Specialty cements

A number of cementitious materials used for cementing wells do not fall into any specific API or ASTM classification.These materials include:

• Pozzolanic portland cements
• Pozzolan/lime cements
• Resin or plastic cements
• Gypsum cements
• Microfine cements
• Expanding cements
• Refractory cements
• Latex cements
• Cements for permafrost environments
• Sorel cements
• Cements for carbon dioxide (CO₂) resistance

Pozzolanic cements

Pozzolanic materials include any natural or industrial siliceous or silico-aluminous material, which will combine with lime in the presence of water at ordinary temperatures to produce strength-developing insoluble compounds similar to those formed from hydration of Portland cement. Typically, pozzolanic material is categorized as natural or artificial, and can be either processed or unprocessed. The most common sources of natural pozzolanic materials are volcanic materials and diatomaceous earth (DE). Artificial pozzolanic materials are produced by partially calcining natural materials such as clays, shales, and certain siliceous rocks, or are more usually obtained as an industrial byproduct. Artificial pozzolanic materials include:

• Metakaolin
• Fly ash
• Microsilica (silica fume)
• Ground granulated blast-furnace slag

Pozzolanic oilwell cements are typically used to produce lightweight slurries. Because the specific gravity of the pozzolanic material is lower than that of the cement, a pozzolan slurry has a lighter weight than a corresponding Portland cement slurry of similar consistency. The lighter weight keeps the formation from breaking down. It is important not to exceed the fracture pressure of the formation while cementing.

Some pozzolanic materials also have a high water demand that effectively gives a higher yield and lighter slurry. They also tend to improve compressive strength over time. The additional binding material also reduces permeability and minimizes attack from formation waters. In most cases, pozzolanic materials can also reduce the effect of sulfate attack, though this is to a certain degree dependent on the slurry design.

Commercial cements such as TXI Lightweight™ for use in oil wells are a special formulation composed of Portland-cement clinker interground with lightweight siliceous aggregate to produce, in effect, a pozzolanic cement.

Pozzolan/line cements

Pozzolan/lime or silica/lime cements are usually blends of fly ash (silica), hydrated lime, and small quantities of calcium chloride. At low temperatures, the initial reactions of these cements are slower than similar reactions in Portland cements, and they are generally recommended for primary cementing at temperatures greater than 284°C (140°F). The merits of this type of cement are:

• Ease of retardation
• Light weight
• Economy
• Strength stability at high temperatures

Gypsum cements

Gypsum cement is a blend of API Class A, C, G, or H cement and a hemihydrate form of gypsum (CaSO₄ •½H₂O). Gypsum cements are commonly used in low-temperature applications for primary cementing or remedial cementing work. This combination is particularly useful in shallow wells to minimize fallback after placement. A high-gypsum-content cement has increased ductility, thixotropy, and acid solubility. It is usually used in situations of high lateral stress, or in temporary plugging applications. A 50:50 gypsum cement is frequently used in fighting lost circulation, to form a permanent insoluble plugging material. These blends should be used cautiously, because they have very rapid setting properties, and could set prematurely during placement. A limitation of gypsum cements is that they are slowly soluble, and they are not stable in contact with external sources of water. This would be a fatal error for an oilfield cement.

Microfine cements

Microfine cements are composed of:

• Very finely ground sulfate-resisting Portland cements
• Portland cement blends with ground granulated blast-furnace slag
• Alkali-activated ground granulated blast-furnace slag

Such cements have a high penetrability and are ultrarapid-hardening. Applications for such cements are in consolidation of unsound formations, and in repairing casing leaks in squeeze operations, particularly “tight” leaks that are inaccessible to conventional cement slurries because of their penetrability. Ultrafine alkali-activated ground blast-furnace slag is the product used in the mud-to-cement technology, in which water-based drilling mud is converted to cement.

Expanding cements

Expansive cements are available for the primary purpose of improving the bond of cement to pipe and formation. If expansion is properly restrained, its magnitude will be reduced and a pre-stress will develop. Expansion can also be used to compensate for the effects of shrinkage in normal Portland cement.

At this time, there is no test procedure or specifications in the API standards for measuring the expansion forces in cement. Most laboratories use the expansive bar test, employing a molded 1 × 1 × 10-in. cement specimen. Ring molds are also available, though they are not as commonly used. The expansive force is measured soon after the cement sets, for a base reference, and again at various time intervals until the maximum expansion is reached. Hydraulic bonding tests have also been used to evaluate the growth of expanding cements.

Calcium aluminate cements

High-alumina cement (HAC) is used in well-cementing operations at temperature extremes in permafrost zones with temperatures at 32°F or below, in-situ combustion well’s (fireflood) where temperatures may range from 750 to 2,000°F, and thermal-recovery wells where temperatures can exceed 1,300°F and temperature fluctuations can be high.

A number of HACs have been developed with alumina contents between 35 and 90%, and there is a move to term these collectively as calcium aluminate cements (CACs) because the reactive phase in all cases is calcium aluminate.

It is the standard type (e.g., Ciment Fondu) that is mostly used in well cementing. These cements can be accelerated or retarded to fit individual-well conditions, but the retardation characteristics will differ from those of Portland cements. The addition of Portland cement to refractory cement causes a flash set, so when both are handled in the field, they should be stored separately.

Latex cement

Latex cement, although sometimes identified as a special cement, is actually a blend of API Class A, G, or H with latex. In general, a latex emulsion contains only 50% latex by weight of solids, and is usually stabilized by an emulsifying surface-active agent. Latexes impart elasticity to the set cement, and improve the bonding strength and filtration control of the cement slurry. Latex in powdered form can be dry-blended with the cement before it is transported to the wellsite, and is not susceptible to freezing.

Permafrost cement

It is normally desirable to use a quick-setting, low-heat-of-hydration cement that will not melt the permafrost. API RP10B, Sec. 14, gives special cementing procedures for simulating arctic conditions and cementing in such environments.

Two cement systems that have been used successfully are calcium aluminate cement blends and gypsum cement blends. Fly ash or natural pozzolan is normally blended (at about 50% by weight) with calcium aluminate cements to lower the heat of hydration, thus preventing permafrost damage. Gypsum-cement blends can be accelerated or retarded, and will set at 15°F below freezing. For surface pipe, these slurries are normally designed for 2 to 4 hours of pumpability, yet their strength development is quite rapid and varies little at temperatures between 20 and 80°F.

Resin or plastic cements

Resin and plastic cements are specialty materials used especially in highly aggressive, acidic environments to:

• Selectively plugging open holes
• Squeezing perforations
• Cementing waste disposal wells

They are usually mixtures of water, liquid resins, and a catalyst blended with an API Class A, B, G, or H cement.

When pressure is applied to the slurry, the resin phase may be squeezed into a permeable zone to form a seal within the formation. These specialty cements are used in wells in relatively small volumes. They are effective at temperatures from 140 to 392°C (60 to 200°F).

Cements for CO₂ resistance

The hydration products of Portland cement are susceptible to carbonation in the presence of moisture. Carbonation is the attack resulting from dissolved CO₂ in formation waters or as a result of CO₂ -injection processes. The CO₂ dissolves in the aqueous pore solution of the hydrated cement, ultimately producing calcium carbonate (CaCO₃).

Carbonation can be minimized by the use of a specially formulated calcium phosphate cement, ThermaLock™, that is resistant to both CO₂ and acid. This cement can be used at temperatures typically ranging from 140°F (60°C) to 700°F (371°C). ThermaLock™ is an ideal cement for environments in which high concentrations of CO₂ are anticipated.

The one disadvantage is that it is more expensive than Portland cement, but it has several advantages, as well, including:

• It greatly reduces concerns on the long-term affects of CO₂.
• It saves on remedial operations, abandonments, and redrilling or recompletion.
• It does not require special cementing equipment or techniques.

External links

American Petroleum Institute (API)

American Society for Testing and Materials (ASTM)

See also

Cement composition and classification

Cementing operations

PEH:Cementing