川南深层各向异性页岩井壁失稳力学机理

川南页岩埋藏深、岩性复杂、质地硬脆、层理发育、地应力复杂，水平井钻井井壁失稳事故时常发生，严重阻碍了深层页岩气高效开发进程。为此，以川南深层龙马溪页岩为对象，实验测定了页岩各向异性弹性和强度参数，通过成像测井分析了层理产状，通过室内实验和测井资料确定了地应力大小和方位，建立了各向异性页岩井壁坍塌压力模型，分析了不同地层条件的坍塌压力，并以川南深层页岩气井为例进行了验证。结果表明，页岩基质黏聚力和内摩擦角为13.00 MPa、39.50°，层理黏聚力和内摩擦角为11.10 MPa、28.50°，层理倾角5°～15°、倾向NE110°～NE120°；水平最大主应力114.69～117.23 MPa、水平最小主应力93.79～94.57 MPa、垂向地应力108.42～112.81 MPa；综合考虑弹性和强度各向异性计算的坍塌压力最高，强度各向异性的影响明显大于弹性各向异性；沿水平最大主应力钻进的水平井稳定性最好，其次为直井和小角度定向井，沿水平最小主应力方向钻进水平井时稳定性最差； L20X井实钻情况与预测结果基本一致，证实了模型的准确性。

Mechanical Mechanisms of Wellbore Instability of Deep Anisotropic Shale in Southern Sichuan

Wellbore instability problem usually occurs during horizontal drilling in deep shale reservoirs in Southern Sichuan basin, due to the deep buried depth, complicated lithology, hard brittle texture, well-developed bedding planes and complicated in-situ stresses, which seriously hinders the efficient development of deep shale gas. Thus, taking the deep Longmaxi shale in Southern Sichuan basin as the research objective, the anisotropic elastic and strength parameters of the shale were determined by experiments, the bedding occurrence was analyzed by using imaging logging data, and the in-situ stresses and its orientation were determined based on the indoor experiments and logging data. The wellbore collapse pressure model of anisotropic shale was established, and the collapse pressure with different formation conditions were analyzed and verified by taking the deep shale gas well in Southern Sichuan basin as an example. The results indicated that the cohesion and internal friction angle of shale matrix are 13.00 MPa and 39.50°respectively, while the cohesion and internal friction angle of bedding plane are 11.10 MPa and 28.50°respectively. The dip angle of bedding plane is 5～15ånd the azimuth angle of the bedding plane is NE110°～NE120°. The maximum horizontal stress, minimum horizontal stress and vertical stress are 114.69～117.23 MPa, 93.79～94.57 MPa and 108.42～112.81 MPa respectively. The collapse pressure is the highest when simultaneously considering the elastic anisotropy and strength anisotropy and the effect of strength anisotropy is obviously greater than that of elastic anisotropy. The stability of horizontal wells drilled along the direction of maximum horizontal principal stress is the best, followed by vertical wells and small-angle directional wells, while the stability of horizontal wells drilled along the direction of minimum horizontal principal stress is the worst. The actual drilling condition of well L20X is basically consistent with the predicted results, which confirms the accuracy of the model presented in this paper.