Losses at 2,900 m: ECD Above the Leak-Off Pressure
At 04:20 the 8½ in hole is at 2,900 m MD and 2,750 m TVD with 1.46 sg water-based mud at 1,150 L/min. The MWD reports an ECD of 1.58 sg. The shoe is at 1,900 m TVD and the leak-off test gave 1.56 sg EMW. Returns at the shakers fall away and the active pit drops 18 m³ in 30 minutes. Mud logging shows no connection gas and the flowline temperature has risen 4°C. The mud engineer raised the weight from 1.44 to 1.46 sg two hours earlier to control a 1.43 sg pore pressure after a small influx. The company man wants to mix a 20 m³ LCM pill, keep the pump at 1,150 L/min and continue drilling to section TD at 2,950 m MD, 400 m away.
1. Assess the loss situation using the pressure numbers. What is the likely cause, and what are the viable options without exposing the well to a kick?
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The static mud weight of 1.46 sg gives only 0.03 sg overbalance against the 1.43 sg pore pressure. Circulating at 1,150 L/min adds enough annular friction to raise the ECD to 1.58 sg, which is 0.02 sg above the 1.56 sg leak-off at the shoe. The falling pit and reduced returns are therefore consistent with induced losses caused by excessive ECD, not with a dry formation or a kick.
Options:
- Reduce the pump rate. Annular friction falls roughly with rate to the power 1.8: at 900 L/min the friction term would be about 0.64 of its present value, so ECD would fall from 1.58 to about 1.54 sg. That is below the 1.56 sg leak-off and still above the 1.43 sg pore pressure.
- Stop drilling, pick up off bottom and monitor the pit while treating the loss zone with a correctly sized LCM pill.
- Keep the static mud weight at 1.46 sg for now. Dropping to 1.44 sg would leave only 0.01 sg overbalance, and 1.43 sg would be on balance.
- Do not add more barite. It would raise ECD further and worsen the losses.
Continuing at 1,150 L/min is not viable: losses will grow, the cuttings bed and stuck-pipe risk increase, and a loss-induced kick becomes more likely.
2. Set out the immediate actions and monitoring plan for the next two hours.
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Immediate actions:
- Stop drilling and pick up off bottom; keep the string moving and monitor torque and drag for signs of stuck pipe.
- Reduce the pump rate to about 900 L/min in a controlled step and record the new ECD and return rate.
- Keep the annulus full and record pit volume every 10 minutes; do not allow the pit to fall without accounting for every cubic metre.
- Notify the toolpusher, company man and mud engineer; call a safety briefing.
- Mix and spot the LCM pill at the loss zone, then verify returns before resuming circulation.
Monitoring:
- Watch ECD, pump pressure, pit volume, return flow, gas, connection gas and flowline temperature.
- Record the volume and rate of any losses and the volume of pill returns.
- If losses continue, consider a second pill, a lower pump rate or a controlled reduction in mud weight while keeping at least 0.02 sg overbalance.
- Do not resume drilling until losses are controlled and the ECD is below the leak-off pressure with a clear margin.
3. How would you distinguish continuing losses from a kick once the loss rate has been reduced? Explain why the earlier barite addition made the situation worse.
Read the worked answer
With the pump off, a well that is only losing fluid will stop flowing and the pit will remain static or continue to fall slowly. A kick is indicated by flow from the well with the pump off, a rising pit, or shut-in SIDPP and SICP above zero. A loss-induced kick can be hidden because the falling pit masks the influx volume, so a flow check and a shut-in pressure reading are essential after every loss-control step.
Check for:
- flow with pumps off, however small;
- pit gain after the loss has been controlled;
- gas readings and a rising flowline temperature;
- SIDPP and SICP if the well is shut in;
- pump pressure and ECD behaviour when circulation resumes.
The barite addition raised the static weight from 1.44 to 1.46 sg but also raised the circulating ECD, which was already at the shoe limit. The extra weight increased the overbalance against the loss zone and made the losses worse rather than controlling the small influx. The correct response was to reduce annular friction and treat the loss zone, not to add weight to a well that was already overbalanced.