Is the long-term sequestration of CO2 in and around deep, abandoned coal mines feasible?

Abstract

Many of the largest fixed sources of CO2 emissions are major power stations located on or very close to major coalfields. Even when local coal reserves are exhausted, coal- and/or biomass-fired power generation often persists at such sites, as they occupy pivotal positions in the national power grids that developed around them. To date, strategies for CO2 sequestration from such power plants have focused on long-distance transport by pipeline to depleted hydrocarbon reservoirs and similar deep saline aquifers. Yet where abandoned coal mines extend more than about 800 m below ground level, the void space represented by the old mine voids themselves, and roof strata that have been rendered more permeable by void collapse, could represent convenient auxiliary loci for CO2 sequestration. Furthermore, the geochemical nature of coal and coal-bearing strata may offer mechanisms for entrapment of injected CO2 in or on the solid phase that are not available in potential storage zones considered to date. Engineering evaluation of this possibility requires consideration of the likely fate of CO2 in free, adsorbed, dissolved, and mineralized forms, and of the geotechnical integrity of enclosing strata and abandoned mine infrastructure that could serve as seals to trap injected CO2 in place. A protocol for assessing these factors has been developed, based on critical evaluation of mining records, hydrogeological conditions, and geotechnical data, resulting in a quantitative assessment of the capacity for CO2 sequestration represented by deep abandoned coal mine workings. Preliminary application of the decision logic is illustrated for the Daning coal mines in China.