Revisiting Thermal Well Integrity Through a Closer Look at Casing-Cement-Formation Interaction

Thermal wells (observation, productions or abandoned) are highly susceptible to leaks due to cement degradation resulting from thermo-chemical-mechanical loads. Therefore, successful application of thermal processes requires a close look at well integrity assessment and prediction for long periods of time. During the life of the well, the casing-cement-rock system is experiencing a cumulative damage which can, for instance, explain the sustained casing pressure that appears after years of good well integrity. Our industry is striving to implement the highest well integrity standards during the field development and beyond. Deficiencies in well construction and well abandonment are the most common causes of defects inside the composite system of rock, cement and casing. These defects can be measured using standard wellbore measurement tools, and some solutions exist to cure them. Long time exposure of the well to several loads may change the well response due to parameter degradation, with typical loading mechanisms being related to chemical and mechanical interactions. A large number of studies have been conducted to investigate chemical and thermal degradation process of Portland cement systems. The degradation process is very complex and is affected by temperature, pressure, chemistry of the products and time. A literature review of various investigations on well degradation processes shows that most of the experiments are time limited, and therefore the well integrity is limited to the frame in which we can extrapolate the results. This paper will point out the future need for research to improve the prediction of well integrity. The results of the work show that the major unknowns are long-term properties of the downhole environment like casing, cement, and the interaction between casing-cement-formation. Some preliminary laboratory testing also point out the importance of specific cement testing, beyond those recommended by various regulations and recommended practices. As an example, a theoretical demonstration of why fully cemented casing strings may additionally damage the cement sheath during heating and cooling cycles is provided in this paper, pointing out a novel testing procedure.