关键词: 煤层气, 水力压裂, 液氮伴注, CO2驱替, 工业试验

Abstract: Coalbed methane development in China is generally faced with the problems of low pressure, low permeability, low saturation and other natural attributes of coal reservoirs, aiming at this problem, the technology of hydraulic fracturing assisted by liquid gas injection is put forward. The mechanism of improving the critical desorption pressure by liquid nitrogen injection and the mechanism of CO2 displacing coal seam methane are described respectively, combined with the ground coalbed methane industrial test of Luling Coal Mine, the test results of liquid nitrogen and liquid CO2 displacement with hydraulic fracturing were analyzed. The results show that, (1) after injecting liquid nitrogen, nitrogen molecules will occupy the space of methane molecules in the coal seam and provide external energy for methane gas, at the same time, it can reduce the molecular pressure of methane, improve the critical desorption pressure, promote its faster desorption of methane gas in coal seam, and promote gas production, it only took 190 days to reach the peak gas production of 3145.2m3/d in NO.2 well, and the average gas production during the stable production period was 1400m3/d. (2)The strong adsorption of CO2 can replace the methane with the adsorbed CH4, which promotes the rapid change of coalbed methane from the adsorption state to the free state, and achieves the effect of massive desorption of coalbed methane,CO2 can also effectively reduce the partial pressure of free methane and promote gas production under the condition of constant pressure, the theoretical critical desorption pressure ratio was 3.29, and it only took 124 days to reach the peak gas production of 3351.9m3/d in NO.3 well, and the average gas production during the stable production period was 800m3/d. Comparative analysis shows that the advantages of liquid nitrogen injection technology are obvious, and there is no CO2 outburst risk in the subsequent coal mining process.