Dual-gradient drilling (DGD) technology could significantly enhance deepwater well economics by reducing the number of casing points and increasing production rates.
BP, the world's second largest oil company, believes the application of this technology will enable the penetration of ultradeep exploration targets and achieve the desired casing size across production intervals.
It believes that limited to conventional single-gradient drilling techniques, many deep geological objectives will remain unreachable or be drilled with no contingency casings to combat such problems as shallowwater flow, lost circulation and well control incidents.
Technology requirements
DGD consists of hardware and software equipment, operating procedures and professionals trained to manage wellbore pressures.
The technology provides the required hydrostatic pressures at total depth while reducing the level of excessive hydrostatic pressure imposed on shallower formations. Reducing the pressure on these formations and at the previous casing allows drilling to proceed with a greater kick tolerance and greater safety margins.
According to BP, the key to this technology is a subsea pumping system installed at the seafloor that provides the energy necessary to lift mud from the annulus back to the surface. These subsea pumps control the pressure of the annulus at the mud line, typically maintained slightly (50 psi) above the pressure of the surrounding seawater.
BP's Gulf of Mexico deepwater development business unit is committed to the use of DGD on its Crazy Horse development project. Said to be at least a 1 billion-boe find, Crazy Horse is in the Boarshead basin in Mississippi Canyon Block 778, in more than 1,829m
(6,000ft) of water.
DGD is, according to BP, expected to produce cost savings of around US $9 million per well. This could potentially increase the project value by $150 million to $250 million. This is dependent upon the final number of wells drilled and the specific cost of integrating a system into the rig of choice.
Several benefits can be achieved using DGD, including:
• a reduction in the number of casing strings required to reach the geological target. Up to three casing strings can be eliminated, saving about $2 million to $3 million per casing string;
• an increase in the margin between pore pressure and fracture gradient, thereby reducing instances of lost circulation;
• an improvement in operational efficiency;
• a significant reduction in overall well trouble time by allowing use of new gauge hole drilling technologies (for example, rotary steerable and multilateral technologies);
• the drilling of larger bores through pay sand for high-rate, large-tubing completions; and
• less harm to the environment. The riser is no longer filled with mud, therefore an emergency disconnect will result in a minimal release of mud to the environment. In addition, shallow casing strings will not have to be "pushed" by employing riserless drilling with heavy mud techniques.
BP claims it is committed to delivering the first commercial DGD system for use on the Crazy Horse project by the end of the fourth quarter of 2002. The company's DGD expert, Andy Frazelle, said, "We are actively participating in two projects involved in the development of DGD - the subsea mudlift drilling (SMD) joint industry project and DeepVision."
Subsea mudlift
The Conoco-led SMD project uses positive displacement chamber pumps designed and manufactured by Hydril, and project funding is provided through BP's High Value Wells Technology theme. Additional participants are Chevron, Texaco, Diamond Offshore, Global Marine and Schlumberger.
This innovative project involves a pumping system to be used in a 1,150ft (351m) shallowwater test in March. The field trial is designed to be a cost-effective test of the SMD pump system and further proof of the concept.
The Hydril diaphragm pumps are powered hydraulically by seawater from the rig through a high-pressure line attached to the riser. This approach simplifies equipment requirements by minimizing electric cables, surface-mounted reels, subsea electric motors and subsea hydraulic supplies.
DeepVision
A limited liability company comprised of Transocean Sedco Forex and Baker Hughes Inc. runs DeepVision. The technology uses a centrifugal pump designed by DeepVision in cooperation with National Oilwell. BP's Gulf of Mexico development business unit has committed the financial resources necessary for project finance together with Chevron, Transocean Sedco Forex and Baker Hughes. This project is earmarked to conduct component testing during the second quarter of 2001.
DeepVision uses electrically powered, staged centrifugal pumps. These pumps are capable of reducing the mud line inlet pressure to below that of an equivalent seawater column. National Oilwell has fabricated and tested three of these pumps on their flow loop. The next phase requires the manufacture and testing of a complete pump, motor and all peripherals required for a commercial system.
And the future?
BP plans to continue the development of DGD technology through its involvement in these projects following two distinct solution paths until proof of concept is achieved. Maintaining this involvement minimizes the risk to delivery, provides maximum assurance on product development, maintains
development momentum and provides safeguards against early selection of a suboptimal solution.
The timeline for delivery of a DGD system is 18 months upon placement of an order. It is, Frazelle said, anticipated that the rig selected to receive the first unit will be chosen in the first quarter of this year, and the placement of an order for a DGD system will occur at the end of the second quarter. n
Editor's note: Information for this article came from BP's in-house publication Well Connected and author Andy Frazelle.