Impact of Poro-Elastic Coupling and Stress Shadowing on Injection-Induced Microseismicity in Reservoirs Embedded With Discrete Fracture Networks

Abstract

We propose that slip triggering mechanism of injection-induced microseismicity in naturally fractured reservoirs can be significantly impacted by two mechanical effects: poro-elastic coupling and stress shadowing. We numerically investigate these impacts through 2D finite element modeling of coupled single-phase fluid flow and geomechanics in reservoirs embedded with stochastic discrete fracture networks (DFN). We assume that the reservoir is a dual-porosity double-permeability medium with inter-porosity mass exchange neglected and Darcy flow throughout the reservoir, and that it is linearly elastic. Fluid injection is modelled by prescribing either fluid flux or pressure, and a full degree of poro-elastic response is enforced by prescribing zero displacements at the reservoir boundaries to allow occurrence of a reactive stress field from reservoir surroundings (in contrast to a constant stress boundary condition in previous studies). Following a sequential coupling scheme, we utilize FEM to solve the one-way coupled conservation of mass and linear momentum, where the fluid pressure gradient is passed as an equivalent body force vector to obtain the induced deformation, strain and stress at selected time steps. We show that the fluid pressure distribution is strongly controlled by the DFN, and induces highly heterogeneous strain and stress field, both compressional and dilatational, within the reservoir. Finally, we compute the effective normal stress (referred hereon as ENS) and shear stress resolved on each fracture before and after injection, and determine the location of the stress state in relation to the shear failure line. We observe that, (1) most fractures are affected by both decreased ENS and shear stress, thus their likelihood of re-activation is reduced, and (2) some fractures experience increased ENS and decreased shear stress, also moving away from re-activation. We attribute the first observation to poro-elastic coupling: reservoir surroundings respond to elevated fluid pressure to increase the total in-situ stresses within the reservoir, and the second to stress shadowing: the stress state of a region between highly pressured fractures evolves to inhibit shear failure within that region. We are investigating stimulation scenarios with mixed stress and displacement boundary conditions to allow different degrees of poro-elastic coupling and stress shadowing, thereby promoting/demoting additional shearing during fluid injection.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016