More than half of all recoverable oil reserves are found in carbonate rocks. Most of these fields are highly fractured and develop different zonations during primary and secondary recovery stages; therefore, they require a different development approach than conventional reservoirs. Experimental results for water-alternating-gas injection [WAG] and foam-assisted water-alternating-gas [FAWAG] under secondary and tertiary recovery conditions were used to investigate these enhanced oil recovery [EOR] methods in gas invaded reservoirs. The relative permeability curves of the cores and the fitting foam parameters were derived from these experiments through history matching. These findings were then used in a quarter
5-spot, cross-sectional, and a sector model of a carbonate reservoir where a double five-spot setup was implemented. The fracture and matrix properties' impact on the recovery was illustrated through the cross-sectional model.
The gas mobility reduction effect of the FAWAG was more noticeable than that of WAG. The apparent viscosity of the gas was increased due to the Foam presence, which caused a diversion of the gas from the fractures into the matrix blocks. This greatly enhanced the sweep efficiency and led to higher oil recovery. The gas front was much sharper, and gravity overrides by the gas were much less of a concern.
The properties of the fracture network also have a significant effect on the recovery. Oil recovery was found to be most sensitive to fracture permeability. At the same time, sweep efficiency increases substantially, improving the recovery rate in the early injection stages and differs slightly at the ultimate recovery. However, a lower fracture permeability facilitates gas entry into the matrix blocks.
The results of the reservoir sector model were similar to the core and pilot. However, the WAG injection recovered more of the uppermost layers, whereas significant portions of the lowest layer were not effectively recovered. In contrast, FAWAG was more effective in the lowest layer of the reservoir. The FAWAG was a beneficial aid in the recovery of gas invaded fractured reservoirs, increasing the oil recovery factor with respect to WAG.