Mechanisms and characteristics of thermal CO₂ assisted electric heating to improve reservoir heating efficiency

Low-to-medium maturity (LMM) shale oil represents a substantial unconventional resource with the potential to supplement conventional hydrocarbon supplies; however, its economic viability depends critically on effective in-situ thermal conversion. In-situ conversion technology is considered the most promising approach for the green and low-carbon development of such resources. Currently, electrical heating — the predominant downhole heating method — suffers from low heating efficiency and slow hydrocarbon conversion rates. To address these limitations, laboratory-scale simulation experiments are performed to compare pure electrical heating and CO₂-assisted electrothermal heating. Through systematic data analysis, the underlying mechanisms by which thermal CO₂ synergy enhances heating efficiency were elucidated. Subsequently, a coupled thermal-hydraulic numerical model was developed using COMSOL Multiphysics and rigorously validated against experimental data, enabling accurate prediction of reservoir-scale temperature field evolution. Sensitivity analyses were performed on key operational parameters affecting the CO₂-assisted heating process. Results indicate that initial injection temperature, injection rate, reservoir porosity, gas specific heat capacity, and gas type are the primary factors governing temperature distribution. Specifically, increasing injection temperature and injection rate, while decreasing gas specific heat capacity, enhances heating efficiency. Under constant injection rate conditions, lower reservoir porosity favors higher heating efficiency. Among the gases tested, CO₂-assisted heating demonstrated the highest thermal performance, whereas gas thermal conductivity exhibited negligible influence. Over a 500 d heating simulation, the effective heated volume (temperatures ≥300 ℃) reached 371.5 m³ under pure electrical heating, compared to 1293.5 m³ under CO₂-assisted electrical heating. These findings demonstrate that thermal CO₂-assisted electrical heating significantly improves reservoir heating efficiency and expands the thermally affected zone compared to conventional electrical heating, thereby optimizing overall heating and production performance.