Deviated Well: MD Against TVD in the Kill Sheet
A deviated well is being drilled: 12-1/4 in hole at 14,800 ft MD and 10,200 ft TVD, with the 9-5/8 in shoe at 8,500 ft TVD and a leak-off test of 13.8 ppg. The mud is 11.6 ppg, the standpipe pressure is 2,200 psi at 550 gpm, and the well kicks with a 20 bbl pit gain. The well is shut in: SIDPP 380 psi, SICP 720 psi, pit gain 22 bbl. The slow circulating pressure at 25 spm is 480 psi. The derrickhand has already written 12.1 ppg on the kill sheet and the driller is about to bring the pumps up to 25 spm. The remote choke is lined up and the pit and trip tanks are both being monitored.
1. Check the kill sheet. Is 12.1 ppg correct? Show the correct calculation and the consequence of the error.
Read the worked answer
The derrickhand has used measured depth:
11.6 + 380 / (0.052 x 14,800) = 11.6 + 0.49 = 12.09, rounded to 12.1 ppg. Wrong.
The correct basis is true vertical depth, because hydrostatic is generated only by the vertical column:
11.6 + 380 / (0.052 x 10,200) = 11.6 + 380 / 530.4 = 11.6 + 0.72 = 12.32 ppg
MD is used for volumes, strokes to the bit, lag and pipe capacity - never for pressure. The error is 0.22 ppg, which is 0.22 x 0.052 x 10,200 = about 117 psi underbalance at the sand. Pumping 12.1 ppg would leave the well flowing throughout the kill: the drillpipe pressure would not hold on schedule, the pit would keep gaining and a second influx would enter. The kill would have to be restarted with the correct weight, with extra time on the choke and more pressure on the shoe.
2. Using the correct kill weight, calculate ICP, FCP and the maximum casing pressure allowed when the kill mud reaches the shoe.
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Circulating pressures at 25 spm
- ICP = SIDPP + SCR = 380 + 480 = 860 psi
- FCP = SCR x (kill MW / old MW) = 480 x (12.32 / 11.6) = 510 psi
The drillpipe pressure is held at 860 psi when the pump is stable at 25 spm, then bled down on the schedule as the 12.32 ppg mud travels down the string, reaching 510 psi when it reaches the bit.
Shoe limit when the kill mud arrives
- Fracture pressure at 8,500 ft TVD = 13.8 x 0.052 x 8,500 = 6,100 psi
- Hydrostatic of 12.32 ppg mud to 8,500 ft = 12.32 x 0.052 x 8,500 = 5,446 psi
- Maximum casing pressure at the shoe = 6,100 - 5,446 = 654 psi
At shut-in, the shoe pressure is 720 + (11.6 x 0.052 x 8,500) = 720 + 5,128 = 5,848 psi, which is below 6,100 psi, so the shoe is intact. During the kill the casing pressure should fall, and it must be at or below 654 psi by the time the kill mud reaches the shoe; if it is higher, the shoe is being over-pressured - cut the pump rate to reduce the ECD and re-check the choke.
3. Mid-kill, the drillpipe pressure falls 200 psi below the schedule while the casing pressure climbs. What are the likely causes and what do you do?
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Likely causes
- A washout or leak in the drillpipe, standpipe or surface manifold: pressure is lost upstream of the bit, so the drillpipe gauge reads low while the choke operator, still chasing the falling drillpipe pressure, closes in and pushes casing pressure up.
- A pump problem - a failing liner or valve drops the effective rate while the stroke counter lags behind.
- A partly plugged choke, which raises casing pressure independently of the drillpipe reading.
Action
- Check the pump rate first (strokes, SPM and pump pressure) to rule out a rate change; a genuine rate change explains both readings and the schedule must be recalculated.
- Confirm whether the casing pressure is rising because the choke is closing. If the choke is at fault, line up the standby choke.
- If the drillpipe pressure cannot be made to follow the schedule, stop the pump, shut in and re-establish control. A leak in the string means the drillpipe gauge is no longer a faithful bottom hole pressure indicator, so control passes to the casing pressure and the volumetric method until the string can be repaired or the well killed another way.
- Throughout, keep the casing pressure below the 654 psi shoe limit at 8,500 ft TVD and keep the pit level under continuous watch.