At what point do we stop drilling for gas and start hitting magma? Operators in the Deep Shelf play in the Gulf of Mexico seem intent on solving that riddle.

I was recently shown a slide that piqued my interest. Constructed by Paul Kelly, senior vice president of Rowan Companies Inc., the slide shows wells that are on Rowan's radar screen in the Deep Shelf play in the Gulf of Mexico. Of the 17 wells that are either currently drilling or expected to spud soon, 15 are scheduled to reach total depth (TD) at 20,000 ft (6,100 m) or below, five are expected to TD at 25,000 ft (7,625 m) or below, and at least four are scheduled to TD at 30,000 ft (9,150 m) or below.

Excuse me, but didn't we used to think of anything below 18,000 ft (5,490 m) as really, REALLY deep, at least in this part of the world? Or, to put a finer point on it, anything between 15,000 ft to 18,000 ft is considered "Deep Shelf," meaning that anything below 18,000 ft must now be considered "Ultradeep Shelf." So what happens past 35,000 ft (10,675 m)? Will we be finding superlatives greater than "ultra"? (Do those exist, I wonder?) Maybe "super-mega" or "beyond awesome" ... Well, anyway, these are mind-boggling numbers, to be sure. And they're even more mind-boggling considering that some of these gargantuan wells were scheduled to have spudded before this article was printed.

ExxonMobil's Blackbeard West, covering several blocks in the South Timbalier and Ship Shoal regions offshore Louisiana, is the first 30,000+ ft well on Kelly's list. Expected to have spudded on or around Jan. 15, 2005, partners in the project include Newfield, BP and Petrobras. The well is being designed to test objectives that range from 27,000 ft (8,235 m) to more than 30,000 ft - Kelly's stats list its TD at 30,000 ft to 35,000 ft.

What are these companies seeing on seismic that is convincing them to spend upwards of US $100 million per well on these high-risk wildcats? Much of it is proprietary data, though Marc Lawrence, senior vice president and division manager for Fairfield, said that some of Fairfield's Deep Shelf 3-D survey, acquired earlier this year, has recently been licensed. He added that some of the operators are also using an old Western Geophysical 3-D survey and often even use old 2-D data for reconnaissance purposes, though they're not picking drilling targets based on it.

Ian Ashcroft, principal research consultant with Wood Mackenzie, said that the size of the targets perhaps allows the luxury of using older technology. "With some targets you need long-offset amplitude variations with offset (AVO) or 3-D," he said. "But maybe when the targets are this big you can see them on 2-D. You only need to refine your view of the prospect if it's that big."

Still, this isn't the low-hanging fruit that was being plucked on the basis of good seismic in the past. Once again the operators are pushing the technology envelope to chase something exciting. Rowan has constantly upgraded and redesigned its drilling fleet to meet the increasing depth demands of its customers. Its Gorilla-class rigs were designed to operate in harsh environments such as the North Sea and the eastern coast of Canada. With the Gulf's relatively benign weather (after hurricane season, of course) some of these considerations, which add considerably to the dayrate, were scrapped in favor of bulking up the rigs' ability to offer high lift and pump capacity. The result was the Tarzan class, and Kelly said that of the 22 wells below 18,000 ft drilled in 2003, Rowan drilled 64% of them.

Already these rigs are undergoing further modifications as the learning curve kicks into high gear. And meanwhile the companies that provide downhole tools such as logging equipment are scrambling to develop tools that can withstand temperatures upwards of 400?F (204?C) along with potentially corrosive fluids. Rowan has used a mud cooling system that will chill the mud prior to sending these tools downhole.

Further modifications will obviously be needed. "The size of the prize is big, but it is tough to get down there," Ashcroft said. "We can just about drill it now, but it's difficult to evaluate it. There aren't many tools that can be run at those depths.

"They're also going to need specialized downhole completions, H2S treatment facilities and maybe new pipelines to shore."

How low can we go? It depends on the objective. Wells have certainly been drilled deeper than 35,000 ft, though usually not with the goal of producing hydrocarbons in mind. Most agree that the deepest well in the world is on the Kola Peninsula in Russia. It took the Russians 20 years to drill to almost 40,000 ft (12,000 m), not a particularly good rate of penetration. The cost was similar to the Blackbeard estimate; rumors that the well entered the Earth's hollow center and that screams of the damned could be heard through the well bore are probably exaggerated.

It will be interesting to watch events unfold as these wells, which can take up to a year to drill in some cases, unlock the secrets of the Deep Shelf. But it might be years before the potentially vast resources can truly be tapped. "This country needs the Deep Shelf," Lawrence said. "But the model is going to be totally different [from exploration in the Gulf in the past]. It's very expensive, and many of these operators are going to sit back, watch the other guys and learn from their mistakes.

"This is an historical well," he added. "If they can produce at those depths, it will definitely be a world record."

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