Optimization of Admixture for Thermal Expansion Cement Used for Cementing Heavy Oil Thermal Recovery Well

In heavy oil thermal recovery, the high temperatures, 300℃, for instance, will deteriorate the strengths of set cement, and generate over high interior pressure inside the cement sheath because of the difference between the swelling capacities of the casing string and cement sheath. The strength deterioration and the over high interior pressure are the main factors contributing to damage to the integrity of the airtightness of the cement sheath. Several methods can be used to maintain the airtightness of the cement sheath, for example, controlling the strength deterioration of the set cement at elevated temperatures, or adding admixtures to the thermal expansion cement to improve the expansion behavior. Factors affecting the coefficient of thermal expansion of a set cement, the main parameter characterizing the thermal expansion behavior of the set cement, have been studied by adding various admixtures into cement slurries at different temperatures. Mechanisms of these factors affecting the coefficient of thermal expansion have also been investigated. It was found that some admixtures, such as silica powder and hollow micro spheres, reduce the coefficient of thermal expansion of set cement to varying degrees, while other admixtures, such as latex and carbonaceous materials can increase the coefficient of thermal expansion of set cement. Carbonaceous materials, as industrial waste, are cheap and their use in oil well cement not only reduces the cost of well cementing, but also improves the thermal expansion behavior and the airtightness of set cement, thereby prolonging the lifespan of a thermal recovery well.