Impacts of Petrophysical Heterogeneity on Efficacy of CO2 Enhanced Oil Recovery Targeting Sandstones with Spanning Permeabilities: An Experimental Investigation

Summary Petrophysical heterogeneity plays a crucial role in carbon dioxide (CO2)-enhanced oil recovery (EOR) in unconventional reservoirs. To analyze how heterogeneity affects CO2 injection and EOR recovery potential, we designed a suite of cyclic CO2 injection experiments on a group of sandstone samples with spanning permeabilities (K = 0.01 to 10 md). By integrating mercury intrusion capillary porosimetry (MICP) and nuclear magnetic resonance (NMR) measurements, we established relationships between capillary-pore-throat heterogeneity and hydrocarbon recovery with a heterogeneity-based CO2 (HET-CO2) recovery model. Through this work, several important observations were made: The three permeability level samples have average recoveries of 72.75%, 51.24%, and 27.74%. Capillary heterogeneity analyses indicate that weakly adsorbed oil in intragranular characterized mesopores is the predominant producer (>50%) in low- to ultralow-permeability samples (K < 1 md). However, pore heterogeneity analyses further imply that the CO2 injection should focus on recovering more adsorbed oil from mesopores for ultralow permeability rocks with K = 0.1 to 1 md in this area, where the T2 response of these pores at 3–20 ms and pore radius at 0.18–1.20 μm was identified by NMR and MICP. Throat heterogeneity results show that the critical extractable pore radius ranges differently (16–35 nm), mainly influenced by the rock type related to throat tortuosity affecting discharge of oil-soluble gas at the puff stage. The HET-CO2 model predicts that recovery factor in heterogeneous formations with a critical pore radius is related to the saturation of minimum hydraulic flow unit (HFU). This model can be used in optimizing development plans for better extraction of bound oil. The findings from this work can further facilitate EOR efficacy prediction and optimization of CO2 enhanced recovery techniques for heterogeneous reservoirs.