Diffusion Behavior of CO2 in Bulk Fluids- and in Porous Media under Reservoir Conditions: A Micro-CT Study

Background: CO2 injection into the geological formations is an efficient method to enhance oil recovery and increasing CO2 storage efficiency. CO2 diffusion process plays an integral role in the mechanisms of CO2-EOR and CO2 storage processes. Numerous authors have extensively studied CO2 molecular diffusion process under ambient conditions but a systematic study on CO2 diffusion process under reservoir conditions is unclear and lacking in the current literature [1, 2]. We studied the diffusion phenomena of CO2 and oil swelling effect due to CO2 miscibility, at reservoir temperature and pressure conditions using micro-CT technique.

Methods: A series of CO2 injection experiments were conducted at high pressure (2- 8 MPa; using high-precision Cetoni pump), into the fluids with and without the presence of porous media (sintered-Glass beads, sandstones). The experiments were carried out using, n-octane, n-decane, and crude oil as fluids. We employed micro-CT technique to improve our fundamental understanding on multiphase transport phenomena in CO2 injection and to quantify fluid-fluid saturation, interface migration, and volume expansion effect.

Results: We observed that the CO2 diffusion was instantaneous and faster into fluids without the presence of porous media. The diffusivity of CO2 into the fluid causes the volume expansion or swelling effect of the fluid immediately, and decreases with time until reaching a stable state. We also found out that the swelling effect increases with increasing fluid viscosity and the volume expansibility was directly proportional to initial CO2 pressure. CO2 diffusion into fluid saturated porous media was slower (tortuosity of porous media impede CO2 diffusivity). We noted that the interplay of CO2 dissolution or CO2 diffusion process depend on the initial CO2 saturation in the porous media. CO2 dissolution process facilitates the solubility trapping of CO2 whereas a dominant CO2-diffusion process leads to ex-solution process of CO2.

[1] Amarasinghe, et al. (2021). CO2 dissolution and convection in oil at realistic reservoir conditions: A visualization study. Journal of Natural Gas Science and Engineering, 95. [2] Liu, Y., et al. (2016). Experimental study on CO2 diffusion in bulk n-decane and n-decane saturated porous media using micro-CT. Fluid Phase Equilibria, 417