Drilling advances

Vol. 232 No. 7

A deepwater drilling method even Mother Nature could bless

Some of you may remember the commercial from the 1970s that warned, “It’s not nice to fool Mother Nature.” The ad may have been peddling a substitute for butter but, more than three decades later, in an admittedly circuitous way, its premise also could apply to a potentially game-changing deepwater drilling technology that is being prepped for its long-awaited debut in the Gulf of Mexico early next year.

David Dowell, drilling advisor for Chevron North America E&P, told the quarterly meeting of the Drilling Engineering Association (DEA) in Houston that the operator is moving closer to sending its much-anticipated dual-gradient drilling (DGD) system out for a real-world road test. “We plan to start drilling in the second quarter,” Dowell said of the technology that supporters contend will remove much of the influence of water depth on well design.

 For the past three years, Chevron has spearheaded a development team that includes GE and Norway’s AGR Subsea to refine dual-gradient drilling for the construction of wells in as much as 10,000 ft of water. The team actually refined technology that was first examined in 1996 and was field proven but later was shelved, as industry conditions at the time precluded the enormous investment required for its full-scale deployment.

Unlike the typical single-gradient drilling method, dual gradient introduces fluid with a seawater-equivalent density of 8.6 lb/gal in the riser, with weighted mud only coming into play from the mudline down. The result is formation pressures “that are closer to the way Mother Nature laid down these formations and what it normally sees,” Dowell said.

With “safer and cheaper” the mantra of the day in deep water, he said DGD more than fits the bill on both counts, pointing out its capacity to deliver near-immediate response to changing pressures and the opportunity for eliminating two and possibly more casing strings. Dowell said Chevron is confident the optimism of DGD enthusiasts will be realized shortly after the newbuild Pacific Drilling drillship Santa Anna sails into the Gulf of Mexico in January 2012. The rig was still under construction in early 2010 when Chevron inked the contract that led to the addition of pumps, riser modifications and other changes that were needed to accomplish dual-gradient drilling.

Since drillers first began poking into deeper waters, wells have been drilled using a single pressure gradient, namely a marine riser filled with weighted, and usually premium, synthetic-based drilling fluid that basically does nothing but take up space and puts unnecessary hydrostatic pressure on the wellbore. There’s also the matter of cold water temperatures, which create havoc with the rheological profile of static fluid.

Enter dual-gradient drilling. Here, the weighted mud is removed from the riser and replaced with a fluid closer to the density of seawater, which generates less pressure, not to mention being not so premium in cost. In this scenario, the seawater-equivalent fluid density in the riser and the surrounding pressure at the mudline combine to create the hydrostatic head at the seafloor. The actual working fluid is confined strictly to the wellbore where it belongs, much like in a shallow water or onshore well. “Basically, what you are doing is introducing two different fluids in the wellbore. One is the equivalent of seawater in the annulus above the mudline, and then mud is run in the lower hole and pumped back to surface, which sets up the dual gradient. This does several things for us,” Dowell told the DEA.

One thing it promises to do is allow casing, particularly in the upper hole sections, to be set deeper in the well, conceivably eliminating at least two strings. “When you can eliminate a casing string, it gives you better clearances and the cement integrity goes way up,” Dowell said. By separating the static and lower-weight non-drilling fluid in the riser from the higher-density circulating mud in the wellbore, it becomes much easier to reduce bottomhole hydrostatic pressures to pre-determined levels and maintain them, he said.
“We’re able to react to changing pressures in a matter of minutes rather than hours,” Dowell added. “Also, if I can match the bottomhole pressure at any point in the well, the pressure gets considerably lower and a lot of the lost-circulation problems that we fight constantly will go away, as long as we don’t go crazy and try to drill to China before setting pipe.”

Along with a new subsea rotating device that provides the interface between the riser and the drilling fluid, a solids processing unit and other specialty technologies, the heart of the system is GE’s 450,000-lb mud lift pump. As for the rig itself, modifications included six pumps, including three for the seawater, which will provide the power, and three for the active drilling fluid. The riser also required a major makeover, with two new 6-in. lines, one for seawater and the other for returning spent mud to the surface.

In the beginning, plans to use only inactive seawater in the riser had to be modified, Dowell said, as it was determined that the risks of hydrates, corrosion, drillstring harmonics and other issues required some type of fluid other than pure seawater. Consequently, he said Chevron has asked M-I Swaco to develop an alternative, which basically will entail spent synthetic-based mud, modified slightly and reduced through a centrifuge to the 8.6-lb/gal density requirement.

While the technology will first be field tested in an unidentified development well in the deepwater Gulf of Mexico, Dowell said its primary advantages will be realized in exploration drilling. “The versatility you get with this technology is unmatched, which makes it ideal for exploratory drilling, where we always run into things we don’t expect.”