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After primary and secondary recovery of tight reservoirs, it becomes increasingly challenging to recover the remaining oil. Therefore, improving the recovery of the remaining oil is of great importance. Herein, molecular dynamics simulation (MD) of residual oil droplet movement behavior under CO2 displacement was conducted in a silica nanopores model. In this research, the movement behavior of CO2 in contact with residual oil droplets under different temperatures was analyzed, and the distribution of molecules number of CO2 and residual oil droplets was investigated. Then, the changes in pressure, kinetic energy, potential energy, van der Waals' force, Coulomb energy, long-range Coulomb potential, bond energy, and angular energy with time in the system after the contact between CO2 and residual oil droplets were studied. At last, the g(r) distribution of CO2–CO2, CO2-oil molecules, and oil molecules-oil molecules at different temperatures was deliberated. According to the results, the diffusion of CO2 can destroy residual oil droplets formed by the n-nonane and simultaneously peel off the n-nonane molecules that attach to SiO2 and graphene nanosheets (GN). The cutoff radius r of the CO2–CO2 is approximately 0.255 nm and that of the C–CO2 is 0.285 nm. The atomic force between CO2 and CO2 is relatively stronger. There is little effect caused by changing temperature on the radius where the maximum peak occurs in the radial distribution function (RDF)-g(r) of CO2–CO2 and C–CO2. The maximum peak of g(r) distribution of the CO2–CO2 in the system declines first and then rises with increasing temperature, while that of g(r) distribution of C–CO2 changes in the opposite way. At different temperatures, after the peak of g(r), its curve decreases with the increase in radius. The coordination number around C9H20 decreases, and the distribution of C9H20 becomes loose. [ABSTRACT FROM AUTHOR]

CO 2 -enhanced oil recovery (CO 2 -EOR) technology has shown great application potential in the development of tight reservoirs. Since the proportion of nano-pores in tight oil reservoirs exceeds 77%, it is necessary to further study the occurrence state of crude oil in nano-pores. In this paper, the molecular dynamics simulation was applied to study the adsorption and diffusion behaviors of multi-component crude oil in quartz (hydrophilic) nano-pores. Besides, the density discretization method was used to investigate the occurrence characteristics of crude oil, the proportion of movable fluid in nano-pores, and the effects of CO 2 and polar molecules (C 3 H 6 O) on the adsorption characteristics of crude oil. The simulation results showed that the adsorption state of crude oil in the quartz nano-pores was in the form of 4 adsorption layers with a thickness of 0.45 nm each. In nano-pores, the oil molecules with longer molecular chains were more likely to aggregate and adsorb on the quartz surface. Among them, polar oil molecules have the strongest adsorption capacity on the quartz surface. Moreover, the crude oil potential energy and the self-diffusion coefficient gradually increased from the vicinity of the quartz wall and tended to be stable in the free layer. Meanwhile, the content of movable crude oil gradually augmented with the increasement of nanometer pore width and temperature. Furthermore, the pressure had little effect on the density distribution of crude oil in the pores. In contrast, the temperature had a more significant effect on the density distribution of the adsorption layer. At last, due to the more substantial adsorption capacity of CO 2 on the rock surface, the crude oil adsorbed initially on the rock surface would be stripped off by CO 2 , converting from irreducible oil to moveable oil. [Display omitted] • Distribution profile of adsorbed and free crude oil in various nanoslits. • The self-diffusion coefficient and potential energy of crude oil at different positions. • Temperature has a great influence on the occurrence state of crude oil. • Effects of different proportions of CO 2 on the occurrence state of crude oil in nanoslits. [ABSTRACT FROM AUTHOR]

Abstract: Alkaline agents have an appeal for enhanced oil recovery because of their low cost and favorable performance. In this paper, sodium carbonate (Na2CO3) and sodium hydroxide (NaOH) are used as the alkaline chemicals, at the same Na2O content, to investigate the oil/water interfacial reactions between the Daqing crude oil and the alkaline solutions. Moreover, oleic acid or the mixture of ethyl acetate and phthalic acid diethyl ester were added into the crude oil, respectively, to facilitate the direct observation of the interfacial reactions and to compare the functional effectiveness of alkalis. The results showed that: Na2CO3 reacted slowly and partly with the acid components in crude oil, while NaOH did it very fast and completely. Interestingly, Na2CO3 is better than NaOH in lowering the oil/water interfacial tension (IFT), due to its buffer effect. These help the optimum formulation design of flooding alkali, which should also be of great importance for tertiary oil recovery. [Copyright &y& Elsevier]