Abstract
For many decades, the upstream oil and gas industry has been injecting liquids and slurries into the subsurface to mitigate formation compaction and also to dispose of drill cuttings, mud and other wastes. The technical principals involved and the expertise gained over the years can be used to give back something to New Orleans that it has been steadily losing for centuries – its elevation.
The subsidence of New Orleans is principally due to natural dewatering and decomposition of the fluviodeltaic sediments underlying the city. As the sediments compact, the city sinks lower and lower, and becomes at the mercy of flooding as with Hurricane Katrina on August 29, 2005. The situation will become even worse if global warming causes sea level rises and more extreme weather conditions.
Injection to halt compaction by increasing fluid pressure in the reservoir is the accepted technique to mitigate surface subsidence in many areas of the world where fluid withdrawal for upstream petroleum development or excess usage of groundwater has caused the ground surface to drop. Case studies from California and the Netherlands show that subsidence rates of several feet per year covering large areas can be stopped. These techniques tend to be based on either volume balancing (balancing fluid withdrawal with injection of replacement fluid) or changing the zones of fluid extraction. However, these techniques will only cause cessation of surface subsidence. Unless misapplied, they will not raise the ground level.
What is needed for a long-term solution is a technique that will either restore ground levels back up to their original elevation or raise the ground levels to higher elevations. One possible technique is to inject water-based slurries into key shallow layers beneath the area of concern at rates and pressures that will result in the formation of horizontal sand-propped pancake-shaped openings. The technique of injecting water-based slurries to create artificial sand-filled fractures is widely used to increase hydrocarbon productivity but these fractures are generally designed to be vertical and are created in zones where the maximum stresses are vertical. When the principal stresses are horizontal (e.g. close to the surface or in underpressured zones), a horizontal pancake-shaped opening can be created. By displacing the layers above them, these sand-filled pancakes will raise the ground mechanically and may at the same time reduce the rate of any natural dewatering.
Provided that the injection slurry can be contained adequately in the subsurface and the application covers a large enough area, New Orleans could be mechanically raised using standard oil industry techniques. Appropriate short- and long-term monitoring using InSAR, GPS, and microseismic (all standard oil industry techniques) should be done to demonstrate that the pancake body stays within zone and does not cause problems. Oversight by government agencies and adherence to the highest environmental health and safety standards would be essential in order to get community buy-in and acceptance of the technique.
The same technique can also be applied to other areas at risk and, once expertise is gained, could be used to build underground levees or platforms that would be a significantly cheaper and longer-lasting solution than a never-ending process of raising buildings, dredging channels, and building a series of walls and levees on the ground that fail to address the underlying geologic problems.
