Study on CO2 Foam Acid Phase Evolution and Diverting Acidizing Performance in Heterogeneous Carbonate Reservoirs

A coupled numerical model was developed to investigate the phase evolution and diverting acidizing performance of CO2-foamed acid in heterogeneous carbonate reservoirs. The model integrates CO2 phase evolution, gas–liquid two-phase Darcy flow, acid–rock reactions, heat transfer, pore-structure evolution, and foam mobility control. By dynamically updating CO2 thermophysical properties in response to local temperature and pressure variations, the model captures the coupled effects of phase behavior on foam quality, apparent viscosity, flow resistance, and acid redistribution between high- and low-permeability layers. The effects of reservoir temperature, foam quality, and permeability contrast on wormhole propagation, foam distribution, and stimulation depth in the low-permeability layer were systematically evaluated. Under the simulated conditions, low-permeability-layer stimulation initially increased with reservoir temperature, reached its maximum near 393 K, and declined at 413 K. This non-monotonic behavior reflects the balance among CO2 phase behavior, foam stability, and acid–rock reaction rate: moderate temperatures promote the formation of a favorable foam region and effective diversion, whereas excessive temperatures weaken foam stability and accelerate acid consumption near the wellbore. Foam quality also exhibited a non-monotonic influence on diversion performance. Foam qualities of 60–80% provided strong mobility control and effective acid redistribution, while a foam quality of 90% restricted liquid-acid transport because of excessive near-wellbore foam accumulation. In contrast, the pure-acid system preferentially entered the high-permeability layer and broke through at approximately 0.3 PV. Increasing permeability contrast weakened foam retention and intensified preferential channeling; at a permeability contrast of 8, the wormhole length in the low-permeability layer was less than 50% of that obtained at a contrast of 4. These results demonstrate that phase-dependent foam resistance can redirect acid from preferential high-permeability channels toward low-permeability regions. This study defines an effective operating window for CO2-foamed-acid diversion and provides a theoretical basis for designing diverting acidizing treatments in heterogeneous, high-temperature carbonate reservoirs.