Comparative Study of Nitrogen, Carbon Dioxide, and Hydrogen Pore Occupancy, Connectivity and Ostwald Ripening in Carbonate Rocks

Abstract High-resolution three-dimensional micro-CT imaging is used to investigate nitrogen, carbon dioxide, and hydrogen injection in reservoir carbonate rocks. The images were analysed to ascertain gas saturation, pore occupancy, ganglia size and connectivity across the sample. The experiment studies a water-wet reservoir carbonate rock that is filled with synthetic brine before and between each gas injection. The experimental conditions involve unsteady-state gas injection at a pressure of 9 MPa, a temperature of 60°C, gas injection at a rate of 0.005 mL/min, and brine injection at a rate of 0.1 mL/min to remove all the gas in the sample. The sample is firstly saturated with brine, followed by N2 injection, and then brine injection. Images are taken throughout each injection phase and after waiting 24 hours after gas injection. This procedure is replicated for the CO2 and H2 cycles. The saturation of gases in the sample after injection were as follows: N2 around 25%, CO2 approximately 21%, and H2 around 18%. While the N2 saturation slightly decreased after 24 hours, the CO2 and H2 saturation remained relatively stable over time. The saturation profile reveals that N2 and H2 experience more significant changes during the waiting period caused by Ostwald ripening compared to CO2 as the gases propagate to the centre of the sample. Capillary pressure measurements based on interfacial curvature at around 80% brine saturation indicate a significant decline for N2, from 1.46 kPa to 1.18 kPa after 24 hours. CO2 experiences a decrease in pressure from 1.17 kPa to 1.05 kPa over 24 hours, while H2 shows a minimal decline in capillary pressure with time remaining at approximately 1.04 kPa. Pore occupancy and ganglia analysis indicate N2 occupying pores mostly above 25 µm radius, and the saturation increase in the larger pores after 24 hours. Throat radii larger than 18 µm show an increase in N2 saturation, indicating improved connectivity, as supported by the normalized Euler number. CO2 predominantly occupies pores from 30 to 40 µm with minimal change. H2 occupies pores larger than 19 µm with minimal changes with time and throats larger than 8 µm with increased saturation with time that suggests enhanced connectivity. Overall, this study provides a useful reference for comparing N2 and CO2 displacement and trapping behaviour after waterflooding which have been studied extensively, with those for H2, and is applicable to gas storage projects within carbonate reservoirs.