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Water soluble materials in cementing
Two forms of derivatized cellulose have been found useful in well-cementing applications. They are the single-derivatized hydroxyethyl cellulos (HEC) and twice-derivatized carboxymethyl hydroxyethyl cellulose CMHEC. The usefulness of the two materials depends on their retardational character and thermal stability limits.
This is commonly used at temperatures up to approximately 82°C (180°F) for fluid-loss control, and may be used at temperatures up to approximately 110°C (230°F) BHCT, depending on the co-additives used and slurry viscosity limitations. Above 110°C (230°F), HEC is not thermally stable. HEC is typically used at a concentration of 0.4 to 3.0% by weight of cement (BWOC), densities ranging from 16.0 to 11.0 lbm/gal, and temperatures ranging from 27 to 66°C (80 to 150°F) BHCT to achieve a fluid loss of less than 100 cm3 /30 min.
Carboxymethyl hydroxyethyl cellulose
This is commonly used at temperatures up to 300°F for fluid-loss control and may be used at temperatures up to approximately 350°F, depending on degree of substitution, the co-additives used, and slurry viscosity limitations. CMHEC is more thermally stable than HEC, and is not as susceptible to oxidative attack.
Since the 1970s, a significant amount of work has been performed concerning synthetic copolymers for use in cement slurries. Most of this work has centered on copolymers of acrylamide and/or acrylamide derivatives and their salts; however, several nonacrylamide-based monomers have also been reviewed.
Polyvinyl pyrrolidone (PVP)
This is a nonionic polymer that is typically used as a fluid-loss enhancer in conjunction with sodium naphthalene sulfonate condensed with formaldehyde (SNFC) to improve the performance of other polymers. When used alone, PVP is not very effective as an Fluid-Loss Control Agents (FLA). However, when PVP is used in conjunction with SNFC, the fluid loss is improved through improved particle orientation. PVP/SNFC is particularly advantageous when used in densified cements for both dispersion and fluid-loss control. The use of PVP/SNFC, in conjunction with HEC or CMHEC, results in significant improvement in fluid-loss control. Surfactants are surface-active agents that may be used to modify the interfacial tension between two liquids or between a liquid and a solid. Low-molecular-weight polymers such as SNFC and lignosulfonate are surfactants. The choice of the proper surfactant can have a significant effect on the FLA itself, and its interaction with cement particles. Surfactants can be used to accelerate or retard the solubility or wettability of polymers.
Cement slurries can be lost to the formation and not circulated back to the surface during completion of a wellbore. This is defined as lost circulation. It should not be confused with the volume decrease resulting from fluid-loss filtration. Lost circulation tends to occur in three basic formation types:
- Unconsolidated or highly permeable. It is considered that the particles of a cement slurry can enter an unconsolidated or highly-permeable formation only if the permeability is greater than 100 darcies.
- Fractured, induced or natural. Induced fractures occur in highly incompetent zones (e.g., shale) that break down at relatively low hydrostatic pressures. Natural fractures can be encountered anywhere.
- Cavernous or vuggy. These are usually formed by erosion of the formation caused by the action of subsurface waters and are discovered unexpectedly.
In many cases, lost circulation occurs during drilling with loss of drilling fluids, and actions can be taken at that time to combat the lost circulation. At other times, difficulties may be encountered during drilling, indicating potential lost-circulation problems, and measures can be taken to prevent their occurrence during cementing. Typically, there are two steps in combating lost circulation: reducing slurry density and adding a bridging or plugging material.
Additives categories for lost circulation
Additives for prevention of lost circulation can be separated into three basic groups:
- Bridging materials
- Rapid-setting or thixotropic cements
- Lightweight cementing systems
Bridging materials physically bridge over and/or plug the lost-circulation zone, and are typically available in fibrous, flake, or granular form. Most bridging materials are considered to be chemically inert with respect to cement hydration.
Fibrous materials are, in general, used for controlling lost circulation in highly permeable formations where the fibers form a mat over the surface.
The most common flake material is cellophane. Cellophane flakes act by forming mats or bridges over very narrow fractures. Concentration range of cellophane is usually from 0.125 to 0.5 lbm/sk.
Granular materials are most frequently used and include gilsonite, perlite, and coal. These coarse particles are typically used for large fractures and cavernous or vuggy lost-circulation formations. As the cement slurry enters the formation, these large granular particles, in principle, become trapped and block off the opening. Concentrations vary according to the material used and are typically, 5 to 50 lbm/sk for gilsonite, 0.5 to 1.0 ft3/sk for perlite, and 1 to 10 lbm/sk for coal.
Rapid-setting or thixotropic cements
Rapid-setting and thixotropic cements are the preferred means for lost-circulation control in large cavernous or vuggy formations where bridging materials are no longer effective. These cements are usually designed to set up in the lost-circulation zone, ultimately plugging it off.
Rapid-setting cements include both quick- and flash-setting formulations. These cements generally give thin slurries, but have very rapid setting times. The quick-setting cements will set up while being displaced or shortly after entering the lost-circulation zone, whereas the flash-setting cements form semisolid materials when mixed with water or water-based drilling fluids.
Thixotropic cements have a low viscosity during mixing and placing, but when they enter the formation and are no longer subjected to shear, they gel and become self-supporting. There are a number of thixotropic formulations that include:
- Gypsum cement
- Gypsum Portland cement
- Aluminum sulfate/iron (II) sulfate
- Clay-based systems
- Crosslinked polymer systems
Combination of additives
It is often more effective to solve lost circulation by combining the bridging materials with rapid-setting or lightweight systems. The choice of system and the bridging material depends on the type of formation, the size of the lost-circulation zone, the fracture pressure gradient, and the downhole temperatures and pressure, as well as economics.
Strength retrogression is a normal phenomenon that occurs with all Portland cements at temperatures approximately 230 to 248°F (110 to 120°C), and is usually accompanied by a loss in impermeability. The use of 35 to 40% SiO2 (sand or flour) is used to combat strength retrogression.
In well-cementing applications, the maintenance of a consistent column of cement is critical to assure proper zonal isolation. Because of rheological demands and the need for silica or weighting agents in some applications, this is not always possible with conventional materials. It is necessary, therefore, that an additional additive be incorporated into the cement slurry to address the potential problem of particle sedimentation. This group of additives is known as free-water-control additives.
Sodium silicate may be used to control free water in normal- and low-density cement slurries. Typically, approximately 0.15 to 0.5% BWOC is sufficient to provide free-fluid control.
Biopolymers impart the unique characteristics of thinning at higher shear rates and viscosifying at lower shear rates. This yields slurries that will more readily go into turbulent or upper laminar flow yet have sufficient low shear to prevent sedimentation. Xanthan gum and Welan gum both provide these characteristics, and are typically used at an active concentration of approximately 0.2% BWOC.
Synthetic polymers of high molecular weight, which are resistant to alkaline hydrolysis, have been found to be effective as free-fluid-control additives at temperatures where sodium silicate and biopolymers are not effective. They are typically used at an active concentration of approximately 0.1 to 0.2% BWOC.
Expansive cements are used primarily for obtaining effective zonal isolation by improving the bond between the cement and pipe and the cement and annulus. Good zonal isolation is essential to prevent:
- Loss of production
- Control gas migration
- Provide protection from corrosive formation waters
- Reduce water production
- Improve confinement of stimulation treatments
Poor bonding of cement to pipe and/or annulus is most often a result of a combination of effects from a variety of factors. The root causes are usually associated with:
- Drilling-fluid properties and displacement mechanics
- Casing expansion and contraction caused by thermal stresses or internal pressures
- Fluid loss from the cement
- Hydration volume reduction during setting of cement
The resultant effect of poor bonding is the formation of “microannuli” or small gaps at the cement/casing or cement/formation interface. Expansive cements expand slightly after the cement has set and fill in the void spaces. Because of the restraints imposed by the casing and formation, any additional expansion will occupy the space provided by the internal cement porosity, resulting in a reduction in porosity.
Types of expansive additives or cement
The two principal types of expansive additive or cement are:
- Post-set crystalline growth (or chemical expansion)
- In-situ gas generation
The expansion mechanism is the growth of the crystals within the solid cement matrix. These crystals have a greater bulk volume than the original solids from which they form, and cause a wedging action because of the internal pressure of crystalline growth, forcing the solid matrix apart. Crystal-growth expansion is unilateral in that restraint in one direction does not increase expansion in other directions. The amount of expansion is dependent on a number of factors that include:
- Amount of additive
- Curing time and temperature
- Cement-slurry composition(in some cases)
Cement slurries containing high concentrations of salt (NaCl, KCl, or CaCl2 ) have a long reputation for contributing to expansion. Expansion is caused by the crystal growth of calcium chloroaluminate hydrate (3CaO•Al2O3•CaCl2•H2O) from reaction of the chloride ions with the aluminate phase in cement. There are indications that the temperature limitation for calcium chloroaluminate hydrate is around 51°C (125°F), although salts are reported to be effective, expanding additives up to 204°C (400°F), depending on the system. Salt also contributes to bond improvement by preventing dissolution of the salt formation.
In-situ gas generation
Expansion resulting from in-situ gas-generating additives occurs before set while the cement is still in the plastic state. The most common in-situ gas-generating additive is aluminum powder, although zinc, iron, and magnesium are possible alternatives. The expansion is caused by the reaction with alkali and water present in the cement aqueous phase to produce microsized bubbles of H2 gas. Expansive forces that are a direct function of the gas generated compensate for any volume losses caused by hydration volume reduction or fluid loss, and increase the pressure of the cement against the pipe and formation. In-situ gas-generating additives can be used at temperatures from 16 to 204°C (60 to 400°F). Because of the compressibility of the gas, the amount required is more dependent on the hydrostatic pressure of the slurry than on the downhole temperature. Concentrations generally range from 0.15 to 0.6%, although they can be higher.
Several additives are used that do not fit in any of the preceding categories. These additives can be used frequently (as in antifoam additives) or in more-specialized cases, such as:
- Mud decontaminants
- Radioactive tracers
- Cement for CO2 resistance
Antifoam additives are frequently used to decrease foaming and minimize air entrainment during mixing. Foaming is a secondary effect, often caused by a number of additives. Excessive foaming can result in an underestimation of the density downhole and cavitation in the mixing system.
Effects of foaming
Slurry density is usually measured with a densitometer during mixing to proportion the solids and water to obtain the desired density. When a slurry foams, the entrapped air is also included in the density measurement, and because air compresses under pressure, the actual density downhole becomes greater than that measured on the surface. Another effect of foaming is that, if severe, it can cause cavitation of the pumps and ultimately lead to loss of hydrostatic pressure.
Use and characteristics of antifoam additives
Antifoam additives, in general, modify the surface tension and/or dispersion of solids in the slurry so that foaming is prevented or the foam breaks up. The concentration of foaming additive required to be effective is very small, typically less than 0.1% BWOW. Antifoam additives consist primarily of polyglycol ethers or silicones or a mixture of both, and may also include additional surfactants.
Types of antifoam additives
Polypropylene glycol is the most common polyglycol ether used, and is favored for its low cost. It is effective in most situations, although it typically has to be added before mixing. In some cases, it can interact with other additives, and cause increased foaming. The silicone antifoam additives are a suspension of very fine particles of silica dispersed in a silicone base, and can also exist as an oil-in-water emulsion. They can be used both before and during mixing, and are highly effective as antifoam additives.
Paraformaldehyde or a blend of Paraformaldehyde and sodium chromate is sometimes used to minimize the cement retarding effects of various drilling-mud chemicals in the event a cement slurry becomes contaminated by intermixing with the drilling fluids. A mud decontaminant consisting of a 60:40 mixture of paraformaldehyde and sodium chromate neutralizes certain mud-treating chemicals. It is effective against:
- Ferrochrome lignosulfonate
- Chrome lignin
- Chrome lignite
Mud decontaminants are used primarily in openhole plugback jobs and liner jobs, and for squeeze cementing and tailing out on primary-casing jobs.
Radioactive tracers are added to cement slurries as markers that can be detected by logging devices. They were originally used to determine the location of fill-up or cement top and the location and disposition of squeeze cement, although, temperature surveys and cement-bond logs fulfill this function now. Radioactive tracers are still used in remedial cementing to locate the slurry after placement, and for tracing lost circulation. Radioisotopes are controlled and licensed by the U.S. Nuclear Regulatory Commission, and various state agencies and cannot be used indiscriminately.
Small amounts of indicator dye can be used to identify a cement of a specific API classification or an additive blended in a cementing composition. When the dyes are used downhole, dilution and mud contamination may dim and cloud the colors, rendering them ineffective. Naturally occurring mineral oxides and/or synthetically produced color pigments may be substituted for the dye indicator.
Conventional Portland cement, mixed at normal density, has low ductility, making it somewhat brittle. This makes it susceptible to post-cementing stresses. Synthetic fibrous materials are frequently added to make the cement more ductile, and to reduce the effects of shattering or partial destruction from perforation, drill-collar stress, or other downhole forces. Fibrous materials transmit localized stresses more evenly throughout the cement and, improve the resistance to impact and shattering. Nylon, with fiber lengths varying up to 1 in., has commonly been used, because it is resilient and imparts high shear, impact, and tensile strength. Particulated rubber also acts to improve the ductility of cement and improve on the flexural strength, and it is usually used in concentrations up to 5% BWOC. More recently, aluminum silicate and/or fibrous calcium silicates have been reported to enhance the compressive, flexural, and tensile strengths.