What lies beneath

Dan McConnell

March 1, 2015

Most of the Mexican side of the Gulf of Mexico remains unexplored. Dan McConnell explores the geotechnical knowns and unknowns in the basin.

Acquiring geotechnical data for platform foundation design in Mexico’s Bay of Campeche
Image from Fugro.

With the historic energy reform transforming Mexico’s petroleum sector, what will oil companies and service providers need to know about working in Mexico?

Fugro has been a major supplier of offshore survey and geotechnical services to PEMEX since 1983, providing positioning, geotechnical engineering, integrated geophysical studies, data collection of ocean currents, site characterization, risk modeling, and earthquake engineering.

Working closely with Mexican oil field services provider Grupo Diavaz, Fugro has produced more than 1500 geotechnical investigations and geophysical surveys both in shallow and deepwater offshore Mexico.

Bay of Campeche

Much of the early work consisted of understanding the site conditions and soils in the Cantarell field, one of the world’s offshore “supergiant” oil fields, located in the shallow waters of the Bay of Campeche.

Oil and gas fields in these waters have been under intense development for over 35 years. New operators entering production sharing contracts in these developed areas as a result of the energy sector reforms will need to understand where the existing man-made infrastructure is. Dense developments in these areas require the use of dynamically positioned vessels, instead of anchored work barges, in order to avoid interference. An accurate record of surveyed infrastructure is essential before launching any operations.

In contrast with the US shelf areas, where operators need to be concerned with highly mobile soils and punch-through hazards from shallow sands, the Bay of Campeche soils are calcareous. Foundation designs that are successful in the northern Gulf of Mexico are not adequate for calcareous sediments, in which the bearing capacity of piled foundations can be significantly weaker. Indeed, not only are the soils generally weaker, but in certain layers they are also prone to cementation along certain discrete formations where there is more sand content.

Knowing the distribution of these subsurface layers and the juxtaposition of geotechnical properties is critical for site specific foundation design. The main derivations, pile drivability and end-bearing capacity under vertical and lateral loads, are critical installation and design parameters that are calculated from laboratory analysis of geotechnical samples. Anomalous zones of weakness, such as buried reefs, are also common and must be identified and avoided when designing foundations for structures.

Other conditions affecting foundation design are slope stability, currents and scour, and seismic response, although slope stability is generally not a concern in most of the flat Bay of Campeche; nor is the area, with a few exceptions, prone to soil scour. The response of the structures to ground motions caused by earthquakes, however, is a critical factor for foundation design. Structures also need to be designed to accommodate lateral loads from tropical storms and hurricanes. Storms crossing the Bay of Campeche are less frequent than those that affect oil field structures in the northern Gulf of Mexico, but they still occur and have to be factored into designs.

Pervasive shallow gas has to be carefully mapped and monitored when planning the installation of structures. Out-of-date surveys will not suffice since enhanced oil recovery (EOR) effects need to be anticipated.

New deepwater developments in the Mexican Ridges

Mexico’s first deepwater development will be the Lakach gas field, in 1000m water depth, about 100km off the Port of Veracruz. Here the geologic setting approaches the Mexican Ridges. Although surveys have been carried out in support of the specific development, there are still many unknowns with respect to ground conditions.

Calcareous soils transition to siliciclastics in the Mexican Ridges area, with different angles of repose and slope failure triggers and response. The Mexican Ridges is a compressional, seismically active area, especially just off Veracruz. This seismic activity necessitates fuller understanding of ground movement and slope instability, among other issues. Age dating slope failures and measuring potential run-out of submarine landslides are essential requirements for planning developments in this area.

Shell’s Perdido platform in the Gulf of Mexico in 2010. Photo from Shell.
 

The Perdido

PEMEX’s ultra-deepwater discoveries to date are in the Perdido Fold Belt, a trend that starts on the US Gulf of Mexico and becomes more fully developed on the Mexican side. The Perdido is expected to hold world class reserves. On the US side the discoveries and developments by Shell were some of the most technically challenging for a number of reasons, not only that the discoveries are in water depths of 3000m. It was the need for long term investment and specialized technology to develop these deepwater fields that was part of the drive for energy reform in Mexico.

The geotechnical and geophysical challenges here are similar to those in the US side of the Perdido trend. Gas hydrates were found in the Frio sands at the crest of Chevron’s Tiger Shark exploratory well which, with the right geometries and depths, can present gas hazards*. Operators looking south along the Perdido should expect similar challenges including reactive clays, borehole stability, and overpressured flowing sands in the top sections of the wells.

Much is still to be explored

Although early satellite work to look for natural oil seeps indicates strong possibilities for completely new hydrocarbon plays, most of the Mexican side of the Gulf of Mexico remains unexplored. Will there be analogs to the new discoveries in the US Eastern Gulf of Mexico, or new hydrocarbon plays in the Campeche Knolls regions? Energy reform will accelerate the answers. What is certain is that the Gulf of Mexico is the world’s premier deepwater hydrocarbon province and that new hydrocarbon plays will be found on both sides of the border.

Fugro’s primary operational office in Mexico is in the port complex in Ciudad del Carmen and administrative offices are in Mexico City.

*Subsurface gas hydrates in the northern Gulf of Mexico, Marine and Petroleum Geology 34 2012 - Boswell et al



Dan McConnell
is Director of Business Delivery, Fugro. A marine geologist, McConnell has written numerous articles about deepwater site conditions, frontier marine geochemical surveys, and gas hydrates. McConnell holds degrees in Latin American History and Geology from the University of Texas at Austin.