"CCS appears to be the best use of the Earth's subsurface in terms of reducing greenhouse-gas emissions while still meeting society's energy demands," Grant Ferguson of the University of Saskatchewan told environmentalresearchweb.

Ferguson believes that if greenhouse-gas emissions and electricity production are the primary goals, carbon sequestration should be prioritized where geological conditions are favourable. "Arguments can also be made for pursuing geothermal energy given its renewability but shale-gas developments are more difficult to justify," he said. "Although economics were not analysed in this study, that appears to be the reason why we see a boom in natural-gas production and not carbon sequestration and geothermal energy."

According to a 2012 study by Elliot and Celia in Environmental Science & Technology, extensive shale-gas development could put up to 80% of CCS capacity in the US at risk.

To come up with these rankings, Ferguson calculated the technologies' subsurface energy footprints, in terms of both electricity generation per unit area and per unit volume. For CCS, he considered the amount of electricity generated by the power plant producing the carbon sequestered. Large conventional oil fields and coal required less than two cubic metres to produce one gigajoule of electricity, while enhanced geothermal systems needed more than 800 cubic metres.

Ferguson also looked at the amount of subsurface needed to cause a unit reduction in greenhouse-gas emissions. Geothermal energy and CCS needed larger volumetric energy footprints than shale-gas and coal-bed methane projects for the same reduction in emissions. But, since these developments use different thicknesses of subsurface, on a per area basis CCS produced far greater reductions in emissions than unconventional gas.

"Here in Saskatchewan we have seen a great deal of oil and gas development recently, active CCS pilot projects and some proposals to develop geothermal energy," said Ferguson. "All of these projects can be found in a relatively small area of south-eastern Saskatchewan and I was curious to see which type was in our best interests in terms of climate-change mitigation and energy production."

Underground conditions that host unconventional natural gas are often suitable for CCS reservoirs. Similarly, depleted oil reservoirs and deep saline aquifers could be ideal for both CCS and geothermal projects. While the optimal depths for each development differ, writes Ferguson in Environmental Research Letters (ERL), there is considerable overlap and the same geological units could have multiple possible uses.

As an example, an oil reservoir could produce oil, then gas, then act as a site for storing carbon, as long as the use of fracking was avoided. Alternatively the oil reservoir could support geothermal energy after extraction of the oil, or CCS without the intermediate step of gas production. While the Canadian province of Alberta has legislated on priorities for the sequence of withdrawing oil and gas, such regulation is limited or absent elsewhere.

"I am currently looking at specific reservoirs to evaluate their suitability for CCS or geothermal development and their interplay with the overlying caprocks," said Ferguson. "Research on just how reservoir seals might be affected is key to determining when and where the conflicts...discussed are significant. I am also examining different methods of bringing in the impact of CCS, geothermal and natural-gas developments on groundwater resources."

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