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Stiff Foam Drilling Overview
Stiff foam is a more viscous and stable form of foam that can be generated using surfactants alone. The reduction in horsepower requirements and low water consumption make the stiff foam a more economical option compared to other drilling fluids. In general, stiff foam drilling involves similar equipment and procedures to those in stable foam drilling. However, the flow properties of stiff foam are different from those of stable foam.
In stiff foams, increased viscosity of the liquid phase provides a more stable structure. This structure allows higher quality foams to be generated without rupturing the cell walls.
Rheology
The concentrations of clays and viscosifying agents have a significant effect on the viscosity of the base liquids. According to the model of Hatschek ( 1910), the foam viscosity for qualities between 74 and 96 can be expressed as:
Equation 1
where:
μF = foam viscosity, cp
μL = liquid viscosity, cp
Γ = foam quality, fraction
As seen in this expression, the foam viscosity increases as the viscosity of the liquid phase is increased.
