椭圆形钻孔应力场解析解及水力压裂特性  

Analytical solution of stress field on elliptical borehole and its hydraulic fracturing characteristics

作　　者：尹光志[1,2] 刘超[1,2] 李铭辉[1,2] 张东明[1,2] 邓博知[1,2] 鲁俊 刘玉冰[1,2] YIN Guangzhi;LIU Chao;LI Minghui;ZHANG Dongming;DENG Bozhi;LU Jun;LIU Yubing(State Key Laboratory of Coal Mine Disaster Dynamics and Control,Chongqing University,Chongqing 400030,China;College of Resources and Environmental Sciences,Chongqing University,Chongqing 400030,China)

机构地区：[1]重庆大学煤矿灾害动力学与控制国家重点实验室,重庆400030 [2]重庆大学资源及环境科学学院,重庆400030

出　　处：《煤炭学报》2019年第S01期61-73,共13页Journal of China Coal Society

基　　金：国家自然科学基金资助项目(51874053,51434003);重庆市研究生科研创新基金资助项目(CYB19046)。

摘　　要：地下储层岩石在钻探之前处于原岩应力状态,钻孔成孔是在煤岩有侧限地应力下完成的。在钻孔时,孔周围应力重新分布,使圆形钻孔在地应力作用下呈椭圆状。因此,在进行水力压裂时,增压水可能作用于椭圆形孔壁。通过合理的物理假设,根据复变函数理论,求解出椭圆形钻孔在远场应力和钻孔内压(椭圆长轴两端的局部对称荷载)共同作用下的钻孔周围应力场解析解。该解析解可退化成椭圆形或圆形钻孔内受均布荷载的情况。通过给定应力场和钻孔形状,根据最大周向应力准则,得出在水力破裂时水平应力差和钻孔形状共同影响着破裂压力和水力主裂缝的起裂。当水平应力差较大时,椭圆形钻孔“越扁”,形成水力主裂缝所需的水压越高。当水平应力差为零时,椭圆形钻孔“越扁”,形成水力主裂缝所需的水压则越低;当水平应力差较小(本文为5 MPa)且钻孔越扁时,最大周向应力可能出现在平行于最大水平主应力方向上,使得水力主裂缝沿着最小水平主应力方向起裂。此外,由于物理假设的局限性,该解析解可能更适合于均质性较好的砂岩或具有横观各向同性特性的页岩。不同储层岩石(如砂岩、页岩和原煤)因地应力作用所形成的椭圆形钻孔在均布荷载作用下的破裂压力特性和水力裂缝扩展规律还需要通过实验和现场进一步验证。Subsurface reservoir rocks have been in the state of in-situ stress prior to drilling.The borehole is performed under confined pressure,which results in stress redistribution around the borehole.Thereby,the borehole shape may change from a circular borehole to an elliptical borehole under in-situ stress.Therefore,the pressurized water caused by hydraulic fracturing technology may act on the elliptical borehole wall.The analytical solution of stress filed around elliptical boreholes under far-field stress and borehole internal pressure(local symmetric stress on both ends of the elliptical long axis)is obtained by using complex variable function under reasonable physical assumptions.The analytical solution can be degenerated into the uniform stress on the elliptical or circular borehole wall.Assuming that the shape of the borehole and stress field are given,the horizontal stress differences and the borehole shapes jointly affect the breakdown pressure and the initiation of the main hydraulic crack based on the maximum circumferential stress criterion.When the horizontal stress difference is large,the flatter the elliptical borehole is,the higher the water pressure is needed to form the main hydraulic crack.When there is no horizontal stress difference,the water pressure needed to form the main fracture is lower.When the horizontal stress difference is small and the borehole is flatter,the maximum circumferential stress may appear in the direction parallel to the major horizontal stress,which makes it possible for the hydraulic main fractures to crack in the direction parallel to the minor principal stress.In addition,due to the limitations of physical assumptions,the analytical solution may be more suitable for sandstones with better homogeneity or shale with transversely isotropic properties.The breakdown pressure properties and hydraulic crack propagation of elliptical boreholes formed by in-situ stress in different reservoir rocks(sandstone,shale and raw coal)under uniform load need to be further verified by exp

关 键 词：椭圆形钻孔 解析解 水力压裂 预测 

分 类 号：TD311[矿业工程—矿井建设]