Abstract
Carbon capture and storage (CCS) is currently a main way to reduce anthropogenic CO2 emissions. However, the supplies of ideal reservoirs for the geological CO2 storages are substantially less than the demands if the CO2 storage is treated as a usual method to reduce the anthropogenic CO2 emissions and implemented worldwide. On the other hand, the tight/shale formations with micro/nanopores are extensively distributed, which are usually fractured by providing the outer pressures (e.g., injecting CO2) or after the tight/shale oil/gas productions. Hence, it is necessary to study the potential CO2 storages in the fractured tight formations. In this paper, thermodynamic phase behaviour for the CO2 storage process in the fractured tight formations (i.e., nanoproes) with adsorptions are studied.
More specifically, first, an analytical equation of state (EOS) is modified and applied to calculate the thermodynamic phase behaviour of confined pure/mixing fluids in nanopores considering the effects of pore radius and molecule‒molecule (m‒m) interactions. Second, a new empirical correlation for predicting the adsorption thickness in nanopores is initially developed. The modified EOS coupled with the new adsorption thickness correlation and the fracture geometry equation is used to calculate the phase behaviour of confined pure and mixing CO2 streams in fractured nanopores with adsorptions. The phase behaviour, which include the pressure‒volume (P‒V) diagrams, pressure‒temperature (P‒T) diagrams, and critical properties, of 12 pure substances of CO2, N2, and alkanes of C1‒10 and 12 CO2-dominated binary and ternary mixtures are specifically studied.
A new empirical correlation for the adsorption thickness is developed, which is coupled with a modified van der Waals EOS (vdW-EOS) and the fracture geometry equation to calculate the phase behaviour of confined pure and mixing CO2 streams in fractured nanopores with adsorptions. The calculated pressures for all cases in nanopores with and without the adsorptions and fractures are reduced with the system volume increases while they are increased by increasing the system temperature with constant compositions, which are always larger than those in bulk phase at small volumes but in good agreement at large volumes. In comparison with the N2 or C1‒10, the pure CO2 is more easily transited to be a liquid or supercritical phase. Moreover, any additions of N2 or C1‒10 into the pure CO2 increase the vapour/bubble pressures to different extent, especially in nanopores. In the P‒V diagrams of pure and mixing CO2 streams, the pressures are more sensitive to the volume change in pure gas phase but the liquid‒gas or liquid‒liquid phase are inert to the volume expansions. The pressures in the P‒T diagrams are monotonically increased with the temperature in bulk phase and nanopores. The pressure-transition pressures of the C1‒10 become more sensitive to the temperature with the carbon number increase. In comparison with the N2 or C1‒10, the pure CO2 is more easily transited to be a liquid or supercritical phase. Any additions of N2 or C1‒10 into the pure CO2 lead the system pressures to increase more with the temperature to different extent, especially in nanopores. Also, the pressures in nanopores become even higher than those in bulk phase for the CO2-dominated binary and/or ternary mixtures. The critical properties with the adsorptions and fractures are decreased with the pore radius reduction to some certain level, which agree well with the results without the adsorptions or fractures, and reversed to increase afterwards. The bottom pore radius for the coincidences of the critical properties with and without adsorptions and fractures vary for different substances. The critical properties, in comparison with the pure CO2 case, are substantially decreased by adding N2 or C1‒10, wherein the N2 exerts the strongest effect while the effects of the alkanes are weakened with the carbon number increase. Furthermore, the critical properties of the binary and ternary mixtures with and without adsorptions and fractures are similar at rp = 0.4‒1000 nm. Finally, for CO2 storage projects in the deep tight formations, three optimum strategies are obtained from this study and listed as follows: lager pore radii (by natural or manmade fractures), purified CO2 streams, and low temperatures.
At the current stage that the experimental methods are incapable of fully exploring the nanoscale phase behaviour, this study is critically important because the CO2 storage is the key part to mitigate the greenhouse gas emissions and control the climate change to a manage and meanwhile, its applications in the tight/shale formations with micro/nanopores are inevitable in the near future.