Geochemical reactions may decrease effectiveness of carbon storage schemes

sc604 — Mon, 05 Jan 2015 12:14:21 +0000

Geochemical reactions taking place in aquifers – underground layers of water-bearing porous rock –may lead to carbon dioxide being ‘pooled’ for hundreds or even thousands of years, and may force a rethink of how these underground reservoirs are used in carbon capture and storage (CCS) schemes.

̽��ֱ��new research, from the ̽��ֱ�� of Cambridge, has shown that aquifers rich in silicate minerals may delay, or even prevent, CO2 from being carried to greater depths, where it may be less likely to leak out of the aquifer and into the atmosphere. ̽��ֱ��results are published in the journal Nature Communications.

Saline aquifers have been considered the safest and most efficient option for CCS schemes, where anthropogenic carbon emissions are trapped and stored underground so that they do not enter the atmosphere.

Both dissolution in the formation water and transport to depth decrease the risk of CO2 escaping: dissolution reduces the risk of potential upward leakage through fractures in the cap rock of the reservoir, while transport to greater depth increases the rate of dissolution and of potential incorporation of the CO2 into the rock minerals.

“Our research has found that CO2 may not behave as expected when stored in aquifers, challenging some of our previous assumptions about CCS schemes,” said Dr Silvana Cardoso of the Department of Chemical Engineering and Biotechnology, who led the research.

It has been thought that once the CO2 is dissolved in the aquifer water, making it denser, convection streams develop and carry the mixture to deeper parts of the aquifer.

Dr Cardoso and her co-author, Jeanne Andres, a former PhD student in the same department, found that chemical reactions between the rock formations and the dissolved CO2 may delay, or even prevent, the CO2 from reaching greater depths by decreasing the strength of the convection streams.

̽��ֱ��researchers used a combination of simple laboratory experiments and mathematical analysis to establish the basic interaction between fluid flow and chemical kinetics in a deep porous medium. Their study assessed the impact that the natural chemical reactions between the dissolved CO2 and the rock formation have on the convection streams which carry the CO2 to greater depths.

̽��ֱ��researchers found that the behaviour of carbon dioxide depends strongly on the chemical composition of the rock formation: while the streaming of dissolved carbon dioxide persists in carbonate rocks, the chemical interactions in silicate-rich rocks may curb this transport drastically and even inhibit it altogether. For example, for a rock matrix rich in calcium feldspar, the convection streams may be completely shut off just two months after the onset of motion. After this, the carbon dioxide will be transported to depth by much slower diffusional processes.

These results challenge current views of carbon sequestration and dissolution rates in the subsurface, suggesting that pooled carbon dioxide may remain in the shallower regions of the formation for hundreds to thousands of years, while deeper regions of the reservoir can remain virtually carbon free.

̽��ֱ��findings could have important practical implications for storage of carbon dioxide in saline aquifers, enabling informed screening of the most effective sites.

“Screening of new sites will need to include not only the size and location of the reservoir, but also the mineralogy of the rock,” said Cardoso. “ ̽��ֱ��present study simply shows that mineralogy has a strong impact on where the CO2 ends up. Which specific mineralogy is best remains to be studied.”

New research shows that the natural reactions taking place in some of the underground reservoirs used to store carbon dioxide may prevent carbon emissions from being transported to greater depths, where it may be less likely to leak into the atmosphere.

CO2 may not behave as expected when stored in aquifers, challenging some of our previous assumptions about CCS schemes

Silvana Cardoso

Evolution of the pink diffusive boundary layer formed by geochemical reactions

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Chemistry curbs spreading of carbon dioxide

gm349 — Fri, 06 May 2011 10:59:18 +0000

̽��ֱ��findings may have implications for carbon sequestration in saline aquifers – one of the many methods being explored to mitigate rising CO2 levels in the atmosphere.

Depending on the strength of the reaction between dissolved CO2 and porous rock, the new research shows that distinct scenarios of CO2 transport may occur in deep saline rock formations.

Jeanne Andres, a Schlumberger Foundation PhD researcher at the Department of Chemical Engineering and Biotechnology at the ̽��ֱ�� of Cambridge, said: “If one knows the physical properties of the aquifer, one can now calculate the movement of CO2 across it, and when it will begin to mix with the brine. In theory, one can manipulate the strength of reactions, thereby engineering the movement of CO2 – keeping it in one area or moving it to another within the aquifer - to enhance its storage underground.”

With weak reactions, the CO2 will spread from the top throughout the depth of the aquifer, but with stronger reactions, the CO2 remains near the top of the reservoir, leaving the deeper part inactive.

̽��ֱ��strength of these reactions can vary significantly among deep saline reservoirs - rock formations possess a wide range of chemical reaction rates depending on the mineralogy (e.g. calcite, dolomite, etc) as well as other factors such as temperature and pressure,. With the new insight this research provides, it would now be feasible to consider creating and injecting compounds which could alter the strength of reactions in the aquifer.

To arrive at their conclusions, the researchers established that the basic interaction between fluid flow and the rate of chemical reactions (chemical kinetics) in a deep porous medium is governed by a single dimensionless number, which measures the rate of diffusion and reaction compared to that of the natural mixing of fluids (convection).

As applied to the storage of CO2 underground, the scientists demonstrate how this new parameter controls CO2 flow and mixing in briny porous rock. Through numerical simulations, the researchers found that above this parameter’s critical value, reaction stabilizes the CO2 system and convection no longer occurs. Below the parameter’s critical value, stronger reactions result in longer delays in the onset of convective mixing throughout the reservoir.

For systems with similar convective mixing strengths, stronger reactions, indicated by rising values of the new parameter, can increase the minimum rate at which pure, lighter CO2 dissolves into the brine, enhancing storage and reducing the risk of leakage.

Dr Silvana Cardoso, Reader in the Department and project leader, said: “This research shows how rigorous mathematical analysis coupled with strong physical understanding can help us grasp the complex interactions of flow and reaction in a carbon reservoir. Such knowledge will be valuable in guiding future approaches to carbon storage.”

̽��ֱ��paper ‘Onset of convection in a porous medium in the presence of chemical reaction’ was published in the journal Physical Review E.

̽��ֱ��presence of even a simple chemical reaction can delay or prevent the spreading of stored carbon dioxide in underground aquifers, new research from the ̽��ֱ�� of Cambridge has revealed.

In theory, one can manipulate the strength of reactions, thereby engineering the movement of CO2 – keeping it in one area or moving it to another within the aquifer - to enhance its storage underground.

Jeanne Andres

Jeanne Therese H. Andres

CO2 fingers - Strong chemical reactions between dissolved carbon dioxide and porous rock (top) may stop CO2 fingers from spreading from the top throughout an aquifer’s depth, in contrast to systems with no reaction (bottom).

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̽��ֱ�� of Cambridge - Silvana Cardoso

Geochemical reactions may decrease effectiveness of carbon storage schemes

Chemistry curbs spreading of carbon dioxide