SANS from CO$_{2}$-saturated coals at conditions relevant to subsurface sequestration

Carbon dioxide (CO$_{2})$ is the greenhouse gas which makes the largest contribution to global warming and roughly one third of the United States' CO$_{2}$ emissions are generated by fuel-burning power plants. Capture and storage of CO$_{2}$ in underground geologic structures may significantly reduce CO$_{2}$ emissions to the atmosphere. Sequestration of CO$_{2}$ in unmineable deep coal seams is particularly attractive as many coal-burning power plants are located near sites potentially suitable for geological storage. It is widely assumed that CO$_{2}$ can be captured and retained in coals by virtue of several of mechanisms, such as fluid trapping of an ``immobile phase'' inside the pore space, adsorption to the pore surface and chemical bonding inside the organic coal matrix in the vicinity of pore walls. We report the results of the first small-angle neutron scattering (SANS) studies of several coals saturated with CO$_{2}$ at temperatures and pressures similar to those found in deep coal seams which are likely to be used for industrial-scale underground storage of CO$_{2}$. We found that the porous coal matrix may work to create absorbed fluid phase with the physical density much exceeding the density of the bulk fluid at the same thermodynamic conditions. Fluid densification is different in different coals which may explain the observed differences in sorption capacity and migration rates.