Well Design PE 413 Classification – Additives – Calculation of Drilling Cements

1. Well Design PE 413 Classification – Additives – Calculation of Drilling Cements

2. 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.

3. Classification of Drilling Cements

4. Classification of Drilling Cements

5. Classification of Drilling Cements

6. Classification of Drilling Cements

7. Classification of Drilling Cements

8. 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

9. Cement Additives

10. Cement Additives Accelerators 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.

11. Cement Additives Retarders Cement-thickening time is slowed primarily to allow the slurry to be pumped and displaced into position before setting.

12. Cement Additives Temperature Effect on the Thickening Time Thickening time is a function of both temperature and pressure, and these effects must be predicted before additives are selected

13. Cement Additives Density Reducing Additives - Extenders 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 water-content increase. However, we shall see later that high strength is not always required

14. Cement Additives Density Reducing Additives - Extenders

15. Cement Additives Density Increasing 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.

16. Cement Additives Filtration Control Additives Fluid loss, or the premature escape of mix water from the slurry before chemical reaction occurs, can cause many downhole problems, including Differential sticking of casing and decentralization Formation damage by filtrate (if not controlled by mud cake) Loss of pumpability Cement bridging above gas zones and gas cutting from hydrostatic pressure loss Improper or premature dehydration during squeezing

17. Cement Additives Filtration Control Additives

18. Cement Additives Friction Reducer 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.

19. Cement Additives Lost Circulation Materials "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 perlite-semiexpanded) which form bridges, and laminated materials (cellophane flakes) which form flake-type mats.

20. Cement Additives Lost Circulation Materials

21. Cement Additives Lost Circulation Materials

22. Cement Additives Compressive Strength Stabilizers 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

23. Cement Additives Summary

24. Calculation Basic Calculations 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.

25. Calculation Basic Calculations

26. Calculation Basic Calculations

27. Calculation Basic Calculations 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.

28. Calculation Basic Calculations 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

29. Calculation Basic Calculations 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

30. Calculation Density Calculations 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.

31. Calculation Density Calculations 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: Vwater = 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 mwater = (4.5 + 0.0036x)8.33 (lbm)

32. Calculation Density Calculations x = 18.3 lbm hematite per 94 lbm cement.