Her-Yuan Chen

Coupled Fluid Flow and Geomechanics in Reservoir Study - I. Theory and Governing Equations

The purpose of this study is to examine Biot`s two-phase (fluid and rock), isothermal, linear poroelastic theory from the conventional porous fluid-flow modeling point of view. Not`s theory and the published applications are oriented more toward rock mechanics than fluid flow. Our goal is to preserve the commonly used systematic porous fluid-flow modeling and include geomechanics as an additional module. By developing such an approach, complex reservoir situations involving geomechanical issues (e.g., naturally fractured reservoirs, stress-sensitive reservoirs) can be pursued more systematically and easily. We show how the conventional fluid-flow formulations is extended to a coupled fluid-flow-geomechanics model. Consistent interpretation of various rock compressibilities and the effective stress law are shown to be critical in achieving the coupling. The {open_quotes}total (or system) compressibility{close_quotes} commonly used in reservoir engineering is shown to be a function of boundary conditions. Under the simplest case (isotropic homogeneous material properties), the fluid pressure satisfies a fourth-order equation instead of the conventional second-order diffusion equation. Limiting cases include nondeformable, incompressible fluid and solid, and constant mean normal stress are analyzed.

Coupling Fluid-Flow and Geomechanics in Dual-Porosity Modeling of Naturally Fractured Reservoirs

The purpose of this study is to formulate a coupled fluid-flow/geomechanics model of a naturally fractured reservoir. Fluid flow is modeled within the context of dual-porosity (more generally, overlapping-continuum) concept while geomechanics is modeled following Biots isothermal, linear poroelastic theory. The development follows along the line of the conventional and existing porous fluid-flow modeling. The commonly used systematic fluid-flow modeling is therefore preserved. We show how the conventional fluid-flow dual-porosity formulations are extended to a coupled fluid-flow/geomechanics model. Interpretation of the pore volumetric changes of the dual continua, fractures and matrix-blocks, and the associated effective stress laws are the most difficult and critical coupling considerations. New relations describing the dual rock volumetric changes are presented. These relations allow a smooth and consistent transition between single-porosity and dual-porosity concepts and are in terms of measurable quantities. Reduction to the single-porosity is presented to demonstrate the conceptual consistency of the proposed model.

A Two-Domain, 3D, Fully Coupled Fluid-Flow/Geomechanical Simulation Model for Reservoirs With Stress-Sensitive Mechanical and Fluid-Flow Properties

A 3D finite-difference, fully implicit model has been developed to represent the physical phenomena occurring during the production from reservoirs with stress-sensitive mechanical and fluid-flow properties. The model considers two different physical domains: (i) an inner domain representing the reservoir, where fluid-flow and rock deformation occur, and (ii) an outer (surrounding) domain representing the extended stress-disturbed region caused by reservoir depletion. The inclusion of the surrounding domain leads to realistic modeling of the actual geomechanical boundary conditions taking place in the subsurface. The reservoir is treated as a multiphase poroelastic system consisting of a deforming solid skeleton and a moving compressible pore fluid. Non-linear elastic deformation is assumed for both domains. The governing equations describing the deformation of the surrounding domain, the deformation of the reservoir solid skeleton and the motion of the pore fluid are fulty coupled. Simulation results clearly show that the permeability of stress-sensitive reservoirs may significantly change through the reservoir producing life. Different reservoir conditions yield different degrees of permeability reduction. In general, permeability decreases as production time increases. Zero-displacement boundary conditions yield less permeability reduction than constant stress boundary conditions. The permeability reduction decreases as the rock elastic moduli of the outer domain increase. The arch effect has a strong influence on reservoir permeability behavior.

Flow-Induced Stress Reorientation in a Multiple-Well Reservoir

Flow-induced stress changes in reservoirs can result in reorientation of principal stresses. Stress reorientation has a direct effect on fracture orientation and thus drainage or flood patterns. The full-length paper studies stress reorientation in a multiple-well reservoir to determine controlling parameters. The study adopted an analytical-based, coupled fluid stress model. Multiple wells and variable flow rates are handled by the principle of superposition. Principal stresses are time- and space-dependent and are determined by initial stress anisotropy, permeability anisotropy, flow mode, and flow rate. Well interference is identified as an important factor affecting stress reorientation.

Fully Coupled Fluid-Flow/Geomechanics Simulation of Stress-Sensitive Reservoirs

In this study, a 3D finite-difference model for the simulation of stress-sensitive reservoirs has been developed.