煤层气井水力压裂液氮伴注与CO2驱替技术研究

Liquid nitrogen injection and CO2 displacement in hydraulic fracturing of CBM wells

在我国煤层气的开发中普遍面临煤层具有的低压、低渗、低饱和度等自然属性问题,针对此问题,提出利用液态气体伴注辅助水力压裂改造煤层技术。文章阐述了液氮伴注技术提高煤层临界解吸压力机理和CO2驱替煤层甲烷机理,结合芦岭煤矿地面煤层气工业试验,进行了液氮伴注辅助水利压裂、液态CO2驱替煤层甲烷试验以及效果分析。结果表明:注入液氮后氮气分子会挤占煤层甲烷分子的空间,为甲烷气体提供外部能量,同时能够降低煤层甲烷分子分压,提高其临界解吸压力,促使煤层更快的解吸出甲烷气体,提高产气量,试验2号井,达到产气峰值3145.2m^3/d仅用190d,稳产期平均产气量为1400m^3/d;CO2具有的强吸附性能够与吸附态煤层甲烷发生置换作用,促使煤层甲烷更快的由吸附态变为游离态,实现煤层甲烷大量解吸的效果,同时CO2在等压条件下还能够降低游离甲烷分压,进一步提高产气量,试验3号井,实际/理论临界解吸压力比值为3.29,达到产气峰值3351.9m^3/d仅用了124d,稳产期平均产气量为800m^3/d。对比可知:液氮伴注技术优势明显,且在后续煤矿工作面回采过程中无新的CO2突出风险。

As the coalbed methane development in China is facing the problems of low pressure,low permeability,low saturation and other natural attributes of coal reservoirs,the 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,combining with the ground coalbed methane industrial test of Luling Coal Mine,the test results of liquid nitrogen and liquid CO2 displacement with hydraulic fracturing are analyzed.The results show that,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,accelerate methane gas desorption in coal seam,and promote gas production,it only took 190 days to reach the peak gas production of 3145.2 m^3/d in No.2 well,and the average gas production during the stable production period was 1400 m^3/d.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 massive desorption of coalbed methane,CO2 can also reduce the partial pressure of free methane and promote gas production under constant pressure,the theoretical critical desorption pressure ratio was 3.29,and it only took 124 days to reach the peak gas production of 3351.9 m^3/d in No.3 well,and the average gas production during the stable production period was 800 m^3/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.