Cement Additives.doc

Cement AdditivesAlmost all cement used in oil and gas wells is Portland cement. However, neat cement is seldom used throughout a job since various additives are usually necessary to modify the properties of either slurry or set cement. With basic cements (API Class G or H) and the use of additives, cement slurries can be tailored for any specific requirement.Most additives in current use are free-flowing powders that are dry-blended with the cement prior to its transportation to the well. When necessary, some powdered additives can be dispersed in mixing water at the site. Liquid additives are more commonly used offshore and in remote land locations where dry cement blending and storage are a problem.Properties that are modified by additives are shown below:For the slurry: thickening time (acceleration, retardation) density (extenders, weight increase/reduction) friction during pumping fluid loss (by filtrate) lost-circulation resistance (whole slurry loss)For set cement: compressive strength strength retrogression (loss with time) expansion/contractionAccelerators Cement-setting time is accelerated to reduce WOC time and to increase early strength. This is desirable for surface pipe, in shallow (cooler) wells, and for setting plugs. General pressure recommendations are as follows: pipe support and zonal isolation 100 psi (690 kPa) drilling out 500 psi (3450 kPa) perforating bullets 500 psi (3450 kPa) hollow carrier or expendable jets 2000 psi (13,800 kPa) whipstock plug 2500 psi (17,200 kPa) or greater (or harder than formation)The most common accelerators are calcium chloride, sodium silicate, sodium chloride (low concentrations), seawater, hemihydrate forms of gypsum, and ammonium chloride. Table 1 shows typical amounts used per sack. TypeAmount used per sack (% by weight)AcceleratorsCalcium Chloride (CaCl2) (flake, powder, an hydrous)2 4Sodium Chloride (NaCl)3 - 10 (water)1.5 - 5 (cement)Hemihydrate forms of20 - 100Gypsum (plaster of Paris)Sodium Silicate (Na2SiO3)w1 - 7.5Cements with dispersants and reduced water0.5 - 1.0Sea Water RetardersCalcium-Sodium Lignosulfonate0.1 - 1.0Calcium Lignosulfonate0.1 to 1.0Calcium Lignosulfonate0.1 to 2.5plus organic acidCMHEC0.1 to 1.5Saturated Salt15 to 17 lb/skTable 1. Commonly used accelerators and retarders. (Source: Halliburton Services).Retarders Cement-thickening time is slowed primarily to allow the slurry to be pumped and displaced into position before setting. Retarder additives include calcium lignosulfonate, organic blends, carboxymethylhydroxyethyl cellulose (CMHEC), borax, sodium chloride (in high concentrations), and most fluid-loss agents (see Table 1).Thickening time is a function of both temperature and pressure, and these effects must be predicted before additives are selected TypeAmount used per sack (% by weight)AcceleratorsCalcium Chloride (CaCl2) (flake, powder, an hydrous)2 4Sodium Chloride (NaCl)3 - 10 (water)1.5 - 5 (cement)Hemihydrate forms of Gypsum (plaster of Paris)20 100Sodium Silicate (Na2SiO3)1 - 7.5Cements with dispersants and reduced water0.5 - 1.0Sea WaterRetardersCalcium-Sodium Lignosulfonate0.1 - 1.0Calcium Lignosulfonate0.1 to 1.0Calcium Lignosulfonate plus organic acid0.1 to 2.5CMHEC0.1 to 1.5Saturated Salt15 to 17 lb/skTable 1: Commonly used accelerators and retarders. (Source: Halliburton Services). Thickening time can also be shortened by interruption of pumping (loss of agitation). API tests may be done in this manner to simulate actual interruptions during squeezing. An increase in water volume increases the thickening time of unretarded cement (Classes A, C, G, and H). With retarded cements (Classes D, E, and F), however, increased water or solids may decrease thickening time by reducing the concentration of retarder. The thickening time of a slurry under realistic conditions must be established to ensure adequate pumping time for slurry placement. Excessive thickening time must be avoided to prevent: delays in resuming drilling operations settling and separation of slurry components formation of free-water pockets loss of hydrostatic head and gas cutting Density-Reducing Additives Slurry density may be reduced with extenders such as bentonite, pozzolan, diatomaceous earth, and anhydrous sodium metasilicate. Table 1 shows typical additive concentrations. Low-density slurry is frequently preferred, to decrease the likelihood of breaking down the formation and causing lost circulation. In addition, low-density slurries cost less per cubic foot because yield per sack is increased. Density decrease results in large part from increased water content. Extenders, with their high surface area to tie up water, permit water addition without separation. Cement strength is reduced approximately in proportion to water-content increase. However, we shall see later that high strength is not always required. TypeAmount used per sack(% by weight)Density reducers/extendersBentonite2 to 16Attapulgite1/2 to 4Diatomaceous Earth (Diacel D)10, 20, 30 or 40Pozzolan, Artificial (fly ash)74 lb/skNatural hydrocarbons:Gilsonite1 to 50 lb/skCoal5 to 50 lb/skPozzolan-Bentonite CementVariableSodium Silicate1 to 7.5 lb/skExpanded Perlite5 to 20 lb/skHollow SpheresVariableDensity increasersSand5 to 25Barites10 to 108Ilmenite (iron-titanium oxide)5 to 100Hematite4 to 104Salt5 to 16Friction Reducers0.05 to 1.75 Table 1: Materials used to vary slurry density.( Source: Halliburton Services). For years, bentonite has been the most commonly used additive for filler-type cement. In addition to its effect on density, yield, and cost, bentonite increases viscosity and gel strength, which reduces settling of high-density particles (e.g., weight material, cement), or floating of low-density particles (e.g., perlites, pozzolan, gilsonite, crushed coal, hollow spheres). Bentonite also reduces API fluid loss. However, cements containing bentonite are more permeable and have lowered sulfite resistance. Pozzolans increase slurry viscosity and provide low permeability. Sodium meta-silicate provides a very low density slurry with early compressive strength; this material and calcined shale-cement (a special cement, not an extender) are becoming popular, particularly in offshore applications. Very light slurries (less than 8 lb./gal.) have been made using hollow spheres. These new cements are useful in underpressured, hot geothermal wells and other special applications. Density-Increasing Additives High density cement sluries are often necessary to offset the high pressures that are frequently encountered in deep or abnormally pressured fromations. Density may be increased with weight material such as sand, barite, hematite or ilmenite, and/or salt dissolved in the mix water, as shown in Table 1. TypeAmount used per sack (% by weight)Density reducers/extendersBentonite2 to 16Attapulgite1/2 to 4Diatomaceous Earth (Diacel D)10, 20, 30 or 40Pozzolan, Artificial (fly ash)74 lb/skNatural hydrocarbons:Gilsonite1 to 50 lb/skCoal5 to 50 lb/skPozzolan-Bentonite CementVariableSodium Silicate1 to 7.5 lb/skExpanded Perlite5 to 20 lb/skHollow SpheresVariableDensity increasersSand5 to 25Barites10 to 108Ilmenite (iron-titanium oxide)5 to 100Hematite4 to 104Salt5 to 16Friction Reducers0.05 to 1.75Table 1: Materials used to vary slurry density.( Source: Halliburton Services). Available densities and effects on compressive strength are shown in Table 2. MaterialSpec. Gravity (lb./gal)Max. DensityEffect on compressive strengthOttawa Sand2.6318NoneBarite4.2519ReduceCoarse Barite4.0020NoneHematite5.0220NoneIlmenite4.4520NoneDispersant17.5IncreaseSalt18ReduceTable 2: Densities of weight materials and their effect on compressive strength. A density of 22 lb/gal can be obtained with hematite or ilmenite plus friction-reducing additives. Fine barite requires a large amount of water, which reduces compressive strength and retards thickening time. Slurry weighted with solids must have adequate viscosity and gel strength to carry and suspend high-specific-gravity solids. In addition, some additives (e.g., fluid-loss agents, retarders, water) tend to significantly thin or thicken a slurry. High slurry densities (up to 17.5 lb/gal) may be obtained by (1) using heavy additives and/or (2) adding dispersants to achieve pumpability at lower-than-normal water/cement ratios. The latter is more expensive, but it yields the highest compressive strength. Pretesting of such high-density slurries should include measurement of density, thickening time, compressive strength, settling, free water, and viscosity. Filtration-Control Additives Fluid loss, or the premature escape of mix water from the slurry before chemical reaction occurs, can cause many downhole problems, including differential sticking of casing and decentralization formation damage by filtrate (if not controlled by mud cake) loss of pumpability cement bridging above gas zones and gas cutting from hydrostatic pressure loss improper or premature dehydration during squeezingFiltration-control additives in present use and their recommended concentrations are listed in Table 1. TypeAmount used per sack (% by weight)Fluid-loss additives0.5-1.5%Organic polymers (cellulose), form micellesOrganic plymers (dispersants), size distribution and form micelles0.5 - 1.25%Carboxymethyl hydroxyethyl cellulose, from micelles0.3 - 1.0%Latex additives, form films1.0 gal/skBentonite cement with dispersant12-16% gelTable 1: Materials to reduce filtrate loss, friction. These materials function by forming micelles or films, and/or by improving particle-size distribution, which holds liquids. A neat Class G or H slurry has an API 30-minute filter loss of over 1000 ml. Figure 1 shows the effectiveness of high-molecular-weight synthetic polymer in reducing filter loss. Figure 1Friction Reducers Friction reducers or dispersants are commonly used to lower viscosity, yield point and gel strength of the slurry to reduce friction in pipe, and thus allow turbulent flow to occur at reduced pump rates. For example, to achieve turbulent flow with 7 5/8 in. casing in a 8 5/8 in. hole requires a rate of over 600 gal/min. With 0.5, 0.75, and 1.0% friction reducing additives (FRA), the required rate is only 530, 300, and 210 gal/min, respectively. These additives also permit slurries to be mixed at lower water/cement ratios so that higher densities may be achieved. Some common dispersants are alkylaryl sulfonate, polyphosphate, lignosulfonate, salt, and organic acid. Table 1 shows typical concentrations. Turbulent-flow additives tend to cause settling and excessive free water. These effects should be tested in the lab prior to field use. TypeAmount used per sack (% by weight)Friction reducers/ dispersantsPolymer: blend0.5 to 0.3 lb/skPolymer: long chain0.5 to 1.5 lb/skCalcium lignosulfonate0.5 to 1.5 lb/sk(organic acid)Sodium Chloride1 to 16 lb/skOrganic acid0.1 to 0.3 lb/skTable 1: Materials to reduce filtrate loss, friction. Lost-Circulation Materials Lost circulation or lost returns refers to the loss to formation voids of either whole drilling fluid or cement slurry used during the course of drilling or completing a well. It should not be confused with volume decrease caused by filtration. Drilling fluids or slurries are usually lost to either natural or induced formation fractures. These fluids may also be lost through highly permeable formations those starting at about 5 darcies for drilling fluid with a maximum particle size of 0.002 in. (300 mesh). Cement, with its larger particle size (neat cement has 2.6 to 18% particles larger than 200 mesh) is less susceptible to loss in permeable formations. The best time to treat the formation to reduce such fracture or formation permeability is during drilling, when high concentrations of bridging materials and various types of plugs (pills) may be utilized. During primary cementing, concentrations of such materials must be carefully controlled to avoid bridging the casing or liner-borehole annulus, or plugging downhole equipment such as bottom wiper plugs, small-diameter stage tools, and float equipment. Types of lost-circulation additives available for cement are blocky-granular materials (walnut shells, gilsonite, crushed coal, perlite-expanded and perlite-semiexpanded) which form bridges, and laminated materials (cellophane flakes) which form flake-type mats. In laboratory studies, granular material was found to be best suited for bridging fractures ( Figure 1 , Figure 1the performance of LCM materials in sealing simulated Type materialGeneric nameType particleVolumes used, typical rangeGranularGilsoniteGraded5-50 lb/skPerliteExpanded1/2-1 cu ft/skWalnut shellsGraded1-5 lb/skCoalGraded1-10 lb/skLamellatedCellophaneFlakes1/8-2 lb/skFibrousNylonShort fibers1/8-1/4 lb/skTable 1: Ranges of lost-circulation material (LCM) volumes used per sack. Fibrous materials (such as nylon fibers) are used in drilling fluid for sealing large openings but are not normally used in cement because they tend to plug surface and downhole cementing equipment. Also, most other fibrous materials contain organic chemicals that can seriously retard cement-thickening time. Ranges of lost-circulation material (LCM) values used per sack are listed in Table 1. Salt Salt has many different properties. In addition to its uses as a dispersant and in slurry densification, it may be used as a cementing additive in the following ways: Bonding to Salt Formations: Saturated-salt slurries are the best overall choice for cementing across salt zones because they do not dissolve the salt zone and thus give a better formation-to-cement bond. Protecting Clay and Shale Formations: Small amounts of sodium chloride (NaCl) and potassium chloride (KCl) help protect clay and shale formations that are otherwise susceptible to crumbling and sloughing. Acceleration: Salt may be used as an accelerator. KCl in small amounts also promotes early-strength development. Retardation: Saturated salt is an effective retarder in circulating temperatures up to about 230 to 260 F (110 to 127 C) Expansion: Salt can be used to cause linear expansion of cement to occur long after the cement has set. This effect is minor but beneficial in obtaining a better formation-to-cement bond. Compressive-Strength Stabilizers Four variables composition, temperature, pressure, and time affect compressive strength. However, at high temperatures, cement compositions may retrogress (lose strength) after reaching a high value and never attain the strength reached at lower curing temperatures ( The effect of curing: Figure 1 , pressure and Figure 2 , temperature on cement strength. Figure 1The high temperatures cause strength retrogression). Figure 2This strength retrogression is accompanied by increased cement permeability, e.g., a neat retarded cement with 0.02+ md permeability at 290 F (143 C) after three days may have 8+ md at 320 F (160 C) after seven days. Retarded cement (used in high-temperature applications) and high-water-content cement seem particularly subject to strength retrogression. For cement types used in deep and/or hot wells, the phenomenon begins at around 260 F (127 C), and becomes severe at 290 F (143 C). Silica flour in high percentages inhibits strength retrogression and produces compressive strength far in excess of that of neat cement. Silica flour also reduces permeability of set cement; for instance, its addition to cement cured at 350 F (177 C) reduces permeability to less than 0.001 md. Usually 30 to 40% silica flour is used. Silica sand ground to 200 mesh reacts with cement in the same way as fine-ground 325-mesh silica flour. Sand is used when high density is desired, and flour when low density is adequate. The different densities are achieved because of the different water requirements of the sand and the flour. Compositions containin