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Abstract

Recent studies have estimated that oceans have naturally sequestrated, by dissolving and mixing with deep waters, about 40% of the anthropogenic CO₂ emitted since the start of the industrial revolution. Moreover, the International Maritime Organization has recently announced that storage of CO₂ under the seabed would be allowed starting in 2007. To date, almost all studies, simulations and technical papers concerning carbon sequestration have focused on storing supercritical CO₂ in deep saline aquifers or depleted oil and gas reservoirs. However, a critical limiting factor for such carbon sequestration is the need for proper physical trapping and the necessity for monitoring the upward migration due to buoyancy effects and mobility of supercritical CO₂.  Carbon sequestration in deepwater sub-seabed formations provides an attractive alternative.

This paper presents a feasibility study of carbon sequestration in deepwater formations in the Gulf of Mexico with the existing technologies available in the offshore industry. We describe each step of the carbon capture and storage process and discuss the technical limitations when trying to capture CO₂ from industrial processes, transport it offshore via tanker, drill a CO₂ injector well and then, inject the CO₂ from floating facilities such as drill ships or semi-submersible vessels. Due to high pressures and low temperatures reigning at water depths deeper than 9,000 feet, the liquid CO₂ injected in the first few hundred feet of deposits will have a higher density than the surrounding formation pore-fluid and therefore will be buoyantly trapped. In addition, CO₂ hydrates that may form and fill up pore spaces will act as an additional trapping mechanism. Finally, at these great depths, the CO₂ that could leak will dissolve by reacting with ocean waters and forming mainly bicarbonate compounds.

Because oceans cover about 70% of the Earth’s surface with an average water depth of 12,500 feet, deepwater sub-seabed sequestration provides an enormous storage capacity to counteract increasing world consumption of fossil fuels. However, large time and space-scale simulations need to be performed to estimate the impact of the change in geochemistry in the deepwater seabed region. Also, the injection of liquid CO₂ will force and displace formation fluid into the seabed surface zone, which will change the ocean chemistry.