olelog

Emission to the atmosphere of greenhouse gases, mainly resulting from combustion of fossil fuels, is considered a major player in global warming. Carbon dioxide (CO₂) is the main greenhouse gas, accounting for around 65%. Carbon capture and storage (CCS) is an approach to mitigate global warming by capturing carbon dioxide (CO₂) from large point sources such as fossil fuel power plants and storing it instead of releasing it into the atmosphere.

Since 1996 the Norwegian oil and gas company Statoil has been injecting 1 million ton CO₂ per year into a salt water containing sand layer, called the Utsira formation, which lies 1 km below the bottom of the North sea. Other countries in Northern Europe have very advanced plans (some under construction) to store CO₂ in terrestrial reservoirs, typically consisting of sand or sandstone (aquifers) sealed by an overlying layer of clay or other impermeable rock.

There are, however, concerns about putting large amounts of CO₂ into the ground. You don’t want any leakage so that it seeps up to atmosphere, which is missing the point by the injection. In the case of deep ocean storage, there is a risk of greatly increasing the problem of ocean acidification, a problem that also stems from the excess of carbon dioxide already in the atmosphere and oceans. (So far there has been no leakage from the Norwegian Sleipner project).

It would be better if the CO₂ through chemical reactions were bound in minerals. E.g. a reaction between calcium (Ca) and CO₂ to form (stable) calcium carbonate (CaCO₃).

According to a paper published online (and with open access!) deep-sea basalt offers a unique environment for CO₂ storage (or CO₂ sequestration as it is often called). The paper by Goldberg et al. is titled Carbon dioxide sequestration in deep-sea basalt (PNAS of 22 July 2008, vol. 105. no. 29). Oceanic crust is mainly composed of basalt (welling up at mid-ocean ridges), which means that vast volumes of seawater-filled pore space are available. Within deep-sea basalt aquifers, the injected CO₂ mixes with seawater and reacts with basalt, both of which are rich in elements with which the carbon dioxide can react. The release of Ca² and Mg² ions from basalt will form stable carbonate minerals as reaction products.

Important mechanisms for trapping CO₂ injected within deep-sea basalt further include
i) blanketing deep-sea sediments, which form a low-permeability stratigraphic barrier impeding vertical fluid migration;
ii) the formation of CO₂ hydrate, which is denser and less soluble than liquid CO₂ in sea-water of 2°C;
iii) gravitational trapping at water depths of at least 2,700 m, where injected CO₂ is denser than typical seawater, causing it to sink.
All three of these mechanisms are simultaneously available within ocean crust, providing independent protective barriers that could safely isolate the oceans, oceanic ecosystems,
and the atmosphere from leakage of CO₂ escaping from deep-sea basalt aquifers.

The deep-sea basalt region of the Juan de Fuca Plate off the coast of Oregon and Washington is suggested as CO₂ storage region for US. Here an area of 68,000 km2 has water depths of at least 2,700 m and a covering sediment thickness of at least 200 m. The total storage capacity for injected CO2 in this area is 208 gigaton of carbon.

• http://www.pnas.org/content/105/29/9920.abstract
• http://environmentalresearchweb.org/cws/article/futures/35017

A small fact sheet in pdf format about the Norweian Sleipner project and the Utsira fomation can be downloaded from:
http://www.bellona.org/factsheets/1191928198.67

See also my post on Carbon Dioxide Storage

Arizona geology has a recent post on geologic sequestration of CO2 here.