to the reservoir thickness), reactivation was promoted but dynamic rupture size decreased.