# Microscopic Experiments to Assess the Macroscopic Sweep Characteristics of Carbon Dioxide Flooding
## 1 Introduction  
Alongside the rapid development of conventional oil reservoirs worldwide, the reserves and annual production of conventional and high-quality oil reservoirs are declining, while the reserves and production of tight oil and gas are steadily increasing. These resources have now become a significant portion of the global oil and natural gas production. The Lunnan oilfield in the Tarim Basin is one of the important onshore oilfields in China, known for its substantial geological reserves [1]. However, the increase in carbon dioxide emissions is exacerbating global climate change and increasing the frequency and intensity of extreme weather events such as droughts, floods, and storms. Rising carbon dioxide levels contribute to global warming, accelerating the melting of polar ice caps and leading to a rise in sea levels. In some regions, higher temperatures and insect pests may severely impact global grain production, potentially causing a decline in yields. As the Lunnan oilfield develops, traditional water flooding technology faces challenges in maintaining a satisfactory recovery rate. Carbon dioxide flooding technology leverages the supercritical properties of carbon dioxide by injecting it into underground oil formations to improve the fluidity and recovery of crude oil. Compared to traditional water and polymer flooding, CO\({}_{2}\) flooding offers environmental benefits, enhances recovery, and reduces production costs. Consequently, CO\({}_{2}\) flooding technology has been implemented to enhance oil recovery in the Lunnan oilfield in the Tarim Basin [2; 3; 4; 5]. Some significant achievements have been made in the study of CO\({}_{2}\) flooding. In the oil field displacement schemes being implemented in the United States, the volume of injected carbon dioxide typically accounts for about 30% of the hydrocarbon pore volume, with enhanced oil recovery ranging from 7% to 12%. In Daqing oilfield, a field test of CO\({}_{2}\) immiscible flooding was conducted using high-purity carbon dioxide (96%) as a byproduct from the hydrogenation workshop in a refinery. Although gas channeling caused by reservoir heterogeneity affects the sweep efficiency, it still achieves the overall effect of reducing water cut and improving oil recovery. [PERSON] et al. (2020) studied the underground diffusion behavior of CO\({}_{2}\). Through numerical simulation, they concluded that, when the temperature of the underground environment exceeds the critical temperature (31.1 \({}^{\circ}\)C) of CO\({}_{2}\), CO\({}_{2}\) exists in a supercritical state. When the critical pressure (72.9 MPa) is exceeded, CO\({}_{2}\) remains in a supercritical state, and its diffusion and migration are influenced. CO\({}_{2}\) can move and diffuse more easily in reservoirs with high porosity and high permeability [6]. [PERSON] et al. (2021) studied the interaction between crude oil and CO\({}_{2}\). By analyzing the composition, properties, and temperature of crude oil, they found that there are complex interactions between crude oil and CO\({}_{2}\). They concluded that the density and viscosity of crude oil can be altered due to the dissolution of CO\({}_{2}\), which impacts the recovery efficiency and recovery rate of the reservoir. In the process of injecting CO\({}_{2}\) to extract crude oil, the ideal driving pattern is the formation of two-phase flow. However, the deposition of asphaltenes may negatively affect the permeability of the core. The oil-vapor interfacial tension is also influenced by the acting pressure and temperature when CO\({}_{2}\) interacts with crude oil, which directly relates to the ease or difficulty of extracting the crude oil [7]. [PERSON] et al. (2022) investigated the effect of CO\({}_{2}\) flooding on the ripple characteristics and recovery rate of underground oil reservoirs. Through laboratory and numerical simulations, they concluded that CO\({}_{2}\) flooding can significantly improve the ripple characteristics and increase the recovery rate of underground oil reservoirs.