γγ 2 = max mud density during the next drilling phase
7. CEMENTING CONSIDERATIONS
7.1.1. API Specification
Portland cement is the most widely used in cementing operations in the oil industry and an API specification (10) was established. API 10 consists of eight classes of cement, A through H, to provide standard to suit a range of well conditions. The API classification system is shown in table 7.a below:
API
Class Mixing Water Slurry Weight Well Depth Static BHP Temperature
gal/sk ltrs/sk lbs/gal kg/ltrs ft m oF oC
A 5.2 19.7 15.6 1.87 0-6,000 0-1,830 80-130 27-77
B 5.2 19.7 15.6 1.87 0-6,000 0-1,830 80-130 27-77
C 6.3 23.8 14.8 1.77 0-6,000 0-1,830 80-170 27-77
D 4.3 16.3 16.4 1.97 6,000-12,000 1,8303,660 170-260 77-127
E 4.3 16.3 16.4 1.97 6,000-14,000 1,8304,270 170-290 77-143
F 4.3 16.3 16.4 1.97 10,000-16,000 3,050-4,880 230-320
110-160
G 5.0 18.9 15.8 1.89 0-8,000 0-2,440 80-200 27-93
H 4.3 16.3 16.4 1.97 0-8,000 0-2,440 80-200 27-93
Table 7.A - API Cement Specification
Class A Is intended for use when no special properties are requires.
Class B Has the same properties as class A except has a moderate to high sulphate resistance (MSR and HSR).
Class C Is intended for use when conditions require high early strength.
Classes D, E and F are referred to as retarded cements developed for higher temperature and pressures conditions.
Class D Intended for use in moderately high temperatures and pressures and is available in both MSR and HSR.
Class E Intended for use in high temperature and pressure conditions and is available in both MSR and HSR.
Class F Intended for use in extreme high temperature and pressure conditions and is available in both MSR and HSR.
Classes G and H were developed in response to the improved technology in slurry acceleration and retardation by chemical means. These are the most widely used cements today.
Class G, H Intended for use as a basic well cement to cover a wide range of well depths and temperatures and is available in both MSR and HSR. Types G and H are essentially identical except that H is significantly coarser than G, evident from their different water requirements.
The following table 7.b shows the various properties of neat slurries and API cement.
API Class Water Slurry Weight Slurry Volume
gal/sk ltrs/sk lbs/gal kg/ ltrs Ft3/sk m3/sk ltrs 3/sk
A & B 5.2 19.7 15.6 1.87 1.18 0.033 0.33
C 6.3 23.9 14.8 1.77 1.32 0.037 0.37
G 5.0 18.8 15.8 1.89 1.15 0.033 0.33
h 4.3 16.3 16.4 1.97 1.06 0.030 0.30
D, E & F 4.3 16.3 16.4 1.97 1.06 0.030 0.30
Table 7.B - Properties of Neat Slurries and API cement.
table 7.d below shows the typical compressive strengths and thickening times of API cements.
table 7.d Definitions
* Determined by Wagner turbidmeter apparatus
** Based on 250ml volume percentage equivalent 3.5ml is 1.4%
+ Bearden unit of slurry consistency (Bc)
Bc Bearden units of consistency on a preserved consistometer
ABc Beaden units of consistency on an atmosphere pressure consistometer
The relationship between Bc and ABc is approximately Bc x 0.69 = ABc This relationship is valid for units of consistency less than 30Bc
*** Thickening time required are based on 75% values of total cement times observed in the casing survey, plus 25% safety factor
++ Maximum thickening time required for Schedule 5 is 120 mins
Well Cement Class A B C D E F G H
Water % by weight of well cement 46 46 56 38 38 38 44 38
Soundness (autoclave expansion), Maximum %
0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80
Fineness *(Specific surface) Minimum m2/kg 150 160 220 - - - -
-Free-Water content, Maximum ml - - - - - - 3.5** 3.5**
Compressive Strength Test 8-hours Curing time Schedule
Minimum Compressive Strength, psi (MPa)
- 100 Atmos 250 200 300 - - - 300 300
-Compressive Strength Test 12-hours Curing time Schedule
Minimum Compressive Strength, psi (MPa)
8S 290 3,000 - - - - - - -
-(143) (20,700) - - - - - - -
-Compressive Strength Test 24-hours Curing time Schedule
Minimum Compressive Strength, psi (MPa)
- 100 Atmos 1,800 1,500 2,000 - - - -
-Pressure Temperature Thickening Time Test Specification Test
Schedule Number
Maximum Consistency 15 to
30 min Straining Period B + Minimum Thickening Time (min***)
1 30 90 90 90 - - - -
-Table 7.C - Physical Requirements for API Portland Cements
Concentration of Additives
The concentrations of most solid cement additives are expressed as percentage by weight of cement (BWOC). This method is also used for water. For example, if 30% silica sand is used in a blend, the amount for each sack of cement is 94lbs x 0.30 = 28.2lbs of silica sand. This results in 94 + 28.2 = 122.2lbs total mix. The true percentage silica sand in the mix is 28.2/122.2 = 23.07%.
Salt is an exception and is added by weight of mix water (BWOW). Weighting materials are often added on a lbs/sk basis for convenience as it eliminates the need to convert from percentage BWOC to lbs in the bulk plant.
Liquid additive concentrations are most commonly expressed in gal/sk of cement. For example, according to table 7.d, liquid sodium silicate has an absolute volume of 0.0859gal/lbs. If a concentration of 0.4lbs/sk is prescribed, the weight of the material is 0.4/0.0859 = 4.66lbs/sk.
Material Absolute Volume SG
(gal/lbs) (m3/t)
Barite 0.0278 0.231 4.33
Bentonite 0.0454 0.377 2.65
Coal (ground) 0.0925 0.769 1.30
Gilsonite 0.1123 0.935 1.06
Hematite 0.0244 0.202 4.95
Limenite 0.0270 0.225 4.44
Silica Sand 0.0454 0.377 2.65
NaCl saturated 0.0556 0.463 2.15
Fresh Water 0.1202 1.000 1.00
Table 7.D - Absolute Values of Common Cementing Materials 7.1.2. Slurry Density and Weight
The slurry density is calculated by adding the masses of the components and dividing it by the total of the absolute volumes occupied, i.e. divide the total weight in lbs/volume in gals.
additives
The yield of a cement is the volume occupied by a unit plus all the additives and mix water.
Cement is measured is sacks therefore the yield is expressed in cubic feet per sack (ft3/sk).
This is now used to calculate the number of 94lbs sacks required to achieve the required annulus volume.
As there are 31.51 cubic feet per cubic metre, divide the cubic feet by 31.51 to obtain the amount of cement in cubic metres.
Example calculation:
A slurry is composed of G class cement and 50% water, 94 x 0.50 = 47.0lbs water.
Component Weight (lbs) Absolute Volume
(gal/lbs) Volume (gal)
The total volume of mix water required is the gals calculated above, 5.65 multiplied by the number of sacks of cement to be mixed.
Additives are treated in the same manner as above, however if any have a volume less than 1% then they are generally ignored.
An example calculation with additives is as follows:
A slurry is composed of class G cement + 35% silica flour + 1% solid cellulosic loss additive + 0.2gal/sk liquid PNS dispersant + 44% water.
Component Weight (lbs) Absolute Volume
(gal/lbs) Volume (gal)
Cement 94 0.0382 3.59
Silica flour 32.9 0.0454 1.49
Cellulosic Fluid Loss Additive
0.94 0.0932 0.088
Liquid PNS Dispersant 1.97 0.1014 0.20
Water 41.36 0.1202 4.97
The yield is:
In well cementing, Portland cement systems are designed for temperatures ranges from below freezing to 700oF (350oC) in thermal recovery and geothermal wells. They also encounter pressures ranging from ambient to 30,000psi (200Mpa) in deep wells.
Accommodation of such variations in conditions was only possible through the development of cement additives. They modify the properties of the cement system allowing successful placement of the slurry between the casing and the formation, rapid compressive strength development and adequate zonal isolation for the life of the well.
It is not possible to detail all of the 100 or more additives in use today but the categorisation of these additives and some of those used by Eni-Agip are described below.
There are eight recognised categories:
• Accelerators
• Retarders
• Extenders
• Weighting Agents
• Dispersants
• Fluid Loss Control Agents
• Loss Circulation Control Agents
• Speciality Additives
Details of all of these additives are given in the ‘Drilling Fluids Manual’.
7.2.1. Accelerators
Added to cements to shorten the setting time and/or accelerate the hardening process.
They are also required to counter the effect of other additives added to the slurry such as dispersants and fluid loss control agents.
Calcium Chloride is undoubtedly the most efficient and economical accelerator. It is generally added in concentrations of 2-4% BWOC (Refer to table 7.e) but over 6% its performance becomes unpredictable and premature setting may occur.
CaCl2 %BWOC 91oF 103oF 113oF
0 4:00 3:30 2:32
2 1:17 1:11 1:01
4 1:15 1:02 0:59
Table 7.E – Calcium Chloride Thickening Time on Portland Cement