Interim solution reins in environmental problem,
cuts reliance on fluid weight and mud cake
William FurlowTechnology Editor
Shallow Water Flow Diverter JIP
A joint industry project (JIP) is underway to develop a subsea diverter to overcome the water flows from shallow zones problem of hampering deepwater exploration in the Gulf of Mexico. Allen Gault, Staff Engineer for Conoco, was elected project administrator after drawing the high card among the four original partners.
Gault said the diverter will take the form of a modular tool that can be used with different methods to control shallow water flows. As solutions go, back pressure might be an acceptable solution in situations where the flow zone is close to the previous casing shoe, but this approach might not be appropriate if two shallow water flow zones are exposed that had significantly different pore pressures and fracture gradients.
An operator using seawater as a drilling fluid would have to use the wellbore filter cake to prevent the higher pressure zone from charging up the lower pressure zone. If this were the case, Gault said, the operator would have to rely on mud weight to maintain fluid overbalance downhole and control the flows. The shallow water flow diverter envisioned by Gault's JIP team would also assist in the dynamic kill process.
Diverter designThe design of this first diverter will be very straightforward and would apply to the first scenario above. Ultimately, the answer would be to use the diverter in conjunction with a mud-lift system, similar to that proposed for riserless drilling. This technology is several years away, so a scaled down version that can be developed within two years is envisioned.
Gault said the JIP has four founding members - Amoco, Shell, Conoco, and Chevron - but is looking for a total of eight, with a participation of $120,000 each. The deadline for JIP participation is March 31, he said.
As Gault envisions it, the diverter would be installed on the structural casing, providing back pressure so the operator, drilling with seawater, could pass through a shallow water flow zone while maintaining an overbalance fluid condition. The back pressure on the drilling fluid would be regulated by choke valves on a pair of 10-in. flow lines at the top of the diverter.
These chokes could be adjusted using a remotely operated vehicle. Gault said the center of the diverter would turn with the drill string, while the outer portion would remain in place over the annulus and casing string.
The total outlay for this joint industry project will be around $800,000, depending on the number of participants, Gault said. Shallow water flow control costs are about $7 million, according to DeepStar data.
To ensure the new device will work with any system being used in deepwater, Gault said a total of six vendors have been invited to participate in the equipment design portion of the project: FMC, Dril-Quip, Hydril, Shaffer, Cameron, and ABB Vetco Gray. These six will be given an opportunity to present their concepts, either as individuals or in partnership.
Ready in one yearIf everything goes as planned, Gault said the first diverter prototypes will be ready for field tests within a year. He said it is not certain at this point if there will be more than one design to test, it depends on the work done by the equipment providers. But Gault is confident that this will be the first step toward a more integrated diverter system to overcome shallow water flow problems in deepwater.
If successful, this JIP would offer an attractive advantage over the current solution which involves pumping heavy mud down hole to maintain overbalance. Basically, the drilling fluid is run through the drillstring and out the annulus, carrying the cuttings onto the sea floor.
This has been successfully applied on many occasions, but the environmental risks alone preclude its use in the Gulf of Mexico. Regardless of the environmental issues, this is an expensive approach. The operator is out of pocket the full cost of the mud.
Also, the use of one uniform weight of cheap mud carries its own set of risks in these delicate formations. There is no opportunity to alter the weight or other properties of this mud because it is transported in bulk and pumped downhole one time. Though it has worked in the past, it is a sledgehammer approach and has, at best, limited application.
The ultimate solution, Gault contends, is a combination of the two approaches - using a heavy mud in combination with a diverter and a mud-lift apparatus. This version of the diverter would feature flowlines attached to and controlled by the surface, so that the choke valves could be readily adjusted, and the mud would be pumped back to the surface. Gault said this approach would rely on the principles of riserless drilling and he predicts it is the best long-term solution to this deepwater problem.
In the meantime, however the Shallow Water Flow Diverter JIP will offer a quick, inexpensive solution that could be available within the year, and will give operators some relief while more advanced solutions are developed.
More information on the diverter JIP can be obtained by contacting Allen Gault (E-mail:firstname.lastname@example.org) and Tel: US (281) 293-3338.
Copyright 1998 Oil & Gas Journal. All Rights Reserved.