There are more than 1,200 heavy oil deposits worldwide containing approximately 13,000 billion bbls of oil. Various steam injection methods are used to mobilize the viscous fluid to flow so that it can be produced to surface. During the production process, the cement sheath maintaining wellbore zonal isolation is subjected to large mechanical strains imposed by temperature variations. Under these extreme conditions, mechanical and thermal cement properties are key factors to maintain wellbore integrity and to achieve the desired reservoir productivity. Significant heavy oil reserves are found in unconsolidated sandstone formations and carbonate formations. To avoid fracturing of these formations during cement placement, systems with densities below 1 800 kg/m3 are required. Further, during exposure to steam temperatures, formation type may affect cement structure and hence its material properties.
A new steam resistant cement system was designed to have tailored mechanical and thermal properties for slurry densities in the range of 1,400 to 1,700 kg/m3. To simulate the influence of formation composition on the material properties, cement samples were aged in autoclaves containing either water or water-saturated sand. Aging was continued up to six months at temperatures of 250 to 344 degC. A two-fold improvement of the material properties in the conditions simulating contact with the formation (water-saturated sand) is observed compared to standard curing conditions.
Cement sheath integrity is simulated for the new and conventional steam resistant cements for typical conditions of heavy oil wells, including the effect of the formation. The higher durability of the new cement material compared to conventional systems is confirmed. Operators can count on cement sheath integrity as they apply thermal processes to produce heavy oils.