Drilling Break With Falling Standpipe Pressure
An 8½ in hole is being drilled at 3,420 m TVD with 1.32 SG water-based mud. The overbalance is estimated at about 120 psi. Over the last 3 m the ROP jumps from 6 m/hr to 22 m/hr, torque falls from 8 kN·m to 5 kN·m, and standpipe pressure drops from 3,200 psi to 2,850 psi at a constant 520 gpm. The shaker hand reports heavier cuttings and a change in cuttings shape; the mud logger reports connection gas of 4.5% and background gas rising from 0.6% to 1.8%. The pit level has gained 9 bbl in 12 minutes. The company representative is off location. The toolpusher says to keep drilling the last 40 m to section TD because 'it is just a fast formation and we will case it off anyway'.
1. Explain what each of the drilling and mud indicators is telling the driller, and why they reinforce one another.
Read the worked answer
Each indicator is consistent with a permeable formation being drilled faster than expected while formation fluid enters the hole.
- ROP increase with falling torque suggests the bit has entered a softer or more porous zone with lower confined strength, not a bit or parameter change - the parameters are unchanged.
- Standpipe pressure falling at constant pump rate means the annular or bit path has become easier to flow through. In a rock with lower resistance, or with gas beginning to occupy the annulus and lighten the column, less pressure is needed.
- Cuttings change and rising gas show reservoir-type material and formation gas reaching surface.
- 9 bbl pit gain in 12 minutes is unaccounted volume - the well is taking nothing and giving back fluid.
Individually, faster drilling is normal. Taken together with a pit gain and rising gas, the most probable explanation is an influx. The pit gain is the decisive one: it is a direct observation of volume entering the well.
2. What should be done now, and how should the toolpusher's instruction be handled?
Read the worked answer
The correct action is to stop drilling, stop rotating, lift the string off bottom, shut the pumps down, and flow check. If the well flows, shut in, record SIDPP, SICP, pit gain and time, and notify the company representative and toolpusher formally.
The toolpusher's reasoning has a factual weakness: a 'fast formation' does not cause the pit to gain 9 bbl, and it does not cause total gas to triple. The proposal assumes the observation is benign, but does not explain the volume gain. Drilling 40 m further means circulating more gas up the annulus while the bottom-hole pressure is already compromised, and it delays the only measurement that would settle the question - a shut-in pressure.
Disagreeing with a supervisor is uncomfortable at Level 1, but the expected behaviour is to state the observations plainly, request the flow check, and escalate if refused. The driller is the person who must act; the decision cannot wait for the company representative to return.