Search

Abstract Shale gas is a potential substitute for the gradually depleted conventional oil and gas resources. Since studies on microscopic behavior of shale gas in nano−slits and the exact mechanical mechanisms behind it are still in urgent demand, this paper uses the model of graphite layers to describe microscopic details of shale gas occurrence behavior in organic−rich nano−slits from the viewpoint of molecular interactions by molecular simulation. "Dual adsorption mechanisms" and the consistency theory of gas distributions affected by wall effects under the condition of no overlapped wall force field are proposed to clarify the formation mechanisms of shale gas in the nano−slits. Results also show the critical channel width for all gases to be affected by the nano−scale effects is between 1 and 2 nm, and the regions (counted from the gas zone) for overlapped wall force field in 0.5 and 1 nm slits are about 0.32 and 0.42 nm, respectively. Furthermore, the effects of overlapped wall forces, channel size and pressure variations on gas aggregation and its movability etc., interaction mechanisms inside the nano−slits and their causalities, the development enlightenment for Knudsen layer (KL) and the whole gas have been explicitly depicted and clarified, which is expected to be a useful reference not only for shale gas evaluation and exploitation, but also for widespread research of gas occurrence phenomena in carbon−based materials in the field of industry. Highlights • Systematic interactions for partial shale gas and its relationship with microscopic gas behavior in nano−slits are studied. • "Dual adsorption mechanisms" and the consistency theory of gas distributions are proposed. • The enlightenment for gas development is presented from the viewpoint of microscopic mechanical mechanisms. [ABSTRACT FROM AUTHOR]

• The mass transfer experiment of CO 2 -oil system was studied. • The oil phase swelling effect was characterized through the modified PR-EOS equation. • Diffusion coefficient of CO 2 and swelling factor of oil was calculated by using a variety of methods. • Effects of CO 2 on the potential energy and self-diffusion coefficient of oil. The oil volume expansion and CO 2 diffusion are one of the main mechanism of CO 2 -enhanced oil recovery (CO 2 -EOR). This paper established a series of experiments, numerical simulations, and molecular dynamics (MD) simulations to describe mass transfer behaviors between oil–gas phases. Expressly, the CO 2 diffusion coefficient and light oil swelling factor are signification parameters to quantify and analyze these behaviors. In detail, the CO 2 diffusion coefficient and the oil swelling factor were obtained from the traditional pressure decay method and the advanced MD simulation. Synthetically, the pressure decay method and MD simulation results were mutually verified. The results showed that the equilibrium pressure was proportional to the mole fraction of CO 2 and inversely proportional to the mole fraction of light oil. The equilibrium time was proportional to the mole fraction of CO 2 and light oil. At the temperature of 333.15 K and the initial pressure of 7.5 MPa, the CO 2 diffusion coefficient in light oil was positively correlated with the relative molar proportion of CO 2 , while the light oil swelling factor was vice versa. In addition, the closer to the CO 2 -light oil interface, the greater the light oil's potential energy and self-diffusion coefficient, and the stronger the transport ability. [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]