基于PPCZ模型的KGD水力压裂数值模拟

Pore pressure cohesive zone模型以Barenblatt裂尖黏聚力模型为基础,考虑了裂尖断裂过程区对裂缝扩展的影响,对模拟水力裂缝扩展问题具有一定优势。采用该数值模型对KGD水力压裂问题进行研究,验证了模型模拟KGD水力裂缝的能力,分析了裂尖断裂过程区及流体黏度μ对裂缝扩展的影响,揭示了排量Q_0、黏度μ和注入时间t三者之间的联系。研究表明:(1)断裂过程区对压裂结果有较大影响,其内在原因是:随着裂缝刚度K_(nn)的变化,缝内流体不同程度地渗透到裂尖断裂过程区,改变了裂尖断裂的难易程度和缝内流体压力的响应行为;(2)流体黏度增加导致缝内净压力和缝宽的增加、缝长的减小;(3)对于两种主控型水力裂缝,排量Q0、黏度μ和注入时间t之间存在着内在联系:μQ_0不变、Q_0t不变情况下,最终可得相同的缝压、缝宽和缝长值;(4)相比于弹性状态,弹塑性状态下的岩体需要更高的缝内净压力,才能使得岩体开裂扩展。数值模型具有模拟不同类型水力压裂的能力。

Numerical simulation of KGD hydraulic fracture based on PPCZ model

Based on Barenblatt’s cohesive zone model,the pore pressure cohesive zone model takes into account on the effect of fracture process zone,which has some advantages for simulating the hydraulic fracture propagation.This model was used to study the KGD hydraulic fracturing problem.Its capability of simulating the KGD hydraulic fracture was verified.The influence of fracture process zone and fluid viscosity μ on the fracture propagation was analyzed.The relationship between fluid displacement Q0,fluid viscosity μ and injection time was revealed.Research shows that ① the fracture process zone has an impact on the fracturing result.As the crack stiffness Knn changes,the fluid penetrates to fracture process zone to different extents,which changes the ease of fracture and the response of fluid pressure within the fracture.② An increase in fluid viscosity results in an increase in net pressure and crack width,but a decrease in crack length.③ The final geometry of hydraulic fracture will not change in μQ0 unchanged,and  Q0t unchanged conditions.④ Compared with the elastic state,the elastoplastic state of the rock requires a higher net pressure to initiate and propagate.Pore pressure cohesive zone model has the ability to simulate different types of hydraulic fractures.Conclusions can be further developed for practical engineering applications.