A novel approach to characterize gas flow behaviors and air leakage mechanisms in fracture-matrix coal around in-seam drainage borehole

Abstract

Modeling gas flow behaviors in coal is the basis for quantitatively predicting gas production and planning the life cycle for coalbed methane recovery. Air leakage is a tricky problem encountered in underground borehole gas drainage, which seriously affects gas extraction quality. However, seldom studies consider the air flows in coal associated with drainage air leakage, which may yield some misleading guidance for in-situ drainage operation. This work first presents two theoretical models (single-phase flow model (S-model)/compositional flow model (C-model)) to characterize gas flow behaviors through the fracture-matrix system of coal. The S-model only regards the methane flows, while the C-model couples the methane flows and air flows in coal to model drainage borehole leakage. Then the C-model is numerically solved using the finite difference method and validated to be reliable for evaluating actual gas drainage. The results reveal: (a) The S-model will overestimate methane production, while the C-model can accurately predict drained gas concentration, methane production and drainage air leakage; (b) Air leakage flux is mainly dominated by the fractures gas flux, and slightly affected by the matrix flows; (c) Poor gas deliverability of coal matrix incurs a slow diffuses/releases rate of adsorbed methane, forming a weak gas source for the fracture flows, which is the main culprit of rapid attenuation of methane production; (d) the drained gas concentration is collaborative determined by the fracture flow flux and the matrix flow flux; (e) Sealing up the coal wall around the borehole will linearly reduces air leakage flux and promote the drainage concentration.