Foam Drilling | What is Foam Drilling?
Foam drilling is a drilling technique that involves the use of a polymer foaming agent, water, and compressed air. In certain areas, it’s more effective than mud drilling, and it generally results in more efficient spoil disposal. Trenchless operators have access to a variety of drilling techniques.
Foam Drilling Overview
Foam is composed of a continuous liquid phase that surrounds and traps the gaseous phase. Its main characteristics are a relatively low density and extremely high viscosity. This combination of properties gives foam a major advantage over mist and dry air drilling.
The high viscosity allows efficient cuttings transport with relatively low gas injection rates. The low density ensures that the underbalanced conditions are established in the most practical circumstances. The large liquid lifting capacity of foam drilling is one of its greatest benefits.
The gaseous phase used in foam drilling is typically nitrogen. Due to its continuous liquid phase a foam formed with nitrogen will not easily permit downhole fires. A surfactant solution is mixed with the gas flow to generate the foam. Typically, a stable foam is not completely formed until the liquid and gas phases are mixed thoroughly at the bit.
Freshly generated bubbles are typically very small and they are spherical in shape. Therefore, these types of foams are called sphere foams. Polyhedron foam is composed of equal sized bubbles in pentagonal dodecahedra (twelve-sided) shape. This shape configuration minimizes the liquid volume required to form films between bubbles.
Foam Quality
A foam’s quality is its gas volume fraction expressed as a percentage. An 85 quality foam is composed of 85 percent gas by volume. If a foam has greater a percentage of gas than of liquid, it is referred as high quality foam.
If the foam quality exceeds some critical percentage, the foam becomes unstable and starts to break. Depending on the composition of the liquid phase, this threshold value varies between 95 -99.65 percent. Polymeric viscosifiers are needed to achieve stable foams at high qualities of around 99 percent.
Low quality foams decay more slowly than coarse and high quality foams. Bubbles grow to the top of the foam while the liquid accumulates to the base of the foam. If we stir a foam mixture, spherical bubbles tend to re-distribute and keep their walls, whereas the high quality, polyhedron foam will decay more quickly.
The pressure within a gas bubble is inversely proportional to its size. Therefore, smaller bubbles merge into larger ones. Strengthening the bubble walls results in a more stable foam. This strengthening is achieved by the use of surfactants.
Surfactants are composed of a hydrophilic group of molecules attached to a long hydrophobic tail. Most of the time they concentrate at the liquid gas interface in foams. Depending on their type, they either strengthen or weaken the bubble walls.
Ammonium salts are the most widely used types of surfactants. They are expensive, but they form stable foams for use at very low temperatures. Alpha olefin sulphonates, on the other hand, are inexpensive, perform well in fresh water and are not affected by hydrocarbon contamination.
Increasing the foaming agent’s concentration up to a critical value typically increases the foam stability, which is measured by its half-life. Beyond the critical concentration, the foam half-life decreases with further increase in concentration. Brine and hydrocarbon presence in foam can significantly reduce the half-life. In some extreme cases, fluids will not foam at all in the presence of even modest concentrations.
Increasing temperature also increases the rate of decay. For example, to keep the half-life at 400°F the same as at 70°F, the foaming agent concentration has to be increased by 10 times.
After the foam returns to surface, it is necessary to kill it in order to avoid large foam accumulations. Several different chemicals are used to accelerate the decay of the foam. For example, iso-octanal alcohol is an effective defoaming agent that is used to kill foams made with anionic foaming agents. One can also destroy foam mechanically. This technique is particularly effective on high-quality foams.
