分支井渗流-应力耦合场分析

摘要

There is very strong fluid-solid coupling during seeping in porous media.Taking full account of fluid-solid coupling in the COMSOL software for multilateral wells,the distribution characteristics of coupling seepage and stress field are analyzed,and the dangerous section of sidewall is recognized according to calculating the value of fail coefficient by the Coulomb criterion.Through calculating the minimum value of fail coefficient and the volume integration in which fail coefficient is less than -2.5 MPa,the impact of the build-up rate on sidewall failure is studied.The results show that the changes of porosity is very small obtained in the simple seepage field by using unique compressibility factor,while the porosity and permeability of the coupling seepage field change remarkably in the vicinity of the bottom.Therefore,fluid-solid coupling seeping is a true reflection of underground seepage.In the stress field,both the maximum valve of Von Mises equivalent stress and the minimum valve of coefficients fail emerge in the side tracking point of the lower lateral,which means that the sidetrack window is the dangerous section of sidewall.Finally,the conclusion of smallest range of rock failure drilling lower lateral with build-up rate of 15°/30m is obtained,which provides a theoretical basis for the designing and building of the multilateral wells.%在多孔介质的渗流过程中,存在着极强的流固耦合作用。针对广泛使用的分支井型,在利用COMSOL Multiphysics软件时充分考虑流固耦合作用,分析耦合渗流场和应力场的分布特点,同时通过运用三维库伦准则计算失效系数fail值来判断井壁岩石的危险截面,并进一步计算出不同造斜率情况下失效系数fail的最小值和失效系数fail≤-2.5MPa区域的体积积分值,从而研究采用不同造斜率侧钻分支井眼对井壁失效区域大小的影响。结果表明,在单渗流场分析中采用定压缩系数方法得到的孔隙度其变化微乎其微,而耦合渗流场中的孔隙度、渗透率在井底附近区域变化明显。因此,考虑流固耦合作用下的渗流场能更真实地反映地下渗流情况。在耦合应力场分析中,Von Mises等效应力的最大值和失效系数fail最小值均出现在侧钻分支井窗口处,即窗口处井壁岩石为危险截面,进而得出在取造斜率为15°/30m进行侧钻分支井眼的情况下井壁岩石失效范围最小的结论,为分支井的设计和施工提供了理论依据。