Gas drainage performance evaluation in coal under non-uniform stress with different moisture content: Analysis, simulation and field verification

Abstract

Moisture has a non-negligible effect on the adsorption characteristics of coal reservoirs and the migration of gas. In this study, we analysed the adsorption behaviours of coal samples with different moisture contents in the 3# coal reservoir of the Xinjing Coal Mine in the Yangquan Mining Area. We then built a one-way sequential coupling process for a geomechanical and multi-physics simulator. The process was used to simulate the gas changes in the process of gas drainage in coal reservoirs with different moisture content, under a condition of a non-uniform stress distribution in the overlying strata. The result show that under the action of gas drainage, the relative gas contents of coal reservoirs with higher moisture contents decrease faster, and the gas pressures decrease more. That is, the gas pressure drop in coal reservoirs with 11.6% moisture content is 1.95 times that in coal reservoirs with 1.09% moisture content, and 2.44 times that in coal reservoirs with 0.54% moisture content. The pressure difference between the matrix gas pressure (p1) and fracture system gas pressure (p2) provides the driving force for the gas diffusion and seepage in coal reservoirs. The engineering practice results also validate that the moisture content has a better correlation with the relative gas content change and coal samples with higher moisture contents have relatively lower gas contents. Therefore, it is necessary to consider the moisture factor when establishing the coupling model or scheme, which is of great significance for accurately predicting the production of coalbed methane.