A GENERAL FRACTAL-BASED MULTI-SCALE QUIN-MEDIUM MODEL FOR GAS TRANSPORT IN DEEP SHALE RESERVOIR

Abstract

The complexities of deep shale reservoirs are expressed in terms of multi-scale pore structure, abundant micro-fractures, strong stress dependence and multiple gas transport mechanisms. The current macro-scale numerical simulation models have not fully considered the influence of multi-scale pore-fracture structure on gas transport in deep shale reservoirs. In this work, we propose a general fractal-based multi-scale quin-medium model for gas transport in deep shale reservoirs. The quin-medium is composed of organic matter, inorganic matter, micro-fracture medium, induced hydraulic fracture network and hydraulic fracture. The shale matrix is modeled by multiple interacting continua (MINC) with intersected organic matter, inorganic matter and micro-fracture medium. Organic matter is distributed at the center of MINC while the micro-fracture medium is embedded into inorganic matter with layered distribution. The organic/inorganic pore structure and micro-fracture are characterized by fractal geometry, respectively. The apparent gas permeability in each medium is derived considering gas slippage and stress-dependence while the adsorbed gas surface diffusion and Langmuir volumetric strain are additionally considered in organic matter. The apparent gas permeability models are further substituted into the quin-medium continuity equations and the hydraulic fracture is described by an embedded discrete fracture model. The influences of pore structure fractal parameters, gas transport mechanisms and stress dependence on deep shale gas productivity are analyzed in detail. We found that the influence of stress dependence on deep shale gas productivity is more pronounced than the gas transport mechanism. Furthermore, the pore structure fractal parameter of inorganic matter has a more notable influence on deep shale gas productivity than that of organic matter and micro-fracture medium.