The influence of shale swelling on casing deformation during hydraulic fracturing

Abstract

Casing deformations during hydraulic fracturing are a serious problem in the Weirong shale gas field. This study focuses on the mechanism of shale swelling-induced casing deformation. First, the characteristics of the casing deformations in the Weirong shale gas field were analyzed. Subsequently, the mechanism of shale swelling during immersion was investigated and numerical simulations were performed to investigate the effect of shale swelling on casing deformation. Finally, we discussed various countermeasures to suppress casing deformation. The results demonstrate that clay-swelling during immersion leads to the propagation and expansion of fractures along the beddings, thereby causing shale swelling. Furthermore, the casing stress increases with the increase in shale swelling. The swelling formation distributions have a significant effect on the type of casing deformation. Symmetric swelling formation distributions cause extrusion deformation, and asymmetric swelling formation distributions cause bending deformation, which is similar to the type of casing deformation observed in the WY 23–5 well in the Weirong shale gas field. Furthermore, shear deformation can also be induced if the dip angle of the swelling formation is less than 90°. Increasing the casing thickness and steel grade, and adjusting the Poisson's ratio of the cement sheath does not have a significant effect on suppressing shale swelling-induced casing deformation. The effect of cement quality on casing deformation depends on whether the cement void can provide a sufficient area for stress release. Perforations provide artificial stress release areas and reduce the casing stress, in spite of local stress concentration near the perforation points. The findings obtained herein provide a new understanding of the casing deformations in the Weirong shale gas field, and provide practical solutions to suppress casing deformation. • Casing deformation characteristics of shale wells in Weirong shale gas field were analyzed. • The mechanism of shale swelling-induced casing deformation was investigated. • Casing stress increases with the increase in shale swelling. • Swelling formation distribution affects the type of casing deformation. • Countermeasures to suppress the casing deformation were discussed.