Microscopic mechanisms of CO2 injection mitigating condensate dropout and fracture network effects in fractured condensate gas reservoirs

Condensate gas reservoirs are important to global gas development. However, during their exploitation, pressure depletion induces condensate dropout and retention, impairing gas-phase permeability and reducing recovery efficiency. CO2 injection is a key strategy for mitigating condensate dropout through miscible displacement, solubility enhancement, pressure maintenance, and carbon sequestration. A microfluidic platform was employed to visualize the effects of CO2 injection on condensate flow dynamics under varying depletion pressures in fractured reservoirs with three distinct fracture network patterns, including the Parallel Unilateral-Flow, the Crossed Unilateral-Flow, and the Parallel Bilateral-Flow Fracture Networks. The regulatory roles of the fracture network patterns and injection timing in CO2 sweep efficiency were also examined. Experimental results indicate the following: (1) When pressure was depleted from 40 to 30 MPa, the oil content ratios of the Parallel Unilateral-Flow Fracture Network increased from 20.000% to 31.986%, the second pattern increased from 19.866% to 31.891%, and the third pattern increased from 19.735% to 28.164%, correlating closely with the pressure–volume–temperature phase behavior. (2) CO2 injection reduced condensate dropout, with an injection pressure of 40 MPa yielding higher efficiency than 30 MPa by enhancing gas-phase solubility and reducing interfacial tension, promoting droplet fragmentation. (3) The Parallel Bilateral-Flow Fracture Network exhibited the most effective mitigation due to superior fracture connectivity and a regular primary fracture distribution, while flow path interference limited Crossed Unilateral-Flow performance. This study developed a high-temperature, high-pressure microscopic visualization platform to evaluate the effect of CO2 injection on condensate flow in fractured reservoirs, providing theoretical and technical insights for enhanced condensate gas recovery.