Abstract:
We present a finite element method for the one-way coupled poromechanical modeling of injection-induced shear re-activation in a porous medium embedded with a highly conductive pre-existing discrete fracture network (DFN). The fluid problem is formulated over an integrated matrix-fracture domain by permitting two sets of flow constitutive laws and by admitting discontinuities in normal fluid fluxes across fractures to account for matrix-fracture mass exchanges. Based on a transversal uniformity assumption, a novel hybrid-dimensional approach is proposed where factures need not be explicitly meshed along normal directions, but are modeled as linear line elements tangentially conforming to the edges of linear triangular elements representing the porous matrix. Fracture nodes holds no additional degrees of freedom. A dimensional transformation matrix is introduced during finite element interpolation, leading to three additional equi-dimensional modification terms to the mass and stiffness matrices to account for contribution of fractures to flow. The gradient of the modeled fluid pressure is then passed as an equivalent body force vector to the solid problem to solve for induced poroelastic stresses by assuming a single solid constitutive law for the medium. Finally, Coulomb stresses on fractures are calculated for determining onset of shear re-activation.
Introduction
Fluid injection into naturally fractured geological media can induce seismicity over a wide range of scales. An understanding of the physical processes of induced seismic and micro-seismic events helps to better assess potential seismic hazards associated with, e.g., CO2 sequestration and wastewater injection, as well as to assist stimulation of hydrocarbon and geothermal reservoirs with ultra-low permeability. The triggering mechanism of fluid injection induced shear re-activation on pre-existing fractures is fundamentally a coupled hydro-mechanical process. The presence of a discrete fracture network (DFN) imposes significant challenges on numerical modeling of this coupled process, due to not only the geometric complexity, but also two sets of material properties and constitutive laws for both the fluid and the solid, as well as discontinuous changes in modeling targets.
