二连盆地褐煤断裂行为加载率效应及对压裂裂缝调控的启示

The loading rate effect of lignite fracture behavior in Erlian Basin and its enlightenment on the regulation of fracturing fractures

室内试验尺度对煤岩断裂行为及加载率效应的认识同样有助于地下储层压裂过程中压裂裂缝延展规律分析及压裂造缝形式的调控。利用SHPB试验对采取自内蒙古二连盆地褐煤半圆弯曲试样在层理平行、层理垂直及层理斜交方向上的断裂行为及加载率效应开展研究,据此构建研究区褐煤储层水力压裂造缝模式。结果表明:二连盆地褐煤试样冲击下断裂过程包括弹性变形、断裂及卸载阶段;随加载率提升,试样断裂强度、断裂韧度随之线性增大,研究区褐煤断裂性能加载率效应非常显著。随加载率增大试样破碎程度加剧,层理平行型试样冲击下以层理剪切破碎形式为主,垂直层理及斜交层理试样以冲击压碎式破坏为主并形成大量煤粉,对煤层气排采较为不利。冲击下褐煤岩断裂方位基本与加载方位一致。同时随加载率增大,研究区褐煤断裂过程耗能线性增加。以上表明二连盆地褐煤断裂行为受外部加载率深刻控制。最后提出一种基于加载率效应的煤储层压裂造缝模式调控程序。其中深部应力主控型褐煤储层压裂,压裂中主要采取由近到远逐级增大加载率,而浅层由天然裂缝主控的褐煤储层压裂,压裂中由近到远逐级减小加载率的泵注程序,使压裂裂缝初期由应力主控,后期转为天然裂缝主控。形成近井地带主干裂缝,中端较复杂裂缝及远端复杂裂缝的“分段式”压裂造缝的效果。

The understanding of coal fracture behavior and loading rate effect on the scale of laboratory test is also helpful for the analysis of the fracture propagation law and the regulation of the fracturing fracture formation in the process of underground reservoir fracturing.The fracture behavior and loading rate effect of lignite semi-circular samples taken from Erlian Basin in Inner Mongolia were studied by SHPB test in the direction of bedding parallel,bedding vertical and bedding oblique direction.The results show that the fracture process of lignite samples from Erlian Basin under impact includes elastic deformation,fracture and unloading stages;with the increase of loading rate,the fracture strength and fracture toughness of the samples increase linearly,and the effect of loading rate on the fracture behavior of lignite in the study area is very significant.With the increase of loading rate,the degree of sample breakage intensifies.The bedding parallel type specimens is mainly in the form of bedding shear fracture under impact,and the vertical bedding and oblique bedding specimens are mainly in the form of impact crushing and form a large number of fractures.Coal fines is unfavorable for coalbed methane drainage.The fracture orientation of lignite under impact is basically consistent with the loading direction.Meanwhile,with the increase of the loading rate,the energy dissipation of the lignite fracture process in the study area increases linearly.The above shows that the fault behavior of lignite in Erlian Basin is deeply controlled by the loading rate.Finally,this paper proposes a control program for coal reservoir fracturing mode based on loading rate effect.Among them,the deep stress-controlled lignite reservoir fracturing is mainly used to increase the loading rate step by step from near to far,while the shallow lignite reservoir controlled by natural fractures is fracturing,and the fracturing is from near to far.The pumping procedure of decreasing the loading rate step by step makes the fracturing fractures mainly controlled by stress in the initial stage,and turned into the main control by natural fractures in the later stage.The effect of“staged”fracturing is to form main fractures in the near-wellbore zone,complex fractures in the middle and complex fractures at the far end.