High Rank Coal Pore Fracture Structure and its Impact on Reservoir Characteristics in the Southern Qinshui Basin

Although the southern Qinshui Basin is the most successful area for coalbed methane (CBM) development in China, the production of CBM wells in different blocks in the area is significantly different. One of the key reasons is the difference in pore structure in various-ranked coal. In this study, No. 3 coal seam of Sihe and Zhaozhuang blocks in southern Qinshui Basin was selected as the research object to investigate the high rank coal pore fracture structure and its impact on reservoir characteristics. Mercury intrusion porosimetry (MIP), low-temperature liquid nitrogen adsorption (LTN(2)A), scanning electron microscopy (SEM), and isothermal adsorption tests were conducted. The results show that the Sihe No.3 coal seam was mainly composed of open cylindrical and flat pores with a high proportion of transition pores (10-100 nm), large specific surface area, good connectivity, strong adsorption capacity, high gas content, and reservoir energy. Zhaozhuang No.3 coal had high proportion of mesopores (100-1,000 nm), small specific surface area, poor pores connectivity, weak adsorption capacity, poor gas content, low reservoir energy, and critical desorption pressure. The proportion of cylindrical pores, parallel plate pores, and wedge-shaped pores closed at one end was high. The anomalies in pore morphology and pore structure characteristics of coal reservoir were the main factors that caused variation in gas production of No.3 coal seam in Sihe and Zhaozhuang blocks.

虽然沁水盆地南部是中国煤层气开发最成功的地区，但该地区不同区块的煤层气井产量差异显著。其中一个关键原因是不同煤阶煤的孔隙结构存在差异。本研究选取沁水盆地南部寺河和赵庄区块的3号煤层作为研究对象，对高煤阶煤的孔隙裂隙结构及其对储层特征的影响进行研究。进行了压汞孔隙度测定（MIP）、低温液氮吸附（LTN(2)A）、扫描电子显微镜（SEM）以及等温吸附试验。结果表明，寺河3号煤层主要由开放的圆柱状孔和平板状孔组成，过渡孔（10 - 100nm）比例高，比表面积大，连通性好，吸附能力强，含气量高，储层能量大。赵庄3号煤中孔（100 - 1000nm）比例高，比表面积小，孔隙连通性差，吸附能力弱，含气量低，储层能量低，临界解吸压力低。一端封闭的圆柱状孔、平行板状孔和楔形孔比例较高。煤储层孔隙形态和孔隙结构特征的异常是导致寺河和赵庄区块3号煤层产气差异的主要因素。