页岩储层各向异性对水平井坍塌压力的影响

为揭示层理页岩强度各向异性对水平井井壁坍塌压力的影响规律，对取自四川盆地龙马溪组页岩钻井不同层理角度的岩芯，通过岩芯观测、偏光显微镜、扫描电镜试验分别从宏观和微观角度研究了页岩层理特征，并基于单轴抗压强度研究了页岩强度各向异性特征.层理结构发育导致页岩呈现出显著的各向异性特征，粗略地将页岩视为各向同性体及简单采用Mohr-Coulomb准则作为破坏判据，使得预测的维持井壁稳定的坍塌压力不能满足安全钻井的需要.该文在研究层理地层岩石力学特性的基础上，采用横观各向同性地层井周应力分析模型，研究了弹性模量及地应力各向异性比对井周应力的影响；并结合考虑中间主应力的Mogi-Coulomb判据，分析了井壁坍塌压力对弹性模量E和Poisson(泊松)比ν各向异性比值变化的敏感性.实例分析结果表明:弹性模量和水平地应力各向异性比值的变化会对井周应力产生较大影响；Poisson比各向异性对井壁坍塌压力没有明显影响，而弹性模量比值的变化对井壁坍塌压力影响较大；横观各向同性地层中井壁坍塌压力与储层水平和垂直方向的硬脆性强弱关系有关，弹性力学参数各向异性既有可能利于井壁稳定，也可能更易导致井壁的剪切破坏；在实际工程中应根据储层岩石水平和垂直方向的硬脆性强弱关系，确定安全钻井液密度窗口.该结果对现场施工具有很好的指导意义.

Impacts of Rock Anisotropy on Horizontal Wellbore Stability in Shale Reservoir

In order to understand the effects of strength anisotropy of bedding shale on the collapse pressures of horizontal wells,the cores of different bedding angles were drilled out of the Longmaxi group,Sichuan Basin,and the macroscopic and microscopic shale characteristics were studied based on the polarizing micrographs and the scanning electron micrographs.Besides,the anisotropic shale strengths were investigated through the uniaxial compressive strength tests.Given the anisotropic physical and mechanical properties of the bedding shale,the practice that the shale was roughly simplified as an isotropic body in the previous borehole stability design made the predicted collapse pressure for maintaining borehole stability incapable of meeting the need for drilling safety.Hence,a calculation model for the stress field around the borehole in anisotropic formation was established.With a transverse isotropic formation model,the effects of the isotropic surface and the elastic parameters' anisotropic ratios on the well circumferential stresses were analyzed;meanwhile,the Mogi-Coulomb criterion was adopted to estimate the wellbore stability,and the sensitivity analysis was carried out to consider the impacts of different mechanical properties including Young's modulus and Poisson's ratio on the collapse pressure.Results show that the formation anisotropy increases the circumferential stress distribution heterogeneity and worsens the wellbore stress situation significantly;the degree of anisotropy affects the collapse pressure a lot.The sensitivity analysis also show that Poisson's ratio anisotropy does not markedly change the collapse pressure for low anisotropy degrees;however,the effect of the elastic modulus anisotropy is notable.The work is useful for the in-situ wellbore design before drilling.