Managing Differential Pressures Encountered While Milling with Coiled Tubing
When milling with coiled tubing, it is important to anticipate problems associated with sudden pressure changes in
the wellbore that can occur when communication is established across the obstruction (plug, packer, bridge, etc.).
Differential pressures can be either positive (“kicks”) or negative (“lost circulation”).
Kicks
Pressure surges, or “kicks,” can occur when the bit breaks through the obstruction into an area of higher pressure
trapped below. A kick introduces the hazards of buckling the coiled tubing, which may part the pipe and/or damage
the bottomhole assembly, and of lodging the coiled tubing (and BHA) in debris blown up the hole.
Lost Circulation
Sudden pressure losses in the well may occur on breakthrough into a low-pressure zone below the obstruction. Any
loose fill in the annulus will be blown downward and pack off around the tool assembly, so it is important to
circulate the hole clean before milling. Lost circulation can also lead to kicks, especially if differentially pressured
zones are present. In these situations, fluid in the annulus flows into the deeper low-pressure zone, which reduces
hydrostatic control on the upper zone and causes a kick. This introduces all the hazards normally associated with a
kick and may also cause differential sticking.
Preventing Pressure Changes
The most effective way to minimize the hazards of differential pressure is to prevent or eliminate the conditions that
cause differential pressure. This may require, for example, that a strong upper zone be depleted until the flowing
pressure of the upper zone is less than or equal to the static pressure of the zone below the plug. In this case, the
well should be allowed to flow until just before intervention with coiled tubing. Similarly, a weak upper zone
should not be drawn down excessively if a high-pressure zone is isolated below it.
Milling Fas-Drill® Bridge Plugs
The relief port on a Fas-Drill® Bridge Plug is located within three inches of the top of the plug. To allow pressure to
equalize across the port, the operator should mill the top three inches of the plug, then pick up the coiled tubing and
wait until the pressure stabilizes. Realistically, it is impossible to see 3 inches on a coiled tubing depth indicator, so
the operator must rely on changes in tubing pressure to indicate breakthrough. However, a positive pressure change
may also indicate a motor stall. The operator, then, must be able to differentiate the pressure characteristics of a
kick and a stall. Pressures should stabilize across a Fas-Drill® within 5-30 minutes.
Managing Pressure Changes
It is not always possible or feasible to prevent kicks and lost circulation. In these cases, sudden pressure changes
must be managed as they occur.
Before Milling
In either condition of differential pressure, it is imperative that the hole be circulated clean before milling. This
will reduce the risk of sticking the tubing in sand or fill. A good practice is to circulate bottoms-up twice from
the top of the obstruction. Hydrostatic control is also important in this situation. If the wellbore pressure
exceeds the closure stress of a fractured zone, the fracture may produce proppant, which can bridge around the
tubing.
After Milling
When an operator suspects that he has taken a kick, he needs to continuously watch tubing weight, pump
pressure, and wellhead pressure. If WHP first increases and then shows a gradual decrease (with no decrease in
pump pressure), this is a probable indication of solids traveling up the annulus and increasing hydrostatic
pressure. If this is indicated, the operator needs to pull slowly out of the hole, pumping at high rate, adjusting
the choke as necessary to maintain a constant WHP and return rate. He should continue in this manner until the
tubing is out of the hole or at least two annular volumes have been circulated. If excessive drag develops, he
must maintain a reasonable ejection rate and avoid overpulling the pipe. Attempts to “run away from” drag will
result in stuck pipe and may lead to much more serious consequences.
