TABLE OF CONTENTS The term stable foam from the mathematical modeling point of view describes a special class of aerated drilling fluids and work over completion fluids. This class of drilling fluid is made up of a specific mixture of incompressible fluids injected with compressed air or other gases. To create a stable foam drilling fluid, the incompressible component is usually a mixture of fresh water with a surfactant foaming agent. However, stable foam drilling fluids can use formation water with dissolved salts. The surfactant foaming agent usually comprises about 1 to 5% by volume of the treated water being injected (depending on the surfactant product).
The term "stiff foam" refers to the use of viscosified (aqueous polymer) water instead of fresh nonviscosified water as the incompressible fluid component (typical viscosity additives are polyanionic cellulose, xanthan gum polymers, and carboxymethyl cellulose). In essence, stiff foam is drilling mud with a surfactant additive. The subject of stiff foam is beyond the scope of this chapter.
Figure 10-1 shows a schematic representation of a direct circulation foam drilling operation. In a typical deep drilling operation, the foam mixture is injected into the top of the inside of the drill string (i.e., direct circulation, see Chapter 6). The mixture of the incompressible fluid (with surfactant) and compressed air (or other gas) flows as an aerated fluid mixture down the inside of the drill string to the bottom of the string just above the drill bit nozzles. The nozzles in the drill bit are usually required...
