The list of wells drilled using dual gradient includes one drilled in the “eastern Gulf of Mexico,” which could be more precisely described as offshore Cuba.
From a drilling standpoint, the well stands out as the only one using dual-gradient drilling done in water so deep, at 7,500 ft, and it was completed on time by Petronas in a formation where conventional methods had failed, said Robert Ziegler, head of deepwater drilling technology at the Malaysian national oil company.
The well offered a difficult test for a joint industry project that paid much of the cost of the well managed by Petronas. The company is one of the most active users of managed-pressure drilling methods offshore, with as many as five rigs using the technique in southeast Asian waters, but it never required dual gradient in that part of the world.
Petronas chose a form of dual gradient drilling known as EC-Drill because it was suited for working in a low-pressure formation, where tight pressure control was needed to avoid formation damage and fluid loss. The system reduced the pressure exerted by the fluid returning up the riser by using a pump connected to the riser about 1,000 ft below the drilling rig. By adjusting the pump speed, it could quickly change the bottomhole pressure. Sensors in the riser allowed drillers to track those changes as they occurred.
Speeding the pump lowers the fluid column and reduces effective pressure; slowing it can raise the column and the pressure. Either change can be made in minutes. Making a similar change by adding drilling mud would take hours to circulate throughout the well, Ziegler said.
While drilling, Ziegler said the crew observed that changing the pump rate could affect drilling performance. Increasing the pressure significantly slowed drilling, while decreasing the pressure speeded penetration equally, he said.
One question facing the EC-Drill method has been: Will natural gas escape from the drilling mud and accumulate in the open space inside the riser? The answer on that well was “no gas was recorded in the top of the riser at any stage during the operation,” according to an SPE paper.
During cementing, the managed-pressure control systems allowed a pressure reduction that made it easier to get the mud to rise to the planned height, Ziegler said.
The EC-Drill equipment was installed for the job using a 2-month trip by the semisubmersible rig, Saipam’s Scarabeo-9, from Singapore to the Gulf of Mexico. It successfully drilled three wells at the prospect, said Roger Stave, senior technology adviser at Enhanced Drilling, which provided the drilling equipment.
“We were able to reach TD (total depth), thanks to the system’s ability to remove the weight that would have been added by the weight of a fluid in the riser in a weak formation,” Ziegler said. “There were no well control incidents and no lost circulation in a place where others were fighting losses.”
