Production of CO2 is a natural consequence of burning any fossil fuel. In the immediate future, the primary alternative to controlling the produced CO2 is sequestering it in geologic media, particularly because of the value added benefits like enhanced and incremental recovery of coalbed methane (CBM), or oil, associated with the alternative. This paper presents the findings of a basic laboratory study carried out to investigate the various properties of coal that are impacted by injection of CO2 into deep coal. The first part of the study included measurement of permeability of coal cores at in situ conditions. This was followed by injection of flue gas, consisting primarily of nitrogen and CO2, to avail of the benefits associated with both CO2 and N2 injection, since injection of nitrogen alone has also been shown to enhance methane production from coal. The results suggest a decrease in permeability of coal with CO2 injection, whereas the permeability loss with flue gas injection is minimal. The experimental results also indicate that there is significant volumetric strain induced in the coal matrix when exposed to CO2. The critical factors affecting the permeability loss appear to be the injection pressure and composition of the gas injected.
The concentration of atmospheric carbon dioxide (CO2) has been rising continuously with increased consumption of fossil fuels. Accounting for 81% of the greenhouse gases in the US, and regarded as the most prominent greenhouse gas, its concentration over the last 100 years has increased from 280 to 370 particles per million (ppm) [1]. Hence, in accordance with the objectives of the United Nations Framework on Climate Change (UNFCC), in addition to other avenues like fuel switching and efficient energy management, possibilities of largescale sequestration of CO2 are being explored to stabilize its atmospheric concentration. Sequestering CO2 in deep coal seams is an option with significant potential since it may lead to the value added benefit of enhanced recovery of methane contained in coal, partly offsetting the cost of sequestration and making it economically feasible [1, 2]. Another advantage of sequestering CO2 in deep unmineable coal seams is that it remains there permanently as sorbed gas, or physically trapped between the cleats, due to its very long retention time of 105 to 106 years [3]. Coordinated research efforts to understand and explain the CO2 storage process by studying the naturally occurring CO2 reservoirs are underway in US, Canada, Japan, China, Europe and Australia [4].
Latest estimates show that deep coals have a storage capacity of 220 gigatons (Gt) worldwide, of which 25 to 30 Gt can be stored profitably in US coals, based on the assumption that 2 molecules of CO2 can be stored for every methane molecule in place [5]. The enhanced coalbed methane (ECBM) recovery potential, associated with sequestration in US coals, is also estimated to be over 150 trillion cu ft (TCF). Finally, it is estimated that an additional 80 Gt can be sequestered at an additional cost of $5/ton, excluding the CO2 capture and transportation costs [5].
