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

doi:10.11885/j.issn.16745086.2021.04.28.05

Abstract

Abstract: 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.

Key words: deep shale gas, wellbore stability, anisotropy, transverse isotropy, collapse pressure

CLC Number:

TE21

LI Zhengtao, ZHANG Zhen, WU Pengcheng, MA Tianshou, FU Jianhong. Mechanical Mechanisms of Wellbore Instability of Deep Anisotropic Shale in Southern Sichuan[J]. Journal of Southwest Petroleum University(Science & Technology Edition), 2021, 43(4): 11-25.

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