A coupled methane/air flow model for coal gas drainage: Model development and finite-difference solution

Abstract

Gas drainage through underground boreholes is intensively associated to safety mining and clean energy capture. The key factor restricting the improvement of underground gas drainage is air leakage around borehole, which always leads to the rapid decay of drained gas concentration and methane production rate. Although tremendous work has been implemented on coal-gas interactions, few studies address the air leakage phenomenon in underground gas drainage operations. This work first presents a coupled compositional flow model by integrating the methane-air mixture flow in fracture, methane flow within the matrix, mass transfer between fractures and matrixes, and permeability evolution induced by gas depletion. Then, a numerical model and a simulator are developed using the finite difference method (FDM) to solve the compositional model and are successfully validated against two sets of in situ gas drainage data. Subsequently, the effect of parametric variations on gas drainage performance is quantified through a series of simulations. The simulated results reveal that: (1) A higher drainage pressure corresponds to a higher drained gas concentration and a longer time for air leakage into the borehole; at the beginning of drainage, the methane flow rate increases linearly with the decrease of drainage pressure. (2)The efficiency of lowering the drainage pressure to enhance methane production is getting weaker with the drainage pressure decreasing. Lowering drainage pressure will transfer more air to the drainage borehole, decrease the concentration of drained gas and pure methane production. Therefore the drainage pressure should be optimized by comprehensively balancing methane production, gas concentration and costs; (3) Increasing the sealing area on the coal wall around the borehole will prevent air from flowing into coal, and linearly promote the concentration of drained gas and methane production; (4) A longer borehole sealing length corresponds to a higher concentration of drained gas and a slower decay of methane production rate, while the increase of drained methane flowrate results from increasing borehole sealing length is non-linear, and an optimal borehole sealing length could be evaluated using the proposed model and simulator to promote the gas extraction efficiency. This coupled model and the numerical results improve the understanding of the methane-air flow behaviors and air leakage phenomenon in coal during underground gas drainage, and suggest a powerful tool for evaluating the drainage gas production objectively and optimizing the gas extraction system scientifically.