Implementation of Advanced Cementing Techniques to Improve Long Term Zonal Isolation in Steam Assisted Gravity Drainage Wells

Abstract

Abstract Steam Assisted Gravity Drainage (SAGD) development of the Athabasca oil sands has led to a unique approach to horizontal drilling combining many existing and new technologies. From a cementing perspective, preventing degradation of the cement sheath due to induced stress cracking and preventing gas migration are the two main challenges that must be addressed in order to achieve the objective of long-term zonal isolation across the length of the wellbore. 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 have 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. Changes in downhole stress conditions that occur during the life of a SAGD well are particularly extreme. Stresses in the cement sheath induced by extreme temperature cycling can result in severe mechanical damage and ultimate failure of the cement sheath, potentially leading to leakage of steam from the reservoir. Along with the requirement of maintaining cement sheath integrity, special cement design and placement considerations must be taken to prevent annular gas influx and migration. Annular gas migration prior to the setting of the cement can lead to channels that would not only provide a pathway for gas but also a potential conduit for steam to escape into shallower zones or to surface. A new approach to cement design was undertaken to fulfill the requirements of cementing PanCanadian's (now EnCana Corp.) SAGD wells in Christina Lake. 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 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 is designed to be gas tight to prevent gas migration. This paper presents data on the new cementing techniques and technologies used as well as case history results on the six wells cemented at Christina Lake.