Cement composition and classification

Almost all drilling cements are made of Portland cement, a calcined (burned) blend of limestone and clay. A slurry of Portland cement in water is used in wells because it can be pumped easily and hardens readily, even under water. It is called Portland cement because its inventor, Joseph Aspdin, thought the solidified cement resembled stone quarried on the Isle of Portland off the coast of England.

Proportioning of materials

Portland cements can be modified easily, depending on the raw materials used and the process used to combine them.

Proportioning of the raw materials is based on a series of simultaneous calculations that take into consideration the chemical composition of the raw materials and the type of cement to be produced: American Society for Testing and Materials (ASTM) Type I, II, III, or V white cement, or American Petroleum Institute (API) Class A, C, G, or H.

Classification of cement

The basic raw materials used to manufacture Portland cements are limestone (calcium carbonate) and clay or shale. Iron and alumina are frequently added if they are not already present in sufficient quantity in the clay or shale. These materials are blended together, either wet or dry, and fed into a rotary kiln, which fuses the limestone slurry at temperatures ranging from 2,600 to 3,000°F into a material called cement clinker. After it cools, the clinker is pulverized and blended with a small amount of gypsum to control the setting time of the finished cement.

When these clinkers hydrate with water in the setting process, they form four major crystalline phases, as shown in Table 1 and Table 2.

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  Table 1-Typical Mill Run Analysis Of Portland Cement

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  Table 2-Typical Composition And Properties Of API Classes of Portland Cement

Portland cements are usually manufactured to meet certain chemical and physical standards that depend upon their application. In some cases, additional or corrective components must be added to produce the optimum compositions. Examples of such additives are:

• Sand
• Siliceous loams
• Pozzolans
• Diatomaceous earth (DE)
• Iron pyrites
• Alumina

Calculations also take into account argillaceous or siliceous materials that may be present in high proportions in some limestones, as well as from the ash produced when coal is used to fire the kiln. Minor impurities in the raw material also must be taken into account, as they can have a significant effect on cement performance.

In the U.S., there are several agencies that study and write specifications for the manufacture of Portland cement. Of these groups, the best known to the oil industry are ASTM, which deals with cements for construction and building use, and API, which writes specifications for cements used only in wells.

The ASTM Spec. C150 provides for eight types of Portland cement: Types I, IA, II, IIA, III, IIIA, IV, and V, where the “A” denotes an air-entraining cement. These cements are designed to meet the varying needs of the construction industry. Cements used in wells are subjected to conditions not encountered in construction, such as wide ranges in temperature and pressure. For these reasons, different specifications were designed and are covered by API specifications. API currently provides specifications covering eight classes of oilwell cements, designated Classes A through H. API Classes G and H are the most widely used.

Oilwell cements are also available in either moderate sulfate-resistant (MSR) or high sulfate-resistant (HSR) grades. Sulfate-resistant grades are used to prevent deterioration of set cement downhole caused by sulfate attack by formation waters.

API classifications

The oil industry purchases cements manufactured predominantly in accordance with API classifications as published in API Spec. 10A. The different classes of API cements for use at downhole temperatures and pressures are defined next.

Class A

• This product is intended for use when special properties are not required.
• Available only in ordinary, O, grade (similar to ASTM Spec. C150, Type I).

Class B

• This product is intended for use when conditions require moderate or high sulfate resistance.
• Available in both MSR and HSR grades (similar to ASTM Spec. C150, Type II).

Class C

• This product is intended for use when conditions require high early strength.
• Available in ordinary, O , MSR, and HSR grades (similar to ASTM Spec. C150, Type III).

Class G

• No additions other than calcium sulfate or water, or both, shall be interground or blended with the clinker during manufacture of Class G well cement.
• This product is intended for use as a basic well cement. Available in MSR and HSR grades.

Class H

• No additions other than calcium sulfate or water, or both, shall be interground or blended with the clinker during manufacture of Class H well cement.
• This product is intended for use as a basic well cement. Available in MSR and HSR grades.

Properties of cement covered by API specifications

Chemical properties and physical requirements are summarized in Tables 3 and Table 4, respectively. Typical physical requirements of the various API classes of cement are shown in Table 5.

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  Table 3-Chemical Requirements For API Cements

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  Table 4-Physical Requirements For API Cements

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  Table 5-Physical Requirements Of Various Types Of Cements

Although these properties describe cements for specification purposes, oilwell cements should have other properties and characteristics to provide for their necessary functions downhole. (API RP10B provides standards for testing procedures and special apparatus used for testing oilwell cements and includes:

• Slurry preparation
• Slurry density
• Compressive-strength tests and nondestructive sonic testing
• Thickening-time tests
• Static fluid-loss tests
• Operating free fluid tests
• Permeability tests
• Rheological properties and gel strength
• Pressure-drop and flow-regime calculations for slurries in pipes and annuli
• Arctic (permafrost) testing procedures
• Slurry-stability test
• Compatibility of wellbore fluids.

See also

Cementing operations

PEH:Cementing

External links

SPE Cementing Monograph