Experimental Study on Gas-Water Seepage in High-Rank Coal and Its Research Significance

The gas-water seepage parameters of coal reservoir have to do with the development effect of coalbed methane (CBM) wells, and determine the output characteristics of fluid in coal. The gas-water seepage parameters of coals in Jincheng coal mining area under the condition of gas-driven water was tested by using an unsteady-state method. The variation law of the relative permeability (Kr) of gas-water in coal with gas saturation is analyzed, and the gas-water seepage "triangle area" is proposed to clarify the seepage characteristics of coal reservoir. The impact of wettability and pore structure on fluid seepage and the output law of gas-water in coal during CBM drainage are revealed. It is indicated that the gas-phase permeability (Kg) of four specimens ranges from 0.021 x 10-3 to 0.068 x 10-3 mu m2, and the water-phase permeability (Kw) ranges from 0.001 x 10-3 to 0.041 x 10-3 mu m2. With increase in gas saturation (Sg), the relative permeability of the gas-phase (Krg) increases and that of the water-phase (Krw) decreases. The characteristic parameters of the gas-water seepage curve exhibit that Krg at the irreducible water is high, the relative permeability at the crossover point is low, Krw at the residual gas is high, the span of two-phase flow is narrow, and the saturation of residual gas is low. The high-rank coal is strong hydrophilic. With increase in vitrinite reflectance, the wetting angle increases linearly, and Krw increases. With increase in porosity, the saturation of irreducible water in two-phase flow decreases and the span of two-phase flow increases. The irreducible water saturation increases with increase in specific surface area; the span of two-phase flow is decreasing. Gas-water seepage in the dewatering and depressurization process of CBM wells will greatly reduce the drainage efficiency. Based on the current situation of CBM well drainage, the conditions and evaluation models of gas-water production are given, and the laws of gas-water seepage in CBM well production are revealed. After coal reservoir pressure drops to the critical desorption pressure, the water yield in CBM well goes down rapidly, while the gas yield goes up rapidly, and Kw drops continuously, which is caused by the gas-water seepage in coal.