页岩层理弱面对裂缝穿层扩展的影响数值模拟研究

Numerical simulation of the effect of weak shale bedding on fracture propagation through strata

层理弱面是影响页岩油气水力裂缝穿层扩展的关键因素之一,而层理弱面强度主要由黏结强度、内摩擦角以及抗张强度决定。为探究层理弱面对水力裂缝穿层扩展的影响机理,基于三维块体离散元法建立了含层理弱面页岩压裂数值模型,研究了黏结强度、抗张强度以及内摩擦角对水力裂缝穿层和裂缝扩展形态的影响。数值模拟结果表明:层理黏结强度对水力裂缝穿层扩展存在显著影响,黏结强度较低时水力裂缝主要为横向扩展,并伴随着较大的层理激活程度;而高黏结强度下水力裂缝穿层能力较强,主要呈近圆形扩展;层理激活主要为剪切破坏,即层理抗张强度对水力裂缝扩展无明显影响,在实际工程中可忽略不计;水力裂缝的穿层能力随内摩擦角的增大而增强,在低内摩擦角下水力裂缝易沿层理扩展。

Bedding strength is one of the key factors affecting the propagation of hydraulic fractures through beddings,which is mainly determined by cohesion,internal friction angle and tensile strength.However,there are few researches on this.Therefore,in order to further study the influence mechanism of bedding strength on the propagation of hydraulic fractures through beddings.Based on the three-dimensional block discrete element method,this paper simulated the propagation morphology of hydraulic fractures under different cohesion,tensile strength and internal friction angle.The simulation results show that bedding cohesion has a significant influence on the propagation of hydraulic fractures through beddings.Under low cohesion,hydraulic fractures mainly shows transverse propagation,accompanied by a larger degree of bedding activation,however,at high bonding strength,the permeability of hydraulic fractures is strong,and the propagation is nearly circular.The tensile strength of bedding has no obvious effect on the propagation capacity of hydraulic fracture and can be ignored in practical engineering.The penetration of hydraulic fractures increases with the increase of internal friction angle,and the hydraulic fractures are easy to diffuse along bedding at low internal friction angle.