OK, We Can't Get API Cement; What Now?
Authors: Rogers Murray James , Dillenbeck Robert Lee , GoBoncan Gil 
Published: December 2007

APIwellcement is difficult to obtain in many parts of the world. In some 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 API well cement is forcing operators to consider using Non-API cements in applications beyond their previously recognized limits. While the use of ASTM construction cements in applications such as shallow/cool casings is not new, their migration of use into areas many consider HTHP applications, is new.

Because they are not just "re-named API well cements", using Non-API cements in more rigorous applications presents significant new challenges. These include determining best slurry density, viscosity/gellation issues, thermal stability, physical response to current cementing additives, corrosion resistance, and reproducibility of data. To successfully apply non-API cements in more challenging environments requires moving beyond historical "comfort areas". Engineers must ask difficult questions, such as "Why use slurry densities &gt; 15.0 ppg (1.8 s.g.) if not required for well control"?, or "What strengths are really needed to provide isolation and casing support?" Also of concern is if ASTM cements perform adequately at elevated temperatures. Answering these questions, and changing old paradigms on the way cements are designed, is the purpose 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 &gt; 300oF (149°C). This information includes thermal-stability, strength development, mechanical properties, and gas control results. Data will be presented to support conclusions illustrating thesuitability of these systems for applications previously deemed impossible and help change paradigms regarding the ability to successfully utilize non-API cements for use in more widespread critical applications.

Introduction

Although cementitious materials have been written about and used since the 1st century BC, it was not until 1824 that Portland cement, named after the Isle of Portland, was patented. The advent of modern day hydraulic cement, which sets by means of hydration in the presence of water and not by dehydration, was developed when English Engineer Joseph Aspdin blended limestone with various materials and heated them to produce the desired proportions of CaO, Al2O3, SiO2 and Fe2O3. Over a century later 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 oil well cements in 1936, it was not until 1952 whenthe first specification (API std. 10A) and recommended practices (RP 10B) were established1. Although the API has defined nine classes of cement, only Classes "A", "B", "C", "G" and "H" are available from producers in the U. S2. These documents have gone through numerous changes and modifications over the years, but the basic premise of the original is still being used today.

The raw material used to produce Portland cement is a blend of calcareous (sedimentary and metamorphic limestone) and argillaceous (clays, shale's, marls, etc.) minerals. These raw materials are finely ground and provide the main constituents of calcium, silica, aluminum and iron required to feed the kiln and produce the clinker. The clinker is then ground to obtain the finished Portland cement. Both ASTM and API well cement start with the same raw materials and undergo similar processes. As described by Myers3 there can be large variations in ASTM cement'schemical, physical and microscopic properties, not only from plant to plant, but also from day-to-day operations in the same plant. The same thing can be said for some API well cements.