Well cementing

Polyacrylamides are used in many other oilfield appUcations. These include cement additives for fluid loss control in well cementing operations (127), viscosity control additives for drilling muds (128), and fracturing fluids (129). Copolymers [40623-73-2] of acrylamide and acrylamidomethylpropanesulfonic acid do not degrade with the high concentrations of acids used in acid fracturing. 

Retarders and Accelerators. Materials that control hardening of cement may be either organic or inorganic. Retarders are often incorporated in oil well cementing and hot-weather concrete appHcations, whereas accelerators may be useful for cold-weather concrete appHcations in which higher rates of reactivity are desirable. In most cases, these admixtures are used in low concentrations, suggesting that they act by adsorption. 

Oil well cements are manufactured similarly to ordinary Portland cements except that the goal is usually sluggish reactivity. Eor this reason, levels of C A, C S, and alkafl sulfates are kept low. Hydration-retarding additives are also employed. 

Oil well cements (78) are usually made from Pordand cement clinker and may also be blended cements. The American Petroleum Institute Specification for Materials and Testing for Well Cements API Specification 10) (78) includes requirements for nine classes of oil well cements. They are specially produced for cementing the steel casing of gas and oil wells to the walls of the bore-hole and to seal porous formations (79). Under these high temperature and pressure conditions ordinary Pordand cements would not dow propedy and would set prematurely. Oil well cements are more coarsely ground than normal, and contain special retarding admixtures. 

The constant value of 0.25 for Poisson s ratio versus depth reflects the geology and the rock mechanics of the mature sedimentary basin in the West Texas region. Since mature basins are well cemented, the rock columns of West Texas will act as compressible, brittle, elastic materials. 

In addition to primary cementing of the casing and liner, there are other important well cementing operations. These are squeeze cementing and plug cementing Such operations are often called secondary or remedial cementing [161]. 

Well cementing materials vary from basic Portland cement used in civil engineering construction of all types, to highly sophisticated special-purpose resin-based or latex cements. The purpose of all of these cementing materials is to provide the well driller with a fluid state slurry of cement, water and additives that can be pumped to specific locations within the well. Once the slurry has reached its intended location in the well and a setup time has elapsed, the slurry material can become a nearly impermeable, durable solid material capable of bonding to rock and steel casing. 

Through the past half century the well cementing industry has considered cement compressive strengths of about 500 psi to be acceptable. However, such low compressive strengths plus some of the past cementing practices may not be adequate for future wells. 

There are two basic oil well cementing activities primary cementing and secondary cementing. 

The American Petroleum Institute (API) has nine classes of well cements. 

Class H Intended for use as a basic well cement from surface to 8,000 ft (2,440 m) depth as manufactured, and can be used with accelerators and retarders to cover a wide range of well depths and temperatures. 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. Available in moderate and high- (tentative) sulfate-resistant types. 

A properly designed cement slurry will set after it has been placed in its appropriate location within the well. Cement strength is the strength the set cement has obtained. This usually refers to compressive strength, but can also refer to tensile strength. Cement having a compressive strength of 500 psi is considered adequate for most well operations. 

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