Casing Shear Deformation Mechanism and Prevention Measures of Multifault Slip Induced by Shale Gas Volume Fracturing

Abstract

Casing deformation often occurs in the process of shale gas volume fracturing, affecting the efficient development of shale gas. According to the actual deformation morphological characteristics of the casing and the mechanism of shear deformation, the three-dimensional physical model and finite element model of casing-cement sheath-formation under the interaction of multifault slip were established. The influence rules of sensitive factors such as fault slip distance, fault angle, elastic modulus of cement sheath, cement sheath defect, and casing eccentricity on casing deformation were studied. The prevention and control measures to reduce casing shear deformation were put forward. The results show that (1) the diameter shrinkage of the casing is more obvious when the relative slip distance between two faults exceeds 60 mm; (2) when the fault angle θ is from 60° to 75°, the shear deformation extent of the casing is large. Before fracturing construction, fault identification technologies should be used to detect the fault distribution in shale reservoirs and effectively prevent and control the risk points of casing deformation; (3) ultra-flexible cement sheath with elastic modulus less than 1 GPa can effectively reduce the shear deformation of the casing; (4) lack width of cement has a greater effect on reducing casing deformation than lack angle. Not sealing the well sections near the fault and leaving a certain buffer area for fault slip can delay or reduce the shear deformation failure of the casing; and (5) casing eccentricity makes casing more prone to failure. Therefore, it is necessary to ensure well-cementing quality.