Analytical Chemistry of Portland Cement and Its Oilfield Admixtures

Abstract

Su":,mary. The successful cementing of boreholes in earthen formations requires careful control of the properties of cementing slurries and of the final set product. Accurate, dependable, time-efficient chemical analyses are prerequisites in determining quality of cement, additives, field blends, slurries, and set cement. A review highlighting chemical, spectrochemical, microscopic, and thermal methods of analysis of portland cement and oil,field cementing systems is presented. IntroductionAnalytical chemistry can be defined as the science of determining the chemical composition of matter both qualitatively and quantitatively. In simpler terms, the analytical chemist answers the two key questions: "What is it?" and "How much?" 1 The emphasis of this paper is therefore limited to the determination ofthe chemical composition of portland cement and portland cement blends with common oilfield admixtures. Such physical measurements as surface area, particle-size distributions and rheology behavior of slurries are not discussed in detail. BackgroundPortland cement was invented in the early 1800's and was named for its visual similarity to building stone obtained from the Isle of Portland off the English coast. 2 It is generally defined as the product obtained upon mixing and firing specified raw materials, basically calcium carbonates and aluminum silicates, at a given set of conditions. The major constituents of anhydrous portland cement are tricalcium silicate (C 3 S), dicalcium silicate (C 2 S), tricalcium aluminate (C 3 A), and tetracalcium alumino ferrite (C 4 AF) (C=CaO, S=Si0 2 , F=Fe203, and A=AI 2 0 3 ). Upon mixture with water, the various phases react to form a complex series of calcium silicate hydrates. Depending on the desired qualities of the "paste" or "slurry" thus formed and of the final set product, various materials may be added to the cement and water at the time of mixing. Properties typically controlled include hydration rate, viscosity, and ultimate strength of the set material. In response to the rather stringent controls required for oilfield applications, API developed standard specifications for oilfield cements in terms of both slurry performance and chemical composition. 3 The chemical requirements are highlighted in Table 1.The in-depth chemistry of portland cement and its interactions with admixtures are quite complex and well beyond the scope of this paper. For further information, the reader is referred to Ref. 4, which provides an excellent overview of calcium aluminosilicate chemistry as it applies to oilfield applications,The challenge for the analytical chemist is to determine the identity of any or all of the various phases and/or admixtures and then to quantify the amount present. Since the invention of portland cement, numerous methods have been developed and are still being developed toward this end. First, the techniques applicable to portland cement are reviewed and organized by technique. Second, analyses for the various common oilfield admixtures in the cement are describe...