Quantifying the Impacts of Reservoir Geochemistry and Pore Structure on the CO₂ Diffusion and Leakage in Organic-Rich Mudrock Formations and Caprocks

Depleted gas reservoirs often have complex geometries with varying porosity and permeability distributions. Quantifying CO₂ diffusion in such heterogeneous reservoirs remains challenging, especially when considering the interactions between CO₂ and different rock types and pore structures. To ensure the long-term effectiveness and safety of CO₂ storage, it is essential to understand and quantify the diffusion of CO₂ within the storage reservoirs. Therefore, the objectives of this paper are to (i) investigate the impact of pore size and pore geometry of kerogen structures on the diffusion of CO₂, (ii) investigate the impact of water and hydrocarbon saturation on the CO₂ diffusivity in organic-rich mudrocks, and (iii) quantify the coupling effect of CO₂ adsorption/diffusion in kerogen nanopores on the apparent gas permeability.

We used a realistic kerogen molecular model of type IIC. The kerogen model was developed using the restrained simulated annealing process to investigate the impact of different pore geometries (i.e., cylindrical and channel-shaped pores) on the adsorption and diffusion behavior of CO₂ molecules. Meanwhile, we modeled a variety of water and methane saturation cases for kerogen structures. The developed kerogen structures were then used as inputs to molecular dynamics (MD) simulations with CO₂ particles. Next, we quantified the CO₂ diffusion and transport coefficients under different water and methane saturation conditions. Finally, we quantified the surface, matrix, and bulk gas diffusion coefficients across the kerogen structures. Once the actual diffusivity value is determined, we can use it to calibrate the apparent gas permeability models which are used to describe the gas flow in nanopores.

Results showed that the pore size and geometry of kerogen structures affect the diffusion regime and magnitude of the injected CO₂. We identified three diffusion regimes inside the kerogen nanopores. The bulk diffusion regime lies far away from the kerogen surface, the surface/adsorbed diffusion regime lies along the kerogen surface, and finally, the sorption/matrix diffusion regime is inside the kerogen matrix. The CO₂ bulk diffusion is the dominant regime for both the channel and cylindrical-shaped kerogen pores. For the case of channel-shaped pores, the average bulk, surface, and sorbed diffusion coefficients are 1.3×10⁻⁷ m²/s, 4.6×10⁻⁸ m²/s, and 9.8×10⁻⁹ m²/s, respectively, at 20% methane saturation. These values dropped to 9.5×10⁻⁸ m²/s, 4.0×10⁻⁸ m²/s, and 1.2×10⁻⁸ m²/s for the same corresponding regimes respectively at 20% water saturation. Changing the kerogen pore geometry to a cylindrical shape reduced the CO₂ bulk and surface diffusion by 40% and 25% compared to the channel-shaped pores for the same gas saturation.

These findings highlight the significant impact of reservoir geochemistry and pore geometry on the diffusion behavior of CO₂ in organic-rich mudrocks. Therefore, carrying out reservoir geochemical screening is essential for prospective CO₂ storage projects. The outcomes of this paper enhance our understanding of the diffusion process, which is crucial for reliable assessment of the amount of CO₂ that can be safely and securely stored in subsurface reservoirs. The diffusion of CO₂ in organic-rich mudrock formations can also impact the geological sealing properties of the rock. If CO₂ can diffuse through the kerogen pores, it may increase the risk of leakage, potentially compromising the integrity of the storage site.