Inclusion of Hydro-Mechanical Behaviour of Discrete Fracture Networks in Coupled Reservoir Geomechanical Simulations
Creators: Sanchez Juncal, Abel
Publisher: name: University of Alberta Library
Year: 2023

<p>Discontinuities are one of the most prominent features of the Earth’s upper crust, these are the result of the action of different geological, mechanical, thermal and chemical activities over millions of years. The term rock mass was created to describe the in situ medium containing intact rock material and rock structures such as joints, faults, fractures, veins, bedding planes and folds. Traditionally, rock masses are considered as a continuous, homogeneous, isotropic and linear elastic material in engineering practice. However, they commonly occur as discontinuous, inhomogeneous, anisotropic and non-elastic materials in nature. Although it is well known that discontinuity networks influence the flow and geomechanical behaviour of fractured reservoirs, their effect and impact on deformability and permeability properties are not usually taken into account in complex reservoir simulations. Therefore, the determination of hydraulic and geomechanical properties is of great importance in characterization of rock mass formations. Inclusion of fracture patterns described by discrete fracture networks (DFN) in coupled reservoir geomechanical simulations is necessary to capture the influence of discontinuities in the reservoir life cycle operations.<br>
A coupling methodology is presented here to include the hydro-mechanical behavior of fracture networks in coupled reservoir geomechanical simulations whereby several commercial simulators are involved and linked together following an explicit sequential coupling scheme built into a coupling simulation platform developed by the reservoir geomechanics research group (RGRG) at the University of Alberta. A porosity correction strategy based on the fixed stress split method has been implemented in the sequential scheme. The generalized tensorial form of the Biot effective stress coefficient, disregarded in most numerical coupling methods, is rigorously included in the thermo-poromechanical coupling formulation. The small and large scale structural features present in the reservoir such as joints and major faults are represented by a DFN. The complex fluid flow processes are captured by the reservoir flow simulator, CMG – STARS. A continuum geomechanical simulator, Itasca – FLAC3D, is used to calculate deformation changes due to new stresses induced by changes in temperature and pore pressure in the reservoir at each simulation stage. A virtual rock mass numerical laboratory, VRM lab, is developed by means of a discontinuum geomechanical simulator, Itasca – 3DEC, to determine through a numerical homogenization process the hydraulic and mechanical equivalent anisotropic properties for the characterization of the equivalent continuum representative of the fractured rock formation. During the numerical simulation, the equivalent permeability, anisotropic elastic parameters as well as the Biot effective stress coefficient tensor can be updated at specific simulation stages in all the fractured reservoir discretization regions or only in those where the change in the effective stress field reaches a certain tolerance to account for effect of pore pressure changes developed during the activities of the reservoir production operations. The explicit sequential coupling scheme implemented in the RGRG coupling platform and the mechanical and hydraulic modules of the VRM lab have been compared and successfully verified against analytical solutions. This research shows the importance of conducting reliable equivalent characterizations of fractured media and properly modelling the effect of material anisotropy in thermo-poromechanical coupled simulations, which is required to correctly model stress-sensitive reservoirs involving anisotropic porous formations. The methodology presented here has the goal of providing some insights in the influence of geomechanics in the overall behaviour and management of fractured porous formations sensitive to stress changes.</p>