高煤阶煤层气富集机理的深化研究

Comprehensive study on the enrichment mechanism of coalbedmethane in high rank coal reservoirs

高煤阶煤层气资源相对丰富是中国煤层气资源的重要禀赋之一,实现高煤阶煤层气大规模商业化开发突破是中国煤层气产业的世界贡献。以沁水盆地为代表的高煤阶煤层气生产基地是目前中国煤层气产量的主体来源,基于沁水盆地等高煤阶煤层气勘探开发工程实践数据和大量研究工作积累,开展了高煤阶煤层气富集机理的系统深化研究。研究工作表明:区域岩浆热变质作用成因高阶煤是高煤阶煤层气富集的基本地质背景,沁水盆地南部(晋城)、鄂尔多斯盆地东南缘(韩城—延川南)、沁水盆地北端(寿阳—阳泉)、黔北—川南(织金—筠连)等中国高煤阶煤层气富集区煤储层均以区域岩浆热变质成因为主;高煤阶煤层气富集规律是煤层含气量和渗透率在不同埋深和构造条件下耦合配置的结果,在沁水盆地高煤阶煤层气向斜富集模式、褶曲翼部斜坡带富集模式、构造高位富集模式分别对应于煤层埋深约500 m以浅、500~800 m埋深、约800 m以深;决定高阶煤储层渗透率的主控地质因素构成高煤阶煤层气富集关键主控地质因素,主要为煤层埋深、地质构造(含构造应力场)、煤体结构等;煤层渗透率随埋深呈负指数关系减小,煤层拉张裂隙发育且与现今最大主应力方向一致的构造部位煤层渗透率高,不同煤体结构高阶煤中碎裂煤渗透率最高;埋深、地质构造、煤体结构等关键主控地质因素通过煤储层地应力变化、裂隙发育与应变破坏等方式控制高阶煤储层渗透性,这是高煤阶煤层气富集的主要机理,在地应力场作用下煤岩应变破坏过程对煤层中裂隙发育的自然改造作用是高阶煤储层原始渗透率变化的根本原因。

The relative enrichment of high-rank coal coalbed methane(CBM)is one of the important natural endowments of China’s CBM resources.China’s CBM industry has made a world-class contribution on the breakthrough of large-scale commercial exploitation of high-rank coal CBM.The high-rank coal CBM production base,represented by the Qinshui Basin,is the main source of CBM production in China.Based on the practical data and a lot of works in the exploration and development of high-rank coal CBM in the Qinshui Basin,a systematic and in-depth research on the enrichment mechanism of high-rank coal CBM is carried out.The research works show that:①regional magmatic thermometamorphism is the basic geological background of high-rank coal CBM enrichment.The high-rank coal CBM enrichment areas in China consist of the southern Qinshui Basin(Jincheng),the southeast of the Ordos Basin(Hancheng-Southern Yanchuan),the north of the Qinshui Basin(Shouyang-Yangquan)and the Northern Guizhou-the Southern Sichuan(Zhijin-Junlian).The coal reservoirs in these areas are all affected by regional magmatic thermometamorphism.②The enrichment laws of high-rank coal CBM are the results of the coupling configuration of gas content and permeability under different buried depths and structural conditions.In the Qinshui Basin,the high-rank coal CBM enrichment mode in syncline,ramp of fold limb and relatively high structural part correspond to coal seams at the depth of<500,500-800 and>800 m respectively.③Depth,structure(tectonic stress field)and coal-body structures,which can determine the permeability of high-rank coals,are the key controlling geological factors for the CBM enrichment in the high-rank coal reservoirs.The permeability of coals exponentially decreases with depth.The coals characterized by a high permeability are distributed in the structure position where the tensile fractures develop and is consistent with the direction of the current maximum principal stress.The cataclastic coal has a highest permeability compared to undeformed coal,granulated coal and mylonitized coal.④The control of these key geological factors on coal permeability by means of in-situ stress,fracture and strain failure is the fundamental mechanism of the CBM enrichment in the high-rank coal reservoirs.The transformation of fracture in the coal reservoirs during a strain failure process under the action of the geostress field is the intrinsic reason for the permeability change of high-rank coals.