Number of Sacks of Cement
Although cement and other dry chemicals are delivered to the rigsite in bulk tanks the amount of dry cement powder is generally quoted in terms of the number of sacks (sxs) of cement required. Each sack of cement is equivalent to 1 cu. ft of cement.
The number of sacks of cement required for the cement operation will depend on the amount of slurry required for the operation (calculated above) and the amount of cement slurry that can be produced from a sack of cement. The amount of cement slurry that can be produced from a sack of cement, known as the yield of the cement, will depend on the type of cement powder (API classification) and the amount of mixwater mixed with the cement powder. The latter will also depend on the type of cement and will vary with pressure and temperature. The number of sacks of cement required for the operation can be calculated from the following
No. of Sacks = Total Volume of Slurry
Yield of Cement 3. Mixwater Requirements
The mixwater required to hydrate the cement powder will be prepared and stored in specially cleaned mud tanks. The amount of mixwater required for the operation will depend on the type of cement powder used. The volume of mixwater required for the cement slurry can be calculated from:
Mixwater Vol. = Mixwater per sack x No. sxs 4. Additive Requirements
Their are a variety of additives which may be added to cement.. These additives may be delivered to the rigsite as liquid or dry additives. The amount of additive is generally quoted as a percentage of the cement powder used. Since each sack of cement weighs 94 lbs, the amount of additive can be quoted in weight (lbs) rather than volume. This can then be related to the number of sacks of additive. The number of sacks of additive can be calculated from:
Number of sacks of additive = No. sxs Cement x % Additive Weight of additive = No. sxs of Additive x 94(lb/sk)
The amount of additive is always based on the volume of cement to be used.
The volume of mud used to displace the cement from the cement stinger or the casing during the cementing operation is commonly known as the displacement volume. The displacement volume is dependant on the way in which the operation is conducted.
a. Stinger Operation :
The displacement volume can be calculated from the volumetric capacity of the cement stinger and the depth of the casing shoe. The cement is generally under displaced by 1-2 bbls of liquid.
Displacement Vol. = Volumetric capacity of stinger x Depth of Casing — 1bbl
b. Conventional Operation :
In a conventional cementing operation the displacement volume is calculated from the volumetric capacity of the casing and the depth of the float collar in the casing.
Displacement Vol. = Volumetric Capacity of Casing x Depth of Float Collar
c. Two-stage Cementing Operation:
In a two stage operation the first stage is firstly displaced by a volume of mud, calculated in the same way as a single stage cement operation described above. The second stage displacement is then calculated on the basis of the volumetric capacity of the casing and the depth of the second stage collar.
The amount of mud to be pumped during the displacement operation may be quoted in terms of a volume (bbls, cuft etc.) or in terms of the number of strokes of the mud pump required to pump the mud volume. It will therefore be necessary to determine the volume of fluid pumped with each stoke of the pumps (vol./stroke). The number of strokes required to displace the cement will therefore be calculated from:
Number of strokes = Volume of displacement fluid/Vol. of fluid per stroke
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The duration of the operation will be used to determine the required setting time
for the cement formulation. The duration of the operation will be calculated on the basis of the mixing rate for the cement, the pumping rate for the cement slurry and the pumping rate for the displacing mud. An additional period of time, known as a contingency time, is added to the calculated duration to account for any operational problems during the operation. This contingency is generally 1 hour in duration. The duration of the operation can be calculated from:
Duration = Vol. of Slurry + Vol. of Slurry + Displacement Vol. + Contingency Time (1hr.)
Mixing Rate Pumping Rate Displacement Rate
EXAMPLE OF CEMENT VOLUME CALCULATIONS
The 9 5/8” Casing of a well is to be cemented in place with a single stage cementing operation. The appropriate calculations are to be conducted prior to the operation. The details of the operation are as follows:
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9 5/8" casing set at: 12 1/4" hole: |
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13800′, 13810′ |
13 3/8" 68 lb/ft casing set at : 6200′
TOC outside 9 5/8" casing: 3000′ above shoe
Assume gauge hole, add 20% excess in open hole
The casing is to be cemented with class G cement with the following additives:
0. 2% D13R (retarder)
1 % D65 (friction reducer)
Slurry density = 15.9 ppg
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Casing/Casing Annulus |
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Casing/Hole Annulus |
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(0.3132 ft3/ft) |
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• к |
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Shoe @ 6200′ |
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13 3/8 |
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TOC @ 10800′ |
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Float Collar @ 13740′ |
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Rathole |
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3 |
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(0.8185 ft3/ft) |
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9 5/8” Shoe @ 13800′ 12 1/4” Hole @ 13810′ |
1. Slurry Volume Between The Casing and Hole:
9 5/8" csg/ 12 1/4" hole capacity= 0.3132 ft3/ft annular volume = 3000 x 0.3132
= 939.6 ft3
plus20% excess =187.9ft3
= 1127.5ft3 => 1128 ft3
2. Slurry Volume Below The Float Collar:
Cap. of 9 5/8, 47 lb/ft csg = 0.4110 ft3/ft
shoetrack vol. = 60 x 0.411
Total = 25 ft3
3. Slurry volume in the
Cap. of 12 1/4" hole rathole vol.
plus 20%
Total
rathole
= 0.8185 ft3/ft = 10 x 0.8185 = 8.2 ft3 = 1.6 ft3
= 9 .8 ft3 => 10 ft3
Total cement slurry vol. = 1128 + 25 + 10
4. Amount of cement and mixwater
Yield of class G cement for density of 15.9 ppg = 1.14 ft3/sk mixwater requirements = 4.96 gal/sk
No. of sks of cement = 1163 = 1020 sx
1.14
Mixwater required = 1020 x 4.96 gal
= 5059 gal = 120 bbls
5. Amount of Additives:
Retarder D13R (0.2% by weight)
= 02 x 1020 x 94 (lb/sk) = 192 lb 100
Friction reducer (1.0% D65 by weight)
= 1. x 1020 x 94(lb/sk) = 959 lb 10
6. Displacement Volume:
Displacement vol.
(add 2 bbl for surface line)
= 1008 bbl = vol between cement head and float collar = 0.4110 x 13740 = 5647 ft3 = 1006 bbl
For Nat. pump 12-P-160, 7" liner 97% eff, 0.138 bbl/stk
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No. of strokes |
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= 7300 strokes |
= 1008
0.138
EXERCISE 1 Cementing Calculations — Stinger Cementation
The 20" casing of a well is to be cemented to surface with class ‘C’ high early strength cement + 6% Bentonite using a stinger type cementation technique. Calculate the following for the 20" casing cementation :
a. The number of sacks of cement required (allow 100% excess in open hole).
b. The volume of mixwater required.
c. An estimate of the time taken to carry out the job.(Note: use an average mixing/ pumping time of 5 bbls/min.)
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0 — 400 ft. 0 — 500 ft 500 — 1500 ft. 1530 ft. 5" 19.5" drillpipe 13.1 ppg 1.88 ft3/sk 1.36 ft3/sk |
30" Casing 20" Casing 94 lb/ft 20" Casing 133 lb/ft 26" Open hole Depth Stinger
Class ‘C’ Cement + 6% Bentonite
Density
Yield
Mixwater Requirements
EXERCISE 2 Cementing Calculations — Two Stage Cementation
The 13 3/8" casing string of a well is to be cemented using class ‘G’ cement. Calculate the following:
a. The required number of sacks of cement for a 1st stage of 700 ft. and a 2nd stage of 500 ft.(Allow 20% excess in open hole)
b. The volume of mixwater required for each stage.
c. The total hydrostatic pressure exerted at the bottom of each stage of cement (assume a 10 ppg mud is in the well when cementing).
d. The displacement volume for each stage.
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1500 ft 0 — 1000 ft 1000 — 7000 ft. 7030 ft. 1500 ft. 60 ft. |
20" Casing shoe 13 3/8" Casing 77 lb/ft
13 3/8" Casing 72 lb/ft
17 1/2" open hole Depth Stage Collar Depth Shoetrack
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Cement stage 1 (7000-6300 ft.) Class ‘G’
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Cement stage 2 (1500-1000 ft.) Class ‘G’ + 8% bentonite
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volumetric capacities
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ft3/ft 0.0997 |
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bbls/ft 0.01776 0.1480 0.1463 |
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0.8314 0.8215 |
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Drillpipe 5" drillpipe : Casing 13 3/8" 72 lb/ft 13 3/8" 77 lb/ft |
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Open Hole 26" Hole 17 1/2" Hole |
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0.6566 0.2975 |
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3.687 1.6703 |
Annular Spaces
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0.2681 0.1237 0.3730 0.1816 1.5053 0.6946 2.0944 1.0194 |
26" hole x 20" Casing:
17 1/2" hole x 13 3/8" Casing: 30" Casing x 20" Casing:
20" Casing x 13 3/8" Casing:
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