Unstable foams

Types of Operations 840. Equipment 844. Well Completion 847. Well Control 852. Air, Gas, and Unstable Foam Calculations 853. 

The volumetric rate of flow requirements for air drilling operations and unstable foam drilling operations are quite large, on the order of 1,000 actual cfm to possibly as high as 4,000 actual cfm. 

Air (or Gas) Downhole Motors. Some positive displacement mud motors can be operated on unstable foam. In general, these mud motors must be low-torque, high-rotalional-speed motors. Such motors have found limited use in air and gas drilling operations where directional boreholes are required. Recently a downhole turbine motor has been developed specifically for air and gas drilling operations. This downhole pneumatic turbine motor is a high-torque, low-rotational-speed motor. 

Air and gas, unstable foam and stable foam techniques are used almost exclusively for onshore drilling operations, rarely in offshore applications. Aerated mud, however, is used for both onshore and offshore drilling operations. 

Pertinent engineering calculations can be made for air and gas drilling operations and for unstable foam drilling operations. 

Using the data and results from the Examples on pp. 856 to 859, determine the approximate amount of surface injected water and foaming agent needed to saturate the air at bottomhole conditions and to provide an unstable foam in the annulus of the borehole. 

The downhole turbine motor designed to be activated by the flow of incompressible drilling mud cannot operate on air, gas, unstable foam or stable foam drilling fluids. These downhole turbine motors can only be operated on drilling mud or aerated mud. 

The positive displacement motor of the Moineau-type design can be operated with unstable foam (or mist) as the drilling fluid. Some liquid must be placed in the air or gas flow to lubricate the elastomer stator as the metal rotor rotates against the elastomer. Positive displacement motors have been operated quite... 

Figure 9 teaches that large pore-body to pore-throat radii ratios lead to a more unstable foam. The effect is more dramatic for higher capillary numbers. 

May be able to handle unstable foams (to be confirmed by testing). 

Laboratory experiments indicated formation of unstable foams from Troll crude, suggesting that foaming in the separation facilities will not be a problem. 

In summary, the results of our thin film drainage study as well as our investigation of oil spreading mechanisms and frequency dependence of dynamic interfacial tension all suggest that the C 2 0S system, which displays the m.ost unstable foam behavior in the presence of oil, should not perform as effectively as the Ci6A0S system in oil displacement experiments in porous media. 

Low-foaming surfactants can be produced by changing the structure of the surfactant molecule so that it retains its surface activity but produces an unstable foam. We mentioned above that if the surfactant is rapidly diffusing, it can destroy the elasticity of the surface film and thus prevent or minimize foaming. 

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