Coupled Response Of A Fault Zone Network To Hydraulic Stimulation, A Numerical Investigation

Abstract

In the context of Enhanced/Engineered Geothermal Systems, stimulation techniques are required to improve the reservoir injectivity/productivity. Among other techniques, hydroshearing aims at reactivating preexisting fault zones (FZs) by increasing the well net pressure. The net pressure reduces the effective normal stress acting on the FZs intersecting the well, which may then shear under the in situ stress state. Once shearing has occurred, the success of the stimulation is mainly conditioned by two mechanisms: - The dilational opening of the shearing FZs (caused by the irregular nature of their walls), - The incomplete closing of the FZs at well shut-in (resulting from the misalignment of their walls after shearing). The conjugation of these mechanisms result in an irreversible opening of the stimulated FZs, and contribute to enhancing the well injectivity. The potential for a single fault zone to be successfully reactivated mostly depends on its orientation towards the in situ stress state. However, when a reservoir incorporates several FZs, interactions between those structures also affect the result of the hydraulic stimulation. We propose a numerical model to investigate the hydromechanical (HM) response of a fault zone network to a hydraulic stimulation. This model, based on the Distinct Element Method (DEM), explicitly accounts for the FZs as hydromechanically active joints. The first step of the simulations is to build the model geometry from in situ data (UBIs, flow logs…). The typical geometry for DEM models is an assembly of blocks (rock matrix) interacting through joints (fault zones). In the next step, the physical description of the system must be provided. In our case, the following features are addressed: - Deformation of the rock matrix (elastic blocks), - Deformation of the FZs, including irreversible phenomena (elasto-plastic joints), - Opening of the FZs under shearing (dilation), - Fluid circulation through the FZs (where matrix permeability is assumed negligible compared to FZs transmissivity), - Impact of the in situ stress state through the applied boundary conditions.

AAPG Datapages/Search and Discovery Article #90345 © 2018 AAPG European Region, Geothermal Cross Over Technology Workshop, Part II, Utrecht, The Netherlands, April 17-18, 2018