Effects of Supercritical Carbon Dioxide on Well Cements

Abstract

ABSTRACT Due to the widespread manifestation of large scale miscible CO2 enhanced recovery projects, particularly in West Texas and Gulf Coast regions, considerable concern has developed regarding the performance and/or durability of hydrated cement located across producing and injection intervals in CO2 related wells. It is known that carbonation affects the micro-structure of cement affecting both porosity and compressive strength. The CO2 reactivity of a cement is characterized not only by its chemical composition, but also by the properties of the CO2 medium itself, that is, partial pressure, temperature and relative humidity. However, a clear understanding of this phenomenon and its effects on portland cement is still not completely substantiated, giving rise to contradictory opinions in this particular area of research. This led to the need and development of a laboratory program for examining the effects of supercritical CO2 on preset cement, as well as the influence of carbonation on the early stages of the cement hydration process. This article presents the findings of a comprehensive study which show that after prolonged exposure to CO2 under supercritical conditions, the hydration products formed in the hydration of common portland cement undergo decomposition into calcium carbonate and a siliceous residue. Cement samples exposed to the lower extremes (temperature and pressure) of a supercritical CO2 environment exhibited greater reactivity under dynamic conditions as compared to static conditions, while increasing CO2 pressure increased the degree of reaction regardless of the carbonation conditions employed.