Speeding Up Mineral Reactions To Fight Global Warming

The more familiar method of carbon dioxide sequestration involves capturing and pumping the gas into underground reservoirs where geological traps that have sealed natural gas for millions of years will serve to lock captured CO2 for eons as well.

Nature has published a paper by Jürg M. Matter and Peter B. Kelemen that proposes another method of CO2 sequestration, locking CO2 as carbonate minerals by reaction with a host rock.

Anthropogenic greenhouse-gas emissions continue to increase rapidly despite efforts aimed at curbing the release of such gases. One potentially long-term solution for offsetting these emissions is the capture and storage of carbon dioxide. In principle, fluid or gaseous carbon dioxide can be injected into the Earth's crust and locked up as carbonate minerals through chemical reactions with calcium and magnesium ions supplied by silicate minerals. This process can lead to near-permanent and secure sequestration, but its feasibility depends on the ease and vigour of the reactions. Laboratory studies as well as natural analogues indicate that the rate of carbonate mineral formation is much higher in host rocks that are rich in magnesium- and calcium-bearing minerals. Such rocks include, for example, basalts and magnesium-rich mantle rocks that have been emplaced on the continents. Carbonate mineral precipitation could quickly clog up existing voids, presenting a challenge to this approach. However, field and laboratory observations suggest that the stress induced by rapid precipitation may lead to fracturing and subsequent increase in pore space. Future work should rigorously test the feasibility of this approach by addressing reaction kinetics, the evolution of permeability and field-scale injection methods.

The advantage here is that the gas is converted into a solid phase and so exists in a more stable form. The principle may be sound but I'm going to think out aloud on bridging the very long distance between scientific possibility and its practical realization in the field for a country like India.

First come the political and economic issues with a scheme of this nature. Those hurdles are immense. India's stated position is of no mandatory limits on emissions, so there is no political urgency in examining CO2 sequestration projects like this one. And this method is going to be expensive as well. Transporting CO2 from emission points to suitable storage sites and then drilling and pumping gas in hard rock ain't going to come cheap. It might work if the price of emitting CO2 is very high. But under the current scenario in India with no limits or penalties on emissions there will be no economic incentives for polluters to even explore a scheme like this.

Besides, there are other urgent practical matters that need to be carefully thought out before rock masses can be given out for CO2 storage.

There is no problem in India finding a suitable host rock. In Maharashtra and adjoining states of Madhya Pradesh, Andhra Pradesh there is half a million square km of Deccan Basalts, a rock rich in magnesium and calcium that will provide plenty of reaction surface and storage volume. This potential reservoir is up to 2 km thick in the central portions and along the western ghats. Other areas of the country too have mafic igneous rocks of the right composition that may extend to even deeper crustal levels.

However... where do you drill and store and to what depth before the project becomes too expensive?...

A crustal thickness of up to 1 km underlying cultivated areas must be kept untouched only for groundwater storage. That may seem excessive since groundwater is extracted mostly from the upper few tens to a hundred feet or so. But the Deccan Basalts and other mafic crystalline rocks have deeper aquifers too. Already these are being exploited by groundwater bore-wells that are reaching a few hundred meters in many parts of the country as demand for the groundwater increases.

Water in these crystalline rocks is stored and moves along fractures and cracks. Fracture systems in these rock masses penetrate to great depths and must be kept open if the rock is to act as an effective aquifer.  The danger is that gas pumped to depths even greater than 1 km may seep up through these deep penetrating fractures depositing carbonate minerals and plugging up the shallow permeability pathways for groundwater.

You could in principle go to very great depths say much more than 2 km, but at some point drilling through the hard crystalline rock will make the project uneconomic.

So then.... if the choice is between pumping CO2 into the crust to economically viable depths to keeping aquifers healthy, then the health of the aquifers must win out. Timely groundwater access improves crop yield and farmer income. The relation is direct and the benefits immediate. Risking reducing the  porosity and permeability of these rocks with carbonate minerals to attain a more nebulous benefit (for people who depend on groundwater) of lowered atmospheric CO2 content will be a suicidal course to take and I unfortunately mean that literally.  Groundwater irrigates a major share of cultivated areas and access to it offers the best chance of alleviating grinding farmer poverty in face of uncertain rains and lower crop yields due to increased temperatures.

So, no playing around with rock masses under cultivated areas...that might leave sections of the crust underlying wastelands, forests and urban areas as potential sites. Of these wastelands appear as the most promising sites in terms of the least political and civic resistance to such a project. This provided field experiments demonstrate the feasibility of this project and the political and economic conditions allow its implementation.

India's National Action Plan for Climate Change (15 MB) has very little to say about carbon sequestration. None of the eight missions or strategies meant to deal with climate change include carbon capture and sequestration.

It is under the political radar and it is expensive and India can best mitigate the effects of global warming by keeping the shallow crust fractured and permeable so as to act as an effective aquifer for groundwater.

My feeling is for India at least in the near term this method of preventing atmospheric CO2 buildup is a non-starter.