Thermal Stability of the Cement Sheath in Steam Treated Oil Wells

Considerable work has been done on the phase equilibria and physical properties of thermal cements heated directly to high temperatures. In thermal recovery oil wells the cement sheath is, however, cured for days to years at temperatures of 40°C or less before it is subjected to steam injection temperatures of up to about 320°C. The effects of this alternative curing regime have not been well documented. Here, the impact of pre-curing samples for 1, 14, and 28 days at 35°C before heating to 230°C on phase assemblages, morphologies, and physical properties of thermal cement were investigated. Control samples included thermal cements heated directly to high temperature as well as samples cured only at 35°C. Xonotlite formed the predominant phase in the pre-cured thermal cements instead of the tobermorite found in the control sample heated directly to 230°C. Although all pre-cured samples were predominantly xonotlite, their morphological characteristics of the phase assemblages were found to vary distinctly with pre-curing time. Significant differences between the pre-cured and directly cured samples were also observed in terms of porosity, permeability, and mechanical behavior.