Study evaluates deepwater gulf overpressure

By OGJ editors
HOUSTON, Feb. 15
-- A recent study of 149 deepwater wells in the Gulf of Mexico found dramatic differences in the overpressure of the Lower Tertiary Wilcox play.

IHS Inc. and GeoPressure Technology Ltd. made the study to gain an understanding of overpressure in the deepwater gulf in terms of its distribution and potential impacts on future exploration of the region’s petroleum system.

The wells studied were in the Alaminos Canyon, Keathley Canyon, Walker Ridge, Garden Banks, Green Canyon, and Atwater Valley areas. Water depths ranged from 4,000 to 10,000 ft. Some of wells had vertical TDs greater than 30,000 ft and reservoir pressures in the Wilcox greater than 20,000 psi.

Forty-nine of the wells studied penetrated the Wilcox reservoir with the remainder encountering either Lower Miocene sediments or were used to supplement the fracture pressure and mud-weight analysis.

IHS estimates that the upside recoverable oil potential of the Lower Tertiary Wilcox trend is 15 billion bbl.

The study found that all the wells with formation pressure data indicated overpressure with overpressures ranging widely from 9.1 ppg to 15.7 ppg equivalent mud weight across the Miocene and Wilcox. The highest Wilcox overpressures were primarily in eastern Keathley Canyon and across the Walker Ridge areas.

The study analyzed the Wilcox data with respect to compartmentalization vs. lateral drainage as an explanation for the overpressure variation observed and also found that the overpressure in the Miocene affected the subsalt play.

The study noted that the area includes high-profile discoveries including Petroleo Brasileiro SA's Cascade, Chevron Corp.'s St. Malo, and more recently BP PLC’s 2009-10 Tiber well in northwest Keathley Canyon, which are expanding the extent of the play. The play also has had problematic and costly dry holes such as Unocal Corp.'s Sardinia on Keathley Canyon Block 681 and ExxonMobil Corp.'s Hadrian well on Keathley Canyon Block 919.

Sam Green, principal technical author of the study for GPT attributed the dry holes to the industry not fully understanding the play in terms of the petroleum system and its impact on the distribution of hydrocarbons.

“Part of this problem lies in the fact that the majority of this play is subsalt, with the inherent problems of using seismic data to visualize structures below the salt canopy. Even if traps are identified, and the risks assessed for reservoir quality and charge, variable overpressure regimes exist in the subsalt, which can make drilling these prospects problematic,” Green said.

Green added, “As part of this study, we developed a unique set of algorithms to calculate overburden gradients and fracture gradients across the many protraction areas, which aids engineers in the well design and planning process.”

According to IHS, the US Gulf of Mexico had 6,269 actively producing wells at the end of October 2010. Of these 486 were deepwater wells (wells in 1,312 ft or greater water depth).

So far in 2011, the US Gulf of Mexico has produced 820 million boe, of which 516 million boe or 70% has come from deepwater wells, IHS noted.

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