Coiled Tubing Milling: Apparent Weight Loss and Pressure Rise
A 1.75-in, 0.156-in wall coiled tubing string is milling a composite bridge plug at 12,800 ft MD in 4½-in tubing. The well is filled with 9.0 ppg brine and has 2,600 psi shut-in tubing pressure. At 12,800 ft, the formation pore pressure is 5,900 psi and the leakoff pressure is 6,700 psi. You are pumping at 2.2 bbl/min with 4,100 psi surface pump pressure; estimated annular friction is 450 psi. The CT burst rating is 7,000 psi. The weight indicator shows 34,000 lb. During milling, the weight indicator drops 3,000 lb and pump pressure rises 600 psi. Returns at the shaker are steady, but you see a slight increase in returns. The plug is not yet tagged.
1. Calculate the bottomhole pressure at 12,800 ft and state whether the current operation is within the leakoff limit. What happens if pump pressure rises another 600 psi?
Read the worked answer
Hydrostatic = 9.0 ppg × 0.052 × 12,800 ft = 5,990 psi.
Bottomhole pressure = hydrostatic + annular friction = 5,990 + 450 = 6,440 psi.
Leakoff is 6,700 psi, so the current margin is only 260 psi. The operation is within the limit but close to it.
If surface pump pressure rises another 600 psi, some of that increase may appear as additional annular friction, pushing BHP toward or above 6,700 psi. That could initiate losses. Losses would lower the fluid column and reduce hydrostatic pressure, which can then allow the well to become underbalanced. The slight increase in returns should be investigated; if it turns into a reduction in returns, losses are occurring.
2. Interpret the simultaneous 3,000 lb weight drop and 600 psi pump-pressure rise. What are the likely causes?
Read the worked answer
The string is being set down, but the bit is not taking weight efficiently. A 3,000 lb weight drop means the surface is slacking off while the downhole assembly is not supporting the string; this can indicate buckling, the mill spinning on the plug, or the string hanging up above the mill. The 600 psi pressure rise indicates increased resistance to flow or motor torque.
Likely causes:
- The mill has stalled or the motor has stalled, so pump pressure rises and weight is no longer being transferred to the plug.
- The CT is buckling in the hole, so surface weight goes into a helix instead of weight on the mill.
- The plug has started to spin or has broken up, changing the load path.
- Nozzles or the motor are partly plugged, raising pump pressure independent of weight.
The combination should be treated as a stall/buckling event, not as normal milling progress.
3. What actions would you take, and what pressure-control concerns apply if you decide to pull out?
Read the worked answer
First, pick up 5–10 ft to remove set-down weight, reduce pump rate, and see whether pressure falls and weight returns. Re-establish circulation and tag the plug gently. If pressure remains high, stop pumping and check for a plugged nozzle or motor. Do not continue to slack off weight into a stalled mill.
If pulling out:
- Monitor wellhead pressure and keep the BOP/stuffing box competent.
- Maintain hole fill. If losses occurred, the fluid level may have dropped, reducing hydrostatic pressure and changing the well's overbalance.
- Watch for swabbing while pulling; the well has only 260 psi leakoff margin, so small pressure changes matter.
- Do not pull the CT out of the well without confirming the well is static and the barriers are in place.