C AF4 =3.04F . . . (4.4)
These equations are valid as long as the weight ratio of Al2O3 to Fe2O3 present is greater than 0.64.
Example 4.1 Calculate the percentages of C3S, C2S, C3A, and C4AF from the following oxide analysis of a stan- dard Portland cement.
Oxide Weight Percent
Lime (CaO or C) 65.6
Silica (SiO2 or S) 22.2 Alumina (A12O3 or A) 5.8 Ferric oxide (Fe2O3 or F) 2.8
Magnesia (MgO) 1.9
Sulfur trioxide (SO3) 1.8
Ignition loss 0.7
Solution. The A/F ratio is 5.8/2.8 = 2.07. Thus, using Eqs. 4.1 through 4.4 yields C3S = 4.07(65.6)–7.6(22.2)–6.72(5.8) –1.43(2.8)–2.85(1.8) = 50.16 %. C2S = 2.87(22.2)–0.754(50.16) = 25.89%. C3A = 2.65(5.8)–1.69(2.8) = 10.64%. C4AF = 3.04(2.8) = 8.51%. 4.2 Cement Testing
API Spec. 10A/ISO 10426-1 (2002), RP 10B-2/ISO 10426-2 (2005), RP 10B-3/ISO 10426-3 (2004), RP 10B-4/ ISO 10426-4 (2004); other API, ISO, and ASTM standards; and some nonstandard publications such as SPE
papers may present state-of-the-art, recommended procedures for testing well cements. All of these tests were devised to help drilling personnel determine if a given cement composition will be suitable for specifi c well con- ditions. Cement slurry design specifi cations almost always are stated in terms of these laboratory tests. The test equipment needed to perform many of the types of tests include:
· A pressurized mud balance for determining the slurry density · An HP/HT fi lter press for determining the fi ltration rate of the slurry
· A rotational viscometer for determining the rheological properties of the slurry
· A consistometer for determining the thickening rate characteristics of the slurry under downhole pressure and temperature conditions
· Cement HP/HT curing and strength testing machines for determining the compressive strength of the cement · A graduated cylinder for determining the free fl uid of the setting cement
· An HP/HT SGS testing device to measure the time period for the fl uid cement slurry to convert into a high- enough SGS to inhibit formation-fl uid infl ux and migration [see more in ISO 10426-6 (2008)]
· A triaxial load cell for determining the ductility of the cement
The fi rst six types of tests are commonly performed for well cementing operations. The seventh type is needed for cement placed across potential fl ow zones to contain formation fl uids during cement curing periods, and the eighth one is needed to ensure sustainable cement sealing and support integrity under life of the well-load cases. Unlike drilling-fl uid testing, routine testing of the cement slurry normally is not done at the rig site except for ce- ment slurry density, which is required to calibrate density measuring devices on the cement mixing and pumping equipment. However, it is imperative for the drilling engineer to understand the nature of these tests if he or she is to interpret cement specifi cations and reported test results properly.
The pressurized mud balance, fi lter press, and rotational viscometer used for cement testing are basically the same equipment described in Chapter 3 for testing drilling fl uids, except that an HP/HT fi lter press is used instead of the low-pressure version. An HP/HT version of the rotational viscometer is often used for applications in deep, hot hole sections. When measuring the density of cement slurries, entrained air in the sample is more diffi cult to re- move. The pressurized mud balance shown in Fig. 4.2 can be used to minimize the effect of the entrained air. Ce- ment slurry density should be determined by use of a pressurized mud balance described in ISO 10426-2 (2005).
4.2.1 Cement Consistometer. The pressurized and atmospheric-pressure consistometers used in testing cement are shown in Figs. 4.3a and 4.3b. The pressurized consistometer consists essentially of a rotating cylindrical slurry container equipped with a stationary paddle assembly, all enclosed in a pressure chamber capable of withstanding temperatures and pressures encountered in well cementing operations. The cylindrical slurry chamber is rotated at 150 rev/min during the test. The slurry consistency is defi ned in terms of the torque exerted on the paddle by the cement slurry. The relation between torque and slurry consistency is given by
Bc= −T
78.2
20.02 , . . . (4.5)
Cementing 143
(a) (b)
Fig. 4.3—Examples of (a) pressurized and (b) atmospheric consistometers. Courtesy of Halliburton.
where T = the torque on the paddle in g-cm and Bc = the slurry consistency in API consistency units designated by Bc. The thickening time of the slurry is defi ned as the time required to reach a consistency of 100 Bc. This value is
felt to be representative of the upper limit of pumpability. The temperature and pressure schedule followed during the test must be given with the thickening time for the test results to be meaningful. API periodically reviews fi eld data concerning the temperatures and pressures encountered during various types of cementing operations and publishes recommended schedules for use with the consistometer. API Spec. 10A (2002), RP 10B-2 (2005)/ISO
10426-2 (2003), RP 10B-3 (2004)/ISO 10426-3 (2003), and RP 10B-4 (2004)/ISO 10426-4 (2004) provide proce-
dures for a number of schedules for simulating various casing and liner cementing operations. While some stan- dards provide “test schedules” for testing thickening times for different well depths and temperature gradients, the test schedule for a given job needs to be calculated using the actual well conditions and the anticipated pump rates.
The atmospheric-pressure consistometer is frequently used to simulate a given history of slurry pumping before performing certain tests on the slurry, such as tests for free fl uid, rheology, fl uid loss, and compressive strength. For example, the rheological properties of cement slurries are time dependent because the cement thickens with time. The history of shear rate, temperature, and pressure before measuring the cement rheological properties using a rotational viscometer can be specifi ed in terms of a schedule followed using the consistometer. The consistometer also is sometimes used to determine the maximum, minimum, and normal water ratios [% BWOC (by weight of cement)] for various types of cements and is most often used to condition the slurry for the free fl uid content test. In the water-ratio tests, the sample is placed fi rst in the consistometer and stirred for a period of 20 minutes at 80°F and atmospheric pressure. The minimum water content (or water ratio in % BWOC) is the amount of mixing water per sack of cement that will result in a consistency of 30 Bc at the end of this period. The normal water content is
the amount of mixing water per sack of cement that will result in a consistency of 11 Bc at the end of the test. The free fl uid (original name: water) content is determined by pouring a 250-mL sample from the consistometer into a
glass graduated cylinder and noting the amount of free supernatant water that separates from the slurry over a 2-hour period. The maximum water content is defi ned as the amount of water per sack of cement that will result in 3.5 mL of free water. However, these water-ratio tests often can have varying results when additives are used in the cement slurry. A consistometer designed to operate only at atmospheric pressure is frequently used in conjunction with the determination of the slurry rheological, free fl uid, and fi ltrate loss properties and water content.
Example 4.2 The torque required to hold the paddle assembly stationary in a cement consistometer rotating at 150 rev/min is 520 g-cm. Compute the slurry consistency.
Bc= −T = − = 78.2 20.02 520 78.2 20.02 22 consistency units
4.2.2 Strength Tests. The standard tests for cement compressive strength are published in API Spec. 10A (2002)/ ISO 10426-1 (2002), RP 10B-2 (2005)/ISO 10426-2 (2003), RP 10B-3 (2004)/ISO 10426-3 (2003), and RP 10B-4
(2004)/ISO 10426-4 (2004) for drilling cements. The compressive strength of the set cement is the compressional force required to crush the cement divided by the cross-sectional area of the sample. Test schedules for curing strength test specimens are recommended by API. These schedules are based on average conditions encountered in different types of cementing operations and are updated periodically on the basis of current fi eld data. The compressive strength of the cement is usually about 12 times greater than the tensile strength at any given curing time. Thus, frequently only the compressive strength is reported.
Nondestructive Sonic Strength Testing of Cement. A sonic nondestructive testing procedure is used to corre- late cement compressive strength to sonic travel time and is performed by a testing device commonly called an ultrasonic cement analyzer (UCA). Strength correlations are specifi c to certain cement slurry compositions, and some cement compositions may not fi t the correlations supplied by the UCA manufacturer. Custom correlations may be needed for some cement system formulations. The UCA test is the most frequently used test for compres- sive strength. More information on the UCA test can be found in ISO 10426-2 (2005).
4.2.3 Nonstandard Tests and Modeling. Parr et al. (2009) used a variety of nonstandard tests and numerical models to fi nd the root cause for abnormally high cement-displacement pressures in liner cementing. The test and mathematical model conclusions are listed next.
1. Slurry dewatering and fi lter-cake buildup were successfully simulated to show an annular restriction effect caused by a high-permeability formation interval.
2. Solids settling was demonstrated within the drilling mud and spacer fl uids. The mud was shown to build a soft layer of low-mobility solids, but not a hard layer of solids. The effect of the low-mobility solids on cement placement would be to make the mud diffi cult to remove completely from the hole, thereby allow- ing the mud and cement to mix.
3. Using ultralow shear and HP/HT downhole conditions, the static-gel-strength (SGS) development of the drilling fl uid was measured, showing that mud erodibility was low, meaning that subsequent mud displacement by cement would be diffi cult.
4. Film buildup on interior liner pipe walls was measured and shown to be minor relative to issues with cement placement.
5. Results of mud/spacer/cement compatibility laboratory test data and numerical modeling showed the means by which mixing of mud and cement as incompatible fl uids might occur and contribute greatly to the abnormal cementing job pressures.
4.2.4 Permeability Testing. Routine permeability testing of cement has been abandoned by the oil and gas industry. Further, the old method of cement permeability testing (Bourgoyne 1991) is not commonly practiced today. The following discussion explains this change in procedure.
In Sutton et al. (1984a, 1984b), the authors discuss the time period for natural gas to migrate through cement with a very high (e.g., 12 md) permeability in a long cement column. For example, the time of gas migration through 2,000 linear ft of 12-md cement was found to be in excess of 72 years. The realization that cement permeability was relatively unimportant for annular gas fl ow was recognized as early as the early 1960s (Goode 1962). This is one reason why the API RP-65 Task Group focused on other reasons for both short-term (during well construction) and long-term (most cases in < 10 years) annular fl ows. Instead of cement permeability, the main focus is on some of the following reasons or causes for fl ows:
· Poor cement placement
· Cement channeling through mud, leaving bypassed mud that forms a gas fl ow path
· Lost circulation during cementing, resulting in cement channeling or top of cement below a fl ow zone · Poor removal of mudcake that later converts into an annular fl ow path
· Poor control of cement SGS development that causes an underbalanced condition before cement sets · Formation of a microannulus at the cement/pipe and/or cement/borehole wall interfaces
· Stress cracking in hard-set, brittle cements that are not designed for certain cyclic well loads (temperature and pressure changes)
The API RP-65 Task Group did not propose any maximum value for cement permeability in RP 65-Part 2 (2010), as it is not a concern and routinely is much lower than the 12-md example. Gas migration travel-time
Cementing 145
periods calculated with typically low cement permeability values may be several hundreds or thousands of years, depending on cement column lengths and differential pressures. Further, geochemical reactions over these time periods have been shown to deposit scale that seals the cement pores indefi nitely. However, when these calculations predict problems, special cements (such as those used in low pH, corrosive environments) are used to ensure sealing by the cement.
For instance (Sweatman et al. 2009b), most scientists agree that 1,000 years of CO2 containment in an injection reservoir is suffi cient to ensure permanent sequestration. Well-abandonment cement plugs with relatively short lengths of Portland type cements can seal CO2 inside the well under the most severe corrosive conditions for much greater time periods than 1,000 years, regardless of CO2-induced degradation. Consequently, the consensus of opinion has now shifted from cement permeability to other issues such as cement slurry placement assurance (100% mud removal, etc.) and long-term mechanical integrity (stress resistance) to prevent annular leak path- ways like channels, microannuli, and cracks.
Accordingly, the API Task Group discounted cement permeability as a cause for natural gas migration occur- rences. This is why routine permeability testing of cement has been abandoned by the oil and gas industry. During the industry’s evaluation, the old API cement permeability test under ambient pressure and temperature conditions was found technically invalid because it could not simulate the downhole conditions that affected cement perme- ability, such as geochemical effects and confi ning stresses in the annulus. Consequently, new laboratory tests have been developed to measure gas fl ow through cement permeability under downhole conditions. Discussion of these tests is considered to be beyond the scope of this textbook, but details may be found in many references, such as papers concerning sealing CO2 injection wells with cement [e.g., Carey et al. (2006), Huerta et al. (2008), Kutchko et al. (2009), Rodot and Garnier (2009), Santra et al. (2009), and Sweatman et al. (2009b)].
4.3 Standard and Nonstandard Drilling Cements
API Spec. 10A (2002) and its equivalent standard, ISO 10426-1 (2006), have defi ned eight (six in ISO 10426-1)
standard classes and three standard types of cement for use in wells. The more recent ISO standard drops Classes E and F, and API may likely follow suit in the next edition of Spec. 10A. The six classes specifi ed are designated Class A, B, C, D, G, and H. The intended meanings of the various classes are defi ned in Table 4.1. The three types specifi ed are (1) ordinary, “O”; (2) moderate sulfate-resistant, “MSR”; and (3) high sulfate-resistant, “HSR.” The chemical and physical requirements for the various types and classes are given in API Spec. 10A. The majority of oilwell cements are Class G and Class H.
The physical requirements of the various classes of cement given in API Spec. 10A apply to cement samples prepared according to API specifi cations. To provide uniformity in testing, it is necessary to specify the amount of water to be mixed with each type of cement. These water-content ratios, shown in Table 4.2, often are referred to as the normal water content or “API water” of the cement class. As will be discussed in the next section, Wyo- ming bentonite sometimes is added to the cement slurry to reduce the slurry density, or barium sulfate is added to increase the slurry density. For example, the water content may be increased 5.3 wt% for each weight percent of bentonite added and 0.2 wt% for each weight percent of barium sulfate added.
4.3.1 Construction Industry Cement Designations. Five basic types of Portland cements are used commonly in the construction industry in the USA. The ASTM classifi cations and international designations for these fi ve cements are shown in Table 4.3. Note that ASTM Type I, called normal, ordinary, or common cement, is similar to API Class A cement. Likewise, ASTM Type II, which is modifi ed for moderate sulfate resistance, is similar to API Class B cement. ASTM Type III, called high early strength cement, is similar to API Class C cement. Other types of construction cements can be found in standards by other countries.
4.3.2 Nonstandard Cements. Nonstandard cements are often used for special applications and do not fall into any specifi c API, ISO, or ASTM classifi cation. Some of these cements are dry blends of API, ISO, or ASTM ce- ments and additives for well applications in primary or remedial cementing operations. These cement materials’ quality and uniformity are generally controlled by the supplier and include the following types:
· Pozzolan/Portland cements · Pozzolan/lime cements · Resin or plastic cements · Gypsum cements · Microfi ne cements · Expanding cements
TABLE 4.1—STANDARD CEMENT CLASSES AND GRADES
Class A
The product obtained by grinding Portland cement clinker, consisting essentially of hydraulic calcium silicates, usually containing one or more forms of calcium sulfate as an interground additive. At the option of the manufacturer, processing additives may be used in the manufacture of Class A cement, provided such materials in the amounts used have been shown to meet the requirements of ASTM C 465. This product is intended for use when special properties are not required. Available only in ordinary (O) Grade (similar to ASTM C 150, Type I).
Class B
The product obtained by grinding Portland cement clinker, consisting essentially of hydraulic calcium silicates, usually containing one or more forms of calcium sulfate as an interground additive. At the option of the manufacturer, processing additives may be used in the manufacture of Class B cement, provided such materials in the amounts used have been shown to meet the requirements of ASTM C 465. This product is intended for use when conditions require moderate or high sulfate-resistance. Available in both moderate sulfate-resistant (MSR) and high sulfate-resistant (HSR) Grades (similar to ASTM C 150, Type II).
Class C
The product obtained by grinding Portland cement clinker, consisting essentially of hydraulic calcium silicates, usually containing one or more forms of calcium sulfate as an interground additive. At the option of the manufacturer, processing additives may be used in the manufacture of Class C cement, provided such materials in the amounts used have been shown to meet the requirements of ASTM C 465. This product is intended for use when conditions require high early strength. Available in ordinary (O), moderate sulfate- resistant (MSR) and high sulfate-resistant (HSR) Grades (similar to ASTM C 150, Type III).
Class D
The product obtained by grinding Portland cement clinker, consisting essentially of hydraulic calcium silicates, usually containing one or more forms of calcium sulfate as an interground additive. At the option of the manufacturer, processing additives may be used in the manufacture of Class D cement, provided such materials in the amounts used have been shown to meet the requirements of ASTM C 465. Further, at the option of the manufacturer, suitable set-modifying agents may be interground or blended during manufacture. This product is intended for use under conditions of moderately high temperatures and pressures. Available in moderate sulfate-resistant (MSR) and high sulfate-resistant (HSR) Grades.
Class E
The product obtained by grinding Portland cement clinker, consisting essentially of hydraulic calcium silicates, usually containing one or more forms of calcium sulfate as an interground additive. At the option of the manufacturer, processing additives may be used in the manufacture of Class E cement, provided such materials in the amounts used have been shown to meet the requirements of ASTM C 465. Further, at the option of the manufacturer, suitable set-modifying agents may be interground or blended during manufacture. This product is intended for use under conditions of high temperatures and pressures. Available in moderate sulfate-resistant (MSR) and high sulfate-resistant (HSR) Grades.
Class F
The product obtained by grinding Portland cement clinker, consisting essentially of hydraulic calcium silicates, usually containing one or more forms of calcium sulfate as an interground additive. At the option of the; manufacturer, processing additives may be used in the manufacture of Class F cement, provided
such materials in the amounts used have been shown to meet the requirements of ASTM C 465. Further, at the option of the manufacturer, suitable set-modifying agents may be interground or blended during
manufacture. This product is intended for use under conditions of extremely high temperatures and pressures.