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Dual gradient drilling defies ultradeep challenge
At least three industry R&D projects seeking best technology
by Dev George, Managing Editor
(6/8/01) As exploration and production move farther and farther beyond the land and the continental shelf and into ultra deepwater environments, the challenges of drilling and producing hydrocarbons at great depths becomes exponentially greater. Foremost among these challenges is the problem of overcoming the hydrostatic pressure in the marine riser generated by the mud column, which frequently necessitates the setting of additional casing to maintain a balance between a formation's pore and fracture pressure. And, this in turn, not only adds to the development costs, but often reduces the efficiency of the wellbore by decreasing its diameter and, consequently, its production capacity - sometimes to such an extent that the well becomes uncommercial even before reaching its target.
Single vs. dual-gradient mud system. From the perspective of the well, there is no mud above the mudline in a SubSea MudLift system.
Dual gradient drilling
In the Gulf of Mexico and the Atlantic's Angola Basin in West Africa's Gulf of Guinea, young, rapid depositional basins where the margin between high pore pressures and low fracture resistance pressures require adding casing in both the upper hole and the pressure transition zones, the cost of an ultradeep well (6,000 ft or more) can be US$30-50 million or more, and that without certainty that its objectives will be reached. In fact, the likely bottomhole size is only 6-63/4 inches - too small to permit horizontal or multilateral completions.
Dual gradient drilling (DGD) is perhaps the savior of ultra-deepwater development. Contrary to today's single gradient drilling technology, in which control of bottomhole pressure is achieved with a mud column from the well's bottom back to the rig, dual gradient drilling does the same thing by using the mud as far up from the bottom as the mudline, and then seawater the rest of the way to the surface. Not only does this method eliminate as many as four strings of casing, but it makes it possible to drill in almost any water depth and reach the well's objectives with a bottomhole diameter of about 12 inches - large enough to permit 7-inch casing up to the mudline and provide for both horizontal and multilateral completions. And, there is a probable savings of $5-15 million on the well.
Three approaches to DGD
At least three major industry efforts are focused on providing dual gradient drilling systems to meet the challenge of ultra-deepwater development. Each addresses the means of directing the fluids in the system to the surface in similar but mechanically different ways. Generally, upon reaching the mudline, they send the fluids into pumps that separate and direct them to different risers, the hydrocarbons and water through the drilling riser annulus, the mud and its cuttings to a separate riser to the surface.
Diamond Offshore's New Era to drill first DGD well.
Shell E&P - Subsea Pumping System
Shell E&P has emulated the single gradient drilling system in its Shell Subsea Pumping System (SSPS). Here, however, before being pumped back to the surface, the fluid is processed and anything larger than a quarter inch is separated and discharged near the seafloor, while a mud-gas separator takes off the gas. An electrical submersible pump then provides the pressure to send the remaining fluid to the wellbore.
Conoco JIP - SubSea Mudlift Drilling
Conoco, on the other hand, is leading a joint industry project (JIP) to develop a dual gradient drilling system it has dubbed the SubSea MudLift system. Partners include the project designer Hydril, as well as BP, Chevron, Diamond Offshore, Global Marine, Schlumberger, and Texaco. Unlike the Shell SSPS, the drilling mud is removed from the riser with triplex pumps at the seafloor, then it is filled with seawater to lower the riser load. Furthermore, fewer casing strings are required.
The SubSea Mudlift Drilling System.
DeepVision JIP
DeepVision, another joint industry project focused on perfecting a viable dual gradient drilling system, is lead by Baker Hughes and Transocean Sedco Forex, in cooperation with BP and Chevron. It is very similar to that of the SubSea Mudlift Drilling System, except that it uses centrifugal pumps to separate the mud at the seafloor and send it surfaceward. Both systems profess to be zero discharge.
The two JIPs' dual gradient drilling systems are being tested and will probably be marketed by the beginning of the third quarter 2001, for delivery after about 18 months. The SubSea Mudlift Drilling System is set to be tested aboard Diamond Offshore's semisubmersible New Era on a Texaco well in 900 ft water depth.
Dual gradient riser
Not to be left out of the game, the Petroleum Equipment Division of Stewart & Stevenson introduced the first riser system specifically designed for future dual gradient, ultra-deepwater environments last fall.
Its Dual Gradient Drilling Mud Return Marine Riser System (DMRS) is designed to take advantage of dual gradient drilling and underbalanced drilling without sacrificing the ability to be used as a conventional drilling riser system, by incorporating a large diameter drilling fluid return line and a new Stewart & Stevenson riser isolation tool into the drilling riser system. Both the DMRS riser system and the SSIT riser isolation tool are the subjects of pending patent applications.
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