降压速率对沁水盆地南部高阶煤产气能力的影响

Influence of depressurization rate on gas production capacity of high-rank coal in the south of Qinshui Basin, China

参照山西晋城地区大宁煤矿3号煤层,设计模拟地层条件下解吸仿真实验,进行相同流体饱和状态、不同降压速率下的解吸模拟实验,对产气量及样品两端压力变化进行分析,并对部分实验现象进行机理探讨。实验结果表明,无论快速降压还是慢速降压,高阶煤吸附的甲烷均能有效解吸,解吸效率均能达到90%以上;解吸过程中产气量曲线存在明显拐点,均出现在煤块整体降至解吸压力之后;高阶煤甲烷解吸主要受压差驱动,高压差有利于快速解吸;快速降压出现解吸拐点时间更早,且快速解吸段产气速率更高。实验结论对原公认的煤层气井排采需要坚持"缓慢、长期"的原则提出了质疑,认为对于高阶煤层,快速排水降压可有效提高煤层气开采的经济效益。沁水盆地南部樊庄区块和郑庄区块快速降压排采现场试验结果显示,提高排水降压速率可以显著提高气井的峰值产量并缩短气井达到经济产量的时间;相比慢速排采策略,快速降压排采的气井平均达产时间缩短一半,高峰产气量更高。

A desorption simulation experiment with the condition of simulated strata was designed.The experiment,under different depressurizing rates and the same fluid saturation,was conducted on the sample from 3#coal of Daning coal mine in Jincheng,Shanxi Province.The gas production rate and pressure change at both ends of the sample were studied systematically,and the mechanisms of some phenomena in the experiment were discussed.The experimental results show that,whether at fast or slow depressurizing rate,the methane adsorbed to high-rank coal can effectively desorb and the desorption efficiency can reach above 90%.There is an obvious inflection point on the gas yield curve during the desorption process and it appears after the pressure on the lump of coal reduces below the desorption pressure.The desorption of methane from high-rank coal is mainly driven by differential pressure,and high pressure difference is conducive to fast desorption.In the scenario of fast depressurization,the desorption inflection appears earlier and the gas production rate in the stage of rapid desorption is higher.It is experimentally concluded that the originally recognized strategy of long-term slow CBM production is doubtful and the economic benefit of CBM exploitation from high-rank coal can be effectively improved by rapid drainage and pressure reduction.The field experiment results in pilot blocks of Fanzhuang and Zhengzhuang show that by increasing the drainage depressurization rate,the peak production of gas well would increase greatly,the time of gas well to reach the economic production shortened,the average time for a gas well to reach expected production reduced by half,and the peak gas production is higher.