Michael Sumrow
Drilling Editor
Oil & Gas Journal
NEW ORLEANS, Oct. 1 -- Texaco Inc. is drilling what it says is the first dual-gradient well using a subsea-mudlift drilling system developed by a joint industry project.
The Shasta field No. 8 is in 910 ft of water on Green Canyon Block 136 in the Gulf of Mexico.
Ken Smith of Conoco Inc., the project manager for the joint industry project, presented a review of the subsea-mudlift drilling system field test during the International Association of Drilling Contractors annual conference here.
Goal of the 5-year, $50 million project was to find a way to eliminate the numerous casing strings required to drill many offshore wells with single mud-gradient drilling (OGJ, Aug. 16, 1999, p. 32).
Multiple casing strings are needed due to the narrow margins between formation pore pressures and the fracture gradients. Some offshore wells, particularly in deep water, cannot be drilled using single mud-gradient drilling due to this narrow pore pressure, frac gradient window.
Requiring a large number of casing strings will either result in a hole size at the targeted formation that is too small to perform necessary completion work, or an excessively large initial hole-size that is uneconomic.
To solve the problem, the industry has been trying to find a way of effectively putting the rig on the sea floor. This means eliminating the mud gradient, which normally exists in the drilling riser, and replacing it with a seawater gradient.
Bob Rose, chairman and CEO of Global Marine Inc. said, "This is the biggest change our industry has seen since we put the BOP on the sea-bed."
Smith said, "At the beginning of the project we agreed that we wanted a 'green' system or one in which all fluids, gas, cuttings, everything (from the well) returns to the surface where we can process it."
He said the subsea-mudlift system had to be simple and reliable with proven components. It had to have redundancy, be light, small, and adaptable to a large number of rigs.
He said that a complete dual-gradient solution was required, which is a lot more than just a "widget." It also required knowledge to safely and efficiently use the "widget."
Smith said they did not want the subsea-mudlift pumps to be a hydraulic bottleneck in the wells. The system uses diaphragm pumps. Seawater is pumped from the rig, down to the pumps on the sea floor, which in turn displace the mud back to the rig, discharging seawater into the sea.
One of the first components the team had to develop was a drillstring valve, installed just above the bottomhole assembly. Its purpose is to hold the mud column up, preventing the drilling mud in the drill pipe from U-tubing around and into the drilling riser.
A subsea-rotating device, installed at the mud line, has stripping elements to allow the drill string to pass through and into the well. It serves as nothing more than a mechanical barrier between the mud in the well and the seawater in the riser.
"One thing you can't prove in a shop environment is the procedures," said Smith. There are two volumes to track, both the drilling mud system and the seawater used to power the subsea-mudlift pumps.
He said there are a lot of strange and different drilling and well control operations. He remarked that, "About 20 man years of development effort went into the procedures development."
Smith said, "We have successfully delivered dual-gradient drilling technology. The design is proving to be robust. The procedures are proving to be sound. In our view, it's only going to get better."