Stiff Foam Drilling
Viscosifiers
Stiff foams were originally made by adding bentonite to water (10 to 12 lbs/barrel) to generate the viscosified base liquid. The most commonly used additives are as follows:
| HEC | Hydroxyethylcellulose is a polysaccharide linear polymer based on a cellulose backbone. It is non-ionic. |
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| PAC | Fast-mixing, high-activity, granular polymer. It increases viscosity, lowers fluid loss and prevents bit balling. |
| Cypan | is a synthetic material manufactured from petroleum feedstocks. It is not as complex structurally as the natural polymers. Usually has a straight-chain carbon backbone with different side chains. |
| XC | Polysaccharides manufactured from bacterial fermentation. Has extremely complex structures with high molecular weights. |
| CMC | Carboxymethylcellulose is a polysaccharide linear polymer based on a cellulose backbone. It is anionic with carboxylic acid groups. |
PAC and Cypan are more resistant to minor saline and oil contamination than bentonite-based mixtures. Depending on the possibility of calcium contamination, it may be necessary to add soda ash to the liquid phase to stop the breakage of the base fluid.
Increasing the polymer content increases the cost of consumables, but the higher viscosity that is attained allows lower injection rates, thus reducing the need for additional compression at the wellhead. On the other hand, an excessively stiff foam will be very difficult to break at the pit, causing foam control problems.
Corrosion of downhole equipment is still a concern when drilling with stiff foam. Corrosion inhibitors compatible with the foaming agent and other potential contaminants should be added to the liquid phase.
The high viscosity of the stiff foam can cause foam control problems. Because of its stable structure, it is more difficult to break stiff foams. Chemical defoamers have to be thoroughly mixed with the foam. The consumption of defoamer is also likely to be higher than for unstiffened foam.
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