Stable Foam

The difference between Stiff Foam and Stable Foam is the method used to prepare the foam. Stable Foam, developed by Chevron, is a completion and workover fluid. Its distinguishing characteristic is that it is preformed on the surface in a foam generating unit. Then, it is injected into the drill pipe. The equipment required for making stable foam is: (1)an air compressor, (2) tanks for blending water, detergent and additives, (3) detergent solution pump, and (4) the foam generator and injection manifold.

Stable Foam consists of a detergent, fresh water and gas. Other additives such as viscosifiers, salts, or corrosion inhibitors may be included in the mixture. To be used effectively as a circulating medium, stable foam must be preformed. This means it is made before it encounters any solids and liquids from the wellbore. The foam is circulated only once. Foam systems have characteristics which make them stable and resistant to wellbore contaminants. Impurities introduced during foam generation can destabilize the foam.

Foams can be prepared with densities as low as two pounds/cubic foot by carefully controlling selection and mixing of components. (Bottomhole pressure measurements have indicated actual pressures of 15 psi at 1000 ft and 50 psi at 2900 ft while circulating stable foam.) Viscosity can be varied so that high lifting capacities result when annular velocities equal or exceed 300 feet/minute.

Stable foam should have a gas-to-liquid volume ratio in the range of 3 to 50 ft3/gal, depending on downhole requirements. The water-detergent solution which is mixed with gas to form stable foam can be prepared using a wide range of organic foaming agents (0.1-1 percent by volume). A schematic diagram of a stable foam unit is shown in Figure 1.

Figure 1 Stable Foam Unit

Figure 1 Stable Foam Unit

Table 3 lists typical components, their function and concentration for formulating a preformed stable foam drilling fluid.

Table 3 Stable Foam

Concentration

Additive

lb/bbl

Vol. %

Function

Prehydrated Bentonite

10 - 15

---

Foam Stabilizer

Polyanionic Cellulose

0.5

---

Filtration Agent Drying Agent

Sodium Polyacrylate

1

---

Filtration Agent

Foaming Surfactant

---

1%

Foaming Agent

Potassium Chloride

---

3 - 5%

Borehole Stabilizer

XC Polymer

0.25 - 1

---

Foam Stabilizer

Filming Amine

---

0.5%

Corrosion Protection

VLime

0.25 - 0.5

---

Corrosion Protection y

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