郑庄区块煤储层水力压裂裂缝扩展地质因素分析

Analysis on Geological Control Factors of Hydraulic Fracture Extension of Coal Reservoirs in Zhengzhuang Block

为了研究煤储层水力压裂裂缝扩展规律,以沁水盆地郑庄区块15口煤层气井水力压裂裂缝的微地震监测数据为基础,通过分析压后裂缝方位、长度和高度等参数,从煤岩学特征、岩石力学特征、地应力条件及煤岩裂隙发育特征4个方面综合研究了压裂裂缝扩展的地质控制因素。结果表明:郑庄区块区压裂裂缝方位近NEE向,与区域水平最大主应力方向相近,垂直裂缝发育;演化程度较高的煤层更易形成长缝,且煤岩显微组分含量与压后缝长有一定相关性;裂缝长度随煤岩抗拉强度及弹性模量的增大而减小,煤岩与顶底板岩石力学性质的差异限制了裂缝纵向扩展;随埋深增加,煤储层闭合应力增大,但裂缝长度与煤层埋深负相关性较弱,原生裂隙在一定程度上阻碍了裂缝扩展。

In order to study hydraulic fracture extension rule in coal reservoirs,microseismic data of 15 hydraulically fractured wells had been chosen in Zhengzhuang block,Qinshui Basin.Based on the data of fracture direction,length and height,the geological controlling factors of fracture extension were integratedly investigated from four aspects:coal petrology characteristics,rock-mechanics property,crustal stress conditions and the pre-fractures.The results showed that the hydraulic fractures orientation were nearly NEE,along the direction of the maximum horizontal principal stress.The vertical fractures were well developed in the study area.The hydraulic fractures extended more easily in the relative high coal rank reservoirs.Moreover,the fracture length depended on the coal maceral compositions,and they decreased with increasing tension strength and elastic modulus. Meanwhile, the rock-mechanics property difference between the coal seam and its roof/floor could effectively limit the extension of vertical fractures.The closure stress increased with the coal seam depth,but because of the pre-fractures,there was a low correlation between the fracture length and burial depth.