ABSTRACT:

This work presents an experiment on the damage of cement-shale combination to evaluate the wellbore integrity in shale gas wells. Firstly, a method is developed to prepare the cement-shale sample. Secondly, SEM is used to investigate the interface structure between set cement and shale rock. Triaxial compression test and direct shear test are conducted to study the damage behavior of cement-shale samples. The results indicate that there are more micro cracks and pores near the interface. Stress concentration tends to occur, which makes the interface be the weakest part. In the triaxial compression test, the interfacial debonding occurs due to the difference in deformation between set cement and shale. There are three stages before the shear damage. The shear strength increases with the increasing interface normal stress (radial stress in the wellbore). The shear fracture surface becomes smoother as the interface normal stress increases. The radial stress plays an important role in the failure of the second interface between the cement sheath and the shale rock under either compression failure or shear failure. Finally, a regression model for the interface shear strength is developed based on the Coulomb criterion. The cohesion and internal friction angle can be obtained on the basis of the model.

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