Well Design
– Spring 2012
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Classification of Drilling Cements
API has defined eight standard classes and three standard types of cement for use in wells. The eight classes specified are designated class A to class H.
Three types specified are: Ordinary O, Moderate sulfate-resistant MSR, and high sulfate-resistant HSR.
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Classification of Drilling Cements
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Classification of Drilling Cements
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Classification of Drilling Cements
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Classification of Drilling Cements
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Classification of Drilling Cements
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Cement Additives
For the slurry:
• Thickening time (acceleration, retardation)
• Density (extenders, weight increase/reduction)
• Friction during pumping
• Fluid loss (by filtrate)
• Lost-circulation resistance (whole slurry loss)
For set cement:
• Compressive strength
• Strength retrogression (loss with time)
• Expansion/contraction
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Cement Additives
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Cement Additives
Cement-setting time is accelerated to reduce WOC time and to increase early strength. This is desirable for surface pipe, in shallow (cooler) wells, and for setting plugs. The most common accelerators are calcium chloride, sodium silicate, sodium chloride (low concentrations), seawater, hemihydrate forms of gypsum, and ammonium chloride.
Type: Accelerators
Calcium Chloride (CaCl
2
) (flake, powder)
Sodium Chloride (NaCl)
Sodium Chloride (NaCl)
Amount used per sack
(% by weight)
2 - 4
3 - 10 (water)
1.5 - 5 (cement)
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Cement Additives
Cement-thickening time is slowed primarily to allow the slurry to be pumped and displaced into position before setting.
Retarders
Calcium-Sodium Lignosulfonate
Calcium Lignosulfonate
Saturated Salt Solutions
Amount used per sack
(% by weight)
0.1 - 1.0
0.1 to 1.0
-
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Cement Additives
Thickening time is a function of both temperature and pressure, and these effects must be predicted before additives are selected
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Cement Additives
Slurry density may be reduced with extenders such as bentonite, pozzolan, diatomaceous earth, and anhydrous sodium metasilicate.
Low-density slurry is frequently preferred, to decrease the likelihood of breaking down the formation and causing lost circulation. In addition, low-density slurries cost less per cubic foot because yield per sack is increased.
Density decrease results in large part from increased water content. Extenders, with their high surface area to "tie up" water, permit water addition without separation. Cement strength is reduced approximately in proportion to watercontent increase. However, we shall see later that high strength is not always required
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Cement Additives
Type: Density reducers/extenders
Bentonite
Attapulgite
Diatomaceous Earth (Diacel D)
Pozzolan, Artificial (fly ash)
Amount used per sack
(% by weight)
2 to 16
1/2 to 4
10, 20, 30 or 40
74 lb/sk
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Cement Additives
High density cement sluries are often necessary to offset the high pressures that are frequently encountered in deep or abnormally pressured fromations. Density may be increased with weight material such as sand, barite, hematite or ilmenite, and/or salt dissolved in the mix water.
Density increasers
Sand
Barites
Ilmenite (iron-titanium oxide)
Hematite
Salt
Amount used per sack
(% by weight)
5 to 25
10 to 108
5 to 100
4 to 104
5 to 16
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Cement Additives
Fluid loss, or the premature escape of mix water from the slurry before chemical reaction occurs, can cause many downhole problems, including
1. Differential sticking of casing and decentralization
2. Formation damage by filtrate (if not controlled by mud cake)
3. Loss of pumpability
4. Cement bridging above gas zones and gas cutting from hydrostatic pressure loss
5. Improper or premature dehydration during squeezing
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Cement Additives
Materials to reduce filtrate loss and friction
Fluid-loss additives
Organic polymers (cellulose)
Carboxymethyl hydroxyethyl cellulose
(CMHEC)
Latex additives, form films
Bentonite cement with dispersant
Amount used per sack
(% by weight)
0.5 -1.5%
0.5 – 1.5%
0.3 - 1.0%
1.0 gal/sk
12-16% gel
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Cement Additives
Friction reducers or dispersants are commonly used to lower viscosity, yield point and gel strength of the slurry to reduce friction in pipe, and thus allow turbulent flow to occur at reduced pump rates. These additives also permit slurries to be mixed at lower water/cement ratios so that higher densities may be achieved.
Type: Friction reducer
Polymer: blend
Polymer: long chain
Calcium lignosulfonate
Sodium Chloride
Organic acid
Amount used per sack
(% by weight)
0.5 to 0.3 lb/sk
0.5 to 1.5 lb/sk
0.5 to 1.5 lb/sk
1 to 16 lb/sk
0.1 to 0.3 lb/sk
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Cement Additives
"Lost circulation" or "lost returns" refers to the loss to formation voids of either whole drilling fluid or cement slurry used during the course of drilling or completing a well. Cement, with its larger particle size is less susceptible to loss in permeable formations.
Types of lost-circulation additives available for cement are blocky-granular materials (walnut shells, gilsonite, crushed coal, perlite-expanded and perlitesemiexpanded) which form bridges, and laminated materials (cellophane flakes) which form flake-type mats.
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Cement Additives
Type material Generic name Type particle
Granular
Lamellated
Fibrous
Gilsonite
Perlite
Graded
Expanded
Walnut shells Graded
Coal Graded
Cellophane
Nylon
Flakes
Short fibers
Volumes used, typical range
5-50 lb/sk
1/2-1 cu ft/sk
1-5 lb/sk
1-10 lb/sk
1/8-2 lb/sk
1/8-1/4 lb/sk
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Cement Additives
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Cement Additives
Four variables: composition, temperature, pressure, and time
— affect compressive strength.
However, at high temperatures, cement compositions may lose strength after reaching a high value and never attain the strength reached at lower curing temperatures
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Cement Additives
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Calculation
When the concentration of an additive is expressed as a weight percent, the intended meaning is usually that the weight of the additive put in the cement mixture is computed by multiplying the weight of cement in the mixture by the weight percent given by 100%.
Percent mix = water weight x 100/cement weight
The volume of slurry obtained per sack of cement used is called the yield of the cement.
1 sack = 94 lbm.
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Calculation
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Calculation
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Calculation
Example 1: it is desired to mixed a slurry of class A cement containing 3% bentonite, using the normal mixing water as specified by API. Determine the weight of bentonite and volume of water to be mixed with one 94-lbm sack of cement. Also compute the percent mix, yield, and density of the slurry.
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Calculation
The weight of bentonite to be blended with one sack of class A cement:
94(lbm)0.03 = 2.82 lbm / sag
The normal water content for class A cement is 46%. 5.3% water must be added for each percent bentonite. Thus, the percent mix is:
46 + 5.3 x 3 = 61.9%
The weight of water to be added per sack
0.619 x 94 = 58.186 lbm / sag
The volume of water to be added for one sack
58.186(lbm) / 8.33 (lbm/gal) = 6.98 gal /sag
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Calculation
Yield = total volume of slurry in one sack
Yield = Vcement per sack + Vbentonite per sack + Vwater per sack
Yield = 94(lbm) x 0.0382(gal/lbm) + 2.82 (lbm) x 0.0453 (gal/lbm) + 6.98 (gal)
Yield = 10.69 gal/sack
Density of the slurry = total mass of slurry / total volume of slurry
Density of the slurry = 94 + 2.82 + 58.186 / 10.69 = 14.48 lbm/gal
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Calculation
Example 2: It is desired to increase the density of a class H cement slurry to 17.5
lbm/gal. Compute the amount of hematite that should be blended with each sack of cement. The water requirements are 4.5 gal/94 lbm class H cement and 0.36
gal/100 lbm hematite.
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Calculation
Let x represent the mass of hematite per sack needed to bring the slurry cement density up to 17.5 lbm/gal. The slurry includes: 94-lbm cement, x-lbm hematite, and y-lbm water.
The volume of water needed for mixing 1 sack of cement and x-lbm of hematite:
V water
= 4.5 + 0.36(x)/100 = 4.5 + 0.0036x (gal)
The weight of water needed for mixing 1 sack of cement and x-lbm of hematite m water
= (4.5 + 0.0036x)8.33 (lbm)
Prepared by: Tan Nguyen

Well Design
– Spring 2012
Calculation
 
17 .
5

94
 x
94

0 .
0382



4 .
5

0 .
0036 x

8 .
33 x

0 .
0239

4 .
5

0 .
0036 x
Prepared by: Tan Nguyen