Flowing sands create problems in the US Gulf deepwater
The offshore petroleum industry is now operating in a deepwater region where "terra firma" does not really exist. The mud zone and unconsolidated sediments extend thousands of feet below the water/mud interface. Hardpans, gas hydrates, escarpments, steep slopes, mudflows, landslides, and overpressured flowing sands are all problems that challenge operations in the deep environment. Hardpans: In deep cold waters anaerobic life feeds off the petroleum seeps, firms the bottom, and concentrates
Detection critical to avoiding or dealing with
shallow geotechnical hazards
- Hardpans: In deep cold waters anaerobic life feeds off the petroleum seeps, firms the bottom, and concentrates carbonates similar to reefs.
- Gas hydrates: These frozen water-and-gas mixtures form where natural gas seeps combine with seawater. The friction of drilling through gas hydrates melts the ice, releasing gas that can destabilize the hole.
- Escarpments: These occur where slopes have failed or where salt supports sediment near the mud/water interface.
- Steep slopes: Sloping seabottoms over 10 degrees become unstable and are subject to mudflows and slides. They often occur as active fault scarps.
- Mudflows: These are streams of unconsolidated sediment moving downslope. Quite common in shallower deltaic areas, they gather speed and power as they move through the valleys into deepwater basins and out to the abyssal plain.
- Landslides: These disturbances are a sudden release of sediment from an unstable slope. Excessive sediment loading on the slope can release a slide as can fault movements or earthquakes. Disturbance of the sediments at the top of the escarpment can carry conductor and drill pipe downslope, endangering the rig.
Shallow water sand flowsSeemingly misnamed because of their deepwater occurrence, shallow water sand flows develop in the upper portion of the wellbore. The problem can be illustrated by piercing a balloon with a needle. The needle enters a zone of higher pressure creating an escape path for the pressure.
Shallow water sand flows can occur at any time during the drilling and completion of a well. The most expensive time to discover the problem is after installing subsea wellheads, pipelines, and other facilities. Offshore, the danger zone begins in waters over 2,500 ft and generally develops within the first 2,000 ft of wellbore, where the caisson and conductor are cemented.
According to Kerry Behrens, J. Chance and Associates, problems occur as formation waters in overpressured sands begin to leak up the exterior of the conductor. Leakage moves due to a poor cement job or because of unbalanced pressures.
Acting like a sand frac, the overpressured sands flow upward around the pipe piercing and fluidizing the sediments next to the well. Once the flowing sands reach the seabed, a boiling or bubbling of sediment begins. Hypersaline formation waters flow out of the well carrying sand and other sediments.
If the leaking cement seal is not located and repaired, the production pipe looses support. Like a television tower that looses a guying-cable, the pipe then bends, drops or collapses into the seabed never to be seen again. Production stops and the field quickly goes into the red. Maintaining a production pipe "needle" in the overpressured "balloon" envelope is a major problem.
No firm answersThe industry has been struggling with the issue for several years now with no firm answers yet. Major companies are reluctant to discuss the problem because of financial losses and the potential environmental effects of hypersaline solutions flowing up the well. Ponding of these fluids in deepsea hollows could endanger life in those hollows.
One naturally occurring hypersaline basin in the Gulf of Mexico is the Orca Brine Basin. Operators and contractors do not want to create any similar basins resulting from well control problems.
Fortunately, for well control purposes, the industry has little to fear since most lost wells will self-seal once the pipe conduit fails. Still, the industry does not want the US regulatory agency, Minerals Management Service (MMS), to create any new burdensome environmental requirements.
Suspect sectionsDeepwater operators are scrambling for an answer. Both DeepStar's Well Control Committee and IADC's Deep Water Well Control Taskforce are seeking solutions. Geotechnical and hazard survey contractors are using 3D seismic to identify sections that are suspect for overpressured sands.
Dr. Peter Trabant, a consultant specializing in flowing sands, says it is possible to identify potential problem sections through the use of sequence stratigraphic techniques. By locating genetic sequences with the potential for sands and overpressure, companies can design their casing program to control the pressure at depth and prevent the sands from flowing.
Other interpreters look for the "acoustic void envelope" or "chaotic zones" in the seismic survey as a key to the overpressured sand zones. Areas in the Gulf of Mexico where shallow water sand flows have occurred include southern Green Canyon, Garden Banks, Mississippi Canyon, and Alaminos Canyon.
Disappearing wellsDeepwater production is a hazardous business. One day oil and gas are flowing fine, the next day, the producing well has disappeared. This unfortunate event has happened to several operators in the Gulf of Mexico, West Africa, and other regions.
The problem is an open secret, but few are willing to discuss specific fields at length. One major operator recently lost nine wells in one Gulf of Mexico field and may have to completely reposition and re-drill to extract the reserves.
Overpressured sand is the culprit and well design is critical for adequate control. Prevention through pre-drill study, well design and placement are the best solutions. Operators have been advised to spend more on geotechnical and over-pressured zone surveys before planning exploration and production wells in order to secure long-term service of the conductor and casing strings.
Copyright 1997 Oil & Gas Journal. All Rights Reserved.