Stiff Foam

Stiff Foam is another type of foam classification. It is rheologically more like a mud than ordinary form. It is a low-density drilling fluid used to drill poorly-consolidated formations in which hole-stabilizing materials can be included. The stronger foam supports larger cuttings compared to simple foam. When stiff foam is used, the annular velocity can be from 100 to 200 ft/minute for adequate hole cleaning. Consequently, compressor requirements are much less than for other types of pneumatic drilling.

Stiff foam is formulated by premixing polymers in water in a mud pit. Sometimes small concentrations of bentonite are added. The mixture is transferred to the foam tank and the foaming agent is folded into this mixture with as little agitation as possible. Then, it is injected into the air stream. Enough air must be injected into the mixture to give 100 to 200 ft/minute annular velocity near the surface. Air-to-mud ratios range from 100:1 to 300:1. The foam is very stiff, similar to the consistency of aerosol shaving cream, in order to inhibit air breakout in the annulus. It cannot be recirculated and must be discarded at the surface by running it through a blow line to a sump or reserve pit. Stiff foam was introduced by the U.S. Atomic Energy Commission for drilling large-diameter holes in loosely consolidated formations. The original composition consisted of 10 to 15 lb/bbl of bentonite, 0.2 to 0.5 lb/bbl of guar gum, 1 lb/bbl of soda ash, and 1% by volume foaming agent. Another relatively inexpensive, and somewhat more effective, formulation is: 12 lb/bbl of bentonite, 1 lb/bbl of soda ash, 1/2 lb/bbl Hi-Vis CMC, and 1/2 to 1% by volume foaming agent. This mixture, however, has the disadvantage of being relatively ineffective in the presence of salt water or oil.

Table 2 lists typical additives, their function and concentration for formulating a stiff foam drilling fluid.

Table 2 Stiff Foam

Concentration

Additive

lb/bbl

Vol. %

Function

Prehydrated Bentonite

10 - 15

---

Builds Foam Structure

Carboxymethylcellulose (CMC)

0.5

---

Foam Stabilizer Drying Agent

Guar Gum

0.2 - 0.5

---

Foam Stabilizer

Soda Ash

0.5 - 1.0

---

Calcium Treating Agent

Caustic Soda/Caustic Potash

0.55 - 0.5

---

Corrosion Protection

Foaming Agent

---

1%

Stabilizes Air in Liquid

Potassium Chloride

---

3 - 5%

Shale Stabilizer

Filming Amine

---

0.5%

Corrosion Protection

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  • filip
    What do polymers do for foam drilling?
    4 months ago

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