A new investigation on casing shear deformation during multistage fracturing in shale gas wells based on microseism data and calliper surveys.

Casing shear deformation has become a serious problem in the development of shale gas fields, which is believed to relate to fault slipping caused by multistage fracturing. This paper presents a new investigation aiming at understanding the reduction of casing inner diameter after fault slipping under different conditions, according to microseism data and calliper surveys. A mathematical model, which was verified by measurement results, was established to build a relationship between microseism moment magnitude and slip distance. A 3D numerical model, which was verified by physical model experiments, was developed to quantify the relationships between slip distance and the reduction of casing inner diameter. And then the relationships among microseism moment magnitude, slip distance and the reduction of casing inner diameter were established. The influence of casing internal pressure, slip distance, mechanical parameters of cement sheath, and casing thickness on the reduction of casing inner diameter was analyzed. The mathematical calculation results showed that with the increase of the degree of moment magnitude, the radius and slip distance increase, and with the increase of stress drop, the radius decreases and the slip distance increases. The numerical analysis results showed maintaining high pressure, increasing the elasticity modulus and decreasing the Passion ratio of cement sheath and increasing the casing thickness were beneficial to decrease the reduction of casing inner diameter. Keeping the designed horizontal segment of well trajectory away from the fracture-developed area, or be parallel to natural fracture can decrease the slip distance of fault, which can decrease the reduction of casing inner diameter. Finally, some effective measures to reduce or prevent casing shear deformation were recommended. • The relationship between moment magnitude and slip distance was established. • The reduction of casing inner diameter after fault slipping was quantified. • Decreasing slip distance and increasing the casing thickness benefited protecting the casing. [ABSTRACT FROM AUTHOR]