Severe Casing Failure in Multistage Hydraulic Fracturing Using Dual-Scale Modeling Approach

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Summary Field evidence of production logs after fracturing have documented the existence of abundant natural fractures in Weiyuan shale plays, which is widely acknowledged to have a positive impact on fracture network complexity. On the contrary, cases of severe casing failures have been frequently reported in this field during multistage fracturing practice. Stress interference between two adjacent stages may increase nonuniform loading on the casing string and accommodate failure. To better understand this problem, we establish a coupled 3D reservoir-scale model with complex well trajectory and tie it to a single well-scale model consisting of casing and the surrounding cement sheath. Using this model, we investigate the potential impacts of cement deficiency, clustered perforations, fracture geometry, and spacing strategy on casing integrity. Our simulation results indicate that cement deficiency could intensify the load nonuniformity around the borehole, which poses potential threats to casing failure. When cement deficiency reaches 45° along the minimum horizontal stress, it has the largest influence on the stress level of casing. In addition, perforations could lower the casing strength, but the reduction may not change further when the perforation diameter reaches a certain value. Also, impacts of fracture geometry and spacing on casing deformation are explored. We can conclude that a lower ratio of fracture length to width and reasonable spacing strategy could help reduce the load nonuniformity on casing and thus avoid the casing deformation. The described workflow may be adopted in other areas to predict the possible casing failure problems induced by multistage hydraulic fracturing with cheap computational costs and to anticipate the challenges and address them by revising pumping schedule or spacing strategy.