Water flooding technology is an important measure to enhance oil recovery in oilfields. Understanding the pore-scale flow mechanism in the water flooding process is of great significance for the optimization of water flooding development schemes. Viscous action and capillarity are crucial factors in the determination of the oil recovery rate of water flooding. In this paper, a direct numerical simulation (DNS) method based on a Navier–Stokes equation and a volume of fluid (VOF) method is employed to investigate the dynamic behavior of the oil–water flow in the pore structure of a low-permeability sandstone reservoir in depth, and the influencing mechanism of viscous action and capillarity on the oil–water flow is explored. The results show that the inhomogeneity variation of viscous action resulted from the viscosity difference of oil and water, and the complex pore-scale oil–water two-phase flow dynamic behaviors exhibited by capillarity play a decisive role in determining the spatial sweep region and the final oil recovery rate. The larger the viscosity ratio is, the stronger the dynamic inhomogeneity will be as the displacement process proceeds, and the greater the difference in distribution of the volumetric flow rate in different channels, which will lead to the formation of a growing viscous fingering phenomenon, thus lowering the oil recovery rate. Under the same viscosity ratio, the absolute viscosity of the oil and water will also have an essential impact on the oil recovery rate by adjusting the relative importance between viscous action and capillarity. Capillarity is the direct cause of the rapid change of the flow velocity, the flow path diversion, and the formation of residual oil in the pore space. Furthermore, influenced by the wettability of the channel and the pore structure’s characteristics, the pore-scale behaviors of capillary force—including the capillary barrier induced by the abrupt change of pore channel positions, the inhibiting effect of capillary imbibition on the flow of parallel channels, and the blockage effect induced by the newly formed oil–water interface—play a vital role in determining the pore-scale oil–water flow dynamics, and influence the final oil recovery rate of the water flooding.
Wheat straw produced annually in the Shaanxi Guanzhong region is a potential biomass feedstock for the production of transportation fuels and specialized chemicals through combustion, pyrolysis, or gasification. In this work, the pyrolytic characteristics, evolved gas products, and kinetics of Guanzhong wheat straw and its components were first investigated with a thermogravimetry-Fourier infrared spectroscopy (TG-FTIR) system. A comparative kinetic study was conducted using different model-free methods of Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS), Kissinger, and the Coats-Redfern methods. The main pyrolysis products identified by FTIR include H2O, CH4, CO2, and CO as well as aromatics, acids, ketones, and aldehydes. Kinetic results showed that the pyrolytic apparent activation energy of the straw is approximately 200 kJ/mol obtained via FWO and KAS methods at the conversion range of 0.4 to 0.75, which was 30 kJ/mol higher than the value 171.1 kJ/mol obtained by the Kissinger method. The apparent activation energy of cellulose in its main pyrolysis region is 135.5 kJ/mol and is about three times larger than that of hemicellulose (49.5 kJ/mol). The apparent activation energy of lignin at the temperature range of 45 to 116 °C was 34.5 kJ/mol, while that value at the temperature range of 120 to 252 °C was 6.64 kJ/mol.
Water flooding technology is widely used to improve oil recovery efficiency in oilfields. The capillary barrier effect induced by the complex pore structures in the reservoir rocks is a crucial reason for the trapping of a great deal of residual oil in oil reservoirs after water flooding. However, the formation condition along with the effect on the recovery rate of the capillary barrier under different wettability conditions should be investigated further. To bridge the gap between the microscopic mechanism of the capillary barrier effect and the macroscopic mechanism of oil displacement efficiency, a simple conceptual capillary model is constructed to obtain the formation conditions of the capillary barrier using the analysis method, and its influence on macroscopic oil displacement efficiency in the porous media model with an opening angle of 45° is systematically investigated in this study using direct numerical simulations (DNS) coupled with the volume of fluid method. The results showed that the capillary barrier effect plays a significant role in the formation of the residual oil in the reservoir rock and the contact angle and the opening angle are the primary factors for the formation of the capillary barrier. The capillary force is the driving force when the oil–water interface advances in the throat channel under water-wet conditions, while the capillary force hinders the movement of oil–water movement when the liquid flows out of the throat channel and when θ + β > 90o. Furthermore, the highest oil displacement efficiency is achieved at the intermediate capillary number and in the case that the minimum conditions of occurrence of the capillary barrier phenomenon are satisfied. This is of great significance for controlling the optimized contact angle to further enhance the oil recovery rate of current oil reservoirs using waterflooding technology.
The low oil recovery rate observed in current oil fields is largely attributed to the presence of remaining oil trapped in the pores of porous media during waterflooding. To improve the recovery rate, it is imperative to gain an understanding of the oil–water flow characteristics and displacement mechanisms during waterflooding, as well as to elucidate the underlying mobilization mechanisms of residual oil at the pore scale. In this paper, we explore these issues in depth by numerically investigating the influence of factors such as water injection velocities, oil–water viscosity ratios, and wettability conditions on pore-scale oil–water flow characteristics and oil recovery rate. To this end, we employ a direct numerical simulation (DNS) method in conjunction with the volume of fluid (VOF) method to study the microscopic displacement mechanisms of waterflooding in a reconstructed two-dimensional digital rock core based on micro-CT technology. In addition, the particle tracing method is adopted to identify the flow path and dominant areas during waterflooding in order to mobilize the residual oil within the pores. The findings indicate that the oil–water flow characteristics in porous media are determined by the interplay between capillary and viscous forces. Furthermore, the oil recovery rate is 10.6% and 24.7% lower under strong water-wet and oil-wet conditions than that (32.36%) under intermediate wettability conditions, and the final oil recovery rate is higher under water-wet conditions than under oil-wet conditions. The seepage path and the dominant areas are directly linked to the capillarity formed during waterflooding. The findings of this study are significant in terms of enhancing the recovery rate of residual oil and provide a novel perspective for understanding the waterflooding process.
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