Casing deformation mechanisms of horizontal wells in Weirong shale gas field during multistage hydraulic fracturing

Casing deformation during hydraulic fracturing has become a serious issue hindering the development of shale gas in Weirong field. Better understanding of the casing deformation in this field is needed. Field data shows that casing deformations herein are strongly correlated with lithologic interfaces, whilst most deformations occur in the half of wellbore near heel. The lithologic interfaces and stress accumulation by fracturing in stages are considered as main potential inducers of casing deformations in the field of interest. Numerical simulations in two scenarios were conducted to investigate casing deformation mechanisms. The results show that a large stress concentration occurs at the lithologic interface, which can cause casing deformations. The shale swelling induced by interactions between fracturing fluids and clays can intensify the influence of lithologic interfaces. Higher injection pressure and formation pressure, larger difference in formation mechanical properties can increase stress concentration at the lithologic interface. Moreover, simulation results also show that the accumulation of induced stresses from hydraulic fractures increase the maximum principal stress near the wellbore. As more frac stages are pumped, stress accumulation increases, resulting in a continuous increase in maximum principal stress, which can cause casing deformations when it exceeds the casing collapse strength. This can explain nonuniform distributions of casing deformations along the wellbore, which is a general feature of casing deformations in shale horizontal wells. The stress accumulation effect increases with an increase in the injection rate and formation elastic modulus. Finally, some countermeasures were recommended to avoid casing deformations. This study provides new understanding of casing deformations of shale horizontal wells, which can be beneficial to the solutions of casing deformations in the field of interest and other similar shale fields.