Evaluation of underground coal gas drainage performance: Mine site measurements and parametric sensitivity analysis

Abstract

Underground gas extraction from coal formations has triple-effects involving mining safety, low-carbon gas capture and greenhouse gas control. Air leakage around the drainage borehole is a serious problem that continuously affects gas drainage performance. In this study, mine site measurement of gas drainage data is firstly performed in coal mine, and then a mechanism-based model is proposed to theoretically describe gas desorption and diffusion and flow through coal around the drainage borehole. Further, the propose model is numerically solved and verified with borehole drainage data measured in mine site. Followed this, the validated mechanism-based model is implemented to conduct parametric studies. The results showed that: (a) as the fracture permeability increases from 3 × 10−22 m2 to 3 × 10-14 m2, the air leakage flux increases from 7355 m3/d to 18,303 m3/d, and the gas concentration decreases from 46.9 % to 12.7 %, it indicates that changing the permeability around the borehole may be a wise strategy to control air leakage; (b) the coal matrix parameters (including permeability, sorption constant and radius of matrix) have a dynamic impact on gas drainage performance at the different stage of gas drainage. For example, the increment of methane production induced by increasing the sorption constants does not exceed 3.3 % at the drainage time ∼0.34 day; while the growth increases to more than 19.5 % at the drainage time ∼ 9.75 days; (c) at the initial stage of extraction gas production is mainly determined by the fracture flow. A higher permeability of coal fracture will incur more air leakage flux, reducing gas concentration in drainage borehole; (d) whereas, the matrix parameters dominate gas flow at a later stage. Increasing matrix permeability, sorption property or decreasing the matrix radius will enhance the gas exchange flux from the pore system of coal matrix to the fracture system, subsequently incurring a higher concentration/production of drainage gas. The simulated result and field tests demonstrates that sealing on the coal wall around the borehole can block a portion of air leakage paths and reduce air leakage linearly, which illuminates a more efficient strategy to minimize air leakage for underground gas extraction.