Pore-scale insights into CO2-EOR performance in depleted oil reservoirs by miscibility – compared with WAG injection

Accurate evaluation of effectiveness and performance of CO₂ miscible flooding in depleted oil reservoirs requires a deep understanding of temporal-spatial evolution in the contact-miscibility-displacement processes between the injected CO₂ and highly-disperse residual oil in heterogeneous porous rocks. In this study, we conducted a pore-scale investigation on CO₂ miscible flooding in a heterogeneous high-water-cut sandstone based on our self-developed lattice-Boltzmann models which have been rigorously validated against theoretical and molecular dynamics analyses. Our results reveal that: 1) Prior to CO₂ breakthrough, CO₂-water displacement dominates three-phase CO₂-oil-water flooding process, creating the preferential displacement path. Under compression, pore water has infiltrated into some adjacent pores from the edge of the preferential displacement path, setting the stage for subsequent CO₂ entry, and accordingly creating necessary conditions for oil-CO₂ contact; 2) After CO₂ breakthrough, more CO₂ penetrates the pores outside the preferential displacement path, strengthening CO₂-oil contact, and gradually expanding local miscible zones. This makes the oil miscible with CO₂ continue to be displaced at a gradually decreasing rate until CO₂ flooding reaches stable state. The efficiency of CO₂ miscible flooding is limited mainly by some specific pore structures where too narrow pore entrance impedes either CO₂-oil contact or fluid migration; 3) The performance advantage of immiscible flooding mode is mainly manifested in the post-breakthrough stage. The final residual oil saturation is significantly decreased compared to immiscible flooding mode, only half of the latter. At this stage, adopting the optimized water-alternating-gas (WAG) injection strategy can enhance oil recovery in immiscible flooding systems to the equivalent level with miscible flooding mode. However, its performance for enhancing the occupancy of CO₂ on pore space oscillates periodically, and is generally inferior to miscible flooding, with extended injection duration increasing costs and smaller CO₂ density reducing storage mass.