Characterization of the CO₂ fluid adsorption in coal as a function of pressure using neutron scattering techniques (SANS and USANS)

Small angle neutron scattering techniques have been applied to investigate the phase behavior of CO₂ injected into coal and possible changes in the coal pore structure that may result from this injection. Three coals were selected for this study: the Seelyville coal from the Illinois Basin (R_o = 0.53%), Baralaba coal from the Bowen Basin (R_o = 0.67%), and Bulli 4 coal from the Sydney Basin (R_o = 1.42%). The coals were selected from different depths to represent the range of the underground CO₂ conditions (from subcritical to supercritical) which may be realized in the deep subsurface environment. The experiments were conducted in a high pressure cell and CO₂ was injected under a range of pressure conditions, including those corresponding to in-situ hydrostatic subsurface conditions for each coal. Our experiments indicate that the porous matrix of all coals remains essentially unchanged after exposure to CO₂ at pressures up to 200??bar (1??bar = 10⁵??Pa). Each coal responds differently to the CO₂ exposure and this response appears to be different in pores of various sizes within the same coal. For the Seelyville coal at reservoir conditions (16????C, 50??bar), CO₂ condenses from a gas into liquid, which leads to increased average fluid density in the pores (??_pore) with sizes (r) 1 ?? 10⁵ ??? r ??? 1 ?? 10⁴???? (??_pore ??? 0.489??g/cm³) as well as in small pores with size between 30 and 300???? (??_pore ??? 0.671??g/cm³). These values are by a factor of three to four higher than the density of bulk CO₂ (??_CO2) under similar thermodynamic conditions (??_CO2 ??? 0.15??g/cm³). At the same time, in the intermediate size pores with r ??? 1000???? the average fluid density is similar to the density of bulk fluid, which indicates that adsorption does not occur in these pores. At in situ conditions for the Baralaba coal (35 ^OC, 100??bar), the average fluid density of CO₂ in all pores is lower than that of the bulk fluid (??_pore / ??_CO2 ??? 0.6). Neutron scattering from the Bulli 4 coal did not show any significant variation with pressure, a phenomenon which we assign to the extremely small amount of porosity of this coal in the pore size range between 35 and 100,000????. ?? 2008 Elsevier B.V.