高阶煤的孔隙结构特征及其对煤层气解吸的影响

Pore structure characteristics of high rank coal and its effect on CBM desorption

以沁水盆地高阶煤3^#煤层为研究对象,借助高压压汞实验对高阶煤的孔隙参数进行测试,研究了高阶煤的孔隙结构特征,采用解吸速率实验对高阶煤的解吸速率和解吸量进行分析,并探讨了孔隙结构对煤层气解吸产出的控制规律。结果表明:3^#煤层的孔隙半径较小,煤层孔隙结构复杂;煤层主要以气体吸附孔和气体扩散孔为主,气体渗流孔占比很少,煤层的吸附气体体积大、吸附性能强、气体的扩散、渗流条件差。3^#煤层孔隙结构分形特征曲线呈"两段型",孔径大于940.7 nm时,不具有分形特征;孔径小于940.7 nm时,分形维数介于2.67~2.76之间,具有很好的分形特征。高阶煤的煤层气解吸特征具有快速解吸和慢速解吸2个阶段,快速解吸时间短,解吸量占比低;慢速解吸时间长,解吸量占比高,煤层气解吸困难。煤层的孔隙结构对煤层气的解吸具有重要影响,高阶煤较差的孔隙结构控制着煤层气解吸速率慢、解吸量低、产出程度低,煤层气井生产实践中表现为开始阶段产气量增长快,产气高峰时间短,稳产气量低、生产时间长,煤层气开发难度大。研究结果为高阶煤的煤层气抽采效果评价提供参考依据。

Taking No.3 coal of high-rank coal in Qinshui Basin as a research object,the pore structure characteristics of high-rank coal were analyzed by high-pressure mercury injection experiment,and the desorption rate experiment was used to analyze the desorption characteristics of high-rank coal,and the control law of pore structure on CBM desorption was studied. The results show that:The pore radius of the No.3 coal seam is small and the pore structure of the coal seam is complex. The coal seam is mainly dominated by gas adsorption pore and gas diffusion pore,and the gas seepage pore accounts for a small proportion. The No.3 coal is large in volume of adsorption gas,strong in adsorption performance,poor in gas diffusion and seepage conditions. The fractal characteristic curve of the pore structure of No.3 coal seam shows a "two-section type". When the pore diameter is bigger than 940.7 nm,it has no fractal characteristic. When the aperture is smaller than 940.7 nm,the fractal dimension is between 2.67 and 2.76,with good fractal characteristics. The desorption of CBM in high-rank coal with rapid desorption and slow desorption has two distinct stages. Rapid desorption time is short,desorption rate is low,slow desorption time is long,desorption rate is high,and CBM desorption is very difficult. The pore structure of high rank coal controls the output of CBM. The poor pore structure of high rank coal results in low degree of CBM output and slow desorption rate. In the practice of CBM well drainage and gas recovery,it is difficult to develop CBM due to the rapid growth of gas production in the initial stage,short stable period of peak gas production,low gas production and long production time. The results provide a reference for evaluation of CBM extraction efficiency of high rank coal.