The displacement efficiency of a CO2 flood has two components: microscopic and macroscopic displacement efficiency. This work focuses on the factors that affect microscopic displacement efficiency. The factors are pressure, temperature, oil composition, CO2 purity, fluid properties, and reservoir pore configurations. These factors contribute to the phase behavior of a particular CO2 flood. Mixing as a result of diffusion improves microscopic displacement efficiency as compared to mixing by dispersion, which decreases microscopic displacement efficiency. Phase behavior governs the development and sustenance of CO2 miscibility with crude oil at reservoir conditions. Lower injection rates and higher residence time increases mass transfer between the oil and CO2. Improved transfer leads to oil swelling and viscosity and surface tension reduction that improve microscopic displacement efficiency. Uniform pore geometry and favorable pore structure causes higher microscopic displacement efficiency. The presence of dead-end pores decreases the displacement process. The volume and distribution of water within the pore affects the contact area between CO2 and the crude oil and can impede the miscibility process. As possible, this work quantifies the affect some of the factors have on microscopic displacement efficiency and suggests means of improving the displacement process.