Table Applications of API Cements

API Class Application

Used at a depth range of 0-6,000 ft Used at a temperature of up to 170° F Intended for use when special properties are not required; well conditions permit Ecomonieal compared with premium cements Used at a depth range of 0-6,000 ft Used at a temperature of up to 170°F Intended for use when moderate to high sulfate resistance is required; well conditions permit Economical compared with premium cements Used at a depth range of 0-6,000 ft Used at a temperature of up to 170°F Intended for use when high early strength is required; its special properties are required High in triealcium silicate

Class D used at a depth range of 6,000-10,000 ft; E, at a depth range of 10,000-14,000 ft Class D used at a temperature of I70-260T; class E, at 170-2(J0°F

Intended for use when moderately high temperature and high pressure are encountered; its special properties are required

Available in types thai exhibit regular and high resistance to sulfate

Retarded with an organic compound, chemical composition, and grind

More expensive than portland cement Used at a depth range of 10,000-16,000 ft Used at a temperature of 230-320°F Intended for use when extremely high temperature and pressure are encountered; its special properties are required

Available in types that exhibit moderate and high resistance to sulfate

Retarded with an organic compound, chemical composition, and grind Used at a depth range of 0-8,000 ft Used at a temperature of up to 200°F without modifiers

A basic cemcnt compatible with accelerators or retard ers

Usable over the complete range of classes A to E with additives

Additives blended in at bulk station or at job site Used at a depth range of 12,000-16,000 ft Intended for use under conditions of extreme temperature and pressure: 170-320°F unmodified (no additives)

Usable with accelerators and retarders

Will not set at temperature less than 150°F if used as a neat slurry

Table 9-3 API Cement Composition

API Class

C,S

cas

C3A

C4AF

Fineness, sq cm/g

Water/cement ratio

A

53

24

8

8

1,500-1,900

0.46

B

47

32

3

12

1,500-1,900

0.46

C

70

10

3

13

2,000-2,400

0.56

D

26

54

2

12

1,100-1.500

0.38

G

52

32

8

12

1,400-1,600

0.44

H

52

32

8

12

1,200-1,400

0.38

J

53.8

38.8

1,240-2,480

0.44

SiO,

CaO

0.435

class (Table 9-4). A major factor affecting the slurry yield is the density, since water must be added in significant volumes to achieve low-weight cements that will not fracture shallow, weak zones.

The density of the cement is an important design criteria. It must be sufficient to prevent kicks and blowouts, yet it should not cause lost circulation. In some cases, the height of the cement column must be controlled to minimize lost circulation problems. Example 9.2 illustrates a typical case where cement density must be monitored closely.

Table 9

-4 API Cement Properties

Mix

Slurry

Slurry Approx.

24-hr Comp.

Cement

Water,

Density,

Yield, Thickening

Strength, psi,

Class

gal/sack

lb/gal

cut ft/sack Time, 113°F, hr

;:o°f

A

5.2

15.6

1.18 2'/2

4.000

C

6.3

14.8

1.32 PA

2,700

G

5.0

15.8

1.15 PA

3,000

H

4.3

16.5

1.05 2

A well is being planned for the South China Sea. A partial well diagram is shown in Fig. 9-9. A 16.8-lb/gal pore pressure (BHP) is expected at 14,000 ft, which is the target pay zone. An intermediate casing string is set at 12,000 ft in a 17.8-lb/gal fracture gradient formation. The mud weight is expected to be 0.3 lb/gal heavier than the maximum pore pressure (16.8 lb/gal).

The company's philosophy with respect to cementing is to cement the entire annular area to surface with a slurry that weighs 1.0 lb/gal above the maximum mud weight. Due to pumping time restrictions in this high-BHT well, the fluid must be displaced at 6 bbl/min, which will create an equivalent circulating density of 0.4 lb/gal.

Can the company's standard cementing philosophies be used in this well? If not, compute the amount (height) of cement weighing 1.0 lb/gal in excess of the mud weight that can be used without causing lost circulation (see Fig. 9-9).

Solution:

  1. The amount of mud and cement hydrostatic pressure that the casing seat can withstand is computed as the fracture gradient less the circulating pressure:
  2. 8 lb/gal - 0.4 lb/gal - 17.4 lb/gal (FG) (ECD)
  3. The cement slurry density is equal to the sum of the pore pressure and safety margins for mud and cement:

pore pressure = 16.8 lb/gal mud safety margin = 0.3 lb/gal cement safety margin = 1.0 lb/gal cement slurry density s=. 18.1 lb/gal

Cement Oil Well Diagram
  1. 9-9 Well diagram for Example 9.2
  2. Since the casing seat can withstand a 17.4-lb/gal fluid, the 18.1-lb/gal cement cannot be circulated to the surface without causing lost circulation.
  3. The maximum vertical height of cement that can be used is calculated as:

fracture pressure — circulating pressure =

mud -I- cement hydrostatic pressures

  • 17.8 lb/gal - 0.4 lb/gal) (0.052 x 12,000 ft) =
  • 0.052) (18.1) (X) + (0.052) (17.1) (Y) ]

Where:

X = length of cement column, ft Y — length of mud column, ft And:

10,857 psi - 0.9412X + 0.8892Y 10,857 psi - 0.9412 (12,000 - Y) + 0.8892Y 10,857 psi - 11,294 psi = (-0.9412 + 0.8892)Y - 437 psi = - 0.052Y

Y = 8,403 ft (mud column) .', X - 3,596 ft (cement column above the shoe at 12,000 ft)

6. The total cement column from the hole bottom is:

The mixing water requirements will vary, depending primarily on cement class and slurry density (Table 9-4). Most cement jobs use wellsite water. If water must be transported to the location due to a shortage or poor supply, accurate mixing volume calculations are important to ensure that an adequate supply is available. The volume requirements can be substantial if a low-density gel cement must be used.

Quality of the mixing water is an important parameter in cement planning. The hydration and curing of the slurry will react differently with varying amounts of salt, calcium, or magnesium in the mix water. It is recommended that pilot laboratory tests be performed with an actual sample of the mix water and cement to obtain good estimates of pumping time and compressive strengths. The pilot lab testing becomes more critical in high-temperature deep wells that require a significant amount of time for ccment mixing and displacement.

Thickening time is the amount of time that a cement remains pumpable with reasonable pressures. This is perhaps the most critical property in the displacement process. Factors affecting the thickening time include cement composition and temperature. An upper limit must be placed on the thickening time, however, so that drilling operations can resume.

Hardened cement must develop compressive strength to secure casing in the hole and withstand pressure differentials across the cement. The compressive strength is measured in pounds per square inch and usually increases with higher slurry densities. A 500-psi minimum compressive strength is generally recommended before drilling operations resume, but higher strengths are preferred.

Temperatures affect the compressive strength of the cement. Higher temperatures reduce the time for the cement slurry to reach some compressive levels (Table 9-5), However, at temperatures above 230°F, cement strength begins to decrease. The approach used most commonly for strength retrogression at high temperatures is to use sand (±35%), which reacts with and neutralizes the component causing the retrogression.

Fluid loss is the water lost from the slurry to the formation during slurry placement operations. As the fluid is forced out of the cement, the density of the slurry increases and changes slurry characteristics. If a large volume of water is lost, the slurry becomes too viscous or dense to pump. Therefore, fluid loss control additives are important considerations in slurry design.

Neat cement, or cement with no special additives, has a fluid loss rate in excess of 1,000 cc/30 min. Varying concentrations of fluid loss additives will control these rates. The following values and their interpretation are generally accepted:

Example 9.3 illustrates a typical slurry design case involving cement slurry and water requirements.

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