Abstract
One of the main functions of cement is to prevent fluid communication between formations or to surface throughout the life of the well, including after abandonment. Even when cementing operations has provided a good initial hydraulic seal, changes in downhole conditions can induce stresses to destroy the integrity of the cement sheath resulting in a loss of zonal isolation.
We present an analysis of the mechanical response of set cement in a cased wellbore to quantify this damage and determine the key controlling parameters. The results show that the thermo-elastic properties of the casing, cement, and formation play a significant role. The type of failure, either cement debonding or cement cracking, is a function of the nature of the downhole condition variations. This analysis allows us to propose appropriate cement mechanical properties to avoid cement failure and debonding. We show that the use of high compressive strength cement is not always the best solution and, in some cases, flexible cements are preferred.
A new approach to cement design was undertaken to fulfill the requirement. A cement system with improved flexibility to resist stress cracking was designed and implemented. This system is engineered to have a low Young's Modulus while maintaining relatively high compressive and tensile strengths as compared to conventional cement systems. The permeability of the set cement is very low, yielding a material that is highly resistant to attack by corrosive fluids. Finally, the system can be designed to be gas tight to prevent gas migration.
This paper presents data on the new cementing design methodology and technologies used as well as simulation comparisons and large scale laboratory test results.
