Experimental study on commingled production with constant-rate of a multi-superimposed gas system under different interlayer pressure difference

This paper presents results of an experimental study on gas production characteristics of commingled production of multi-superimposed gas system in eastern Yunnan and western Guizhou. The large-scale multi-field coupling test system was utilized to simulate commingled production with constant rate and three different pressure difference on four coal seams. The coalbed methane exploitation, reservoir pressure, instantaneous production, and productivity contribution rate of the four coal seams were analyzed. The experimental results show that No.1 coal seam shows obvious reservoir pressure rise which increased to 1.1 MPa in 11.5 minutes. The reason is that the excessive interlayer pressure difference causes interlayer interference, resulting in increase of the reservoir pressure of coal seam with low-pressure. However, the pressure increases at the early stage of commingled production and then decreases with time. When the production rate is 11.25 L/min, the initial instantaneous production rate of the four coal seams are -23.4、-1.6、9.3 and 18.3 L/min, respectively. When the gas production capacity of a single coal seam is higher than the constant production rate, part of the gas production of coal seam with high pressure enters into coal seam with low-pressure through gas well. The larger interlayer pressure difference is associated with larger gas volume of reversed gas. When the pressure differences were 0.2、0.3 and 0.5 MPa, the gas production contribution rate of No.1 seam at the 10 min was -3.2%、-10.4% and -16.9%, respectively. The result indicates that, at the early stage of commingled production, greater interlayer pressure difference has greater inhibitory effect on the gas production in low pressure coal seam. The gas production of coal seams with different reservoir pressure presents a "dynamic equilibrium" gas production feature during the stable production period. When the gas production capacity of high pressure coal seam is insufficient, the gas production capacity of low pressure coal seam begins to increase, maintaining stable gas production.