API well cement is becoming difficult to obtain in many parts of the world. In newer operating areas, API monogram well cements have never been available, while in others, plants previously producing API well cements are choosing to drop such production. This lack of quality API well cement is forcing operators to consider using Non-API (ASTM) cements in applications beyond their previously recommended limits. While the use of ASTM cements in certain applications such as shallow/cool casings is not new, the migration of use into areas approaching what many consider to be HTHP applications is.
Because they are not just "re-named API well cements", using ASTM cements in more rigorous applications presents new challenges. These include best slurry density, viscosity/gellation issues, thermal stability, corrosion resistance, and reproducibility of data, to mention a few. Moving beyond historical comfort areas of operators requires a new mind-set be established to successfully apply non-API cements in more challenging environments. Operators and service companies must ask difficult questions such as why use slurry densities in the 1.8 to 1.98 sg, if not required for well control? What strengths are really needed to provide isolation and casing support? Can ASTM cements perform adequately at elevated temperatures? Answering these questions, and changing old paradigms on the way cements are designed is the aim of this paper.
The authors provide information on optimum densities for ASTM cements replacing API Class "H/G", and "C" cements. They also introduce new materials allowing for mixability, performance and safe use of ASTM cements at temperatures >149oC. This information includes thermal-stability, strength development, mechanical properties, and gas control data. Test data will be shown to support conclusions and help change paradigms on designing with ASTM cements for use in more widespread high-temperature applications.
Portland cement, named after the Isle of Portland, has been available since 1824. It is produced by blending the proper proportions of calcareous (sedimentary and metamorphic limestone) and argillaceous (clay, shale's, marls, etc.) minerals. These minerals are then calcined at temperatures between 1400° and 1500°C to form clinker, which is then ground with small amounts of gypsum to control setting and form the finished cement(1, 2, 3). In 1940 the American Society for Testing Materials (ASTM) established specifications for five types of Portland cement: Type I, II, III, IV and V.
Although the American Petroleum Institute (API) first established a committee to study oilwell cements in 1936, it was not until 1952 that the first specification (API std. 10A) and recommended practices (RP 10B) were established(3). Although the API has defined nine classes of cement, only Classes "A", "B", "C", "G" and "H" are most commonly available from producers around the world(_4_). API documents 10A and 10B have gone through numerous changes and modifications over the years, but the basic premise of the originals is still being used today.
Quality and consistency of ASTM and API well cements have always been issues. The first modern cement plants used a wet process to manufacture cements. This process suspends the ground raw materials in a slurried form. As a liquid suspension, it is easier to obtain uniform samples for raw material feed verification. This ensures that the proper proportions of raw materials are present prior to going to the kiln. Although knowing that you are starting with the proper proportions does not guarantee quality, it can provide cement that is more predictable. As described by Myers(_5) there can be large variations in ASTM cement's chemical, physical and microscopic properties from plant to plant. The same can be said for API well cements. Most oilwell cement service company's laboratory engineers prefer predictability over quality, if the quality equates to a cement that also exhibits extreme variability over time. Although the wet process could provide good reproducibility, it was inefficient and very costly, due to the energy required to eliminate the water.