Wet trip and trip-tank gain: swab kick or ballooning?
While pulling out of hole at 8,200 ft TVD with 11.4 ppg water-based mud, the crew pulls five stands dry. Stand six comes out wet with gas-cut mud, and the trip tank shows a 4 bbl gain over the last three stands. With the pumps off, the flow line has a steady flow that is increasing. The planned trip margin is 0.3 ppg. The driller estimates the swab pressure at 150 psi. The annular BOP is closed; stabilised SIDPP is 45 psi and SICP is 95 psi. The last shoe test at 6,000 ft TVD was 13.0 ppg equivalent. The choke line is 200 ft long. The crew has not yet started the pump. The toolpusher wants to know whether this is ballooning or a swab kick, whether the trip margin covered the swab, and what to do next.
1. Is this more likely ballooning or a swab kick? Give the indicators.
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It is more likely a swab kick. Ballooning usually produces flow that decreases and stops when the pumps are off, because mud that was lost to a fracture is simply flowing back. Here the flow is steady and increasing after pumps off, and the trip tank has gained 4 bbl. A wet string with gas-cut mud supports swabbing: pulling pipe can reduce bottomhole pressure and draw formation fluid into the well. However, a wet string alone is not proof; it can be caused by a dirty connection or by mud clinging to the pipe. The increasing flow and trip-tank gain, taken together with a positive SIDPP, confirm that formation fluid has entered the well. The correct first action is a proper flow check with the well shut in and the trip tank lined up, not to continue pulling stands.
2. Calculate the pressure equivalent of the 0.3 ppg trip margin and compare it with the estimated swab pressure; calculate the kill mud weight from SIDPP.
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Trip-margin pressure = 0.3 × 0.052 × 8,200 = 128 psi.
The estimated swab pressure is 150 psi. That is 22 psi greater than the nominal trip margin, so the planned cushion would not cover the swab if the mud weight were only at the planned value. The positive SIDPP of 45 psi also shows that the well is not statically overbalanced; formation pressure is above the current mud hydrostatic. The trip margin has therefore not been effective.
Kill mud weight = 11.4 + 45 ÷ (0.052 × 8,200) KMW = 11.4 + 45 ÷ 426.4 KMW = 11.4 + 0.106 = 11.51 ppg.
The low SIDPP is not reassuring. A small influx can still be a real kick, and the gas in the annulus explains the higher SICP.
3. What immediate actions and monitoring limits apply while preparing to kill?
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Keep the well shut in and do not resume pulling out. Record SIDPP, SICP, pit gain and time, then calculate the kill mud weight and prepare the kill sheet. Line up the choke manifold and check the slow-circulating-rate pressure before starting the pump. The crew should monitor SICP against MAASP and the shoe pressure; with a 13.0 ppg shoe test at 6,000 ft TVD, the allowable annulus pressure at the shoe is limited. They should also watch for migrating gas: if SICP rises while waiting, use the approved procedure to control it rather than simply bleeding to a previous value. The well should be circulated with kill mud, and the trip should not resume until the well is stable and the trip tank is steady. If the rig has no established kill procedure for a trip kick, the company man must be involved before any choke action.