Cementing and Perforation Skin

Strongly recommend reading Chapter 4 in Fundamentals of Drilling Engineering by Mitchell and Miska,

Problem 1
It is desired to reduce the density of a Class A cement to 12.6 lbm/gal by adding bentonite. Using the water requirements for Class A cement and bentonite given in the table below, compute the weight of bentonite that should be blended with each sack of cement. Compute the yield of the slurry. What is the “percent mix” of the slurry? Answer:
Problem 2
Repeat Problem 1 using diatomaceous earth instead of bentonite as the low-specific gravity solid (S.G. ~2.1) and a water requirement of 3.3 gal/10% diatomaceous earth. Answer:

Problem 3
Casing having an OD of 8.625 in. and a weight of 49 lb/ft is to be cemented at a depth of 13,300 ft in a 10.625-in. borehole. A 40-ft shoe joint will be used between the float collar and the guide shoe. It is desired to place 2,500 ft of cement in the annulus. Each sack of Class H cement will be mixed with 4.0 gal of water, to which is added 15% salt (by weight of water). A small quantity of dispersant will be blended with the cement, but this additive has no significant effect on the slurry yield or density. Compute the density of the slurry, the yield of the slurry, the number of sacks of cement required, and the cementing time. Assume that cement can be mixed at a rate of 20 sacks/min and displaced at a rate of 8 bbl/min, and use an excess factor of 1.5. The salt will be added to the water phase and, thus, does not blend with the dry cement. Answer:

Problem 4
You are asked to design a cementing program for a string of production casing. You know from experience that you can use a 15.8 lb/gal cement slurry prepared with Class G cement (neat, i.e., no additives). The surface and intermediate casing have been cemented and production casing has been set at a total depth (TD) of 10,800 ft. The production casing is 32.0 lb/ft, 7 in., L-80 and the openhole is nominally 8.625 inches. Based on the openhole logs, a 20% excess is required in calculating cement volumes. The previous or intermediate casing is set at 7800 ft. Production casing needs to be cemented from 10,800 to 8000 ft. The float collar is 40 feet above the guide shoe. Calculate the cement volume required.

You will find the uploaded Halliburton reference (Petroleum Well Construction – uploaded under “Useful References” – you should also download Halliburton’s eRedBook for casing dimensions etc.) very helpful here. You may or may not need the information below to calculate the volumes. You would definitely need it to formalize slurry design. At the least check to see if the pore pressure is subdued and that you won’t fracture the formation (just do static calculations). The bottomhole static temperature (BHST) is 260°F. The fracture pressure, expressed in equivalent drilling fluid density, is 17 lbm/gal, and the pore pressure is 12 lbm/gal. The cement slurry designed for this application should have a thickening time of 4 hours at bottomhole circulating temperature (BHCT) and a fluid loss of 25 cm3/30 min at BHST and should be able to resist gas migration.

Answer is

Problem 5:
It is desired to increase the density of a Class H cement to 17.5 lbm/gal using barite. Compute the weight of barite that should be blended with each sack of cement. Use the water requirements for Class H cement a (maximum strength) and barite (table is given above). Compute the yield of the slurry. What is the “percent mix” of the slurry? Answer: 29.4 lbm/sack, 1.26 ft3/sack, 44.3%.