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Cement slurry extenders
In many parts of the world, severe lost circulation and weak formations with low fracture gradients are common. These situations require the use of low-density cement systems that reduce the hydrostatic pressure of the fluid column during cement placement. Consequently, lightweight additives (also known as extenders) are used to reduce the weight of the slurry.
Materials used for extender design
There are several different types of materials that can be used as extenders. These include:
- Physical extenders (clays and organics)
- Pozzolanic extenders
- Chemical extenders
Any material with a specific gravity lower than that of the cement will act as an extender. These materials, in general, decrease the density of cement slurries by one of three means. The pozzolanic and inert organic materials have a lower density than cement and can be used to partially replace cement, lowering the density of the solid material in the slurry. In the case of the physical and chemical extenders, they not only have a lower density, but also absorb water, allowing more water to be added to the slurry without producing free fluid or particle segregation. The gases behave differently in that they are used to produce foamed cements that have exceptionally low density with acceptable compressive strengths.
In many lightweight slurries, it is common to use a combination of the different types of material. For example, pozzolanic and chemical extenders are, or can be, used with physical extenders and/or gases. Pozzolan slurry designs almost always incorporate bentonite, and gases generally have a chemical extender to stabilize the foam. Lightweight additives also increase the slurry yield and can result in an economical slurry.
These are particulate materials that function as cement extenders by increasing the water requirements or by reducing the average specific gravity of the dry mix. There are two general classes of materials that fall into this category: clays and inert organic materials. The most commonly used clay material is bentonite, although attapulgite is also used. The commonly used inert organic materials are:
- Ground coal
- Ground rubber
This extender is a colloidal clay mineral composed predominately of sodium montmorillonite [NaAl2(AlSi3O10)•2OH]. The montmorillonite content of bentonite is the controlling factor in its effectiveness as an extender. It is one of two extenders that are covered by an API specification. Bentonite can be added to any API class of cement, and is commonly used in conjunction with other extenders. Bentonite is used to:
- Prevent solids separation
- Reduce free water
- Reduce fluid loss
- Increase slurry yield
Bentonite is typically used at concentrations of 1 to 16% BWOC. It may be dry-blended with the cement or prehydrated in the mixing water. In prehydrating, the effect of 1% BWOC prehydrated is approximately equal to 3.5% BWOC dry-blended, but the yield point is much higher. For best results, the prehydrated bentonite/water mixture should be used for mixing the cement slurry shortly after prehydration has been completed. Laboratory testing is advised to determine the proper gel concentration and mixing procedure for prehydrated bentonite. Tech grade or “mud gel” should not be substituted for cement-grade bentonite. Lignosulfonate is commonly used as a dispersant and retarder in high-gel cements to reduce the slurry viscosity.
Attapulgite (salt gel)
This is a more effective extender than bentonite in seawater or high-salt slurries, but it is not regulated or does not have a specification. Attapulgite, (Mg,Al)2 (OH/Si4O10)•12H2O, is composed of clusters of fibrous needles that require high shear to be dispersed in water. It produces many of the same effects as bentonite, except that it does not reduce fluid loss.
A disadvantage of attapulgite is that, because of the similarity of the fibers to those of asbestos, its use has been prohibited in some countries. Granular forms are available that may be permitted as a replacement.
Expanded Perlite is a siliceous volcanic glass that is heat-processed to form a porous particle that contains entrained air. It is a highly buoyant product that requires the addition of 2 to 6% BWOC bentonite to prevent separation from the slurry. Because of its low crush strength, the water requirement for perlite-containing slurries must be increased to allow for slurry compressibility under downhole conditions. Volume loss must also be taken into effect in fill-volume calculation.
This is an asphaltic material, or solid hydrocarbon, found only in Utah and Colorado. It is one of the purest naturally occurring bitumens. Gilsonite can be used with slurry densities as low as 11 lbm/gal at a normal concentration of 5 to 25 lbm/sack (sk) of cement, and it will plug float equipment and bridge tight annuli. The low densities obtainable with gilsonite result from its low density (1.07 g/cm3). Because gilsonite is an organic material, it is highly buoyant and will float out of the slurry unless inhibited. Bentonite is commonly added at a concentration of 2 to 6% to prevent bridging in the wellbore.
Crushed coal is used for the same purposes as gilsonite (i.e., for light weight and lost-circulation control). It is commonly used at concentrations up to 50 lbm/sk of cement. Its density is slightly higher (1.3 g/cm3), requiring a slight increase in water content. The addition of bentonite to prevent separation is normally not required.
This is a low-cost alternative to gilsonite and may be used in similar applications. The density of ground rubber is slightly higher (1.14 g/cm3). The physical properties are more variable than gilsonite and are dependent upon material source. One major advantage of ground rubber is its low cost. At present, there are no environmental issues with ground rubber when utilized in a cement system.
A number of pozzolanic materials are available for use in producing lightweight cement slurries. These can be either natural or artificial and include:
In comparison with other additives, pozzolanic materials are usually added in large volumes. Fly ash, for example, can be mixed with cement in ratios of fly ash to cement that range from 20:80 to 80:20, based on an “equivalent sack” weight (that is, where a sack of fly ash has the same absolute volume as that of a sack of cement). Pozzolanic materials have a lower specific gravity than that of cement, and it is this lower specific gravity that gives a pozzolanic-Portland-cement slurry a lower density than a Portland-cement slurry of similar consistency. Depending on the density, pozzolanic cements also tend to give a set cement that is more resistant to attack by formation waters.
Fly ash is by far the most widely used of the pozzolanic materials. According to ASTM Standard C618, there are two types of fly ash:
- Class F
- Class C
Class N refers to natural pozzolanic materials. There is, however, a need for a third category, based on the performance of different fly ashes. ASTM Standard C618, classifies fly ashes on the basis of the combined percentages of SiO2 + Al2O3 + Fe2O3 —Class F having a minimum of > 90% and Class C, 50%.
In reality, there is a much greater relationship between CaO content and performance. The CaO content ranges from 2 or 3% to 30% by weight of the fly ash. The “true” Class F fly ash has a CaO content of less than 10%, whereas a “true” Class C has CaO greater than 20%. Fly ashes having CaO between 10 and 20% behave somewhat differently from either the true Class F or Class C. Fly ashes are generally composed of amorphous glassy particles that are spherical in shape.
The ASTM Class F fly ash is the most common used in oilwell cementing. It is this fly ash that is covered by the API specifications. The major advantages of the Class F fly ash are its low cost and abundance worldwide. The performance characteristics of a Class F fly ash vary little from batch-to-batch from a given source. However, the differences between sources can be considerable, because the composition can vary from the true low CaO to 10 to 20% CaO. This produces significant variations in performance characteristics, and, because of this, different sources of Class F fly ashes should be tested before use. Specific gravities also must be determined. Some power plants produce Class F fly ashes with a high-carbon content, because of poor burning. These should be avoided for oilwell cementing because they can cause severe gelation problems.
The use of Class C fly ash, as an extender for well cementing, is relatively limited. This is, in part, because of the limited availability of Class C fly ash, and the considerable variability that exists not only between sources, but also between batches from a given source.
Microspheres are used when slurry densities from 8.5 to 11 lbm/gal are required. They are hollow spheres obtained as a byproduct from power generating plants or are specifically formulated. The byproduct microspheres are essentially hollow fly-ash glass spheres. They are present, typically, in Class F fly ashes, but usually in small amounts. They are obtained in substantial quantities when excess fly ash is disposed of in waste lagoons. The low-density hollow spheres float to the top, and are separated by a flotation process. These hollow spheres are composed of silica-rich aluminosilicate glasses typical of fly ash, and are generally filled with a mixture of combustion gases such as CO2, NOx, and SOx. The synthetic hollow spheres are manufactured from a soda-lime borosilicate glass, and are formulated to provide a high strength-to-weight ratio—they are typically filled with nitrogen. The synthesized microspheres provide a more consistent composition, and exhibit better resistance to mechanical shear and hydraulic pressure.
The primary disadvantage of most microspheres is their susceptibility to crushing during mixing and pumping, and, when exposed to hydrostatic pressures, above the average crush strength. This can lead to:
- Increased slurry density
- Increased slurry viscosity
- Decreased slurry volume
- Premature slurry dehydration
However, crushing effects can be minimized by the suitable choice of microspheres. These effects can be predicted and can be taken into account in slurry design calculations to produce a slurry having the required characteristics for the well conditions. Lightweight systems incorporating microspheres can provide excellent strength development, and can help control fluid loss, settling, and free water.
Microsilica, also known as silica fume, is a finely divided, high-surface-area silica that can be obtained as a liquid or powder. In the powder form, it can be either in its original state, densified, or pelletized. The bulk density of the densified microsilica is 400 to 500 kg/m3. Microsilica typically has a specific gravity of approximately 2.2.
Microsilica is composed primarily of vitreous silica, and has a SiO2 content of 85 to 95%, which makes it considerably purer than the other pozzolanic materials. Microsilica particles are also considered to impart beneficial physical properties to the slurry. Because of their fineness, they are believed to fill in the voids between the larger cement particles, resulting in a dense, solid matrix, even before any chemical reaction between the cement particles has occurred. Rheological properties tend to be improved with addition of microsilica because the tiny spheres can act as very small ball bearings, and/or they displace some of the water present between the flocculated cement grain, increasing the amount of available fluid. Concentrations of microsilica can range from 3 to 30% BWOC, depending on the slurry and properties required.
The physical and chemical properties of the microsilica make it very useful for a variety of applications other than as an extender. These include:
- Compressive-strength enhancement for low-temperature lightweight cement
- Thixotropic properties for squeeze cementing
- Gas migration
- A degree of fluid-loss control
The one disadvantage of microsilica is the cost. Originally considered to be a waste product, with its increased usage in the construction industry over the last decade, it has become more of a specialty chemical. Also, with fluctuations of supply and demand, there is a question of having a constant supply of a good source of the product.
Diatomaceous earth (DE)
DE is a natural pozzolan composed of the skeletons of microorganisms (diatoms) that were deposited in either fresh water or seawater.
Several materials are effective as chemical extenders. In general, any material that can predictably accelerate and increase the concentration of the initial hydration products is effective as a chemical extender.
This is the most commonly used chemical extender for cement slurries. Sodium silicate is five to six times as effective as bentonite on an equivalent concentration basis. Unlike the physical or pozzolanic extenders, sodium silicate is highly reactive with the cement.
Sodium silicate is available in both dry and liquid forms, making it readily adaptable to onshore and offshore applications. The solid form is sodium metasilicate (Na2SiO3), and it is typically dry-blended with the cement at a concentration of 1 to 3.5% BWOC at densities of 14.2 to 11.5 lbm/gal. It is not as effective if dissolved directly in the mix water unless CaCl2 is dissolved in the water first. If a liquid system is desired, it is better to use the liquid form. Liquid sodium silicate is normally used in seawater applications at a concentration of 0.1 to 0.8 gal/sk of cement at densities of 14.2 to 11.5 lbm/gal.
The two main advantages of sodium silicates as extenders are their high yield and low concentration of use.
The hemihydrate form of calcium sulfate (CaSO4•0.5H2O) is typically used as an extender. It is normally used at concentrations of 15% BWOC or less for the preparation of thixotropic slurries for use in applications where there are severe lost-circulation problems or where expansion properties are desired to improve bonding. Typical slurry compositions for lost-circulation applications, BHCT ≤ 125°F (52°C), contain from 8 to 12% BWOC gypsum with good expansion properties (0.2 to 0.4%). For improved bonding applications, where increased expansion (0.4 to 1%) is desired, NaCl is used (≥ 10% BWOW).
- API Spec. 10A, Specification for Cements and Materials for Well Cementing, 23rd edition. 2002. Washington, DC: API.
- ASTM C618, Standard for Testing and Materials. 2000. West Conshohocken, Pennsylvania: ASTM International. http://dx.doi.org/10.1520/C0618-12.