CO2 storage in fractured nanopores underground: Phase behaviour study

In this paper, thermodynamic phase behaviour, implications, and strategies for the CO2 storage process in the fractured tight/shale reservoirs (i.e., nanoproes) with adsorptions are studied. First, an analytical equation of state is modified to calculate the nanoscale phase behaviour by considering the effects of pore radius and molecule molecule interactions. Second, a new empirical correlation for calculating the adsorption thickness in nanopores is initially developed. The modified equation of state coupled with the new adsorption thickness correlation and fracture geometry equation is used to calculate the phase behaviour of confined pure and mixing CO2 streams in fractured nanopores with adsorptions. Third, the pressure volume diagrams, pressure temperature diagrams, and critical properties of 12 pure substances of CO2, N-2, and alkanes of C1-10 and 12 binary and ternary CO2-dominated mixtures are studied. The calculated pressures for all cases in nanopores with and without the adsorptions and fractures are reduced with the system volume increases but increased by increasing the system temperature with constant compositions. The pressures in nanopores are always larger than those in bulk phase at small volumes but in good agreement at large volumes. In comparison with the N-2 or C1-10, the pure CO2 is more easily transited to be a liquid or supercritical phase. Any additions of contaminations (e.g., N-2 or C1-10) into the pure CO2 increase the pressures in the pressure volume or temperature diagram while decrease the critical properties to different extent, especially in nanopores, wherein the N-2 exerts the strongest effect and the effects of the alkanes are weakened with the carbon number increase. Overall, three optimum strategies are determined for CO2 storage projects in the deep tight/shale formations as follows: large pore radii, purified CO2 streams, and low temperatures.