Experimental study on the viscosity reduction mechanisms and displacement characteristics during multiple thermal fluid injection in heavy oil reservoirs

Multiple thermal fluid (MTF) injection, which co-injects steam/water, CO₂, and N₂, is a promising enhanced oil recovery (EOR) method for heavy oil reservoirs, especially thin and layered ones. However, the synergistic viscosity reduction mechanisms and displacement characteristics during MTF injection remain incompletely understood. The PVT viscosity-measuring tests were carried out to compare the apparent viscosity change of the CO 2 /water, N 2 /water, the MTF-heavy oil system under different shearing stages at 100 °C and 20 MPa. Then one-dimensional sand-packed tube with permeability of approximately 2200 mD was used to conduct displacement tests using the injected fluid under 200 °C and 20 MPa. Results reveal that the viscosity reduction process of MTF under shearing exhibited a distinct two-stage evolution, which comprised gas dissolution followed by foamy emulsion formation. The MTF-oil system achieved a viscosity reduction rate of 77%, which is higher than that of the CO₂/water-oil system (63%) and the N₂/water-oil system (37%), attributed to the synergistic dispersion of CO 2 and N 2 microbubbles and the formation of stable water-in-oil emulsions. In addition, MTF flooding attained the highest ultimate oil recovery of 66.5%, outperforming CO 2 -assisted steam flooding (62.8%) and N₂-assisted steam flooding (54.5%), with higher outlet temperatures indicating superior thermal efficiency. The gas-to-water mass ratio (GWMR) is identified as a critical parameter governing the synergy between thermal and chemical mechanisms. The optimal GWMR range is 1:1 to 2:3, within which oil recovery reached 65.0–66.5%. Excessively high GWMR would induce irreversible gas channeling that drastically reduces sweep efficiency, whereas excessively low GWMR results in slower but more uniform frontal displacement with poorer insulation properties. These findings provide a theoretical basis for optimizing MTF injection parameters in field applications.