Experimental and Numerical Study on the Anisotropic and Nonlinear Gas Flow Behavior of a Single Coal Fracture under Loading

摘要

The flow behavior of the coal fracture is one of the important factors that control the production of coal bed methane after hydraulic fracturing operation in a coal seam. In this paper, a gas flow test was conducted in the single coal fracture perpendicular to the bedding planes. The nonlinear flow behavior was studied by applying differing pressure gradients in the gas flow test. A numerical flow simulation was carried out to study the anisotropic flow behavior. The numerical simulation was based on matching of the experimental results and the digital fracture geometry reconstructed from surface laser scanning and a self-developed surface mate algorithm. Based on the experimental and numerical results, a coal fracture permeability model was proposed considering both nonlinear and anisotropic effects. The proposed model was used in a fictive field operation to reveal their effects on the well performance. According to this study, we found that (1) the permeability of the coal fracture has strong anisotropy which is mainly affected by the irregular contact shape; (2) the nonlinear flow effect was obtained after the Reynolds number exceeded 10; (3) both anisotropic and nonlinear effects are dependent on the loading stress; and (4) the anisotropic and nonlinear effects have a strong influence on the fluid flow under field-scale conditions that cannot be neglected when performing production prediction after the hydraulic fracturing operation.