Study on the mechanism and controlling method for annulus pressure buildup problems in shale gas wells

The Annulus Pressure Buildup (APB) problems in shale gas wells is mainly caused by the crack at casing-cement interface. The stresses on casing-cement interface and cement-formation interface are calculated by the established model which the constrain of formation rock is considered. The elastic modulus of cement sheath has great effect on the stress on casing-cement and cement formation interface. The radial stress on interfaces changed linearly with fluid pressure. The cement rock is treated as a porous elastic material and the residual strain in cement sheath is mainly caused deformation or collapse of pore structure in the cement rock due to the large radial stress on interfaces. Ultimately, micro-annulus will be generated on the casing-cement interface due to the residual strain of cement sheath. High flexibility cement rock is proposed to decrease the radial stress on the interfaces which can reduce the residual strain of cement sheath. On the other hand, PABP is proposed to offset the decreased radial stress due to residual strain. A model for calculating the stress on interface by considering residual strain of cement sheath and PABP is established. The radial stress on the interfaces caused by PABP is quantitative calculated. The mechanism on how the method for solving APB problems in shale gas wells is clearly explained. The field application certificated the validity of method for solving the APB problems and it also proved the correctness of the calculation method.