Mechanisms and Mitigation of Casing Deformation in Ultradeep Composite Salt Formations: A Tarim Basin Case Study

Summary

Casing deformation in deep and ultradeep wells is a technical challenge that significantly threatens well integrity and downhole operations. In this paper, taking the Kelasu Structure Belt in the Tarim Basin as an example, we investigate the underlying mechanisms of casing deformation and propose corresponding mitigation strategies. For this study, e developed wa 3D finite element model for the casing/cement/formation system by integrating multifinger caliper logging data from deformed casing intervals with cementing-quality evaluation results while also accounting for irregular borehole geometry and vertical heterogeneity in composite salt formations. The effects of the geological, engineering, and casing factors on the distribution of nonuniform external collapse loads on casing and the deformation of casing are systematically illustrated, and the results reveal that the numerical simulations show an average error of 5.86% and a maximum deformation error of 7.28% compared with the field data from multifinger caliper logging, thus validating the model’s accuracy and applicability in revealing casing deformation mechanisms in ultradeep wells. The formation creep is identified as the fundamental cause of nonuniform external collapse load on the casing. More importantly, this loading is spatially redistributed and intensified by axial lithological variation, circumferential differences in creep rate, and casing bending. Casing deformation mainly results from the coupling of nonuniform external collapse load and bending moment on casing. The proposed mitigation measures include that the cement sheath with an elastic modulus of 6 GPa may reduce the maximum external collapse pressure on the casing by about 13.58 MPa, the centralizer installations at 10-m intervals may obtain an 86.29% reduction in the maximum casing deformation, and the maximum borehole enlargement ratios kept within 5% reduce the maximum deformation by 45.67%.