API Cement Types
The American Petroleum Institute (API) has established standards and a classification system for oil well cements. This ensures that oil well cements are consistent and predictable in their performance under oilfield conditions and when various additives are introduced. The API classification scheme (API Spec 10A) has been adopted by the International Organization for Standardization (ISO) as Standard 10426-1.
International Standardization of Oilwell Cements
API standards for oil well cements, in use for many years, have been converted into international standards. API Specification 10A (Specification for Well Cements and Materials for Use in Well Cementing) and API Recommended Practice 10B (Testing of Well Cements) were converted into international standards ISO 10426-1 and ISO 10426-2 respectively. ISO 10426-1 is now the internationally recognized standard for oil well cements. By agreement between ISO and CEN (the European standards organization), these standards are also available as European standards.
These standards are based on the API standards with no significant technical changes. The global ISO standards can have local designations, for example in the United States, ANSI/API ISO 10426-1 and 10426-2.
There are eight classes of API-ISO Portland cements, designated A through H. They are classified according to the depths for which they are designed and the temperatures and pressures to which they are suited. Within some classes there are three degrees of sulfate resistance: ordinary grade (O), moderate (MSR) and high (HSR), as determined by C3A content. Table 1 summarizes the API classification scheme.
| API Classification | Applications |
|---|---|
| API Class A | Also referred to in the United States as “construction cement” or “common cement.” Intended for use when special properties are not required. May be used “neat” without additives, from surface to a depth of 6000 ft, if no special properties are required. Widely available; performance can be modified by the use of additives. Available in O grade only. |
| API Class B | Intended for use from surface to 6000 ft depth, where conditions require moderate to high sulfate resistance. Available in MSR and HSR grades. |
| API Class C | Intended for use from surface to 6000 ft depth, where early high strength is needed. Available in O, MSR and HSR grades. |
| API Class D* | Intended for use from 10,000 to 14,000 ft depth, under conditions for moderate to high pressure and temperature. Available in MSR and HSR grades. |
| API Class E* | Intended for use from 10,000 to 14,000 ft depth, under conditions of high temperature and pressure. Available in MSR and HSR grades. |
| API Class F* | Intended for use from 10,000 to 16,000 ft depth, under conditions of extremely high temperatures and pressures. Available in MSR and HSR grades. |
| API Class G** | Intended for use in neat slurries from the surface to a depth of 8000 ft. May be modified by additives for use in a wide range of well depths pressures and temperatures. Available in MSR and HSR grades. |
| API Class H** | Similar in form and application to class G, but is usually more coarsely ground. |
*Classes D, E and F are intended for use under moderately high temperatures and pressures, and are available in MSR and HSR grades. These classes are also known as “retarded cements” because they have reduced amounts of the faster-hydrating C3S and C3A components, and increased particle sizes. The technology of chemical additives to retard cement setting has improved so that these cements are rarely found today (they were largely abandoned in the early 1980s).
**Class G and H cements were developed in response to improvements in chemical additives for accelerating and retarding cement hydration. These cements contain no glycols or acetates, chemicals that may be added during the manufacturing process for other classes in order to facilitate the grinding process but which can interfere with various other additives.
Most well-cementing operations worldwide use slurries based on API Classes A, G, H, and C (high-early-strength cement). Class G and H cements are by far the most commonly used well cements today. In the United States, the split is about 80% Classes G and H, 10% Class A, and 10% Class C. Outside of the United States more than 95% is Class G, often imported. Class A or C, or local common cement may also be used if quality and logistics permit.
Service company literature, such as the Halliburton Cementing Tables, describes the various API cement classes, including their chemical requirements, physical specifications, composition, and fineness.
Note that not all countries use the API classification system. Rapid Hardening Portland Cement (RHC) and High Strength Portland Cement (HSC) for example, are two non-U.S. cement classifications that correspond roughly to API class C cement.
API Class J cement, originally intended for high-temperature applications, has been replaced by Classes G and H with additives such as silica.
Effect of Temperature
Temperature is one of the most important factors affecting how quickly cement sets in a well, the key parameter for choosing a cement. Figure 1 shows the effect of temperature on the thickening time of various cement slurries. The reaction of cement with water begins when they are first mixed, and the thickening time is defined as the time it takes the slurry to reach a standard consistency as measured by a consistometer. For example, the thickening time of Class G cement drops from 8 hours to 3 hours as the temperature rises from about 70 °F to 120 °F.
