As an increasing number of thermal wells are drilled in arctic and subarctic regions, such as the north slope of Alaska and northern Canada, there is an urgent need for lightweight cement systems with thermally insulating properties. Significant temperature changes resulting from activities such as shut-in, steam injection, and production can lead to increased temperatures in the wellbore. As the wellbore temperature rises, there is an increased risk of melting permafrost, which can allow the formation to move and result in costly damage to the well. Lightweight and thermally insulating cement would contribute to the life of a well by maintaining low thermal conductivity while providing structural support for the casing strings. This study compares the thermal and mechanical properties of water-extended, foam, and microsphere cements with densities of 1.32, 1.50, and 1.68 specific gravity (SG) (11, 12.5, and 14 lbm/gal). To simulate several different conditions in a well, thermal conductivity of the foam system was measured for dried, as-poured, and saturated conditions. While the amount of air or fluid in the foam samples influenced the measured thermal conductivity, both microsphere and foamed systems appeared to be comparable. Initial findings from mechanical properties testing demonstrated foamed slurries have higher tensile and compressive strengths. Under confining pressure, the foam cement system had a larger failure envelope and would be able to withstand greater downhole pressure increases compared to the microsphere design at the same density. When designing wells in areas with permafrost, including a cement system with low thermal conductivity would help minimize the risk of melting the permafrost and maximizing the longevity of the well. This paper reviews several possible lightweight solutions and presents the thermal and mechanical properties of various foam and microsphere cement designs.

In preparation for decommissioning work in the Southern North Sea sector, the permeability of historically used cement systems was investigated to help determine their effectiveness as well barriers. Because the placement of cement typically involves some degree of contamination with other wellbore fluids, determining the influence of seawater and water-based drilling fluid (WBM) contamination on the permeability of the set cement was of particular interest. This study investigates the historically used API Class G cement at 16 lbm/gal and Class B cement at 15.8 lbm/gal using contemporary material samples. Cores were prepared using neat cement, cement contaminated with up to 50 vol% seawater, and cement contaminated with up to 50 vol% WBM. The permeability to nitrogen flow of the set cores was measured using a Hassler sleeve. To further investigate the structural changes causing the higher permeability observed in the contaminated samples, the structure of the cores was imaged using X-ray computer tomography (CT) scans. All Class B mixtures within the investigated contamination range, all Class G mixtures with seawater contamination, and Class G mixtures up to 30 vol% WBM contamination produced set cores after curing. The neat Class G cores showed approximately twice the permeability of the Class B cement cores, with the contaminated slurries of both cement types showing much higher permeability—approximately one to two orders of magnitude greater. Further analysis using the CT scans revealed irregularities in the structure density in the contaminated samples, which might explain the increased permeability; however, further work is necessary to fully understand the mechanism causing this change. Understanding the potential effects of contamination on permeability and the structure could be useful for gaining insight into barrier performance of the cement sheath in circumstances where contamination is suspected.