Adsorption-Mediated Fluid Transport at the Nanoscale


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Virginia Tech


Injecting CO2 into unconventional reservoirs to enhance oil recovery has been widely studied due to its potential to improve the profitability of these reservoirs. CO2 Huff-n-Puff is emerging as a promising method, but exploiting its full potential is challenging due to difficulties in optimizing its operations. The latter arises from the limited understanding of CO2 and oil transport in unconventional reservoirs. This dissertation used molecular dynamics simulations to study the storage and transport of oil and CO2 in unconventional reservoirs in single nanopores. The first study examined the modulation of oil flow in calcite pores by CO2. It is discovered that CO2 molecules adsorb strongly on calcite walls and can change decane permeability through 8 nm-wide pores by up to 30%. They impede decane flow at moderate adsorption density but enhance flow as adsorption approaches saturation. The second study investigated the CO2 transport in 4 nm-wide calcite pores during the soaking phase of Huff-n-Puff operations. CO2 entering the pore can become adsorbed on pore walls and diffuse on them or diffuse as free CO2 molecules. The accumulation of CO2 follows a diffusion behavior with an effective diffusivity ~50% smaller than bulk CO2. Two dimensionless groups are proposed to gauge the importance of surface adsorption and diffusion in CO2 storage and transport in nanopores. The third study examined the extraction of decane initially sealed in a 4 nm-wide calcite pore through exchange with CO2 and CH4 in a reservoir. The CO2-decane exchange is significantly driven by the evolution of adsorbed oil and gas initially, but a transition to dominance by free oil and gas occurs later; for CH4-decane exchange, the opposite occurs. The net gas accumulation and decane extraction follow the diffusive law, but their effective diffusivities do not always align well with the self-diffusion coefficients of CO2, CH4, and decane in the nanopore. The three studies identified the essential roles of gas/oil adsorption in their net transport in nanopores and, thus, unconventional reservoirs. Delineating these roles and formulating dimensionless groups to gauge their importance help develop better models for enhanced oil recovery from unconventional reservoirs by CO2 injection.



unconventional reservoirs, surface adsorption, diffusivity, CO2, nanopore