Abstract
In recent years, the non-Darcy flow has seen a significant increase in interest in conventional and unconventional gas developments. The nonlinear behavior can be described by adding a quadratic term of the velocity with the permeability-dependent β factor in Darcy’s law. The methods for improving permeability such as the hydraulic flushing hole enlargement (HFHE) technique can remarkably enhance the coal permeability thus changing the β factor. However, few studies have been done previously to seek how this non-Darcy flow impacts the permeability-enhanced gas drainage. In this study, a fully coupled coal deformation and damage, the non-Darcy flow of free methane gas in the fractures, and Fickian diffusion of adsorbed methane in the coal matrix model were developed. The Klinkenberg effect has been taken into account as a reference object. A series of simulation scenarios were carried out to evaluate and compare the influences of the non-Darcy effect, gas-coal interaction effect, and Klinkenberg effect on the HFHE-enhanced gas drainage. Results of the simulation illustrate that the HFHE technique can significantly improve the gas drainage efficiency by permeability enhancement. But the HFHE-enhanced gas drainage is unique to the other gas developments with high or low flow rates; a slight pressure gradient and short drainage time make the non-Darcy effect fail to generate cumulative influence on the HFHE-enhanced gas drainage.
Highlights
Due to the distinctive geological environment and the increasing mining level, most of China’s coal mines in the main coal production bases face serious coal and gas outburst dangers
The governing equations of diffusion and non-Darcy flow were solved by the partial differential equations (PDEs) module, and the deformation equations for the hydraulic flushing hole enlargement (HFHE)-enhanced permeability distribution were solved by the solid mechanics module
The HFHE-induced improvement of gas drainage efficiency is benefited from two aspects
Summary
Due to the distinctive geological environment and the increasing mining level, most of China’s coal mines in the main coal production bases face serious coal and gas outburst dangers. The permeability of China’s coal seams is usually in the magnitude of 10−4~10−3 mD (except for Jincheng coalfield), which is four orders of magnitude lower than the US and three orders of magnitude lower than Australia [1, 2]. The fairly low permeability challenges the feasibility, cost, and performance of coal mine methane (CMM) drainage. Hydraulic flushing hole enlargement technique has played an increasingly important role in solving the problem of poor efficiency associated with gas drainage, especially in soft coal seams [1, 3,4,5]. The HFHE-enhanced CMM drainage technique involves the use of high-pressure water jet in a borehole to flush the surrounding coal body. The enhanced permeability of the surrounding coal is generally recognized as the main reason for the performance improvement of gas drainage
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