Foam Drilling | What is Foam Drilling?

Limitations of Foam Drilling

Foam drilling is mainly carried out due to:

• Its ability to lift large quantities of formation water from the borehole
• Good hole cleaning at lower annular velocities
• Its easy disposal of cuttings compared to mud drilling

Factors that limit the applicability of foam drilling include:

• Corrosion
• Wellbore instability
• Downhole fires in horizontal wells
• Waste water disposal
• Cost of consumables

Corrosion

Oxygen combined with water in the drilling fluid promotes corrosion downhole. Foam has a higher liquid content than mist and the liquid phase is continuous, which encourages corrosion. If saline formation fluid is present in the borehole, it will enhance the electrical conductivity of the foam and accelerate corrosion. Using an inert gas for foam generation should reduce the corrosion problem.

Corrosion reduces the wall thickness of the downhole equipment. Increasing temperature with depth causes further corrosion problems at greater depths.

It is possible to control the corrosiveness of the downhole environment. Adding an effective corrosion inhibitor to the injected fluid will help to slow corrosion and protect the equipment. Corrosion inhibitor effectiveness tests should be performed under downhole temperature and pressure conditions.

Wellbore Instability

Because of its lower annular velocities, foam should have a lower tendency to cause wellbore erosion than dry air or mist. The higher pressures in the wellbore also help to reduce the mechanically induced wellbore instability problems. Additives must be carefully selected to ensure that polymers in the liquid phase of the foam do not lead to chemically-induced instabilities.

Downhole Fires

Foam drilling significantly reduces the chances of downhole fires. The continuous liquid phase acts as a barrier to combustion. Downhole fires have been reported in horizontal wells drilled with foam. This mainly happens due to gravity segregated foam in a long, horizontal section where air forms a continuous phase on the top side of the wellbore. Foams generated by inert gases will completely prevent the chances of downhole fires. However, typically the project costs with inert gases are too high to be implemented economically.

Waste Water Disposal

Foam drilling is capable of lifting large quantities of water inflow to the surface. The cost of water disposal might become a limiting factor while designing foam drilling projects. If it is not possible to build larger pits, then the drilling system should be ready to be converted into mud drilling once the pit level reaches a critical limit.

Cost of Consumables

Cost of consumables can easily become a limiting factor while drilling with foam. The cost of foaming agents can be very expensive if large amounts of saline or liquid hydrocarbon inflows are experienced. However, foam drilling usually provides a flexible and economical way of underbalanced drilling.

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