水力裂缝穿越非连续面扩展时的断裂过程研究

Propagation process of hydraulic fracture crossing an orthogonal discontinuity

非连续面发育是非常规油气储层的显著地质特征之一,水力裂缝能否穿越非连续面扩展会关系到压裂的改造效果。为研究水力裂缝穿越非连续面扩展时断裂过程区(fracture process zone,简称FPZ)发育特征,采用自主设计的可视化压裂试验装置对含预制摩擦界面的砂岩平板试件开展水力压裂试验。基于数字图像相关法实时监测了水力裂缝正交穿越界面扩展过程中的位移及应变场特征。试验结果表明,水力裂缝穿越界面扩展之前,断裂过程区已经开始跨越界面发育;裂缝能否穿越界面扩展在FPZ的初始发育阶段已经注定,不受FPZ内应力软化过程影响。基于Renshaw-Pollard准则建立了考虑FPZ边界范围的裂缝穿越非连续面扩展准则,并通过前人及文中试验数据进行了可靠性验证。相比而言,改进准则更准确地考虑了裂缝前端线弹性断裂力学的适用范围。研究发现FPZ长宽比对裂缝穿越界面扩展准则有显著影响,相同条件下,FPZ长宽比越大,裂缝正交穿越界面扩展所需要的摩擦系数下限值越小。

Massive developed discontinuities are the salient geological features of unconventional oil and gas reservoirs,and the hydraulic fractures’capabilities of crossing the discontinuities concern the stimulation effects of hydraulic fracturing.To study the development of the fracture process zone(FPZ)when the hydraulic fracture propagates through an orthogonal discontinuity,the self-designed visualization fracturing equipment was adopted to carry out hydraulic fracturing tests on sandstone plates with a prefabricated unbounded friction interface.Based on the digital image correlation method,the displacement and strain characteristics during the hydraulic fracture propagation across the orthogonal interface were monitored in real time.The test results show that the FPZ has developed across the interface before the hydraulic fracture extends across the interface.Whether the fracture can propagate through the interface is predetermined at the initial developmental stage of the FPZ and is not affected by the stress-softening process in the FPZ.Based on the Renshaw-Pollard criterion,a criterion considering the FPZ boundary was established for estimating the fracture propagation across the friction interface,and it was verified by test data and existing results.In comparison,the improved criterion considers a more accurate application scope of elastic fracture mechanics at the fracture front.The aspect ratio of the FPZ has a significant effect on the improved criterion,and the lower limit of friction coefficient required for the fracture propagation orthogonally across the interface declines as the aspect ratio of the FPZ rises under the same conditions.