Production characteristics and synergistic effects of multiple gas-assisted steam huff and puff in shallow heavy oil reservoirs

Steam huff and puff in heavy oil recovery frequently encounters reduced sweep efficiency and declining oil production in later cycles, limiting further recovery enhancement. This study aims to evaluate how steam injection assisted by CO₂, N₂, and flue gas can improve multi-cycle production performance and reservoir pore structure, in order to identify an effective and sustainable strategy for enhanced oil recovery. Three-cycle stimulation experiments were conducted for each scheme, with oil, water, solid production, and pressure data recorded. Post-experiment analysis utilized nanoscale X-ray computed tomography and X-ray fluorescence spectroscopy to quantitatively characterize pore structure evolution. Results show that CO₂-assisted steam yielded the highest oil recovery factor after three cycles (33.4%), followed by flue gas. The temperature and pressure distributions remained relatively stable under CO₂ assistance, supporting long-term steady production. N₂ assistance effectively enhanced near-well heating and provided short-term pressure support, but experienced significant heat loss and a 74% pressure decline. Pore-scale analysis indicated that CO₂-assisted steam dissolved cementing materials and expanded pore networks, increasing porosity by 4.75%. In contrast, N₂ primarily propagated seepage channels through pressure-driven expansion. The improved pore structure correlated with enlarged steam-swept volumes, demonstrating that gas-assisted steam enhances displacement efficiency by regulating pressure-phase behavior, improving mobility ratios, and reconfiguring pore connectivity. Based on these mechanisms, a sequential N₂–CO₂ co-injection strategy is proposed, offering a novel and controllable approach for efficient thermal recovery in shallow heavy oil reservoirs.