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ABSTRACT Large hydrocarbon reserves are trapped in fractured reservoirs where fluid flux is far more rapid along fractures and joints than through the porous matrix, even though the matrix pore volume may be a hundred times greater than the fractures’ pore volume. Accurate recovery prediction in these reservoirs is challenging because of complexity and heterogeneity. Transport properties of the interconnected fractures in such reservoirs are severely affected by production and injection activities that change pore pressures, temperatures, saturations, and effective stresses. Reservoir geomechanics thus must take a significant role in the management of such reservoirs, considering coupled flow-geomechanics responses of the reservoir rocks. In this paper, a coupling approach between fluid-flow and geomechanics in an isothermal continuum using a hybrid FDM/DDM is considered to model the influence of reservoir activities, such as fluid injection or production, on the permeability of partially closed fractures among impermeable matrix blocks. Fracture pressure, aperture, and horizontal and vertical stress variation in a single horizontal fracture is shown as an example for the FDM/DDM model. Also, a geomechanical sensitivity analysis is done to evaluate the effect of Young’s modulus, Poisson’s ratio, fracture normal stiffness, and contact porosity on the fracture pressure and aperture changes. INTRODUCTION In reservoir flow analysis, physical processes such as transport (fluid, heat, chemical flux) and geomechanics (i.e. stress-deformation) should be simultaneously considered to account for important coupling effects. Coupled is required to analyze most natural processes, but uncoupled models may meet engineering needs in cases with one dominant physical phenomenon, such as conductive heat flow in hot dry rocks (HDRs) and fluid flow in shallow aquifers with moderate matrix compressibilities. Uncoupled models are insufficient to model oil and gas reservoir behavior in stress-sensitive cases such as tectonically stressed, massively compacting, or fractured reservoirs. In such cases, we must also deal with complexity and heterogeneity. Production and injection rates in naturally fractured reservoirs are sensitive to pressure and temperature variations, and such reservoirs are also characterized by heterogeneity of fabric. In isothermal, non-reactive processes, pore pressure and deformation of fractures and matrix blocks are the dominant physical parameters in coupled fractured reservoir models. Injection and production are associated with pore pressure changes which induce matrix and fracture deformation. As a result, fracture deformation affects fracture permeability more than the other petrophysical properties. Conventional reservoir simulators consider pore compressibility as the only geomechanical parameter with a pressure-dependent porosity and permeability. From the discussion, this assumption is insufficient for fractured reservoir simulation as fracture permeability is a strong function of effective stress and pressure (?T = 0). In this paper, a hydro-mechanical (HM) coupled approach is used to model fracture behavior in an impermeable matrix block under fluid injection. Hydro-mechanical coupling is based on the concept of effective stress, introduced by Terzaghi (1923) in his one-dimensional consolidation theory. His work was generalized by Biot (1941) into a three-dimensional theory of consolidation, later called the “Theory of Poroelasticity”. This work was oriented toward rock mechanics rather than fluid flow, so there remain issues in the application of Biot’s theory to conventional fluidflow models.
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Unconventional and Complex Reservoirs > Naturally-fractured reservoirs (1.00)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Flow in porous media (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)
Abstract Steam injection for EOR involves high temperatures, usually high pressures, large induced stresses, and associated volume changes, including the effects of shear dilation. Conventional reservoir simulation fails to predict associated transient ground surface movements because it does not consider coupled geomechanics effects. We present a fully coupled, thermal half-space model using a hybrid DDFEM method, in which a simultaneous FEM (Finite Element Method) solution is adopted for the reservoir and the surrounding thermally affected zone, and a DD (displacement discontinuity) method used for the elastic, non-thermal zone. This approach provides transient ground surface movements in a natural manner. Introduction Enhanced oil recovery (EOR) methods involving high pressures and steam injection (steamflooding, steam line-drive, cyclic steam stimulation, steam-assisted gravity drainage) are accompanied by large volume changes in the reservoir horizon. Ground movements in excess of 300 mm heave or subsidence are registered during injection and production cycles. Reservoir simulation cannot address this phenomena without due consideration of geomechanics effects. The first attempts to account for coupled pore fluid behavior and soil deformation led to Terzaghi's one-dimensional consolidation theory, still used in soil mechanics. Since Biot's theory of consolidation was introduced to petroleum engineering by Geertsma, with the coined term "poroelasticity", coupled analysis of petroleum geomechanics effects has been widely advocated. Coupled reservoir simulation can be carried out in a loosely coupled fashion or with a tightly coupled scheme, and comparisons of different coupling techniques can be found. In coupled reservoir simulation, computing challenges persist for large-scale 3D applications to real cases where there are a huge number of equations to solve iteratively; e.g. thermoporo- elasto-plastic analyses involving several simultaneous diffusion processes (Darcy, Fourier, Fick). In regions of large pressure, temperature, concentration, or stress gradients, accurate solution requires small-scale discretization. If the problem has strong non-linearity, such as changes in permeability, compressibility, or other properties arising from changes in pressure and effective stress, the computational effort increases by several orders of magnitude because multiple iteration loops are needed. These issues mean that accurate analysis of realistic complex 3D problems is challenging, and will so remain as we seek to solve larger and larger coupled problems involving non-linear responses. Also, more accurate coupled reservoir modeling requires that a sufficiently large domain be analyzed because mixed stress-displacement boundary conditions are difficult to incorporate. In an analytical solution developed by Rothenburg et al. for stress-coupled transient radial flow of a compressible fluid into a fully penetrating well, the stiffness of the overburden is shown to be an essential coupling element which must be taken into account. Settari and Osorio et al. also suggest that the analysis domain should include overburden, sideburdens and underburden for better accommodation of the coupling effects of stress changes and flow. Hettema et al. demonstrate that accurate depletion-induced subsidence modeling requires understanding of the reservoir and surrounding rock mechanical response to the depletion. surrounding rock mechanical response to the depletion. To partly address this di
- North America > Canada (0.46)
- North America > United States (0.46)
ABSTRACT ABSTRACT: Thermal oil recovery processes involve high stresses, pressures, temperatures and volume changes. Traditional reservoir simulation fails to predict associated transient ground surface movements because it does not consider coupled geomechanics effects. We present a fully-coupled, thermal half-space model using a hybrid DDFEM method, in which a simultaneous FEM solution is adopted for the reservoir and the surrounding thermally affected zone, and a displacement discontinuity method (DDM) used for the elastic, non-thermal zone. This approach provides transient ground surface movements in a natural manner. 1 INTRODUCTION Since Biot’s theory of consolidation (Biot 1941) was introduced to petroleum engineering by Geertsma (1966) with the coined term “poroelasticity”, coupled analysis of petroleum geomechanics effects has been widely advocated (Dusseault 1999, 2004, Gutierrez et al. 1998). Coupled reservoir simulation can be carried out in a loosely coupled fashion (Settari et al. 1998, Fung et al. 1994) or with a tightly coupled scheme (Tortike 1987, Li et al. 1992, Lewis et al. 1998), and comparisons of different coupling techniques can be found (Dean et al. 2006, Samier et al. 2006). In coupled reservoir simulation, computing challenges persist in large-scale 3D applications to real cases where there are a huge number of equations to solve iteratively; e.g. thermoporoelastoplastic analyses involving several simultaneous diffusion processes (Darcy, Fourier, Fick). In regions of large pressure, temperature, concentration or stress gradients, accurate solution requires small-scale discretization. If the problem has strong non-linearity, such as changes in permeability, compressibility, or other properties arising from changes in pressure and effective stress, the computational effort increases by several orders of magnitude because multiple iteration loops are needed. These issues mean that accurate analysis of realistic complex 3D problems is challenging, and will so remain as we seek to solve larger and larger coupled problems involving non-linear responses.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Petroleum Play Type > Unconventional Play > Heavy Oil Play (0.46)
- Reservoir Description and Dynamics > Reservoir Simulation (1.00)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Flow in porous media (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)
- Reservoir Description and Dynamics > Improved and Enhanced Recovery > Thermal methods (0.88)