This paper presents a method for improving oil recovery from heterogeneous mixed to oil-wet carbonate reservoirs. In reservoirs where a high-permeability zone is above a low permeability zone, under water flooding the injected water tends to flow through the upper zone along the high permeability layers with no or very slow cross flow of water into the lower zone, resulting in very poor sweep of the lower zone. It has been demonstrated in earlier publications that this water override phenomenon is caused by capillary forces which act as a vertical barrier and counteracts gravity force in cases where permeability varies between layers for mixed to oil-wet reservoirs.

There is significant scope for improving oil recovery from such type of heterogeneous mixed to oil-wet carbonate reservoirs. Gas injection is known to improve displacement efficiency by reducing residual oil saturation. However, for reservoirs of high permeability contrast especially when the high permeable layers are in the upper part of the reservoir, conventional gas injection (immiscible or miscible) becomes less effective because of gravity override and/or viscous fingering caused by unfavourable mobility ratio compounded by geological heterogeneity. The main challenge to gas injection in such reservoirs is to confine the gas into the low permeability zones and improve sweep efficiency. Therefore, for this type of carbonate reservoirs, mobility control is required to enable gas/CO2 EOR due to the geological heterogeneity and gravity override.

This paper presents a new EOR scheme where mobility control of the injected gas is achieved by injecting viscosified water into the upper zone while injecting miscible gas into the lower zone using vertical and/or horizontal wells. A key prerequisite is to have a static model that captures the geological heterogeneity (e.g., vertical permeability contrast, all prevailing rock types) and a dynamic model that incorporates the SCAL derived capillary pressure (both drainage and imbibition) and relative permeability curves. Integrated geological, petrophysical and reservoir engineering effort was devoted to this EOR program that led to history matched sector models which honours the waterflood remaining oil saturation distribution shown in cased hole time-lapse saturation logs. The model forecasts show that significant sweep of the lower zone is achieved compared to both water or gas injection and that the process is stable and robust to reservoir lateral and vertical heterogeneity.

This EOR process has the potential of recovering the oil that is by-passed by waterflood or conventional gas injection schemes. It is particularly suited for layered oil reservoirs where there is an impediment for water to flow from the upper high permeable zone to the lower reservoir due to e.g. (vertical) permeability reduction at the interface or a capillary pressure barrier. It is also applicable for improving oil recovery from the low permeable layers inter-bedded within the more permeable reservoir unit. Additional benefit of this process is to potentially enable economic EOR and CO2 storage in such kind of heterogeneous reservoirs.

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