3. Off-Bottom
An off-bottom well kill (off-bottom well kill means circulating mud of the appropriate weight with the bit at the present depth) will ONLY be attempted if bull heading and stripping to bottom cannot be carried out in a safe or practicable manner.
Due to the fact that this procedure is abnormal in the extreme, every care must be considered independently and a rigid procedure should not be used. However, written guidance is needed and this follows in the form of an outline procedure plus key points which must be considered prior to attempting an off-bottom kill.
3.1 Outline Procedure
1. Record all relevant well control data.
2. Calculate kill mud weight required to kill well with bit at current depth.
3. Is formation strength (at the open hole weak point) capable of withstanding the equivalent mud weight (EMW) resulting from the circulation of the kill mud? If not, then reduce kill mud weight to an acceptable level to prevent open hole fracture whilst at the same time reducing surface pressures enough to permit a stripping operation.
4. Circulate the kill mud (or partial kill mud) maintaining constant pressure (+ safety margin) at the bit using a drill pipe pressure schedule.
5. Once the well is killed (or partially killed), strip into the hole (possibly in stages conditioning the mud) to bottom.
6. Circulate the well to a stable condition with revised drilling mud weight.
3.2. Key Points to be Considered
The following points should be considered and clearly covered in the detail programme put together prior to any off-bottom well killing operation:
- Where is the influx? (If influx is below the bit then SIDPP=SICP providing there is no slug in the pipe and no trapped pressure in the mud column).
- When calculating kill mud densities use sufficient safety margins (regarding P-pore and P-frac) to allow for surging, ECD and temperature effects on mud density.
- Ensure equipment being used is adequate for pressure and temperature ranges anticipated (this includes low pressure gauges when low pressures are expected).
- Allow pressures to stabilise
- Calculate expected gas volumes at surface after expansion and ensure surface equipment adequate.
- Make allowances for gas migration. Migration rate (m/hr) = pressure increase (bar/hr) / Mud gradient (bar/m)
- Ensure a clear record is kept of events (particular volumes of different density mud in the well and bleed off volumes.
- Drill pipe pressure is not a reliable guide to bottom hole pressure during killing operations if the bit is not on bottom.
- Crews cannot be certain how much mud and how much influx is below the bit.
- If sufficient drill pipe pressure is not applied inflow can occur under the bit with a reduction of hydrostatic gradient and an increase in drill pipe pressure. The normal control action by the choke operator, to open the choke to restore the "pre-planned" drill pipe pressure, leads to further inflow.
- The actual response of the well to the kill should be monitored to allow corrective action to be taken. An example is shutting down kill pumps at predetermined intervals during circulation to record static drill pipe and choke pressures.