Chemistry and Classification of Oil well Cements

Learning Objectives After completing this topic "Chemistry and Classification of Oil well Cements", you will be able to: List API cement classes and their application.Describe the mixing quantities and procedures for various classes of cement and how they relate to cement job planning and execution. Introduction https://youtu.be/D2QVAbm1Nfs Chemistry and Classification of Oil well Cements Cement used in oil and gas well cementing is a Portland cement manufactured with materials and methods that have changed little over the last 160 years. The first Portland cement manufacturing process was patented in 1824 by an Englishman, Joseph Aspdin. It was designed to make an artificial version of Portland Stone, a limestone quarried on the Isle of Portland near Dorset, England. Oil well cement differs from the Portland cement used in building or road construction as it is manufactured to perform under the special conditions encountered in wellbores. Such cements set and develop compressive strength as a result of hydration - a chemical reaction between water and the compounds that make up the cement. Cement Manufacture and Chemistry Oil well cement is prepared by sintering (fusing at high temperatures) fixed proportions of powdered calcareous materials (limestone, chalk, seashells, etc.) with aluminosilicates (clays) in a rotating kiln at temperatures of 1300 to 1550°C. A complex series of progressive chemical reactions take place in the kiln. The resulting material, called clinker (Figure 1), is then cooled and ground together with one or more forms of calcium sulfate, usually gypsum. The calcium sulfate prevents the cement from setting up too quickly once it is mixed with water. Figure 1: Clinker In addition to these basic raw materials, other materials such as sand, bauxite (aluminum ore) and iron oxide may be added to the kiln to adjust the composition of the clinker. The properties of the final cement product are determined by its mineralogical composition. Note that cement chemical notation is different than standard chemical notation. Tricalcium silicate (C3S)Has the greatest effect on the overall compressive strength of the cement; it is also responsible for the early strength (1 to 28 days).Dicalcium silicate (C2S)This slow-reacting component accounts for the gradual gain in strength of the cement which occurs over an extended period of time.Tricalcium aluminate (C3A)Exhibits the fastest rate of hydration; the initial set and thickening time of cement, as well as the sensitivity of the set cement to sulfate-containing waters, are influenced by the concentration of this component.Tetracalcium aluminoferrite (C4AF)Hydrates rapidly at lower heat, allowing for lower kiln temperatures.Table 1: Principal components of cement The addition of iron oxide (Fe2O3) to the kiln favors the production of C4AF at the expense of C3A in the clinker. This procedure is used for cements that are resistant to sulfates, which can be common constituents of subsurface formation brines. Medium sulfate resistant (MSR) cement contains 3 to 8% C3A and high sulfate resistant (HSR) cement contains only up to 3% C3A. The compounds formed in the clinker are anhydrous, and when the cement product is brought into contact with water, they form hydrated compounds. After a period of time, the hydrates form an interlocking crystalline structure (Figure 2) which is responsible for the set cement's strength and impermeability. The formation of this structure as the cement hydrates is a very complex process. Figure 2: Calcium silicate hydrate crystals grown after hydration of C3S Water in excess of that required for cement to set is necessary for the formation of a cement slurry (a suspension of solids in a continuous liquid medium) that can be pumped. After the slurry has been placed in position behind the casing and pumping has stopped, the cement begins to set under downhole conditions and much of this water filters out into the formation. API Cement Types The American Petroleum Institute (API) has established standards and a classification system for oil well cements. This ensures that oil well cements are consistent and predictable in their performance under oilfield conditions and when various additives are introduced. The API classification scheme (API Spec 10A) has been adopted by the International Organization for Standardization (ISO) as Standard 10426-1. International Standardization of Oilwell Cements API standards for oil well cements, in use for many years, have been converted into international standards. API Specification 10A (Specification for Well Cements and Materials for Use in Well Cementing) and API Recommended Practice 10B (Testing of Well Cements) were converted into international standards ISO 10426-1 and ISO 10426-2 respectively. ISO 10426-1 is now the internationally recognized standard for oil well cements. By agreement between ISO and CEN (the European standards organization), these standards are also available as European standards. These standards are based on the API standards with no significant technical changes. The global ISO standards can have local designations, for example in the United States, ANSI/API ISO 10426-1 and 10426-2. There are eight classes of API-ISO Portland cements, designated A through H. They are classified according to the depths for which they are designed and the temperatures and pressures to which they are suited. Within some classes there are three degrees of sulfate resistance: ordinary grade (O), moderate (MSR) and high (HSR), as determined by C3A content. Table 1 summarizes the API classification scheme. API ClassificationApplicationsAPI Class AAlso referred to in the United States as "construction cement" or "common cement."Intended for use when special properties are not required. May be used "neat" without additives, from surface to a depth of 6000 ft, if no special properties are required.Widely available; performance can be modified by the use of additives.Available in O grade only.API Class BIntended for use from surface to 6000 ft depth, where conditions require moderate to high sulfate resistance.Available in MSR and HSR grades.API Class CIntended for use from surface to 6000 ft depth, where early high strength is needed.Available in O, MSR and HSR grades.API Class D*Intended for use from 10,000 to 14,000 ft depth, under conditions for moderate to high pressure and temperature.Available in MSR and HSR grades.API Class E*Intended for use from 10,000 to 14,000 ft depth, under conditions of high temperature and pressure.Available in MSR and HSR grades.API Class F*Intended for use from 10,000 to 16,000 ft depth, under conditions of extremely high temperatures and pressures.Available in MSR and HSR grades.API Class G**Intended for use in neat slurries from the surface to a depth of 8000 ft.May be modified by additives for use in a wide range of well depths pressures and temperatures.Available in MSR and HSR grades.API Class H**Similar in form and application to class G, but is usually more coarsely ground.Table 1: API Cement Classifications *Classes D, E and F are intended for use under moderately high temperatures and pressures, and are available in MSR and HSR grades. These classes are also known as "retarded cements" because they have reduced amounts of the faster-hydrating C3S and C3A components, and increased particle sizes. The technology of chemical additives to retard cement setting has improved so that these cements are rarely found today (they were largely abandoned in the early 1980s). **Class G and H cements were developed in response to improvements in chemical additives for accelerating and retarding cement hydration. These cements contain no glycols or acetates, chemicals that may be added during the manufacturing process for other classes in order to facilitate the grinding process but which can interfere with various other additives. Most well-cementing operations worldwide use slurries based on API Classes A, G, H, and C (high-early-strength cement). Class G and H cements are by far the most commonly used well cements today. In the United States, the split is about 80% Classes G and H, 10% Class A, and 10% Class C. Outside of the United States more than 95% is Class G, often imported. Class A or C, or local common cement may also be used if quality and logistics permit. Service company literature, such as the Halliburton Cementing Tables, describes the various API cement classes, including their chemical requirements, physical specifications, composition, and fineness. Note that not all countries use the API classification system. Rapid Hardening Portland Cement (RHC) and High Strength Portland Cement (HSC) for example, are two non-U.S. cement classifications that correspond roughly to API class C cement. API Class J cement, originally intended for high-temperature applications, has been replaced by Classes G and H with additives such as silica. Effect of Temperature Temperature is one of the most important factors affecting how quickly cement sets in a well, the key parameter for choosing a cement. Figure 1 shows the effect of temperature on the thickening time of various cement slurries. The reaction of cement with water begins when they are first mixed, and the thickening time is defined as the time it takes the slurry to reach a standard consistency as measured by a consistometer. For example, the thickening time of Class G cement drops from 8 hours to 3 hours as the temperature rises from about 70 °F to 120 °F. Figure 1: Temperature versus thickening time (after Bearden, 1959) Basic Cement Slurry Formation Cement slurries, consisting of cement suspended in water (Figure 1), are usually prepared in the field by mixing the dry cement and dry additives with water and liquid additives. This can be done using a jet mixer, a batch mixer, or a recirculating mixer. Figure 1: Cement slurry sample Figure 2 illustrates the components and operation of a continuous jet mixer. The benefits of this type of system are its simplicity and dependability, and the fact that it is not volume-limited. The rates of liquid injection and dry cement flow are carefully controlled to achieve slurry with a specific ratio of water to cement. The most important aspect of cement blending is obtaining a consistent slurry with the proper amount of additives and mix water. FIGURE 2: Jet mixer The principal disadvantage of a jet mixer is its inability to closely control slurry density. Various types of batch mixers utilizing either mechanical agitation or circulation are available for mixing up to 100 bbl (16 [latex]m^{3}[/latex]) of cement slurry. This type of mixer can control density within a range of 0.2 [latex]\tfrac{lbm}{gal}[/latex]. For some operations it is useful to prepare the slurry with a jet mixer that discharges into a batch mixer, where final density adjustments may be made before the slurry is pumped into the well. Recirculating-type jet mixers, where a portion of the slurry is re-circulated through the mixer, can also be used to prepare slurries with a more uniform density. However, a perfectly acceptable slurry can be achieved by an experienced operator using a jet mixer, and jet mixers are used for almost all large jobs that require a constant supply of cement slurry at a high rate. Cement Slurry Specifications If the mixture contains no additives, it is termed "neat" cement. Table 1 shows the preferred water/cement ratio for the various API cement classes, as well as the resulting slurry densities. For example, a Class A "neat" slurry of cement and water should contain 46% mix water "by weight of cement" (BWOC). (For reference, the density of fresh water is 8.33 lbm per gallon and seawater is typically 8.54 lbm per gallon.) Cement ClassMix WaterWater/Cement RatioSlurry DensityYield (%BWOC)[latex]\left( \frac{gal}{94\, lbm} \right)[/latex](ppg)[latex]\left( \frac{ft^3}{sk} \right)[/latex]A465.215.61.18B465.215.61.18C566.314.81.32D, E, F384.316.451.05G445.015.81.15H384.316.451.05Table 1: API Cement Slurry Specifications The range of densities of these slurries results from the different degrees of fineness of the various types of API cement, which affect water requirement. Of the commonly used API types, Class C is the most finely ground, Classes A and G are intermediate, and Class H is the most coarsely ground. Slurries prepared with the finer grinds tend to show slightly lower cement-slurry-filtration rates. The optimal water-to-cement ratio for a cement slurry is a compromise between pumpability and strength. For example, for Class G neat cement, the maximum cement strength occurs at a water-to-cement ratio of about 2.8 [latex]\tfrac{gal}{sack}[/latex]. This is the minimum amount of water necessary to fully hydrate and chemically react with the cement ground to a Class G grain size. If too much water is used (to aid in pumping and displacement, for example), lower compressive strength cement and water separation within the cement column will result. The quality of the water used to mix the cement can vary widely. Fresh water, seawater and some brackish waters are used to mix cement slurries. For any source of water, the setting time and pumpability of the resultant cement must be tested before the slurry is mixed. Cement Slurry Calculations The specific gravity of Portland cement compared to water ranges from 3.10 to 3.25 with an average value of 3.14. A sack of Portland Class A-H cement weighs 94 lb, has a bulk volume of 1 [latex]ft^{3}[/latex] (cement plus surrounding air) and absolute volume (only the material) of 3.59 US gal or 0.48 [latex]ft^{3}[/latex] Density of Cement Slurries The density of a cement slurry is calculated by dividing the sum of the weights of the cement-system (cement, water and additives) by the total of their absolute volumes. [latex]\textrm{Density}\ {\textrm{slurry}} = \dfrac{W{cem} + W_{water} +W {add}}{V{cem} + V_{water} + V_{add}}[/latex] This relationship can then be used to calculate the density of a neat cement slurry for one 94 lb sack of Class A or B cement as follows: ComponentWeight(lb)Absolute Volume([latex]\tfrac{gal}{sk}[/latex])Cement943.59Water43.25.19Total137.28.78 Water to Cement Ratio The water requirement is equal to [latex]0.46\, \cdot 94 = 43.2\, lbs[/latex] which represents a water volume of [latex]\frac{43.2\ lb}{8.33\ \frac{lb}{gal}} = 5.19\ gal[/latex]. Density of the Slurry = [latex]\dfrac{94 + 43.2}{3.59 +5.19}= \dfrac{137.2}{8.78} = 15.6\ ppg[/latex] Notice that the density of the cement slurry is nearly double that of water. Slurry Yield Slurry yield is the quantity of slurry that is realized from a single sack of cement, usually measured in cubic foot per sack([latex]\left ( \frac{ft^{3}}{sk} \right )[/latex]) or cubic meters per SI ton([latex]\left ( \frac{m^{3}}{t} \right )[/latex]). For the above example then, Slurry yield = conversion of [latex]\frac{ft^{3}}{sack} \left ( 1\ ft^{3} = 7.48\ gal \right )[/latex] Slurry yield = [latex]\dfrac{8.78\, \frac{gal}{sk}}{7.48\, \frac{gal}{ft^3}} = 1.174\, \frac{ft^3}{sk}[/latex] (generally rounded to [latex]1.18\, \frac{ft^3}{sk}[/latex] When the volume to be cemented in [latex]ft^3[/latex] is known, this relationship can be used to specify the number of sacks of cement required. Or, if the number of sacks of cement required for the job is specified, the slurry volume can be calculated. Mix water Another important measurement for a cement job is the total volume of mix water required for the job the gallons of water per sack of cement multiplied by the number of sacks required. Example: A cement plan for a 1500 ft intermediate string of casing states "cement with 420 sks of Class H cement".The yield of Class H cement as shown in Table 1 is 1.18 [latex]\frac{ft^3}{sk}[/latex]. Here is how the total required slurry and mix water volumes are calculated: Slurry volume = [latex]1.18 \frac{ft^3}{sk} \cdot 420\ sks = 496\ ft^3 = \dfrac{496}{4.21} = 118\ bbls[/latex] The water/cement mix ratio, as shown in Table 1, is used to calculate the volume of mix water required for the job Mix water = [latex]420\, sks\ \cdot\ 5.2\, \frac{gal}{sk} = 2184\ gal = \dfrac{2184}{42} = 52\, bbls[/latex] Managing BottomHole Pressure during Cementing During cementing, the pressure at the bottom of the well must be maintained above the pore pressure of the formation, to avoid inflow of formation fluids, and below the fracture pressure to avoid causing a fracture. If the pressure caused by a column of relatively heavy cement slurry is above the fracture pressure, it can be reduced by one of these methods: Injecting nitrogen to generate a foamed cementInjecting a "lead" lighter cement slurry (for example, Class C cement) that is placed higher up the annular space, followed by a stronger and heavier "tail" cement (for example Class H cement), that is placed at and near the bottom of the casing. Assignment Basic Cement Slurry Formulation Using the data provided, calculate the quantity of cement slurry and post-flush fluid required to cement the intermediate string of casing in the Assignment Well. Include a 25% cement contingency. Casing StringHole Diameter Dhole (in)Hole Depth* (feet)Casing Shoe Depth* L (feet)Depth to Float Collar* (feet)Casing OD (in)Casing ID (in)Conductor [c]3080L4=80N/A2019Surface [s]17.51510L3=1505146513.37512.615Intermediate [i]12.254015400839689.6258.835 Summary Table* All depths measured from Rotary Kelly Busing (RKB) The fluid volume required to cement the intermediate string of casing of the Assignment well is 692 ft3 without contingency. Class H cement and water is being used for this job. Part A: Calculate the number of cement sacks needed, including a 25% contingency. Part B: Calculate the volume of mix water needed. Solution Part A Solution: According to API Cement Slurry Specifications the yield for a Class H cement = [latex]1.05\ \tfrac{ft^3}{sack}[/latex] The cement needed for the intermediate string plus 25% of contingency = [latex]692\ ft^3 \times \dfrac{1.25}{1.05\, \tfrac{ft^3}{sk}} = 824\ sacks[/latex] Part B Solution: According to API Cement Slurry Specifications the water/cement ratio for a Class H cement = [latex]4.3 \tfrac{gal}{sk}[/latex] The amount of mix water needed = [latex]824\ sks \times 4.3 \tfrac{gal}{sk} = 3544\ gallons\ (85\ barrels)[/latex] Assessment 1. API cements are classified according to what oilwell conditions? (Select all that apply.) A .Mix waterB .AdditivesC .Pressure ✔D .Depth ✔E .Temperature ✔ Oilwell cements set and develop compressive strength as a result of ________. A .pressureB .aerationC .dehydrationD .hydration ✔ How is a cement slurry mixture categorized if it contains no additives? A .CommonB .Neat ✔C .PortlandD .Ordinary Water in excess of that required for cement to set is necessary for the formation of __________ that can be pumped. A .a cement slurry ✔B .aluminosilicatesC .interlocking crystalline structures What is the recommended ratio of water to cement for mixing oilwell cement? A .About 5.0 to 6.3 gallons per 94 lbm sack of dry cement, depending on the API Class ✔B .About 2.8 to 7.3 gallons per 94 lbm sack of cement, depending on the API ClassC .About 4.0 to 5.3 gallons per 94 lbm sack of dry cement, depending on the API Class Recommended for You Introduction to Well Cementing