基于位移不连续模型的多裂缝应力干扰极限距离计算

Critical distance of stress shadow of multiple-cracks based on displacement discontinuity model

利用MTS试验装置对"R13区块"的致密砂岩岩心进行小试样试验,获得"R13区块"致密砂岩的岩石力学参数(弹性模量、泊松比等)。忽略地层内水力裂缝面的不规则性,将其简化为互相平行的光滑裂缝面。考虑到地层在裂缝面的不连续性,将裂缝视为连续地层的内部不连续边界,采用位移不连续理论建立边界元模型,研究多裂缝系统周围的地层应力分布和不同簇数、不同布缝方式下的裂缝群缝间应力干扰的极限距离。结果表明,人工多裂缝的出现明显改变了裂缝周围应力场的分布,裂缝群簇数越多影响距离越大,但簇数的增加对影响距离的改变逐渐趋于平缓,呈对数增长,而段内裂缝间距(20~30 m)的改变基本不影响应力干扰的极限距离。基于位移不连续理论的边界元模型能够准确地计算裂缝群周围地层应力场分布和裂缝群缝间应力干扰极限距离。

A small specimen test is conducted in MTS to get the elastic modulus and Poisson's ratio of tight sandstone core in R13field.The irregularity of hydraulic crack surface is ignored,and the cracks are simplified as smooth and parallel to each other.Considering the discontinuity of the multi-cracks,they are treated as discontinuous boundaries of the continuous stra-tum.Then the displacement discontinuity method is used to construct a boundary element model,which is applicable to cal-culate the distribution of stress around the multi-crack system,and determine the critical distance of the crack swarms'stress shadow with different clusters and different ways of cloth sewn.The results show that the appearance of artificial multiple-cracks has significantly changed the distribution of stress field around the cracks.The more the cluster number,the longer the distance takes effect.But the influence of the cluster number increasing on the distance becomes smaller,taking on a log-arithmic tendency.However,the change of crack spacing between20and30meters,in a single stage has little influence on the critical distance of stress shadow.This study indicates that the boundary element model based on the displacement dis-continuity theory can accurately predict the distribution of the stress field around the multiple-cracks and the critical distance of the crack swarms'stress shadow.