Experimental study on gas mass transfer process in a heterogeneous coal reservoir

Abstract Series and parallel seepage simulation experiments of coal matrix pore sample, natural fracture sample and artificial fracture sample with three permeability ranges were designed and performed. These three samples represent the low permeability zone, transition zone and fracturing zone in the development coal seam. In the parallel experiment, fracturing fractures are the dominant seepage channel, and the gas seepage in the matrix and natural fractures is limited. In the series experiment, a high-efficiency mass transfer stage has the displacement pressure difference is greater than 7.3 MPa, which represents the coal reservoir has a strong gas supply capacity. On the contrary, a low-efficiency mass transfer stage appears when the displacement pressure difference is less than 7.3 MPa, suggesting the decline in gas supply capacity. Due to the rapid output of near-well desorbed gas, a short gas production peak will be formed in the early production stage. After that, due to insufficient gas supply on the parallel path, the gas production rate drops rapidly. The second gas production peak appears in the high-efficiency mass transfer stage. Finally, due to the limited expansion of the pressure drop funnel, the gas production gradually decreased and eventually stabilized after entering the low-efficiency mass transfer stage.