Enhanced properties and cost-effective application with incremental improvements in 50:50 poz cementing

The use of admixtures of 50:50 Class H (or Class C): Pozzalon with 2% bentonite have functioned effectively worldwide for nearly 50 years as lightweight slurries for situations where heavier completion cements posed a risk of exceeding low fracture gradients in a particular well bore. Pozzolanic materials are lightweight, and effectively combine with calcium hydroxide that is liberated during the hydration of Portland cement. Historically, the 2% bentonite has been utilized to assist in the specification of relatively high water-to cement ratios, and therefore lighter slurry density, without the generation of excessive free water as the cement progresses through the setting process. The bentonite has performed well in meeting this requirement, but two things remain elusive: First, its presence in typical cement slurries reduces the effectiveness of a given concentration of most commercially available fluid loss additives. Second, while the 2% (by weight of cement) volume may seem of no consequence, the shipping costs associated with moving tons of the material over a long period of time can be significant. This study was carried out to determine whether or not there were other commercially available materials that could substitute for bentonite and yield improved slurry qualities at the same or reduced cost. Extensive testing of 50:50 slurries revealed that small quantities of sodium metasilicate (on the order of 0.5% by weight of cement) could effectively replace bentonite. Free water was controlled to the same degree, and a synergy with a commonly available fluid loss additive was discovered, allowing either a) less total fluid loss additive for a given fluid loss control tolerance, or, b) better fluid loss control for a given concentration of fluid loss additive. The testing procedure is summarized, and relative economics associated with the systems are discussed.