Sequence Effects of Viscosity Reduction and Gel Conformance Control During Immiscible CO2 Huff-n-Puff in Heterogeneous Extra-Heavy Oil

Preferential flow limits productive CO2–oil contact in heterogeneous extra-heavy-oil systems. This study compares viscosity reduction followed by gel conformance control (VR → PA) with the reverse order during immiscible CO2 huff-n-puff. PVT measurements, dissolution tests, chemical screening, and six treatment schemes in parallel artificial cores with independently collected oil production were combined. At 56.6 °C and 12.01 MPa, below the measured minimum miscibility pressure of 35.9 MPa, dissolved CO2 reduced live-oil viscosity from 5505 to 45 mPa·s. Final recovery was 7.16 ± 0.05% for VR → PA, 5.77 ± 0.05% for CO2 alone, and 6.77 ± 0.03% for PA → VR (two independent dual-core models per scheme; mean ± half range). The corresponding gains were 1.39 percentage points over CO2 alone and 0.39 percentage points over the reverse order. After PA in the final VR → PA cycle, the high-permeability oil-production share fell from 69.4% to 60.5%, and low-permeability incremental recovery reached 2.17%. The water and single-agent controls, separate plugging tests, and cycle-resolved responses support a conceptual sequence of mobilization followed by conformance improvement. The sequence ranking is an observation under the tested conditions and does not establish a statistically significant sequence effect. The contribution to CCUS-EOR is improved oil recovery and a treatment strategy for contact management; CO2 utilization and storage enhancement were not quantified because the cyclic tests did not record a complete CO2 balance.