Foam Drilling | What is Foam Drilling?

Foam Drilling Procedures and Operating Considerations

Similar to dry air or mist drilling, the standpipe pressure should be observed carefully. An increase in standpipe pressure typically indicates fluid inflow. The influx can also be confirmed by the changes in foam quality at the blooie line.

The returning foam quality must be examined carefully. A steady discharge of stiff, high-quality foam that collapses rapidly in the flare pit should be expected. If the foam quality is substantially less than 95 percent at the surface, its quality downhole may be too low for effective hole cleaning. Moreover, a wet foam takes longer to decay and requires more defoamer than a high-quality foam.

If the liquid inflow is not compatible with the foaming agent, slugs of liquid and gas may be seen at the blooie line. In this case, it is necessary to increase the concentration of the foaming agent.

Gas inflow typically increases the standpipe pressure. If the rate of inflow is much larger than the air injection rate, it displaces the foam above it. In this case, first the foam is blown out of the well, then the remaining returns show up in the form of either slugs of foam or mist.

Lost circulation can also be experienced when drilling with foam. A loss of returns followed by a surge of foam is an indication of lost circulation. In some cases, the circulation pressures can be sufficient for foam to flow into cavities or fractures. This should be avoided while drilling near surface to prevent contamination of aquifers and streams. Excessive drag or fill might result in poor hole cleaning. A steady return of dry foam is an indication of a very low annular velocity.

In deep wells, it may be necessary to apply a back-pressure, to maintain a reasonable foam quality profile throughout in the annulus. Since the wellbore is full of highly compressible fluids, it is a better practice to apply small adjustments to the choke setting, and then allow the pressure to stabilize before making any additional adjustments. Another way of reducing the foam quality is to increase the liquid injection rate or to reduce the air injection rate.

Connections are made in a similar way as they are made in mist drilling. Generally in foam drilling, most of the foam in the annulus will not collapse while making a connection. Since foam has a high viscosity and low settling velocity, cuttings will be held in suspension in the annulus. If the foam in the annulus collapses during the connection, cuttings will settle rapidly. If this happens the hole should be circulated clean before making the connection. It is always a better practice to wait until returns are established, before drilling after a connection is made.

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During a trip, foam will normally separate into its liquid and gas components. Therefore, the hole has to be circulated clear of cuttings before tripping.

The following tripping procedures should be implemented for efficient operation:

• Circulate the hole and strip the string back through the rotating head.
• Turn on the compressors and jet the blooie line.
• Pull the rotating head rubber.
• Continue tripping with the rotating head rubber removed.
• Close the blind rams and flow the produced gas through an open choke to the flare pit.
• Open the blind rams and close the choke before rerunning the string back into the hole.
• Re-install the rotating head rubber and stop jetting.
• If there is no liquid production, continue the trip to the bottom and establish foam circulation.
• If there is water inflow, first unload the well before resuming drilling.

The well can be unloaded in one stage by applying the following procedure (assuming that the injected fluids won’t be lost into the formation):

• After running to the bottom begin injecting liquid at up to 30 gpm.
• Slowly inject air to create a low quality foam.
• Standpipe pressure should fall after liquids start reaching the surface.
• When foam returns are seen, the air and liquid injection rates should be adjusted to the desired foam quality.
• Once the bit is at the bottom with the hole unloaded there is no need to dry the hole before resuming drilling.