Stiff Foam Drilling
Stiff Foam Drilling Procedures and Operating Considerations
Most polymeric viscosifiers are added slowly to the liquid phase to ensure a dispersed mix. The polymer is added to the water, which circulates through the hopper. Large quantities of liquid may have to be mixed, and this operation can take several hours. Therefore, it is critical to use the tanks effectively during the mixing process.
Stiff foam returns should have a reasonable consistency (similar to shaving cream). If slugs of liquid and air are seen, either the foam is incompatible with the formation fluid inflow or the foamer concentration is not sufficient. In both cases, the foamer concentration should be increased.
If hole cleaning problems are experienced or water inflow occurs, the air component flow rate should be increased. Increasing the concentration of the foaming agent and viscosifiers in the injected fluid might also solve the problem. However, the liquid injection rate should never be reduced.
Limitations of Stiff Foam Drilling
Stiff foams are vulnerable to gas inflows, which can increase or decrease the quality to a point where the foam breaks down and cuttings transport efficiency is lost. This eventually might lead to a stuck drill string. If gas inflow is suspected, the drilling operation must be stopped and the liquid rate should be increased to establish foam returns. If the inflow is too fast, then the drilling operation can be switched to mist drilling.
In terms of downhole fires, stiff foams are more stable than unstiffened foams. Since gas phase rates are low, it is easier to generate stiff foams using inert gases. This completely eliminates the risk of downhole fires and reduces the rate of downhole corrosion.
The cost of the operation can be very high because of the cost of viscosifying agents. However, reduction in compressor power and low water volume requirements can sometimes offset the high cost of consumables. The presence of viscosifiers in waste water restricts its disposal. Because of the high viscosity, it is also difficult to re-inject the killed foam into disposal wells.
