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[Display omitted] • The CO 2 recovery from different types of quartz nanopores was discussed. • The higher the amount of CO 2 dissolved could improve the flow velocity of oil and gas in nanopores. • There were three flow modes in the oil–water two-phase flow process: WDOWF, WFOWF and ODOWF. • The effect of CO 2 on the distribution and flow of oil–water depends on the flow modes of oil–water. Multiphase flow in nanoporous media is ubiquitous in various fields, including geophysics, physical chemistry, and bioengineering. Tight sandstone reservoirs have relatively well-developed nanoscale pores. Understanding the mechanism of multiphase flow in nanoporous media is crucial for enhanced oil recovery. In this study, the pore distribution characteristics of tight sandstone cores were visually observed and analyzed through Wood's metal injection experiments and scanning electron microscopy. Subsequently, molecular dynamics simulations were used to establish a multiple-pore model with/without bound water. The mainstream channel (MC) was selected as the research pore, and multiple groups of molecular dynamics models with different oil–gas–water ratios were established to simulate multiphase flow. The results showed that the micro-nano-scale pore throats of the tight sandstone were well developed, necessitating a study of fluid flow characteristics within the nanopores. In the process of CO 2 injection production, the bound water could occupy dead-end pores (DEP) and small channels (SC), which was beneficial to the oil recovery. When there was no bound water, the MC mainly relied on the CO 2 displacement for oil recovery, while the DEP of the MC, the DEP of the SC, and the SC mainly relied on oil expansion and CO 2 extraction. In the MC nano-confined fluid flow process, the larger the proportion of CO 2 was, the lower the viscosity of the oil was, and the more easily it could flow. However, when water was present, three flow modes formed: water drop oil–water flow (WDOWF), water film oil–water flow (WFOWF), and oil drop oil–water flow (ODOWF), among which ODOWF had the greatest influence on the flow of bound fluid in the nanopores. The injection of CO 2 could improve the flow in the WDOWF and WFOWF modes and had no positive effect on the ODOWF. [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]

[Display omitted] • Revealing the effect of CO 2 on oil potential energy in nanopores. • CO 2 solubility and oil swelling factor explained within nanopores. • Influence of CO 2 on the recovery law of different components in oil. • Analysis of different water film thickness on CO 2 enhanced oil recovery. Understanding the mechanisms of CO 2 extraction or flooding are vital for enhancing oil recovery (EOR) in tight reservoirs. In this study, the CO 2 EOR mechanism in the displacement-affected area (DPAA) and diffusion-affected area (DFAA) of quartz nanopores were thoroughly investigated using molecular dynamics simulation techniques. First, the following two contents were mainly simulated, namely CO 2 flooding oil in the single/double nanopores of DPAA and CO 2 extraction oil in dead-end nanopores of the DFAA with and without the water film. Then, tight oil potential energy, threshold capillary pressure, CO 2 solubility, and oil swelling in nanopores were calculated to clarify the effects of CO 2 on oil transport. Moreover, different CO 2 injection/flowback rates and different water film thicknesses on dead-end nanopores on oil recovery were discussed. In the DPAA, the CO 2 solubility and the oil swelling factor gradually decreased with distance from the CO 2 -oil interface (Y = 0 nm), where the higher the injection rate, the more easily the CO 2 dissolved in the oil. However, the injection rate of CO 2 was inversely proportional to oil recovery. In addition, it took longer for the displacement efficiency in the 6 nm pore of double pores to reach the same displacement efficiency as in the single 6 nm pore. In the DFAA, the effect of flowback rate on the displacement efficiency of oil was relatively low. However, the thickness of the water film was a key factor that affected the oil displacement efficiency in the DFAA. [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]

CO 2 huff and puff (HNP) is one of the most effective methods to improve tight oil recovery after the primary depletion process. The seepage mechanisms between CO 2 and crude oil are complicated in porous media during CO 2 HNP process. Therefore, in this paper, the CO 2 HNP process of Chang-7 tight oil reservoir, Ordos Basin, China, was studied by nuclear magnetic resonance (NMR) technology, microscopic observation and numerical simulation. Experimentally, using NMR technology and microscopy methods, the distribution characteristics of residual oil during CO 2 HNP process were measured intuitively. Numerically, a group of core-scale and field-scale simulations considering molecular diffusion and asphaltene precipitation were established to further verify and elongate the experimental results. The results show that at the initial state, the crude oil in the tight core was mainly distributed in nanopores, sub-micro-nanopores and sub-micropores, where the oil content exceeded at least 73% in these pores. During CO 2 HNP process, the oil recovery was more pronounced for the 1st and 2nd rounds than for 3rd to 5th rounds. Notably, even if the cores with more nano-pores were more favorable for the 4–5th CO 2 HNP rounds, the oil molecules in nanopores were still difficult to be available. Moreover, the CO 2 sweep scope could be divided into displacement affected region and diffusion affected region. CO 2 could effectively drive the crude oil in the displacement affected region. While the oil could be successfully displaced by dissolved gas flooding in the diffusion affected region only under the appropriate conditions. Meanwhile, the core-scale numerical models confirmed that it was necessary to consider molecular diffusion and asphaltene precipitation factors, which would make the simulation results in line with the experiment. In terms of the ultimate oil recovery, the field-scale model only considering the diffusion (2.456%) > the model both considering the diffusion and asphaltene (2.436%) > the model without considering the diffusion and asphaltene deposition (2.412%) > the model only considering the asphaltene deposition (2.388%). • The oil were difficult to be available in nanopores due to the nano-constraint effect. • The CO2 displacement area was associated with diffusion and asphaltene precipitation. • SP-T2 spectrum and microscopic image were applied to obtain CO2 sweep scope. • A group of simulations were established to analyse the effect of CO2 sweep scope. [ABSTRACT FROM AUTHOR]

Tight oil reservoirs are characterized by a low porosity, low permeability, and strong heterogeneity. The macropores, throats, and microcracks in reservoirs are the main seepage channels, which affect the seepage law in the reservoirs. In particular, oil-water two-phase flow in different types of pores requires further study. In this study, two groups of online NMR displacement experiments were designed to study the seepage characteristics of tight oil reservoirs. It was found that the main seepage channels for oil-water two-phase flow are the microcracks, large pores, and throats in the reservoir. The large pores are mainly micron and submicron scale in size. The oil in the small pores is only transferred to the large pores through imbibition to participate in the flow, and there is no two-phase flow. Based on the influence of different pore structures on the seepage law of a tight reservoir, the pores were divided into seepage zones, and a multistage seepage model for tight reservoirs was established. Based on this model, the effects of the imbibition, stress sensitivity, threshold pressure gradient, and Jamin effect on model's yield were studied. The results show that imbibition is no longer effective after a while. Owing to the stress sensitivity, the threshold pressure gradient, and the Jamin effect, oil production will be reduced. As the parameter value increases, the oil production decreases. The production decreases rapidly in the early stage of mining while decreases slowly in the later stage, exhibiting a trend of high yield in the early stage and stable yield in the later stage. [ABSTRACT FROM AUTHOR]