Gravitational Trapping of CO2 in Deep Marine Sediments

At the high pressures and cold temperatures of deep (>2 miles) marine sediments liquid carbon dioxide is denser than the surrounding pore water and will sink, thus becoming gravitationally trapped. The deep ocean sediments offer a barrier preventing active mixing into the ocean while providing a very large stable reservoir for disposal of carbon dioxide emissions. New York State funded a Lenfest Center project based at the Lamont-Doherty Earth Observatory to study the feasibility of this novel method of CO2 sequestration with the broader vision of using the stability and reservoir capacity available in such sediments to dispose of emissions from coastal population centers such as New York City.

One option for storage of carbon dioxide is to dispose of it in deep geological formations and to rely upon overlying impermeable layers to prevent leakage of the buoyant CO2 to the surface. A promising alternative option is to seek out locations where CO2 is denser than surrounding fluids and so is gravitationally stable.

The large hydrostatic pressures and low temperatures (~2ºC) of the deep ocean (>3km) provide such conditions: liquid CO2 is denser than water and is gravitationally trapped. However, a physical barrier is necessary to slow diffusion into the ocean and the attendant ocean acidification and long term atmospheric re-release of CO2. Liquid CO2 injected in the deep ocean would form lakes on the seafloor. Such lakes would form a layer of clathrate hydrate at the interface of the CO2 and ocean water which would slow but not stop such dissolution into the ocean water.

We explored a form of geologic sequestration that takes advantage of the gravitational trapping of deep ocean storage— the injection of CO2 under the deep ocean into marine sediments. Near the surface the CO2 would be denser than the surrounding pore water and would sink. Geothermal heat increases temperatures with depth in marine sediments and will result in positive buoyancy for injected CO2 past some depth, typically several hundred meters below the seafloor. Gravitational forces will therefore trap the CO2 at a neutral buoyancy level between the positive buoyancy of the geothermal gradient and the overlying negative buoyancy zone where the CO2 would be denser than the in situ pore water.

Figure 1. The deep ocean floor (in shades of blue).

The deep ocean covers much of the Earth’s surface (fig. 1). The area available for gravitational sequestration in deep marine sediments can be limited based upon certain parameters, some of which will be practical such as proximity to CO2 sources and the costs associated with transporting CO2. The scientific and engineering limitations on gravitational sequestration in deep marine sediments can be broadly classified into stability issues and CO2 emplacement issues. A tectonic-scale disturbance, such as a volcano, could push injected CO2and surrounding material toward the surface or could change the in situ conditions such that the CO2 would become unstable. This suggests that volcanically active areas such as the Mediterranean and large parts of the western Pacific Basin would be poor candidates for gravitational sequestration.

Getting the CO2 into deep ocean sediments provides many complications, the most prominent of which is permeability. Permeability in deep marine sediments varies with mineral composition and history of the sediment. The majority of tectonically stable ocean floor at necessary depth is composed of calcareous sediments or clay. Clays have permeabilities that are too low for injection eliminating much of the deepest parts of the ocean (>5000 m). Carbonates have marginal permeabilities that will require hydraulic fracturing or other permeability upgrading to achieve reasonable permeabilities. Such upgrading is dependent upon the geomechanical stability of the sediments during hydraulic fracturing. Smaller areas of the ocean floor consist of terrigenous or glacial sediments that may have higher permeabilities that are suitable for injection, but may suffer from other issues related to geomechanical stability. Our research aimed to establish the permeabilities of different sediments and the geomechanical issues associated with each sediment type as well as related chemical and fluid transport processes.

As part of this project Levine is in Cambridge, Massachusetts at the facilities of Schlumberger, a company with specialization in the geochemistry and geomechanics of carbon dioxide sequestration. This industry-university collaboration focuses on laboratory flow-through experiments to demonstrate the viability of CO2 storage in sedimentary reservoirs, including potential offshore sediment types. The experiments aim to simulate the effects of CO2 injection by flooding of water-laden cores at liquid CO2 pressures and temperatures and measuring the combined effects of geochemistry and geomechanics on core samples.

This project combined the geochemical and geophysical expertise of Lamont’s Jürg Matter (Doherty Associate Research Scientist) and David Goldberg (Doherty Senior Research Scientist and Director of the Borehole Research Group), with the engineering focus of the Department of Earth and Environmental Engineering’s Klaus Lackner (Professor and Chair, Department of Earth and Environmental Engineering) and Jonathan Levine, PhD.

Klaus Lackner, Ewing-Worzel Professor of Geophysics,

David Goldberg, Doherty Senior Research Scientist in the Lamont-Doherty Earth Observatory, (PI)

Juerg Matter, Doherty Associate Research Scientist in the Lamont-Doherty Earth Observatory,

Jonathan Levine, PhD 2010, Department of Earth and Environmental Engineering, currently a postdoctoral research scientist at Colorado School of Mines,