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. Plain Language Summary Gas production can cause earthquakes, which can be felt at the Earth's surface. Even though these earthquakes are relatively small, they can sometimes cause damage to housing and infrastructure which may have large societal and economic impact. An example of this problem are the earthquakes in the Groningen gas field in the north of the Netherlands, where the damages due to induced earthquakes have led to a production cap and early phase-out of gas production. An important question is how the earthquakes are made, and how large the earthquakes may become. Here we modeled the production-induced earthquakes with geomechanical modeling, which calculates the effect of gas production (pressure changes) on the forces (stresses) in the subsurface. These altered stresses can exceed the strength of preexisting faults in the subsurface, causing the fault to break and generate an earthquake. The modeling results showed that earthquake size depended on many factors such as the initial stress in the reservoir and the fault behavior. The earthquakes often remained confined within the gas producing interval. The geometry of the gas reservoir and faults played a large role in generating the earthquake. Results are consistent with field observations and help to understand the timing, location, and size of seismic events.