Numerical modeling of microearthquakes by using coupled geomechanics simulation for planar fracture propagation
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Copyright 2016 ARMA, American Rock Mechanics Association. We modeled microearthquakes (MEQs) for tensile failure of vertical fracture propagation by combining geophysics, flow, and geomechanics simulators while accounting for poromechanical effects. Each time fracturing occurred, we calculated the seismic moment tensor from the obtained displacement field and newly fractured area. Then, using this information, we modeled intensity, location, number of the events, and time of MEQs. We simulated various scenarios, taking single phase flow (i.e., water) in order to remove complex responses from multiphase flow. We first studied a synthetic reservoir having one single shale layer with two strong bounding layers and a horizontal well. The simulated MEQs reflected the propagation of the hydraulic fracture, where the magnitudes of most MEQs were between -1 and -3. Then, we simulated a vertical well in the more realistic geological model of the Arch Forth-Worth Basin, where the Barnett shale is located. We studied two scenarios: one in which the fluid was injected into the Lower Barnett shale and it only fractured into the injected layer, and one in which there was fluid migration from the injection point into the Upper Barnett Shale. The magnitudes of MEQ for these two scenarios were similar and mostly between -0.75 and 0.75 in strength. For all cases, the event locations of MEQs corresponded to the fracture propagation. Thus, the forward simulation of microearthquakes can be a useful to detect propagation of hydraulic fractures and stimulated reservoir volume. Additionally, we showed that under some reservoir conditions it is possible for fluid to fracture layers above the injection point without fracturing the targeted layer.
