Possible snags and boons for CO2 disposal

Partial panorama of a colossal mountain of asb...
Asbestos mine tailingsat Thetford in Quebec, Canada.(Photo credit: Wikipedia)

Not many people would like to visit a waste heap at an asbestos mine. That is not because waste heaps are generally boring but all forms of asbestos are carcinogens when inhaled. Encountering pits in the tailings that emits puffs of warm air would cause health and safety alarm bells to ring. Yet that is exactly what has attracted researchers to the huge asbestos mining complex at Thetford in Quebec, Canada: the air leaving the vents can be extremely depleted in carbon dioxide (Pronost, J. and 10 others 2012. CO3-depleted warm air venting from chrysotile milling waste (Thetford Mines, Canada): Evidence for in-situ carbon capture and storage. Geology, v. 40, p. 275-278). More precisely, the depletion – down to less than 10 parts per million (ppm) compared with normal atmospheric levels of 385 ppm – occurs in winter, when the puffing pits emit warm air far above the frigid air temperatures encountered in winter Quebec. The chrysotile must be reacting with groundwater and CO2, and is therefore a potential means of using near-surface natural materials for carbon capture and storage (CCS). The end product is an innocuous carbonate – Mg5(OH)2(CO3)4·4H2O – and dissolved silica. Quite a find, it might seem, as the reaction is exothermic too: CCS plus geothermal energy plus safe decomposition of a major environmental hazard. In fact any magnesium-rich silicates are likely to undergo the same carbonation reaction, especially if ground-up to increase the net surface area exposed to moist air.

Schematic showing both terrestrial and geologi...
scheme for carbon sequestration and storage at a coal-fired power plant. Rendering by LeJean Hardin and Jamie Payne. Source: http://www.ornl.gov/info/ornlreview/v33_2_00/research.htm

The parent asbestos rock at Thetford is a metamorphic derivative from mantle ultramafic rocks in an ophiolite, and the asbestos insulation business, both for extremely hazardous blue (crocidolite) and less dangerous white (chrysotile) asbestos has been hugely profitable since the 19th century. Consequently, wherever there are altered ophiolites, generally in collision-zone orogenic belts, asbestos has been exposed either naturally or through mining and processing. There are many related cancer ‘hot spots’ in populous mining areas of Canada, India, the Alps and southern Africa, and in dry climates even natural exposures pose considerable risk. Could these blighted areas take on a new role in lessening the chance of global warming? About 30 billion tonnes of CO2 are emitted by burning fossil fuels each year. To keep pace, at the current atmospheric concentration of CO, some 75 trillion tonnes of air would have to react annually with about 100 billion tonnes of magnesian silicate, making this form of CCS the largest industry on the planet (http://www.newscientist.com/article/mg21428593.800-stripping-co2-from-air-requires-largest-industry-ever.html).

Another factor tempering somewhat forced optimism for CCS as a way of having our fossil fuel cake and eating it is that direct injection of greenhouse gases into deep storage may have an unforeseen down-side. Deep drilling and injection of fluids may trigger earthquakes. The alarm raised by small yet disturbing seismicity accompanying sites for shale-gas development by ‘fracking’ (http://earth-pages.co.uk/2011/11/04/fracking-check-list/ and http://earth-pages.co.uk/2011/10/14/britain-to-be-comprehensively-fracked/) has died down to some extent following detailed analysis of small earthquakes around drilling sites. It turns out that they are triggered not by the drilling itself but the subsurface disposal of the large amounts of fluids that have to be passed through the oil shales to make the tight rock permeable to gas (Kerr, R.A. 2012 Learning how to NOT make earthquakes. Science, v. 23 p. 1436-1437). Safe subsurface disposal requires injection wells penetrating 1 to 3 km below the surface, often below the cover of sedimentary strata and into crystalline basement. Such hard rocks store elastic strain induced by burial and tectonics, and release it when lubricated by fluids, especially if they contain dormant faults. Once impermeable rock can thus be hydrofractured in the same manner as ‘fracked’ gas-prone shales and old, often unsuspected faults reactivate: a catastrophic prospect for injected CO2. In sedimentary sequences, drilling CCS wells into porous rocks capped by impermeable ones – the scenario for ‘safe’ gas storage – could also induce ‘fracking’ of the sealing rocks and thereby causing leakage (see also http://www.newscientist.com/article/dn21633-fracking-could-foil-carbon-capture-plans.html).