From pore-scale mechanisms to hydraulic-work analysis: a microfluidic study of CO2-EOR via WAG, FWAG, and PWAG

Abstract

To mitigate severe gas channeling and poor volumetric sweep during CO 2 flooding in low-permeability conglomerate reservoirs, WAG and its enhanced variants, FWAG and PWAG, are widely used for mobility control. However, pore-scale evidence remains limited on how flow resistance, flow diversion, and remaining-oil mobilization evolve during successive alternating-injection cycles in heterogeneous pore networks. Here, we use a high-temperature/high-pressure (HTHP) micromodel reconstructed from reservoir casting thin sections to compare CO 2 -WAG, CO 2 -FWAG, and CO 2 -PWAG under reservoir conditions (65 °C, 33 MPa). We introduce cumulative hydraulic work, W h , calculated by integrating the differential pressure over the injected volume, to quantify the magnitude and persistence of hydraulic resistance during each displacement stage. Continuous scCO 2 flooding after waterflooding rapidly forms stable gas channels and leaves substantial bypassed oil. CO 2 -WAG improves conformance through intermittent liquid-phase snap-off and a transient Jamin effect, but the resistance decays as water slugs become unstable. CO 2 -FWAG provides more persistent resistance through foam-lamella regeneration and restructuring, while independently measured AOS-induced oil–water IFT reduction at 65 °C and ambient pressure is consistent with the observed breakup and dispersion of larger remaining-oil bodies. CO 2 -PWAG generates the strongest and most durable resistance in dominant channels because the higher apparent viscosity of the polymer solution sustains viscous flow control and promotes cluster-like remaining-oil mobilization. Across the three schemes, larger hydraulic-work increments were associated with greater incremental oil recovery, indicating that more persistent flow resistance promoted the displacement of previously bypassed oil under the investigated conditions.