Cement Additives in Drilling

Learning Objectives After completing this topic "Cement Additives in Drilling", you will be able to: List the key cement properties that are modified by additives.List the common categories of additives used in cementing operations.Summarize the functions and application of the common additives. Introduction https://youtu.be/u2pl5-yaExI "Neat" cement - that is, cement containing no additives - is seldom used in cementing jobs because it does not have the properties required either by the slurry, the set cement, or both. The Class G or H cement that is most commonly used can be tailored to meet most cementing job specifications with the use of additives. A well designed cement system, consisting of basic cement plus additives, must: Remain pumpable until properly placed in the well.Set uniformly, bonding securely to both the casing and the formation.Achieve the necessary compressive strength within an allotted time period. It must do these things under conditions that can vary from below freezing in arctic permafrost zones to near freezing in deepwater wells to 700 °F in geothermal wells. Pressures can range from ambient to more than 30,000 psi at the bottom of deep, overpressured wells. In addition, cements must be capable of performing consistently in spite of widely variable formation permeability, corrosive subsurface fluids, and variable hole conditions. Most cement additives 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 the mixing water at the site. Liquid additives are more commonly used offshore and in remote land locations where dry cement blending and storage are not as common. The key cement properties that are modified by additives are listed below: Cement slurrySetting or thickening time (acceleration, retardation)Density (extenders, weight increase/reduction)Viscosity (thinners or thickeners)Fluid loss resistance (via filtrate loss)Lost-circulation resistance (via whole slurry loss)Set cementCompressive strengthStrength retrogression (loss with time)Expansion/contraction The performance of additives is influenced by the chemical and physical properties of the basic cement, which can sometimes vary even within an API/ISO classification. Other variables can also influence additive performance: Mixing temperature and pressureAdditive concentrationMixing methodAdditive mixing orderWater-to-cement ratio As a result, practical cement system design involves thorough laboratory testing before the job, using representative samples of the cement and mix water to be used on site. Also important is an understanding of the expected transport and storage conditions for both bulk cement and additives. This subtopic discusses each of the major categories of cement additives, their impact on cement properties, and examples of the most common additive choices. Accelerators Accelerators are chemicals that reduce the setting time of cement and increase the rate of compressive strength development. Accelerators are often used in shallow or cool wells to speed up the set of cement so less rig time is spent waiting on the cement to set. https://youtu.be/CA9GhHjZDCA Setting time too slow Animation 1 Most operators wait for cement to reach a minimum of 500 psi compressive strength before resuming operations, but at temperatures below 100 °F neat cement may require a day or two to develop 500 psi strength. Accelerators reduce the waiting-on-cement (WOC) time. They can also counteract the effects of other additives employed for other purposes that concurrently cause a delay in setting time. Commonly-used Accelerators The most commonly used accelerator is calcium chloride (CaCl2) added at concentrations between 2% and 4% by weight of cement (BWOC). Its popularity is due to the fact that it is both efficient and economical. For example, at 91 °F the addition of 4% BWOC CaCl2 reduces setting time from 4 hours to 1 hour and 15 minutes (Table 1). As the cement sets faster, compressive strength also builds more quickly, reaching 1,320 psi after 6 hours versus only 370 psi without the CaCl2, at 100 °F. Table 1: Effects of adding CaCl2 to neat cement (Nelson and Guillot 2006) CaCl2 \mathbf{CaCl_{2}} concentration (% BWOC) Thickening Time (hr:mm) Compressive Strength (psi) 91 °F (33 °C) 113 °F(45 °C) 100 °F (38 °C) 6 hr 12 hr 24 hr 0 4:00 2:32 370 840 1780 2 1:17 1:01 1110 2370 3950 4 1:15 0:59 1320 2560 4450 Another accelerator option, sodium chloride (NaCl), affects setting time and compressive strength development in different ways depending on concentration and temperature: Accelerator at concentrations up to 15% by weight of mix water (BWOW)No effect at concentrations between 15% and 20%Retarder (slows thickening time) at concentrations above 20% When seawater is used as mix water at offshore locations, its NaCl content (about 2.5%) results in acceleration of setting time. There are also a number of chloride-free accelerators for special situations, including calcium formate, calcium nitrate in conjunction with triethanolamine, and thiocyanate salts. However, these do not generally perform as well as calcium chloride. A complex mixture of calcium nitrate and four other compounds is marketed as an accelerator for deep water and arctic permafrost wells because it can out-perform calcium chloride at low temperatures between 40 °F and 70 °F. Retarders Retarders are used primarily to extend the time that a cement slurry can be pumped and displaced into position before it begins setting. For example, a retarder may be used in deep or very hot wells to prevent setting of the cement before it is properly placed in the annulus. https://youtu.be/mZX85h3LyzA Setting time too fast Animation Setting time is a function of both temperature and pressure. Setting time can also be shortened if pumping is interrupted (loss of agitation accelerates setting). It is important to understand how these parameters are likely to change over the length of the wellbore before selecting specific retarders. If cement setting time is retarded too much it can lead to: Delays in resuming drilling operationsSettling and separation of slurry componentsFormation of free-water pocketsLoss of hydrostatic head and an influx of formation fluids into the slurry Commonly-used Retarders In general, organic materials will retard cement setting time. The most common retarders are lignosulfonates, primarily calcium lignosulfonate. Lignosulfonates are polymers derived from wood pulp. Calcium lignosulfonate is typically added in concentrations of 0.1% to 1.5% BWOC. Its effectiveness is limited in temperatures above 250 °F. When blended with sodium borate the temperature range can be extended to as high as 600 °F. The effect of a lignosulfonate retarder on a 15.8 [latex]\tfrac{lbm}{gal}[latex] (1,900 [latex]\tfrac{kg}{m^{3}}[/latex]) Class G cement is shown in Figure 1. At 125 °F the thickening time can be doubled, from 1.5 hours to 3 hours, with the addition of 0.20% BWOC of retarder. Figure 1: Relationship between thickening time and lignosulfonate concentration on a Class G cement Other retarders include carboxymethyl hydroxyethylcellulose or CMHEC (which also increases the viscosity of the slurry), organophosphates, and sodium chloride (only in high concentrations). Extenders (Density-Reducing Additives) Extenders are designed to reduce the density of a cement slurry, and also to increase its yield (volume of set cement produced from a given volume of dry cement). Extenders offer the following benefits: A decrease in density reduces the hydrostatic pressure of the cement column in the well, thus reducing the likelihood that it will break down weak formations during the placement process. https://youtu.be/CoO3UsbyoyY The lower hydrostatic pressure can reduce the number of "stages" needed to cement the well. (Multiple stage cementing is discussed in "Primary Cementing Design and Operations.") This eliminates the need for special equipment and can reduce the time needed to place the cement.Increasing the yield can improve the economics of the cementing operation. Table 1 lists the three main categories of extenders. Extender typeDescriptionWater extendersClays such as bentonite and other water-viscosifying agents allow the addition of more water than normal to the slurry while still maintaining homogeneity and preventing the separation of a free water phase.Low-density aggregatesThese include a variety of materials that have a lower density than Portland cement (3.20 [latex]\tfrac{g}{cm^{3}}[/latex]). Adding a significant quantity of such material reduces the overall slurry density.Gaseous extendersNitrogen or air can be used to prepare foamed cements, which have very low density but are capable of building compressive strength.Table 1: Categories of extenders Water Extenders The most common extender is bentonite, also known as "gel." Bentonite is a major component of water-based drilling muds, but "mud gel" should not be substituted for cement-grade bentonite. Additives used to improve the performance of bentonite in drilling fluids make it unsuitable as a cement extender (Grant 1990). Bentonite is typically added in concentrations between 1% and 16% by weight of cement (BWOC). Above 6%, the addition of a dispersant is usually necessary to reduce the viscosity-increasing and gelling attributes of the clay. Additional water is typically added to the slurry along with the clay. As the percentage of bentonite increases the slurry density decreases and yield increases (Table 2). % BentoniteWater(gal/sack)Slurry Density(lbm/gal)Yield(ft3/sack)04.9715.81.1447.3614.41.4889.7613.51.821212.1512.72.161614.5512.32.51Table 2: Effect of bentonite extender on cement slurry properties (Nelson and Guillot, 2006) The use of bentonite can compromise compressive strength. Cement with 12% BWOC bentonite has about half the 24-hour compressive strength of the same cement with 4% BWOC bentonite (Figure 1). Bentonite-extended cements are also less resistant to sulfate containing waters and corrosive formation fluids. Figure 1: Effect of bentonite on compressive strength (Nelson and Guillot, 2006) Bentonite 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. Other water extenders include: Sodium meta-silicates that react with lime or calcium chloride in the cement to form a calcium silicate gel and increase viscosity, allowing the addition of extra water.Gypsum, the hemihydrate form of calcium sulfate.Pozzolans, finely divided siliceous material that extend Portland cement but also react with the calcium hydroxide liberated during the cement hydration process to enhance the compressive strength of the set cement. Examples of pozzolans include volcanic ash, diatomaceous earth, finely divided silica, and power plant fly ash. Low-Density Aggregates Low-density aggregates (Figure 2) reduce slurry density because they are lighter than the cement particles. Examples include expanded perlite (crushed volcanic glass expanded via heat), powdered coal, gilsonite (a naturally occurring asphaltic material similar to coal), and glass or ceramic microspheres. Figure 2: Low-density aggregates Expanded perlite is a highly buoyant product that requires the concurrent addition of 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 additional slurry compressibility under downhole conditions. Gilsonite, a mineral found only in Utah and Colorado, can be used to prepare low density cement systems with relatively high compressive strength. Up to 50 lbm of gilsonite per sack of cement can achieve densities as low as 12 [latex]\tfrac{lbm}{gal}[/latex]. Gilsonite can plug downhole equipment and bridge across tight annuli, so bentonite is commonly added to prevent bridging in the wellbore. Powdered coal is sometimes used in the same concentration as gilsonite but the addition of bentonite to prevent separation is normally not required. Very light slurries (8.5 to 11 [latex]\tfrac{lbm}{gal}[/latex]) have been made using hollow glass or ceramic microspheres. These cements are useful for underpressured formations, hot geothermal wells, and other special applications. The synthetic hollow spheres are similar to the natural glass spheres present in fly ash (cenospheres). The primary disadvantage of microspheres is their susceptibility to crushing during mixing and pumping, and when exposed to hydrostatic pressures above crush strength. Gaseous Extenders Foamed cement involves the addition of a gas, usually nitrogen or air, directly into the slurry. Densities as low as 3.5 [latex]\tfrac{lbm}{gal}[/latex] can be achieved (Nelson and Guillot 2006), but this option requires special equipment at the well site to inject the gas and foam generating surfactants into the cement system before it is pumped. Foams are characterized by their quality, the ratio of the volume occupied by the gas to the total volume of the foam (expressed as a percentage). Good foamed cements consist of nearly spherical bubbles separated by thick liquid films. Qualities are usually less than 50%, and recommended qualities typically range from 18 to 38 percent. Figure 3 shows cement samples prepared at atmospheric pressure using a Class H cement with a slurry density of 16.5 [latex]\tfrac{lbm}{gal}[/latex] and four foam qualities: 10%, 20%, 30%, and 40%. Entrained air fractions were mixed using an industry standard foaming agent. Figure 3: Various foam quality cement samples Weighting Materials (Density-Increasing Additives) High pore pressures encountered in deep wells or abnormally pressured formations are controlled by increasing the density - and therefore the hydrostatic pressure - of the drilling fluid in the wellbore. To maintain control of these wells during cementing operations, the density of the cement slurry must likewise be increased. One way to increase the density is to simply decrease the amount of mix water, but this creates other problems and reaches a practical limit at about 18 lbm/gal. Higher densities require adding materials with a high specific gravity. To be effective, these weighting materials must have certain characteristics: They must have a particle-size distribution that is compatible with the cement. Particles that are too large will settle out, while particles that are too small will significantly increase the slurry viscosity.They must not require additional mix water.They must be inert with respect to other additives and the chemical reactions of cement hydration. Also, the weighted cement slurry must have adequate viscosity and gel strength to carry and suspend these high-specific-gravity solids. The most common weighting agents are high specific gravity minerals: ilmenite, hematite, barite, and manganese tetraoxide (Figure 1). These are prepared as powders or granules and have specific gravities between 4.3 and 4.95. Figure 1: Common weighting materials A density of up to 22 [latex]\tfrac{lb}{gal}[/latex] can be obtained with hematite or ilmenite plus friction-reducing additives. Barite can achieve densities of up to about 19 [latex]\tfrac{lbm}{gal}[/latex] but it requires a relatively larger amount of water, which reduces compressive strength and retards thickening time. A 44% BWOC water slurry of Class H cement has a density of about 17.2 [latex]\tfrac{lbm}{gal}[/latex], but with a barite concentration of 80% BWOC the density reaches 18 [latex]\tfrac{lbm}{gal}[/latex] (Figure 2). Figure 2: Relationship between slurry density and weighting material concentration Dispersants Well cement slurries are concentrated suspensions of particles in water whose rheological (flow) properties are related not only to the relative volume fractions of particles and fluid but also to particle interactions. Dispersants act on these particle interactions to reduce viscosity and control rheological properties. Commonly-used Dispersants Polynaphthalene sulfonate (PNS) is the most common and cost effective dispersant for oil well cements. However, it can no longer be used in some marine environments due to its toxicity and non-biodegradability in seawater. For freshwater slurries, 0.2 to 1.0% active PNS BWOC is normal. For slurries containing sodium chloride (which increases flocculation) concentrations as high as 4% PNS BWOC may be necessary. Figure 1: Polynaphthalene sulfonate Fluid Loss Control Additives When the pressure exerted by a column of cement slurry exceeds the pore pressure of a permeable formation, water from the cement slurry filters into the formation, leaving solid cement particles behind. If this fluid loss is not controlled, the slurry density increases – possibly to the point that it can no longer be pumped. Fluid Loss Other related problems can include: Casing being pushed against borehole wallDecentralization of the casing and differential sticking against the borehole wallFiltrate leaking into a productive zone causing formation damageSolid cement forming above a gas zone blocking hydrostatic pressure and causing gas influx Cement slurry fluid loss is measured by a standard API test and expressed in units of volume-per-time. The "API fluid loss" for neat cement slurry is typically greater than [latex]\tfrac{1500\, ml}{30\, min}[/latex]. However, a fluid loss range of [latex]\tfrac{50\ to\ 300\, ml}{30\, min}[/latex] is often required to maintain adequate slurry performance, so fluid loss additives are nearly always needed. There are two classes of fluid loss additives: Particulate materials that reduce the cement filter cake permeability.Water soluble polymers that increase the viscosity of the aqueous phase. The first fluid loss control agent for cement was bentonite. Particulate additives also include carbonate powder, carbon black, microsilica, asphaltenes and thermoplastic resins. Another fluid loss additive is based on crosslinked polyvinyl alcohol (PVA) microgels. Water soluble polymers, first used in drilling fluids in the 1940s, are high molecular weight compounds that can thicken the water significantly depending on polymer concentration. There are two basic categories, natural and synthetic. The most common natural polymers are cellulosic and the most common used as a cement fluid loss agent is hydroxyethylcellulose (HEC). There are also a number of synthetic polymers that include polyvinyl alcohol (PVA), polyacrylamides, and polyethylene imine (PEI). Lost Circulation Prevention Additives Lost circulation (LC) or lost returns refers to the significant and continuing loss of either whole drilling fluid or cement slurry to a formation. It should not be confused with lost filtration. Drilling fluids or cement slurries can be lost to either natural or induced formation fractures or, less commonly, to highly permeable formations, such as carbonate formations with vugs or cavernous porosity. Cement, with its larger particle size, is less susceptible to loss in permeable formations but can still be lost to induced fractures in weak formations. https://youtu.be/0sg5xz62Z2o The best time to reduce the chance of such lost circulation is during drilling, when high concentrations of bridging materials and various types of plugs (pills) can be pumped into the well to fill up the fractures. During primary cementing, concentrations of LC additives must be carefully controlled to avoid bridging the casing-borehole or liner-borehole annulus, or plugging downhole cementing equipment. Types of LC additives available for cement are blocky-granular materials (ground walnut shells, gilsonite, crushed coal, expanded perlite) and laminated materials (mica flakes, cellophane flakes) which form flake-type mats. In laboratory studies, granular material has been found to be best suited for bridging fractures. 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, some fibrous LC materials contain organic chemicals that can seriously retard cement-thickening time. Strengthening Agents Fibrous materials added to cement slurries in concentrations between 0.15% and 0.5% BWOC can increase the cements resistance to stresses from perforating, hydraulic fracturing, and formation movement over time. By transmitting local stresses more evenly throughout the cement matrix, these strengtheners can maintain a competent cement sheath around the casing. Examples include nylon fibers (up to 1 inch in length), metallic microribbons, and particulated rubber. Figure 1: Nylon fibers and metallic microribbons Antifoam Agents A number of cement additives can cause the slurry to foam during the mixing process. This air entrainment can lead to inaccurate density measurements and once downhole, when the air bubbles are compressed, result in a higher density cement than expected, leading to other problems. Antifoam agents adjust the surface tension of the foaming liquid phase or alter the dispersibility of solids in the cement system. Two common antifoam agents are polyethylene glycol (PEG) and silicone emulsions. Very small concentrations can achieve a desired result, typically less than 0.1% BWOW. PEG must be added before foam forms to be efficient. After foam forms, silicone emulsions can effectively remove the foam. Figure 1: Polyethylene glycol (PEG) Special Cement Systems Introduction Although not additives in the strict sense, there are a number of special cement systems that are distinct from the Portland cement-plus-additives systems we have described: Special cement systems include: Latex systemsSalt systemsBlast furnace slag (BFS) systemsEpoxy systemsMulti-particle cement systems Latex Systems Latex systems are used to control problems with gas migration behind pipe. Latex-modified cement systems include a suspension of very small spherical polymer particles stabilized with surfactants. This polymer emulsion usually contains about 50% solids. Latex cements exhibit normal color and consistency, but after curing the hydrated cement particles are connected by a film of latex particles. The addition of latex provides a number of benefits including: Improved slurry pumpabilityDecreased slurry fluid lossDecreased permeability of the set cementIncreased tensile strengthReduced shrinkageIncreased elasticityImproved bonding between cement/steel and cement/formationLess shattering when perforatedIncreased resistance to contamination by well fluidsImproved durability Polyvinyl acetate latex systems are mixed at a concentration of 2.5% to 25% BWOC, and can be used up to temperatures of 122 °F. Styrene butadiene latexes work equally well but are effective up to temperatures as high as 350 °F. Salt Systems Cement systems that contain significant quantities of sodium chloride (NaCl) or potassium chloride (KCl) are sometimes referred to as "salt cement." Salt is found in cement systems for three reasons: Offshore, it is present in the mix water.It is added to achieve certain properties.During cementing of a wellbore across a massive salt formation or fresh-water-sensitive formation, high salt content slurries will prevent dissolution or clay swelling. Seawater will reduce thickening time, increase fluid loss rate, increase compressive strength at low temperatures, and depress the effect of bentonite as an extender. Addition of salt to cement can create complex interactions and therefore behave as an extender or accelerator, depending on concentration and conditions. Salt-saturate cements are useful when completing wells across producing formations that contain montmorillonite, illite, and chlorite—clays that swell on contact with fresh water, reducing permeability. Salt concentrations as low as 10% BWOW can be sufficient to prevent damage. Drilling through massive salt evaporate layers is a challenge because the salt dissolves, changing the cement slurry properties, and deforms plastically, encroaching into the wellbore. Advanced, proprietary additive combinations have been developed that permit cement systems containing anywhere from 5% to 37% NaCl BWOW. There is no official industry consensus regarding slurry design for salt-zone completions, but some experts believe that most salt zones are cemented with systems containing between 8% and 18% NaCl BWOW. Blast Furnace Slag Systems Blast furnace slag (BFS) is a byproduct of steelmaking. When finely ground, it contains more than 95% oxides of calcium, silicon, aluminum, and magnesium. Unlike Portland cement, BFS requires an activator other than water to set. These include caustic soda (NaOH), soda ash (Na2CO3), sodium silicate, sodium sulfate, calcium sulfate, lime, or mixtures of these compounds. (Portland cement itself is an activator.) Compared to Portland cements, BFS cements exhibit better sulfate resistance, slower diffusion of chloride and alkali ions through the set cement matrix, and lower set cement permeability. They are used in situations requiring salt-saturated cements, carbon dioxide resistant cements, and certain squeeze cementing applications. Epoxy Systems Wells that require cements that are particularly resistant to chemical attack (such as chemical waste disposal wells, or carbon dioxide enhanced oil recovery wells), are candidates for epoxy-based organic polymer systems. Such systems are sometimes called "synthetic cements." Epoxy cements are prepared by mixing an epoxy resin with a hardening agent and a solid filler. The compressive strengths range from 8,000 to 10,000 psi, compared to less than 5,000 psi for conventional cements. Multi-Particle Cement Systems Multi-particle cement uses particles of various sizes to create a suspension of cement. The smallest particles – typically silica – are so small they do not settle in water. The intermediate particles are made of cement, and the largest is a fine sand. The particle size is arranged such that the smallest particles fit within the spaces between the intermediate particles, which, in turn, fit within the spaces between the largest particles. The three different-sized particles fill over 80% of the space of the cement, which means that far less water is required to make the system flow. The slurry is also less susceptible to water contamination, and can easily incorporate more exotic particles that effect various unique properties to the cement. Assignment Cement Additives What casing pressure at the point of injection is needed to open a gas lift valve with the following specifications, where the tubing pressure at the point of injection (Pprod) is 420 psi? Part A: The casing will be cemented in a single stage.The density of "neat" Class H cement slurry (no additives) is 16.4 lb/gal.Formation pressure at the casing shoe is approximately 1,125 psi.Fracture pressure gradient is estimated at 0.795 psi/ft. Note that at the conclusion of cement displacement, this formation must be able to support a column of cement that extends from 2500 feet back to the surface. Perform the necessary calculations to answer the following question: Based on the hydrostatic pressure of the Class H neat cement, do you need to add an extender to the Class H slurry? (Assume a safety margin of 0.5 lb/gal Equivalent Density.) Part B: Bentonite is a commonly used extender. Referring to the Slurry Properties of Class H Cement table (Halliburton eRedbookTM), how many pounds of bentonite should be added to each sack of cement? (1 sack of dry Class H cement weighs 94lb.) Solution Part A Solution: Fracture pressure at shoe: 0.795 psi/ft × 2500 ft=1988 psi Hydrostatic pressure of fluid column: 0.052×TVD×Density Safety margin: 0.052 × 2500 × 0.5= 65 psi Fracture pressure including safety margin: 1988 − 65=1923 psi Equivalent density of fracture pressure: 1923/(.052×2500)=14.8 lb/gal Part B Solution: As you solved in Part A, the slurry density for this job must be equal to or less than 14.8lbgal. In the table, the bentonite concentration that will come closest to the required density is 4%. Weight of bentonite per sack of cement: 0.04×94=3.76 lb/sack Assessment 1. Which of the following materials can be used to prevent lost circulation? A .Manganese tetraoxideB .IlmeniteC .Cellophane flakes ✔D .Montmorillonite 2. A common antifoaming agent for cement is _________. A .azodicarbonamideB .ethylene glycol ✔C .titanium hydride 3. The use of cement additives ensures that the cement operation meets critical criteria. Which of the following attributes are critical to a successful cement operation? (Select all that apply.) A .Remains pumpable until properly placed in the well. ✔B .Does not damage the formation. ✔C .Sets uniformly, bonding securely to casing and formation. ✔D .Sets within two hours.E .Does not cool down. 4. The most commonly used cement accelerator is ____________ . 5. Identify some examples of strengthening agents. (Select all that apply.) A .particulated rubber ✔B .nylon fibers ✔C .polyethylene glycolD .metallic microribbons ✔E .silicone emulsions Recommended for You Introduction to Well Cementing Chemistry and Classification of Oil well Cements