Offshore staff
HOUSTON--In a study designed to evaluate and address exploration and production (E&P) drilling risks associated with over-pressured formations, it was found that dramatically different magnitudes of overpressure existed across all 149 deepwater wells studied in the highly lucrative deepwater U.S. Gulf of Mexico Lower Tertiary Wilcox play. Understanding the variation and magnitude of overpressure is of critical importance, according to the authors of the study. They include officials from information and insight provider IHS and pressure consultants GeoPressure Technology Ltd. (GPT), an Ikon Science Co.
TheIHS/GPT Deep-Water Gulf of Mexico Lower Tertiary Wilcox Pressure Study sought 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 study wells came from the IHS Pressure Database, a global dataset of re-interpreted, quality controlled, subsurface formation pressure data.
The study focuses on key wells in Alaminos Canyon, Keathley Canyon, Walker Ridge, Garden Banks, Green Canyon, and Atwater Valley, a combined study area that spans offshore acreage covering hundreds of miles across offshore Texas, Louisiana, and Mississippi in US federal waters. Water depths in the region range from 4,000 to 10,000 ft and the total depths of the wells can exceed 30,000 ft TVD subsea. Reservoir pressures in the Wilcox can exceed 20,000 psi.
The Lower Tertiary Wilcox trend is a highly successful exploration target, with considerable upside potential of up to 15 Bbbl of recoverable oil reserves, according to IHS.
“What we learned from the study is that overpressure variation has a significant impact throughout the play. Understanding the overpressure variation helps to minimize the drilling risk and inform on future exploration targets,” said Mark Diaz, senior geopressure analyst at IHS, and one of the study 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 visualize subsalt structures largely ineffective.”
“This study gives clients a set of analytical tools and supporting insight that enables them to make sound exploration decisions and minimize the potential risks associated with drilling when they have poor visualization of the subsurface,” said Sam Green, Ph.D., principal technical author of the study.
According to the report, all the wells with formation pressure data that were studied indicated overpressure, although the degree of overpressure varied widely. The overpressures in the report ranged from 9.1 ppg equivalent mud weight up to 15.7 ppg EMW across the Miocene and Wilcox.
“Calculating those pressures correctly is incredibly important,” Diaz said, “since excessive pressures impact every aspect of the planning process.”
Analysis of the overpressure in the Wilcox shows that the highest overpressures are primarily in eastern Keathley Canyon and across the Walker Ridge protraction areas. The Wilcox data were analyzed with respect to compartmentalization versus lateral drainage as an explanation for the overpressure variation. Overpressure observations made in the Miocene have a significant impact on the subsalt 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 northwest Keathley Canyon – 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, the report noted.
“Despite its prospectivity, the play clearly is not fully understood in terms of the petroleum system and its impact on the distribution of hydrocarbons,” said Green. “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.”
As part of this study, the authors developed a set of algorithms to calculate overburden gradients and fracture gradients across the many protraction areas to aid in well design and planning.
According to IHS estimates, if the U.S. deepwater GoM were a country, it would rank eighth with respect to barrels-of-oil-equivalent discovered from 2000 to 2009. In terms of oil discovered during this period, the entire GoM, including the deepwater and shelf regions, would rank number three in the world.
As of the end of October 2010, more than 6,200 wells were actively producing in the US GoM, of which 486 were deepwater wells (wells exceeding 1,312 ft of water depth, according to the IHS E&P Well and Production Database). So far in 2011, based on IHS data, the U.S. Gulf of Mexico has produced 820 MMboe. Of that figure, the deepwater Gulf has delivered about 516 MMboe, or approximately 70% of the total production for the entire Gulf.
02/21/2011
