The reservoir proportion with deep and low permeability, where oil and gas exist widely in nanopores, has been discovered increasingly in China. Affected by the nano-confinement effect, the phase behavior of fluid in nanopores varies with nanopore size rather than being constant. In this paper, the density, viscosity, and saturation pressure of pure and mixed fluids in nanopores are studied by molecular dynamics (MD) simulation combined with statistical physics. The feasibility of using the MD method to investigate fluid behavior in nanopores was verified with laboratory data. It was found that the fluids’ phase behavior parameters in nanopores are lower than those in the bulk phase due to the confinement effect. The boundary of confinement effect (BCE) is defined as a size range of nanopores that separates the pore scale into the confinement scale and bulk scale. Each fluid has a specific BCE influenced by the molecular size of fluid. The phase behavior of crude oil in shale and tight oil reservoirs is most affected by the molecular adsorption and interaction energy between the fluid molecule and pore wall. Clarifying a specific BCE in shale reservoirs can significantly enhance the understanding of reservoirs and guide reservoir development strategies.

As for natural gas miscible flooding in low-permeability reservoirs, studying on the phase behavior characteristic of oil-gas miscible zone has a significant meaning. In this paper, based on the reservoir characteristics of Donghe 1CIII oilfield, the evaluation of phase physical properties and minimum miscible pressure was conducted by laboratory experiments using recombined formation fluid and natural gas samples. Then, one compositional model was established to analyze the phase behavior of the miscible zone during natural gas flooding. The result shows that the fluid critical points of the miscible zone change the polynomial in the P-T diagram. More specially, the curve of critical points owns more area with higher pressure, which means that the fluid reaches a deeper miscible degree. Finally, a new chart method for gas channeling identification is proposed to evaluate the production state of the miscible zone in order to identify gas channeling. The phase envelope of the production fluid is concerned with the migration of the miscible zone. In addition, an application case in Donghe oilfield is given by means of the gas channeling-phase diagram identification method. This work could provide a theoretical basis for timely gas channeling prevention and control measures as well as improvement of the gas injection development effect.

Songliao Basin hosts abundant shale oil and gas resources, yet the phase behavior of the fluids is still unclear since the confinement effect of the shale reservoir alters the adsorption effect on the pore walls. In this study, molecular dynamics (MD) simulations were employed to clarify this issue. A binary mixture was built to represent Gulong shale oil and the phase state properties under the confinement effect were evaluated. In addition, a series of pure alkane models were constructed to analyze the influence of hydrocarbon type, temperature, and mineral composition on phase behavior. The simulation results showed that the typical Gulong shale fluid remains liquid state in the nanopores. Owing to the confinement effect, both viscosity and density under nano space decrease sharply compared to those at bulk space. Therefore, field development should somewhat rely on high-pressure stimulation to create flow paths, followed by CO₂ huff-and-puff technology to maintain production pressure. In addition, fluid composition, temperature, and mineral type are the primary factors governing the magnitude of the confinement effect.

Molecular dynamics simulations examined the roles of molecular structure, thermodynamic compatibility, intermolecular association, and mixture composition in the supercritical CO₂ extraction of ten crude oil components at 363.15 K and 15 MPa. Single-component extraction ratios ranged from 80.70% for n-hexane to 5.85% for 2-naphthol. Compounds of similar size differed widely, indicating that topology, aromaticity, and polar functional groups were more informative than molecular size alone. CO₂ solubility parameters obtained from MD agreed with estimates derived from NIST data, and δ_CO2 = 7.28ρ_r captured their reduced density dependence over 303.15–363.15 K. Extraction generally decreased with increasing oil–CO₂ solubility parameter difference. In binary systems evaluated using oil boundaries determined by the half density criterion, higher fractions of nonpolar partners were associated with increased total extraction ratios, whereas higher fractions of polar partners were associated with decreases. Multicomponent systems showed redistribution that depended on the overall composition, and relative diffusion coefficients qualitatively reflected mobility differences. The gas–oil interaction competition factor, R_comp, decreased from 1.985 to 0.087 in the same order as the extraction ratios. Both the thermodynamic and energetic correspondences persisted after excluding 2-naphthol. Configurations and radial distribution functions showed that association in nonpolar hydrocarbons was dominated by dispersion interactions, whereas polar and aromatic components exhibited additional hydrogen bonding, aromatic stacking, and dipolar or electrostatic organization. Local CO₂ enrichment near polar sites alone did not explain overall extraction. Overall, the selective transfer of individual components was consistent with a balance between local CO₂–oil association and collective oil–oil cohesion that depended on mixture composition, while molecular organization may regulate the accessibility of favorable CO₂ contact sites.