煤变质程度对中低阶煤储层孔裂隙发育的控制作用

Coal metamorphism controlling regulation on the development of pores and fractures in low-medium rank coal reservoirs

煤变质程度是控制煤储层物性的关键因素,不同煤阶煤储层孔裂隙发育特征存在较大差异。以鄂尔多斯盆地东缘山西组煤层为例,基于SEM、荧光显微观察、压汞、低温液氮吸附、低场核磁共振和X-CT扫描三维重构等实验技术手段,研究不同煤阶煤储层物性特征及变化规律,揭示煤变质程度对孔裂隙发育的控制作用规律。研究表明:随着变质程度增高,煤中植物组织孔、粒间孔等原生孔隙减少而气孔等次生孔隙增加;孔隙度呈"减小—增大—减小"的波状变化,大中孔比例先减小后趋于稳定,微小孔比例变化趋势与之相反;吸附孔孔径增大,BJH总孔体积和BET比表面积减小,孔隙结构趋于均一而孔隙表面先变粗糙后逐渐光滑;煤中裂隙先减少后增加,裂隙性质逐渐变好,裂隙排列逐渐规则化。

As coal metamorphism is a key factor controlling the physical properties of coal reservoirs,the development characteristics of pores and fractures show significant differences in different ranks.In this paper,based on the methods of scanning electron microscopy（SEM）,fluorescence microscopy observation,mercury intrusion,low-temperature liquid nitrogen adsorption,nuclear magnetic resonance（NMR） and computed tomography（CT）,taking the coal seam of Shanxi Formation in eastern Ordos Basin,the characteristics and variation of coal reservoirs in different ranks are studied and the controlling regulation of metamorphism on the development of pores and fractures in coal are revealed.The results show,as the coal rank increases,the quantity of plant tissue pores and intergranular pores in coal decreases while the quantity of gas pores increases.The porosity of the coals varies in a wavelike fashion of ‘decreasing-increasing-decreasing’ as the coal rank increases.The proportion of mesopores and macropores decreases at first and then tends to be stable and the proportion of micropores and transition pores shows opposite variation with coal rank increasing.During coal metamorphism,the BET surface area and BJH total volume of adsorption pores decrease,the average pore diameter increases,the surface of adsorption pores becomes rough at first and then smooth,and the pore structure tends to be homogenized.As the coal rank increases,the quantity of fractures decreases firstly and then increases,their length increases and the fractures arranges regularly.