Integrated Experimental and Numerical Study on Debonding of Cement-Casing Interface

Abstract

ABSTRACT: During the hydraulic fracturing process, the cement-casing interface may debond to be a severe leakage pathway for fluids. It is of significant interest to understand how stresses cause the debonding of cement-casing interface. A casing-cement interface normal bond parameters test device was fabricated to test normal bond parameters of the interface, including normal bond strength, normal stiffness and normal fracture energy. And a casing-cement interface shear bond parameters test device was fabricated to obtain the shear bond strength, shear stiffness and shear fracture energy of the interface. A 3D finite element numerical model of casing-cement-formation assembly was established to study the casing-cement interface debonding during hydraulic fracturing process, cohesive elements were employed to model the debonding of casing-cement interface, and the parameters obtained from our experiments were assigned to the cohesive elements. The simulation results demonstrate that the debonding of casing-cement interface is highly influenced by the normal bond parameters, shear bond parameters, situ stresses and internal casing pressure. The possibility and degree of casing-cement interface debonding and plastic deformation of cement increases with the increase of internal pressure. 1. INTRODUCTION Well integrity plays an important role in conventional and unconventional oil and gas development. With the development of unconventional resource the application of hydraulic fracturing is becoming more and more widespread (Vengosh et al.,2014). During the hydraulic fracturing process, the casing will be subjected to significant internal pressure fluctuations, the casing-cement and cement-formation interfaces may debond to be a severe leakage pathway for fluids (Xi et al.,2020; Li et al.,2020). Debonding between casing-cement and cement-formation interfaces can be decomposed into normal failure and shear failure. Debonding will occur once the normal and/or shear stress exceed the normal and/or shear bonding strength of the interface. It is of significant interest to understand how stresses cause the debonding of cement-casing and cement-formation interfaces. Since the difference of mechanical properties between cement and formation is higher than that between cement and casing, the casing-cement interface is more inclined to debond than the cement-formation interface. This paper focus only on the casing-cement interface debonding.