层理和临界长度对页岩水力压裂的影响研究

Study on the Effect of Bedding and Critical Length on Shale Hydraulic Fracturing

准确预测水力压裂起裂压力对页岩储层改造工程具有重要意义。页岩层理强度弱于基质,会对起裂压力产生影响,但其起裂机制还需进一步研究;此外现有考虑临界长度的起裂压力预测模型未基于力学原理描述井筒壁面的弹塑性响应,不具备内插外推能力。因此,开展了层理和临界长度对页岩水力压裂起裂压力影响的实验和模型研究。具体结论如下:(1)页岩的起裂压力和临界长度随层理角度变化呈先减后增的变化;(2)层理角度未影响裂缝张拉起裂机制,但通过影响临界长度来影响起裂压力;(3)起裂压力随临界长度增加呈线性增加,随着储层埋深增加塑性增强,必须考虑临界长度对起裂压力的影响。研究成果可为页岩气水力压裂工程设计提供理论指导。

Accurate prediction of hydraulic fracture initiation pressure is of great significance to shale reservoir simulation projects.The bedding strength of shale is weaker than that of the matrix,which will affect the fracture initiation pressure,whereas the fracture initiation mechanism remains to be further studied;in addition,the existing fracture initiation pressure prediction model considering the critical length does not describe the elastic-plastic response of the wellbore wall based on mechanical principles,and does not have the ability of interpolation and extrapolation.Therefore,experimental and model studies on the effect of bedding and critical length upon the initiation pressure of shale hydraulic fracturing are carried out.The specific conclusions are as follows:(1)The fracture initiation pressure and critical length of shale decrease first and then increase with the variation of bedding angle.(2)The bedding angle does not affect the fracture tension initiation mechanism,yet impacts fracture initiation pressure by influencing the critical length.(3)The fracture initiation pressure increases linearly with the increase of the critical length.The plasticity increases with the increase of the reservoir depth,and the influence of the critical length on the fracture initiation pressure must be considered.The above results can provide certain theoretical guidance for the design of shale gas hydraulic fracturing engineering.