DRILLING TECHNOLOGY Shallow gas flows trouble drillers waiting on cement to strengthen

Cautionary warning issued in US Gulf of Mexico in wake of recurring drilling event

An alert cautioning drillers in the US Gulf of Mexico to await the necessary amount of time for cement to develop sufficient compressive strength has been issued by the US Minerals Management Service (MMS). The MMS has recorded a number of similar shallow gas flows resulting from this downhole problem and has pointed to a flaw in its own regulations regarding waiting-on-cement.

The MMS issued Safety Alert No. 165 regarding this problem and sponsored a workshop in May to deal with the emerging problem.

In each event, the diverter system was partially nippled down before the cement had developed sufficient compressive strength and drilling resumed. Shallow gas flowed through the unconsolidated cement and entered the wellbore, creating control problems for the drillers.

MMS regulations require the following:

A 12-hour period for waiting on cement to develop compressive strength before drilling out the casing strings. This waiting time does not apply to the nippling down of the well diverters.
The cement must have a minimum of 500 psi compressive strength in the bottom 500 ft of the well before drilling out the casing shoes.

In many of the cases recorded, diverters had been partly nippled down in less time than 12 hours after the plug was bumped. When gas flow began, drilling crews were able to re-connect the diverters.

Recommendations

The MMS advisory had a number of recommendations for operators and drillers:

Select a drilling location that is not directly above a seismically visible shallow gas accumulation.
When shallow gas is likely to be encountered, the driller should have a complete plan on hand to deal with any number of resulting problems.
Improve the cement properties, including density, fluid loss, transition time, and compressibility, to minimize chances of shallow gas flow.
Plan for optimum cement column length, using the appropriate guidelines.
Consider holding a slight back pressure on the annulus after the cement is in place, and to consider shutting in the well rather than diverting any flow. Based on result of leakoff testing of the conductor casings shoe, the amount of back pressure that could be safely applied should be calculated.

Four incidents

The MMS cited four specific incident where the gas and cement flow got out of control:

Incident No. 1: After the wiper plug was bumped, drilling crews pressure tested the casing and poured sugar water on top of the mud line hanger. "The well was static for four hours. The operator commenced to nipple down the diverter system.
"Two hours later, the well started flowing gas and cement out the open diverter. The diverter was closed, and the loosened bolts that attach the diverter to the conductor casinghead were re-installed. Efforts to stem the flow by pumping saltwater through the conductor casing valve while allowing the well to unload out the two 10-in. diverter lines were unsuccessful.
"About 1/2 hour after the well started unloading, flow began coming from the annulus between the drive pipe and the conductor. The rig was evacuated and several attempts were made to regain control in the following days.
"Six days after the flow began, the flow rate decreased and the rig was able to move off location. Four days later, the drive pipe, conductor, and surface casings fell to the seafloor and the flow cased.
The next day, a subsea inspection by a remotely operated vehicle revealed a 25-ft crater at the base of the drive pipe with no bubbles or flow observed. Five months later, another rig moved on location and successfully completed abandonment of the well."
Incident No. 2: In mid-1994, drilling crews were spotting water mixed with lignosulfonate (ligno water) after washing the annulus through wash ports on the mudline hanger. "After the 20-in. surface casing was cemented, the annulus was washed by circulating through ports on the mudline hanger and the ligno water was spotted.
"After a gyro survey was run, a diverter line was opened to check for flow. Flow was noticed on the annulus and the diverter line was closed. The annulus pressure was alternately bled off and allowed to build up during the next day.
"During the next two days, unsuccessful attempts were made to place heavy mud into the annulus by injection and by circulating through the mudline hanger. The well was finally killed by placing 3/4-in. tubing into the annulus and circulating heavy mud. Operations were resumed three days after the flow began."
Incident No. 3: In another case, Nippling down of the diverter was begun seven hours after the plug was bumped and the float valves checked "The diverter and diverter lines were rapidly reinstalled when flow began.
"The well was shut in with a surface pressure of 50 psi, and was killed by lubricating mud into the annulus between the conductor and surface casings. The annulus was then successfully grouted, and normal drilling operations were resumed 5-3/4 days after flow first began."
Incident No. 4: Two hours after the plug on the surface casing was bumped, drilling crews nippled down the diverter system and installed the casing slips. "A slight gas flow was ignited by a welder making a rough cut on the surface casing 1/2 hour later. The gas was flowing from a casing valve that was open so that the annulus could be monitored.
"The flame was extinguished with no damage or injuries when the casing valve was closed. However, when the valve was later re-opened, mud and gas flowed from the well. The valve was closed and the diverter was reinstalled on the wellhead.
At this point, the seal on the casing slips failed, and 15-20 bbl of mud and gas were discharged through the diverter line.
When fluid slowed to a small stream, a line was connected to the casing valve, and 3-3/4 bbl of mud was pumped between the 13-3/8-in. casing and the 20-in. casing. The well was monitored for 16 hours and 6.9 bbl of mud were recovered.
Final flow rate was 2-1/2 gallons/hour and at this point, the crew prepared to resume operations.