Flow Drilling
Flow Drilling Procedures and Operating Considerations
Before flow drilling begins, the maximum tolerable surface pressure should be determined. It is possible to adjust the surface pressures if they become too high by shutting down the well and circulating a higher density fluid into the annulus. An annular pressure increase during a trip or connection is typically an indication of gas migration up the annulus.
The connection and tripping procedures require a relatively longer time in flow drilling compared with conventional overbalanced drilling. Before a connection is broken, the drillpipe above the top drillstring float must be bled off. The floats in the drillstring prevent back flow from the well. One float in every twelve joints is a rule of thumb used by the industry.
Lower differential pressures within the wellbore are preferred before making a trip. This is achieved by bullheading down the annulus with a denser drilling fluid while trying to retain underbalanced conditions in the well.
After finishing an underbalanced drilling operation, the well needs to be completed again using underbalanced techniques. Otherwise all the benefits of underbalanced drilling could be lost or reduced during the completion phase.
A snubbing unit might be needed if the annular pressures are not low enough while running casing. Foamed cement is a possible method to maintain underbalanced conditions while cementing the casing.
Limitations of Flow Drilling
It is possible that the surface design might not be enough to support the production rate. If this happens, the flow can be choked back up to the limits of the diverting system. Beyond this point, either the drilling fluid density should be increased or the drilling technique should be changed (possibly to snub drilling). If the pore pressure cannot be predicted well enough before the drilling operation, the flow drilling may not be the best technique to use.
Flow drilling is a popular method for horizontal wells since the formation pressures are supposed to be more or less within a constant pressure gradient range. However, it is not suitable for unconsolidated formations since high annular velocities during drilling can create serious wellbore instability problems.
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