# Investigation of the Effect of Capillary Barrier on Water-Oil Movement in Water Flooding
## 1 Introduction  
Crude oil has a significant impact on industrial civilization development and economic prosperity. According to the British oil company's 2020 Statistical Review of World Energy [1], crude oil accounts for approximately 33.1% of global energy consumption. However, the oil reserves of the world are limited, and the overall world oil reserves are approximately 24.46 billion tons as of the end of 2019 based on a proven/probable basis, and the world's oil reserves can last up to 49.9 years at current production levels [1]. Crude oil is recovered from the oil reservoir via the natural energy and pressure of the reservoirs in the primary oil recovery stage. Waterflooding technology is extensively applied to further enhance the oil recovery rate in oilfields in the initial stage of crude oil extraction, which will ultimately result in a high ratio of water to oil in producing wells and a low oil recovery rate, thus reducing the oil recovery rate at a later stage of oil extraction [2]. It is well acknowledged that ~60% ofthe residual oil still remains in the reservoir rock after waterflooding [3]. The crude oil recovery process can be characterized by a typical multiphase flow within porous media. The flowing conditions, the injected fluid properties, and the pore structure characteristics play a significant role in the multiphase flow dynamics in porous media. The configurations of residual oil trapped in the water-wet porous medium mainly include irregular strings, streaks, squirts, and discontinuous oleic saturations [4; 5], and a great deal of residual oil is trapped in dead-end or semi dead-end pores. While under oil-wet conditions, the residual oil is either attached to the pore walls in the form of oil film or it is trapped in dead-end pores [5]. An oil bank created ahead of the frontal zone will also result in discontinuous ganglia of the residual oil within the larger pores [6]. The capillary entrapment and bypassing induced by the heterogeneity of reservoir rock play a crucial role in the residual oil formation, and the competition of viscous and capillary forces at pore scale determines oil-water two-phase flow dynamics during waterflooding [7; 8; 9]. Nanofluids, a kind of fluid wherein nanoparticles are suspended in a base fluid, can enormously remobilize the trapped residual oil after waterflooding via reducing the interfacial tension (IFT), improving rheological properties, and reducing the droplet coalescence compared with chemical flooding [10]. A variety of novel nanofluids are developed to enhance oil recovery. [PERSON] [11] found that a kind of nanofluid prepared using Janus-graphene oxide nanosheets can considerably enhance oil recovery by restoring wettability to mixed-wettability and reducing the viscous fingering phenomenon and revealed that the Janus nanofluid can be a promising nanomaterial for nanofluid flooding. [PERSON] [12] pointed out that microemulsions-based nanofluids can recuperate bypassed oil and remaining oil caught in the reservoir via plunging interfacial tension and modifying the rock wettability, and this can be a potential strategy for improving oil recuperation. Core flooding tests conducted by [PERSON] [13] using hydroxyl-functionalized silicate-based nanofluids indicated that the nanofluid flooding offers \(\sim\)12-14.5% additional oil in addition to waterflooding and revealed that higher capillary forces resulted from interfacial tension reduction can immobilize trapped oil. [PERSON] [14] numerically investigated the effect of the addition of nanoparticles to the displacing fluid on the oil recovery rate and found that the use of nanosuspensions is most effective for low-permeability reservoirs with high oil viscosity. The state-of-the-art nanofluid flooding literatures revealed that nanofluid flooding achieved promising results in the laboratory and is still in the development stage. The capillary effect is one of the important causes of remaining oil.