Mechanism of gas flooding enhanced oil recovery in ultra-low permeability tight oil reservoirs

Abstract

The effective development of ultra-low permeability tight oil reservoirs (permeability < 0.1 mD) represents a major challenge in China's unconventional oil and gas sector. This study focuses on the Chang-7 tight sandstone formation in the Changqing Oilfield and systematically investigates the adsorption–transport behavior and displacement mechanisms of CO2 and N2 in ultra-low permeability porous media, using high-pressure isothermal adsorption experiments, core flooding experiments, and numerical simulation. The results indicate that under conditions of 30 °C and 20 MPa, the Langmuir adsorption capacity of CO2 (1.82 mmol·g−1) is approximately 4.6 times that of N2 (0.40 mmol·g−1). Under a pressure gradient of 5.0 MPa·m−1, the flow velocity of CO2 (4.20 m·d−1) is approximately 2.4 times that of N2 (1.75 m·d−1). At 20 MPa and 30 °C, the CO2 flooding recovery factor (74.8%) exceeds that of N2 (48.3%) by 26.5 percentage points. The high-pressure CO2 intermittent injection strategy achieves a recovery enhancement of up to 24.8%, with the best overall economic performance. These findings provide a scientific basis for the large-scale implementation of gas flooding EOR in ultra-low permeability tight oil reservoirs.