Experiences from five deepwater well kicks
Several deepwater kicks have been investigated over the past few years in water depths ranging from 2,000 ft to 6,100 ft, with interesting results. The well studies, as presented by Gary Nance of Randy Smith Training Schools at the IADC Well Control Conference, were on wells drilled by major operators in the Gulf of Mexico (semisubmersibles and dynamically positioned drillships).
The first kick investigated was called ballooning, because the company drilling representative stated that ballooning masked the kick to the point that it was undetectable. Details of this kick include: water depth of about 4,500 ft; total depth of about 20,000 ft; mud weight of 13.0 ppg; and kick size of about 80 bbls.
The well began to flow on a connection and was subsequently shut in. A concern is that well flow occurred during the previous two connections, as indicated from the log of pvt/flow show. Kick detection was delegated to the shakerman for a judgment call: whether a 1-2 in. stream, common on connections, was increasing or decreasing.
In the trip tank used to monitor connection flow-back, it was physically awkward to see the returning flow. On previous jobs, the rig had been successful in allowing flow-back during connections due to ballooning, so that flow was not necessarily an alarming situation. Interestingly, the drilling program specifically indicated a mud weight of 13.8 ppg for this well depth.
Severe mud loss occurred during well-killing operations and the well eventually "packed-off," sticking the drillstring. Loss of the well and abandonment of potential targets made this a financially costly event.
Change in MWD log
The second kick investigated was the shut-in of a well with a one-bbl pit gain. The reported shut-in drillpipe and shut-in casing pressures were, respectively, 220 psi and 300 psi. The kick was detected because a change in the MWD's (measurement while drilling) resistivity log piqued the company man's interest and a prolonged flow check was called on during a "simulated" connection (one joint down on the stand). This flow check proved positive, hence the very small influx volume.
Details of this kick include a water depth of about 6,100 ft, a total depth of about 16,700 ft TVD/TMD (true vertical depth/true measured depth), a mud weight of about 12.5 ppg SOBM (synthetic oil base mud), and a choke line friction pressure of 300 psi @ 20 spm (strokes per minute).
The effect of the reduced temperature of the mud in the choke and kill line in deepwater was investigated. The cold mud caused a 700 psi "spike" that endured for some time, prior to slowly declining to a choke line friction pressure measurement of 220 psi at 10 spm.
Of special concern was the affect this would have on pump start-up operations. Of course, most well control events in deepwater report significant mud loss during start up operations.
Flow checks prove negative
The third kick investigated exhibited unusual behavior. Several flow-checks all proved negative, but while circulating between the checks the well would flow like "gangbusters." This strange behavior was never understood satisfactorily. Details of this kick included a water depth of about 2000 ft, a total depth of about 19,100 ft, a mud weight of 15.0 ppg SOBM, and a shut-in casing pressure of 420 psi (initially).
The driller noted a substantial increase in the flow returns (normal 45%; instantaneous increase to 53%) while drilling ahead at +/- 19,100 ft. Responding correctly, he shut down pumps and checked for flow. The flow check was positive and the well was shut in with 420 psi on the casing gauge (during the next 24 hr, there was never a reported attempt at measuring the shut-in drillpipe pressure).
A pit gain of 15 bbls was estimated at this point. Complicating matters was the fact that the drillstring had just been slugged with 150 bbls of 16.5 ppg hole-cleaning sweep and the calculated U-tube resulted in 410 psi.
The hole was packing off and actions were ordered to prevent stuck pipe. The well was opened and a stand of pipe was pulled, resulting in an additional 35 bbls of well flow. For the next nine hours, the well was circulated (with the pits gaining constantly), and flow-checked (with no flow indicated) while rotating, reciprocating, and pulling stands of pipe (attempting to keep it free). Finally, there was no doubt that the well was flowing and was shut-in with 1,910 psi on the casing gauge, with an estimated total pit gain of +/- 800 bbl.
This event took several days out of normal operations, which prolonged working the pipe through the annular caused severe wear, to the point that the annular would not fully open. Post-event analysis revealed an accumulation of shale and salt inside the opening chamber, preventing the rubber element from functioning properly.
The fourth kick investigated was nothing out of the ordinary, but is presented in this article because of similarities with the other well kicks investigated. Details of this kick include a water depth of about 3,900 ft; a total depth of about 22,400 ft TVD/TMD, a mud weight of 15.0 ppg SOBM, and a kill mud weight of 15.3 ppg.
The arrangements of shakes and possum belly volumes were such that under normal circumstances 80 bbl of mud volume flowed back into the active pit system during connections. The time for the flow-back volume to occur made this kick difficult to detect rapidly.
The estimated initial kick volume was 12 bbls, but subsequent mishandling/misinterpretation resulted in +/-150 bbl of influx. In four of the five kicks reported, the MWD quit working just prior to the kick. The decision was made to drill ahead without it. Also, the hole was packing off in each case, resulting in the pipe becoming stuck.
Small pressure margin
The fifth kick investigated was where pore pressure equaled the fracture gradient. Deepwater drilling offers many challenges, and one of the most significant is how to successfully drill wells with a small margin between pore pressure gradient and the fracture gradient. Hopefully, the research being done on riserless drilling and related areas will make drilling in extreme water depths more routine. The details of this kick include a water depth of about 4,200 ft, a total depth of about 20,000 ft MD/19,400 ft TVD, a mud weight of 14.1 ppg (SOBM), and a pit gain of 20 bbl.
Casing had just been set at 19,200 ft TVD and a LOT of 15.2 ppg was obtained (194 units of gas was reported from the LOT bottoms-up). Ballooning and hole pack-off problems began immediately from drilling out of casing with gas units increasing to 250 units. The resulting low differential of equivalent mud weight with the well shut in. Fracture opening pressure and minimum horizontal stress (fracture closing pressure) caused 135 bbl of mud loss while attempting to bring the pump on-line for the well kill.
Working the drillstring to prevent stuck pipe (hole packing-off) also contributed to the mud loss while circulating the influx from the well. Essentially, drillpipe pressure control was impossible and the well (and unlogged targets) was abandoned.
In each of these incidents, the driller and drill crew detected the kicks and promptly shut the well in with minimum kick volumes (less than 20 bbl in all but one case). However, ballooning, stuck pipe conditions, and mud compressibility issues influenced personnel to open the well and distract attention from the real problem at hand which was well control.