Oil production characteristics and CO2 storage mechanisms of CO2 flooding in ultra-low permeability sandstone oil reservoirs

Using the ultra-low permeability reservoirs in the L block of the Jiangsu oilfield as an example, a series of experiments, including slim tube displacement experiments of CO₂-oil system, injection capacity experiments, and high-temperature, high-pressure online nuclear magnetic resonance (NMR) displacement experiments, are conducted to reveal the oil/gas mass transfer pattern and oil production mechanisms during CO₂ flooding in ultra-low permeability reservoirs. The impacts of CO₂ storage pore range and miscibility on oil production and CO₂ storage characteristics during CO₂ flooding are clarified. The CO₂ flooding process is divided into three stages: oil displacement stage by CO₂, CO₂ breakthrough stage, CO₂ extraction stage. Crude oil expansion and viscosity reduction are the main mechanisms for improving recovery in the CO₂ displacement stage. After CO₂ breakthrough, the extraction of light components from the crude oil further enhances oil recovery. During CO₂ flooding, the contribution of crude oil in large pores to the enhanced recovery exceeds 46%, while crude oil in medium pores serves as a reserve for incremental recovery. After CO₂ breakthrough, a small portion of the crude oil is extracted and carried into nano-scale pores by CO₂, becoming residual oil that is hard to recover. As the miscibility increases, the CO₂ front moves more stably and sweeps a larger area, leading to increased CO₂ storage range and volume. The CO₂ full-storage stage contributes the most to the overall CO₂ storage volume. In the CO₂ escape stage, the storage mechanism involves partial in-situ storage of crude oil within the initial pore range and the CO₂ carrying crude oil into smaller pores to increase the volume of stored CO₂. In the CO₂ leakage stage, as crude oil is produced, a significant amount of CO₂ leaks out, causing a sharp decline in the storage efficiency.