IN A STUDY to evaluate exploration and production (E&P) drilling risks associated with overpressured formations, it was found that dramatically different magnitudes of overpressure existed across 149 deepwater wells in the US Gulf of Mexico Lower Tertiary Wilcox play.
Understanding the variation and magnitude of overpressure is of critical importance, according to the authors, information and insight provider HIS and pressure consultants GeoPressure Technology Limited (GPT).
It focused on key wells in Alaminos Canyon, Keathley Canyon, Walker Ridge, Garden Banks, Green Canyon and Atwater Valley. Water depths in the region range from 4,000-10,000ft, and wells can exceed 30,000ft total vertical depth sub-sea (TVDss). Reservoir pressures in the Wilcox can exceed 20,000 pounds per square inch (psi).
“Understanding the overpressure variation helps to minimise the drilling risk and inform on future exploration targets,” said Mark Diaz, senior geopressure analyst at IHS, and one of the study’s authors.
“There are a number of operational challenges that exist alongside varying pressure regimes, including high bottom-hole temperatures, complex structural variances and a canopy of salt that sits over much of the play, which makes the use of seismic data to visualise sub-salt structures largely ineffective.”
According to the report, all of the wells with formation pressure data that were studied indicated overpressure, although the degree varied widely. Overpressures in the report ranged from 9.1 pounds per gallon Equivalent Mud Weight (ppg EMW), up to 15.7 ppg EMW, across the Miocene and Wilcox geologies.
Analysis of overpressure in the Wilcox showed that the highest overpressures were primarily in the eastern Keathley Canyon and across the Walker Ridge protraction areas.
The Wilcox data was analysed with respect to compartmentalisation versus lateral drainage, as an explanation for the overpressure variation observed.
The overpressure observations made in the Miocene have a significant impact on the sub-salt play as a whole.
Although a number of high-profile discoveries have been made to date in this play, including Cascade, St Malo and, more recently, BP’s 2009/10 Tiber well in north-west Keathley Canyon, which is expanding the extent of the play, dry holes such as the Unocal Sardinia project in Keathley Canyon (block 681), and the ExxonMobil Hadrian well in Keathley Canyon (block 919) have proven problematic and costly, notes the report.
Despite its prospectivity, the play clearly is not fully understood in terms of the petroleum system and its impact on the distribution of hydrocarbons.
According to GPT: “Part of this problem lies in the fact that the majority of this play is sub-salt, with the inherent problems of using seismic data to visualise 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 sub-salt, which can make drilling these prospects problematic.
IHS says that, if the US deepwater Gulf of Mexico was a country, it would rank eighth in the world in barrels-of-oil-equivalent (boe) discovered from 2000 to 2009.
In terms of oil discovered during the same period, the Gulf of Mexico would be ranked number three in the world.