Abstract
Nonaqueous cement slurries have been used for many years to prevent unwanted water or gas production and to repair holes/cracks or other pathways that could have formed in the casing, cement column, or at the interface. Such slurries were forced or squeezed into flow channels and allowed to contact water being produced or otherwise inherently present. Exposure of cement to water presumably allows for setting of cement, thereby plugging the flow pathway. These nonaqueous cement slurries primarily contained cement, a nonaqueous fluid, and an oil-wetting surfactant. It was generally assumed that contact between cement and water allowed setting of the entire cement mass with ensuing good strength development, even though an efficient dynamic mixing of cement and water under downhole conditions is unlikely. In the laboratory, this is typically demonstrated by mixing (by means of agitation) the slurry and the required amount of water, allowing it to set at well temperatures and measuring strengths.
Laboratory mixing under quiescent conditions by addition of water to the top of nonaqueous cement slurry with no agitation, and allowing it to cure for many days, demonstrated setting of cement only at the interface as a thin solid film, while the remaining slurry was unset. It was not obvious whether the presence of set cement at the fluid interface prevented further ingress of water into the cement slurry, or if the cement particles in the bulk slurry remained too oil wet to allow hydration reactions. It was also not obvious whether a totally quiescent, totally dynamic, or an intermediate level contact between water and cement slurry truly simulated the downhole situation, accounting for the success of the technology. A surfactant combination was designed to allow deeper penetration of water into cement slurry under quiescent conditions. Details are presented.
