1. Well Design
PE 413

2. Introduction History and Overview
The basic principle of oil well cementing involves displacing cement slurry down the casing to a predetermined point in the well. The slurry is formed by mixing water with Portland cement, or with cement blended with additives. This procedure controls gas/oil and water/oil ratios, and is used in various types of liner jobs and remedial work. The casing must be cemented to exclude water and other unwanted fluids. Cement slurry is forced into the annular space between the casing and the wall of the hole, where the cement can set and form a permanent barrier against water and other fluids.

3. Introduction History and Overview
Cement that is pumped down into the annulus is used as a sealant to help protect:
Casing and wellbore from external pressure that could collapse the pipe or cause a blowout
Oil- and gas-producing strata from extraneous fluids
Casing from possible corrosion and electrolysis caused by formation waters and physical contact with various strata
Downhole production and drilling equipment
Pipe from the stresses of formation movement

4. Introduction History and Overview
The cement composition and placement technique for each job must be chosen so that the cement will achieve an adequate strength soon after being placed in the desired location. This minimizes the waiting period after cementing. However, the cement must remain pumpable along enough to allow placement to the desired location. The main ingredient in almost all drilling cements is Portland cement, artificial cement made by burning a blend of limestone and clay. This is the same basic type of cement used in making concrete.

5. Introduction Hydration of Cement
Cement is composed principally of a blend of anhydrous metallic oxides. The addition of water to this material converts these compounds to their hydrated form. After a period of time, the hydrates form an interlocking crystalline structure which is responsible for the set cement's strength and impermeability.

6. Composition of Portland Cement
The principal components of common Portland cement are
50% tricalcium silicate (3CaO·SiO2) - C3S
25% dicalcium silicate (2CaO·SiO2) – C2S
10% tricalcium aluminate (3CaO·Al2O3) - C3A
10% tetracalcium aluminoferrite (4CaO·Al2O3·Fe2O3) - C4AF
5% other oxides

7. Composition of Portland Cement
2(3CaO.SiO2) + 6H2O --> 3CaO.2SiO2.3H2O + 3Ca(OH)2
2(2CaO.SiO2) + 4H2O --> (slow)3CaO.2SiO2.3H2O + Ca(OH)2
4CaO.Al2O3.Fe2O3 + 10H2O + 2Ca(OH)2 --> (slow)6CaO.Al2O3.Fe2O3.12H2O + Ca(OH)2
3CaO.Al2O3 + 12H2O + Ca(OH)2 --> (fast)3CaO.Al2O3.Ca(OH)2.12H2O
3CaO.Al2O3 + 10H2O + CaSO4.2H2O --> 3CaO.Al2O3.CaSO4.12H2O
Oxide
Lime (CaO or C)
Silica (SiO2 or S)
Alumina (Al2O3 or A)
Ferric Oxide (Fe2O3 or F)
Magnesia (MgO)
Sulfur Trioxide (SO3)
Ignition loss

8. Composition of Portland Cement
API uses the following equations for calculating the weight percent of the crystalline compounds from the weight percent of the oxides present.
C3S = 4.07C – 7.6S – 6.72A – 1.43F – 2.85SO3
C2S = 2.87S – 0.754C3S
C3A = 2.65A – 1.69F
C4AF = 3.04F
These equations are valid as long as the weight ratio of Al2O3 to Fe2O3 present is greater than 0.64

9. Example
Example: Calculate the percentages of C3S, C2S, C3A, and C4AF from the following oxide analysis of a standard Portland cement.
Oxide
Weight Percent
Lime (CaO or C)
Silica (SiO2 or S)
Alumina (Al2O3 or A)
Ferric Oxide (Fe2O3 or F)
Magnesia (MgO)
Sulfur Trioxide (SO3)
Ignition loss
65.6
22.2
5.8
2.8
1.9
1.8
0.7

10. Example The A/F ratio is 5.8/2.8 = 2.07.
C3S = 4.07C – 7.6S – 6.72A – 1.43F – 2.85SO3
C3S = 4.07(65.6) – 7.6(22.2) – 6.72(5.8) – 1.43(2.8) – 2.85(1.8) = 50.16%
C2S = 2.87S – 0.754C3S
C2S = 2.87(22.2) – 0.754(50.16) = 25.89%
C3A = 2.65A – 1.69F
C3A = 2.65(5.8) – 1.69(2.8) = 10.64%
C4AF = 3.04F
C4AF = 3.04(2.8) = 8.51%

11. API Tests for Cementing
presents a recommended procedure for testing drilling cements. Cement specifications almost always are stated in terms of these standard tests. The test equipment needed to perform the API tests includes:
A mud balance for determining the slurry density,
A filter press for determining the filtration 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,
Specimen molds and strength testing machines for determining the tensile and compressive strength of the cement
A cement permeameter for determining permeability of the set cement,

12. API Tests for Cementing Mud Balance – Slurry Density Test
The test consists essentially of filling the cup with a mud sample and determining the rider position required for balance. Water is usually used for the calibration fluid. The density of fresh water is 8.33 lbm/gal.

13. API Tests for Cementing Filter Press – Fluid Loss Test
The rate at which a cement slurry loses the water required for its fluidity through a permeable barrier is called filtration rate or fluid-loss rate.
The standard API filter press has an area of 45 cm2 and is operated at a pressure of 100 psig (6.8 atm). The filtrate volume collected in a 30-min time period is reported as the standard water loss.

14. API Tests for Cementing Rotational Viscometer
The mud is sheared at a constant rate between an inner bob and an outer rotating sleeve. Six standard speeds plus a variable speed setting are available with the rotational viscometer.

15. API Tests for Cementing Rotational Viscometer
Flow curves of time-independent fluids
Newtonian fluids:
Power law fluids:
Bingham fluids:
Herschel-Bulkley (Yield power law fluids)

16. API Tests for Cementing Rotational Viscometer
where: ma(cp) - apparent viscosity,
qN - dial reading in degrees,
N(RPM) - rotor speed,
mp(cp) - plastic viscosity,
and ty (lbf/100ft2) - shear stress, and yield stress
g(1/s) - shear rate,
m and mp(cp) - fluid viscosity and Bingham viscosity,
K (lbfxsn/100ft2) - consistency index,
n - flow behavior index.
For Bingham fluid:

17. API Tests for Cementing Rotational Viscometer
The data below are obtained from a rotational viscometer. Determine type of fluid and the rheological model of this fluid.
RPM
Dial Reading 3 10 6 12
100 35
200 48
300 60
600 75

18. API Tests for Cementing Cement Consistometer – Thickening Time Test
A device used to determine the thickening time of cement slurries under simulated downhole pressure and temperature conditions. The thickening time is a measurement of the time during which cement slurry remain in a fluid state and is capable of being pumped. Thickening time is assessed under simulated downhole conditions using a consistometer that plots the consistency of a slurry over time at the anticipated temperature and pressure conditions. The end of the thickening time is considered to be 50 or 70 Bc for most applications.

19. API Tests for Cementing Cement Consistometer – Thickening Time Test
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

20. API Tests for Cementing
Cement Consistometer
The apparatus consists 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 rpm during the test. The slurry consistency is defined in terms of the torque exerted on the paddle by the cement slurry. The relation between torque and slurry consistency is given by

21. API Tests for Cementing
Cement Consistometer
T is the torque on the paddle in g-cm and Bc is the slurry consistency in API consistency units designated by Bc. The thickening time of the slurry is defined as the time required to reach a consistency of 100 Bc. This value is felt to be representative of the upper limit of pumpability.

22. API Tests for Cementing
Cement Consistometer
Consistometer for simulating down-hole conditions
Consistometer for simulating atmosphere conditions

23. Typical thickening time test output
API Tests for Cementing
Cement Consistometer
Typical thickening time test output

24. API Tests for Cementing
Cement Consistometer
The torque required to hold the paddle assembly stationary in a cement consistometer rotating at 150 rpm is 520 g-cm. Compute the slurry consistency.

25. API Tests for Cementing
Cement Permeameter
Cement permeameter is an apparatus for measuring the permeability of a core sample. The permeability of a set cement core to water is determined by measuring the flow rate through the core at a given pressure differential across the length of the core. The permeability then is computed using an appropriate form of Darcy’s law:
Where K(mD) is the permeability, q(mL/s) is the flow rate, m(cp) is the water viscosity, L(cm) is the sample length, A(cm2) is the sample cross-sectional area, and DP(psi) is the differential pressure.

26. API Tests for Cementing
Cement Permeameter

27. API Tests for Cementing
Cement Permeameter
A class E cement core having a length of 2.54 cm and a diameter of cm allows a water flow rate of mL/s when placed under a pressure differential of 20 psi. A second core containing 40% silica cured in a similar manner allows only mL/s of water to flow under a pressure differential of 200 psi. Compute the permeability of the two cement samples.

28. API Tests for Cementing
Cement Permeameter

29. API Tests for Cementing
Strength Testing Machine – Compressive Strength Test
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.