Evaluation of Shale Oil Extraction by Supercritical CO₂ and Analysis of Influencing Factors

To define the influence mechanism of fractures and pressure on the extraction of shale oil by supercritical CO₂, the core extraction experiment by supercritical CO₂ was conducted on the basis of identifying the pore size distribution, specific surface area, and pore volume of experimental shales. The improved magnetic suspension balance high pressure adsorption instrument was used to measure the shale mass change under high temperature and pressure in real time. Combined with the nuclear magnetic resonance (NMR) T₂ spectrum of shale, the extraction efficiency of shale oil by supercritical CO₂ was accurately measured, and the producing characteristics of shale pores and the lower limit of producing pore size in the extraction process were defined. The experimental results show that the target reservoir shale mesopore (pore size of 2~50 nm) is the most developed, accounting for 69.72% and 73.47% of the total pore volume and total specific surface area. However, macropores (>50 nm) are the least developed, accounting for only 4.45% and 10.77% of the total pore volume and total specific surface area. The crude oil mainly exists in the pores with a small pore size of 1.4~120 nm. The extraction effect of CO₂ on the crude oil in the pores with large pore size (>86 nm) is better than that in the pores with small pore size (≤86 nm). Fractures can increase the contact area between CO₂ and shale oil in the matrix, accelerate the mass transfer rate of oil and gas, improve the depth of matrix production, and reduce the shale oil seepage resistance and the lower limit of pore production. However, the CO₂ extraction efficiency is not only related to the number of fractures but also affected by matrix permeability and fracture-matrix connectivity. The lower limit of pore size for CO₂ production decreases with the increase in injection pressure from 6.54 nm at 8 MPa to 3.27 nm at 18 MPa. The research findings provide a reference for enhancing the recovery rate of shale oil by injecting CO₂.