Because supercritical CO2, when injected onshore or in shallow water depths offshore, is mobile and can, therefore, migrate through any conduits or fractures, there is a need for proper physical trapping and also a necessity to monitor the CO2 migration in the injected zone. In addition, public opinion, government regulatory agencies and the lack of space for CO2 injection sites in some of the largest CO2 emitting regions of the world encourage investigating other alternatives such as CO2 sequestration in deepwater sub-seabed formations.
Furthermore, at the high pressures and low temperatures reigning in deepwater sediments where water depths are greater than 9,000 feet (˜2,750 meters), scientists have proposed that the CO2 should become denser than seawater and therefore would remain buoyantly trapped when liquid CO2 is injected within the first few hundred feet of sediments even in the absence of geological seals and traps. Besides, the bulk of the studies and technical papers concerning CO2 sequestration in deepwater sediments have focused on showing the potential and the feasibility of the concept but very little has been published to demonstrate the viability of the injection and long-term storage of CO2 in deepwater sub-seabed formations.
This paper presents the results of several case studies located in the Gulf of Mexico, the Pacific Ocean, the North Atlantic Ocean and the Sea of Japan. Large time-scale reservoir simulations have been conducted for up to 250 years and show that injected liquid CO2 can remain trapped in deepwater sediments under certain sediment physical properties. Therefore, CO2 sequestration in deepwater sediments provide another attractive technical solution when applied under certain conditions of pressure, temperature, sediment type, thickness, permeability and porosity notably for regions where there are few depleted oil and gas fields available for storage or limited space accessible onshore.
Human industrial activity through the consumption and flaring of fossil fuels has resulted in the emission of nearly 30 billion tons of CO2 for the year 2010 according to the Energy Information Administration, seeing a constant increase for CO2 atmospheric concentrations since the beginning of the industrial age. Indeed, the CO2 concentrations have risen from 280 ppm in the 18th century to about 390 ppm in 2010 and are forecasted to continue increasing at a rate of 2 ppm per year. Nonetheless, recent studies have estimated that oceans have naturally sequestrated, by dissolving and mixing with deep waters, about 40% to 50% of the anthropogenic CO2 emitted during the same period of time. Therefore, carbon dioxide capture and storage (CCS) techniques which involve capturing the CO2 coming from the combustion of fossil fuels and some industrial processes; then transporting and injecting this CO2 (either in a supercritical or liquid state) in geological formations need to be investigated to ensure that these methods could help reduce or mitigate CO2 emissions to the atmosphere.