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 CO 2 -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 CO 2 flooding in ultra-low permeability reservoirs. The impacts of CO 2 storage pore range and miscibility on oil production and CO 2 storage characteristics during CO 2 flooding are clarified. The CO 2 flooding process is divided into three stages: oil displacement stage by CO 2 , CO 2 breakthrough stage, CO 2 extraction stage. Crude oil expansion and viscosity reduction are the main mechanisms for improving recovery in the CO 2 displacement stage. After CO 2 breakthrough, the extraction of light components from the crude oil further enhances oil recovery. During CO 2 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 CO 2 breakthrough, a small portion of the crude oil is extracted and carried into nano-scale pores by CO 2 , becoming residual oil that is hard to recover. As the miscibility increases, the CO 2 front moves more stably and sweeps a larger area, leading to increased CO 2 storage range and volume. The CO 2 full-storage stage contributes the most to the overall CO 2 storage volume. In the CO 2 escape stage, the storage mechanism involves partial in-situ storage of crude oil within the initial pore range and the CO 2 carrying crude oil into smaller pores to increase the volume of stored CO 2 . In the CO 2 leakage stage, as crude oil is produced, a significant amount of CO 2 leaks out, causing a sharp decline in the storage efficiency.