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Figure 7-12. Effect of quality of foam on foam viscosity. (From Mitchell.™ Courtesy of Oil and Gas J.)

If a waterbearing sand is encountered when drilling with air, the inflowing water accumulates in the bottom of the hole, and creates a back pressure which increases air volume requirements and reduces the rate of penetration. In addition, it causes cuttings to ball up and adhere to the bit and drill string. Injection of a suitable foamer into the air stream enables the air to carry (he water and the cuttings out of the hole as a foam. Maximum efficiency is achieved when all the inflowing water is converted into foam as it enters the hole, and the foam remains stable just long enough to reach the surface. The choice of surfactants depends on the salinity of the water and on whether or not oil is present. Suitable surfactants include anionic soaps, alkyl po-lyoxyethylene nonionic compounds, and cationic amine derivatives, all of which are commercially available.

The cutting-carrying capacity of foam depends on the square of the annular velocity and on the rheological properties of the foam. The rheological properties depend mainly on the viscosity of the air and the liquid, and on the quality of the foam.16 (see Figures 7-12 and 13). When the foam quality is between 0.60 and 0.96, foam behaves as a Bingham plastic.12, 17 Buckingham's equation (see Equation 5-12 in Chapter 5) may be used to determine flow pressure/flow rate relations if modified to allow for slippage at the pipe wall and for changes in air/water ratios (and hence in foam

QUALITY OF FOAM, fraction

Figure 7-13. Effect of quality of foam on yield stress of foam. (From Mitchell,™ Courtesy of Oil and Gas J.)

QUALITY OF FOAM, fraction

Figure 7-13. Effect of quality of foam on yield stress of foam. (From Mitchell,™ Courtesy of Oil and Gas J.)

viscosity) with pressure. Beyer et al12 have determined the relationship between slippage, shear stress at the wall, and LVF, and between viscosity and LVF, by means of pilot-scale experiments. From these relationships and Buckingham's equation, they developed a mathematical model that describes the flow of foam in vertical tubes and annuli. Computer programs based on this flow model may be used to determined optimum gas and liquid flow rates, pressures, circulation times and solids lifting capacity in prospective workover jobs.'8 Detailed analysis should be made whenever jobs in new fields or under new conditions are being undertaken.

The rheological properties of the foam in normal air-drilling operations are of no great concern because annular velocities sufficient to clean the hole are economically feasible. But high annular velocities are undesirable when completing in easily erodable formations in workover wells where minimum bottom hole pressures are advantageous, and in very large diameter holes. Under these circumstances, a preformed stiff foam, made from a surfactant plus bentonite or a polymer, is used.'1'- li!

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