In the mid-1990s, dual-gradient drilling (DGD) created a stir as a promising technology that could potentially bring deepwater objectives to fruition that previously were inaccessible. However, a lack of cost-effective DGD processes slowed down the technology's initial progress.

The basic concept of DGD is to increase the margin between fracture gradient and pore pressures in deepwater wells using two fluid gradients. This objective is accomplished by rerouting the mud return. Drilling mud is pumped down the drill string as usual, but rather than using the marine riser annulus for mud return, a parasite line is used to circulate the drilling fluid and cuttings from the seabed to the surface. The annulus above the mud line is then filled with seawater to maintain proper hydrostatic pressure at critical depths downhole. Mud will still move through the annulus but in a very limited distance from the bottom of the hole to the pump on the sea floor. This capability would reduce the number of casings needed to reach total depth.

Filling the annulus above the mud line with seawater rather than drilling mud produces a greater spread between the pore pressure and the fracture gradient pressure over greater depth intervals to extend casing shoe depths and reduce the number of casing strings required.

True dual-gradient drilling "tricks" the well bore into thinking the rig is sitting on the seafloor. All the annulus returns see at the mud line is seawater gradient, not typical mud and cuttings gradient.

Potential benefits

Realizing the potential was there to breach the current depth limit in offshore drilling, the industry took the step to develop the technology and related processes with such initiatives as the Hydril-Conoco SubSea Mudlift Drilling Joint Industry Project (JIP), the Shell Dual-Gradient Project and the DeepVision JIP. Among these projects a series of objectives was established as well as the potential benefits that could be realized from DGD, which include the following:

• Reach targets at virtually any depth.

• Reduce casing strings and still maintain viable commercial objective in the reservoir.

• Increase drilling efficiency.

• Complete big-bore wells.

• Decrease riser loads to enable older-generation, lower-day-rate-cost floaters and drillships to operate in deeper environments.

• Reduce mud use.

Although clear objectives were established and research and development produced equipment and methods to implement DGD in field trials, full commercialization has yet to be realized. These solutions lacked the necessary practical and economic sense required for commercial development. The main obstacle to a marketable process proved to be the bottom line. Variations of DGD are much more practical. They won't necessarily eliminate as much excessive overbalance as true DGD, but they don't cost as much, either.

Applying DGD lessons

The true value of DGD may not lie in its actual application at this time but in the technology that it has spurred to achieve results that are similar but less cost-prohibitive. Lessons learned from the industry-initiated DGD projects have been applied to established drilling processes to increase efficiency and rate of success - specifically, the closed, pressurized mud-return systems needed and used for underbalanced drilling (UBD) and managed pressure drilling (MPD).

MPD has taken its cues from DGD for offshore applications. Although MPD or variations of it have been used in land operations for decades, its use has only been applicable in offshore operations since the mid-1990s. This method uses a closed and pressurized mud-return system that restricts the production of hydrocarbons during drilling operations. It also allows for more precise management of bottomhole pressure (BHP) without interruption of operations.

The impetus for offshore MPD came about with the development of different sizes, designs and pressure-containment capabilities of the rotating control device (RCD). The typical application mounts the RCD on the marine riser in the moonpool of a floating rig. If the drilling operation uses a surface blowout prevention (BOP) stack, the operation also includes an annular BOP. With this configuration, the RCD is mounted on top of the annular BOP.

The more robust RCD also became a key factor in successfully initiating UBD operations in deep water with mud and nitrified fluids for increased production from easily damaged reservoirs.

An example of a variation of MPD consists of using a nitrogen production unit to generate nitrogen on a fourth-generation drilling rig with a mud returns booster pump. Nitrogen is injected into the booster pump line downstream of the pump itself. The nitrified fluid travels down the outside of the marine riser in the booster pump line to a point where it is injected into the marine riser. With this process the only additional equipment needed is a nitrogen production unit and an RCD.

Such a method of achieving dual gradient may be called a differentially stuck pipe remediation system. Severe lost circulation often results in the drillstring becoming differentially stuck against the well bore. By achieving a dual gradient, the differential pressure downhole, which was causing the pipe to stick in the first place, is reduced.

The DeepVision approach to controlling BHP employs adjustable-speed centrifugal pumps in the return circuit, using the choke and kill lines. Adjusting the amount of lift to reduce the hydrostatic pressure in the annulus allows for more control of the BHP to obtain and maintain the desired level.

The concept of the return line for drilling fluids was coupled with riserless drilling in the SubSea Mudlift Drilling JIP. Then ARG Norway took it a step further in 2003, when it implemented a riserless drilling operation for BP in the Caspian Sea. For this operation, the riserless drilled mud and cuttings were pumped back to the rig for proper recovery and disposal. The environmental sensitivity in this area prompted the process to reduce cost and provide an alternative to circulating the drill fluid and cuttings on the seafloor.

The practice of seabed circulation of the drill fluid and cuttings is common in the Gulf of Mexico, where deepwater deep wells are the norm and seawater-compatible fluids are used. Pete Fontana, director of project management at Weatherford, tagged this process "mud push."

"Using the same riserless drilling techniques, but with weighted fluid, operators can circulate the mud and cuttings to the seabed and set surface casing much deeper at a lower cost," Fontana explained. "A weighted saltwater slurry is compatible with seawater and deemed acceptable by the regulatory agencies. With this process, you get the benefits of DGD for surface casing by extending the casing point as much as five times the required depth."

The rest of the well can then be drilled with MPD techniques. This course of action is more environmentally friendly, allows for better well control and reaps many of the benefits of DGD in the remainder of the well without the enormous financial investment.

Fontana went on to explain that some of the other objectives of the JIP have been addressed with the advent and successful application of expandable casing. This technology has addressed the goal of reaching targets at virtually any depth and still maintaining a viable commercial objective in the reservoir at a fraction of the cost compared to DGD.

Next steps

The idea of DGD somewhat mirrors what happened with the early space program. For some the idea was to establish colonies on distant planets. Although that objective failed to become a reality in the projected time frame, it still remains a future intent. What was gleaned from the space program includes a variety of practical developments and applications of enabling technology, from the miniaturized computer chip to VELCRO hook-and-loop fasteners.

The prohibitive investment required for true DGD may be deemed to have been advantageous when the resulting inventive solutions are tallied. What the industry gained from the lack of a cost-effective DGD process are equipment and processes that are much more versatile in different conditions and a variety of scenarios. And like the space program, DGD is not dead in the water. Today's benefits from this pioneering work are the new tools and opportunities of MPD, and possibly limited UBD applications in some areas, at a fraction of the cost of the original DGD system.

"There could easily be a resurgence of interest in DGD," Fontana said, "especially if it becomes more economically viable as we drill deeper wells in deeper water." With the need for hydrocarbon fuel not waning anytime soon, it's not a question of whether the industry will drill deeper, but when.

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