Non-hydrocarbon gas injection followed by steam-gas co-injection for heavy oil recovery enhancement from fractured carbonate reservoirs

Considerable heavy oil is accumulated in naturally fractured carbonate reservoirs, with very low oil recovery efficiency through costly production techniques. In this experimental study, the effects of different non-hydrocarbon gases on the performance of gas injection process followed by steam-gas co-injection on heavy oil recovery from low permeable full sized carbonate cores were investigated at reservoir conditions in a long fractured laboratory model. Three different gases: pure CO₂, pure N₂, and their mixtures as synthetic flue gas were used to study the performance of gas injection process. Also, the tests were continued by steam-gas co-injection process that gas was injected with specific steam/gas ratio at steam saturation temperature condition. In the course of experiments, oil and water productions, pressure and temperature were monitored carefully.It was found that during fracture depletion, the piston-like displacement by N₂ injection and oil foaming by CO₂ injection are the most important mechanisms affecting the oil recovery performance before gas breakthrough. The results showed that after gas breakthrough (GBT), the ultimate oil recovery based on residual oil at GBT for CO₂ injection was 58.4% (14.8% by gas injection and 43.6% by steam-gas co-injection), for flue gas injection was 73.8% (9.8% by gas injection and 64% by steam-gas co-injection) and for N₂ injection was 47% (13.5% by gas injection and 33.5% by steam-gas co-injection). The results of oil recovery and production rate indicated that the flue gas injection during gas injection and steam-gas co-injection processes was outperforming for heavy oil recovery enhancement from fractured reservoirs.