Tuning Gas Fingering in SAGD/SAGP: Operating Windows for NCG Timing and Concentration

A Steam-and-Gas Push (SAGP) enhances energy efficiency in Steam-Assisted Gravity Drainage (SAGD) but induces gas fingering instabilities that limit the sweep efficiency. This study systematically investigates the impact of in situ-generated and externally injected Non-Condensable Gas (NCG) on fingering using fine-grid numerical simulations based on the Du-84 heavy oil reservoir. Two novel dimensionless indexes (heat–gas overlap index and Y-index) are introduced to quantitatively diagnose the fingering severity and heat transfer mechanisms. The results indicate that vertical chamber growth is convection-dominated by buoyant gas fingers, while lateral expansion remains conduction-dominated and stable. Reservoir heterogeneity significantly exacerbates fingering. An NCG concentration-dependent mechanism is established: low-dose co-injection (~0.5 mol%) suppresses minor fingering and increases oil production via a thin insulating gas cap. Conversely, excessive NCG (>5 mol%) thickens the gas cap, hindering heat transfer. Based on these mechanisms, a practical NCG operating window is proposed: a mid-stage, low-dose injection maximizes the production benefit (+4.4%), while a late-stage, moderate-dose injection (~5 mol%) enhances the oil–steam ratio (OSR) by 20.5% with minimal production loss (3.8%). This research offers critical guidance for optimizing NCG injections to mitigate fingering and improve recovery in heterogeneous reservoirs.