Kick While Drilling a Vertical Well: Kill Sheet and Pump Start-Up
At 03:40 the driller is drilling ahead at 9,850 ft (vertical well, so MD equals TVD) with 12.4 ppg water-based mud when the return flow increases and the pit level rises 15 bbl in six minutes. The pumps are stopped and the well is shut in hard; the gauges stabilise at SIDPP 420 psi and SICP 590 psi. The 9-5/8 in casing shoe is at 6,200 ft TVD and a leak-off test gave 15.4 ppg equivalent at the shoe. The slow circulating pressure is 420 psi at 30 spm, taken two hours earlier with the same mud. The choke manifold is lined up to the remote choke, the accumulator is at 3,000 psi, and the mud engineer needs 45 minutes to weight up kill mud. The pit gain has held at 15 bbl since shut-in. The toolpusher asks for the kill sheet numbers and the plan for bringing the pumps up.
1. Calculate the kill mud weight, the initial circulating pressure and the final circulating pressure. Explain why SIDPP and not SICP is the basis for the kill weight.
Read the worked answer
Kill mud weight
Kill MW = current MW + SIDPP / (0.052 x TVD)
= 12.4 + 420 / (0.052 x 9,850) = 12.4 + 420 / 512.2 = 12.4 + 0.82 = 13.2 ppg
SIDPP is used because the drillpipe is a clean column of known mud: SIDPP plus the drillpipe hydrostatic equals the formation pressure. SICP contains the influx, whose density and height are unknown, so it cannot be converted into a pore pressure.
Circulating pressures at 30 spm
- ICP = SIDPP + SCR = 420 + 420 = 840 psi
- FCP = SCR x (kill MW / old MW) = 420 x (13.2 / 12.4) = 447 psi
Sanity check: 13.2 ppg is well below the 15.4 ppg shoe limit, so the well can be killed without breaking down the shoe. Note the FCP figure assumes the pump rate is held exactly at 30 spm; any rate change invalidates both numbers and the schedule must be recalculated.
2. Describe how you bring the pump up to 30 spm with the well shut in. What should the casing pressure do, and what is your action if it climbs 150 psi above SICP during start-up?
Read the worked answer
Start-up procedure
- Confirm the choke is lined up to the remote choke and the trip tank is lined up to the bell nipple.
- Open the choke slightly, then start the pump at a low rate (about 10 spm) and bring it up in stages to 30 spm.
- While the rate is increasing, the choke operator holds casing pressure constant at 590 psi.
- When 30 spm is stable, the drillpipe gauge should read ICP = 840 psi; from that point the drillpipe pressure is the primary control and is held on the schedule down to FCP = 447 psi as the heavy mud goes down the string. The casing pressure is then only a check on the shoe.
If casing pressure climbs 150 psi above SICP
The choke is too closed or is partly plugged, and bottom hole pressure is climbing. Open the choke to bring casing pressure back to 590 psi, then check the drillpipe pressure against the schedule. If the choke does not respond, stop the pump, shut in, and switch to the standby choke rather than pumping against a blocked flow path.
3. Give the MAASP now and once the kill mud reaches the shoe, and explain how the choke operator should use these numbers.
Read the worked answer
MAASP = (LOT - current MW) x 0.052 x shoe TVD
- Now: (15.4 - 12.4) x 0.052 x 6,200 = 3.0 x 0.052 x 6,200 = 967 psi
- With kill mud at the shoe: (15.4 - 13.2) x 0.052 x 6,200 = 2.2 x 0.052 x 6,200 = 709 psi
The shut-in SICP of 590 psi is below 967 psi, so the shoe is safe at shut-in and no bleed is required. During the kill the casing pressure should fall as the heavier mud fills the annulus; the 709 psi figure is the ceiling for the moment the 13.2 ppg mud reaches 6,200 ft. If casing pressure approaches it, the shoe is being over-pressured. Reduce the pump rate to cut annular friction (ECD), and if the pressure still climbs, check for a plugged choke or continued gas expansion. If losses start at the shoe, the formation has broken down and the kill plan must be revised - do not simply open the choke, because that removes bottom hole overbalance and invites a second influx.