Conventional well killing procedures are based upon the basis that all of the well control equipment operates as designed and the wellbore is able to withstand the imposed pressures.
Mechanical problems and formation fracturing, however, occur occasionally during well killing operations causing complications with conventional procedures.
Stopping the Killing Operation
Killing operations may have to be stopped because of equipment failure or for checking static pressures at certain stages of the killing process If the killing operation is interrupted the choke must be closed immediately. The standpipe pressure should not be allowed to be bled off to its theoretical value to avoid the possibility of more influx entering the well bore. The closed-in period must be kept as short as possible.
If a long shutdown period is unavoidable, the well must be observed in order to avoid the pressuring effect of the gas due to migration. "Kick control by the volumetric method" should be applied if bleeding off is necessary.
Plugged Bit Nozzles
When plugged nozzles occur whilst pumping at a constant pump rate, Standpipe pressure (Pst)will increase. If, after opening the choke further, Pstremains higher than expected and choke pressure (Pchoke)drops, the nozzles may be partially plugged, or there is a restriction in the annulus. Newly adjusted standpipe pressures will have to be used during further well killing operations. (Always check if the valves upstream of the Pchoke measuring point are fully open).
The Pstgraph needs to be revised according to the procedure used for changing the circulating rate regardless if any change in circulating rate has taken place. Always keep in mind that nozzles can "un-plug" again after a while, and graphs should be re-adjusted accordingly.
If circulation cannot re-establish by increasing the pump pressure, a wireline operation could be planned to try to open the bit with a string shot or perforate as far down the string as possible.
While rigging up to perforate, observe SICP for gas migration. If gas migration is suspected consider using the volumetric well kill technique.
Plugged Chokes
Cuttings, loose formation particles, etc., may plug the choke and cause a sudden rise in pressure in the annulus and drillpipe. If this happens, another choke must be opened, or pumping must be stopped immediately to avoid over-pressuring the well.
In order to take prompt action and to ensure that the correct operational steps are made, it is essential that:
- there is good communication between all persons involved at all stages of the well control operation
- the actual pressures on drillpipe and annulus are always known.
If the chokes become partially plugged and the decision is made not to interrupt the operation for cleaning the chokes, it may be necessary to reduce the pumping speed, because of too high standpipe pressures.
Change in Circulating Kill Rate
During well killing operations the circulating rate may have to be reduced because of higher than preferred pumping pressures or pumping speed may have to be increased if faster pumping is desired.
The procedure to change the circulating rate and to determine the new circulating pressure while circulating out the kick is illustrated in Appendix H.
Complete Power Failure During the Well Kill
The provision of an power supply or emergency generator system linked with the accumulator system to maintain pressure on the BOP accumulator unit, in case of a complete power failure during well control operations. shall be available on each rig.
Choke Washout
When the well is being killed using the Driller's method or the ‘Wait and Weight’ method inability to maintain casing pressure indicates choke washout.
If an abnormal decrease of drillpipe pressure and casing pressure is noticed it is possible that a washed out choke is the cause.
If a washed choke is suspected the well should be shut-in. The secondary circuit using the spare adjustable choke will be prepared. Well control procedures will be re-established using the spare adjustable choke.
Immediate repair action on the washed-out choke should be considered.
Summary Table
Careful consideration shall be given to the available well data before selecting an alternative procedure. The following table outlines the possible problem causes for several unexpected changes in surface measurements.
