Evaluation of the pre-drained coal seam gas quality

Abstract

Coal seam degasification through underground drilling and its efficiency are directly related to the safety of underground coal mining. The major problem of underground coal seam gas drainage is the rapid decay of gas concentration, which may lead to a low utilization ratio and many hazards, such as environmental pollution, spontaneous combustion of coal, gas combustion and gas explosion. Although coal-gas interactions have been comprehensively investigated, fewer studies consider the low-quality phenomenon (low gas flow and concentration) in the process of gas extraction due to the air leakage of the borehole.In this study, a fully coupled coal deformation and compositional flow model, which represents the important non-linear responses of the gas-drained quality due to the effective stress changes, was implemented into a finite element (FE) model to demonstrate the superiority and reliability of the model through a comparison with another theoretical models and a historical data matching. Subsequently, the susceptibilities of gas-drained quality associated with the intrinsic and extrinsic factors, incorporating the gas sorption, the porosity–fracture characteristics of coal, the sealing depth and the leakage flux, are quantified through a series of simulations. The simulation results revealed that (1) increasing/decreasing the CH4 Langmuir volume/pressure sorption parameter can improve the gas-drained quality, and (2) the leakage fracture characteristics around the borehole are the main factors affecting the gas-drained quality, and thus, increasing the coal permeability or extending sealing depth does not necessarily improve the gas-drained quality under the condition of serious leakage. This FE model and its simulation results can improve the understanding of the coal-gas interactions of underground gas drainage, providing a scientific basis for the evaluation of the gas-drained quality, the design and optimization of drainage systems, etc.