Abstract:
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.
Introduction
Hydraulic fracturing has revolutionized the energy industry for the last decade. This process entails pumping large amounts of sand and water down a well, where the increase in pressure creates a network of fractures that allow petroleum engineers to access trapped hydrocarbons. The operation is poorly understood, so the development of successful hydraulic fracturing methods has been too often a trial and error process. This poses monetary and environmental risks, and a general sense of mistrust towards the oil and gas industry. However, the basics of hydraulic fracturing dictates that a large planar fracture is created in a direction perpendicular to the minimum horizontal stress. In addition to this, a significant number of shear failures (micro-earthquakes) are generated in the surrounding intact reservoir rock [1]. Furthermore, depending on the reservoir on which it is performed, hydraulic fracturing can cause fault reactivation and shear slip of existing natural fractures [2, 3].
The simultaneous combination of these phenomena makes the physics of the problem a difficult subject to study. Nevertheless, because the failure induced by perturbation of fluid pressure implies strong interaction between flow and geomechanics [4], we employed a coupled simulator to accurately predict and assess any risk derived from hydraulic fracturing a vertical well.
