The Importance of the Stability of Cement Sheaths: Interaction between Cement, Acid, Carbon Steel and Formation and Treatment Fluids

Abstract

Abstract The stability of cement sheaths during well workover, production and intervention is an important element for well integrity and operational safety. Cement sheaths support casing, provide zonal isolation, seal off annuli, and protect casing from corrosive fluids. Understanding the interaction of the cement with the production and intervention fluids under reservoir conditions is essential to prevent unexpected operational problems. This paper provides a systematic study on the interaction between the cement and many oilfield elements including fresh acid, corrosion inhibitor, carbon steel, hydrogen sulfide, and carbon dioxide. A series of laboratory experiments were conducted by exposing set cement to various fluids at temperatures ranging from 50 up to 200 °F. Both fresh and aged cements were evaluated. Cement degradation during production in sour environments could be severe and consequently provide a source of iron- and calcium-related deposits. The severity of the problem depends on the level of degradation and production conditions. The degradation process can also maximize the exposure of casing surfaces to corrosive fluids leading to an increased concentration of dissolved cations in produced fluids, thus, increased potential for inorganic deposits and water-oil emulsion formation. Severe cement degradation can exacerbate the potential of creating easy pathways for toxic gases to the surface. Several advanced analytical techniques including X-Ray Diffraction (XRD) and Inductively Coupled Plasma (ICP) were used to evaluate the condition of the cement before and after the fluid exposure in our experiments. The results indicated that, unlike what has been reported in the literature, acid can significantly degrade a major portion of the exposed Class-G cement within less than two hours if the exposed cement surface area to acid is considered. The exposure to hydrogen sulfide and carbon dioxide did impact the cement degradation, but not significantly. The degradation was found to be a function of acid concentration, fluid additives, and environment conditions. To determine whether cement catalyzes the corrosion process, the average corrosion rates were also calculated. The results were in agreement with field observations and beneficial to optimize the cement stability and to minimize the related operational problems.