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Improve Kinetics of Oil Recovery in Naturally Fractured Carbonates: Optimization of an Advanced EOR Foam Process

In naturally fractured carbonate reservoirs, Gas Oil Gravity Drainage processes (GOGD) are successfully implemented but oil recovery is limited by a slow kinetics. However a gas EOR process represents a promising alternative to boost this oil production rate. Nevertheless the design of this process should address several technical challenges: the typically unfavorable wettability of the matrix (intermediate to strongly oil-wet), the densely connected fracture network and the high contrast of fracture-to-matrix permeability. 

We propose here the injection of an advanced EOR foam with reduced interfacial tensions. The foam flow in the fracture creates an important viscous drive by imposing a pressure gradient, thus enhancing the oil recovery dynamics compared to GOGD. Besides, the reduced interfacial tension (IFT) between crude oil and aqueous phase allows the aqueous phase to enter the matrix despite the unfavorable wettability. 

In this paper, we demonstrate that a balance exist between IFT and foam strength performances to optimize the process. Three foam formulations are optimized with very different profiles in terms of IFT and foam performances. For their design, priority is given either to ultra-low IFT values (0.001mN/m) or to a strong foam with larger IFT (0.15mN/m) or to a balance between the two first formulations (0.03mN/m). These formulations are compared in vials, sandpack and coreflood experiments (fractured cores). Foams are evidenced as intrinsically less stable in ultra-low IFT conditions: foam stability (in vials) and apparent viscosity (in porous media) in contact with oil are respectively enhanced by a factor 30 and by two decades when IFT rises from 0.001 to 0.1mN/m. Based on sandpack and coreflood experiments, we recommend an IFT of 0.1 to 0.01mN/m as a balance between the viscous drive in fracture and an efficient aqueous phase imbibition in the oil-wet matrix. Simulation work supports this experimental conclusion: the common target of IFT of the order of 0.001 mN/m determined by capillary desaturation curves in SP flooding can be revised to a higher IFT, which is can be deduced from the wettability of the reservoir. 

To ensure an accelerated oil recovery in naturally fractured carbonate reservoirs, we recommend the design of a low-IFT foam formulation with revised IFT performances compared to a classical Surfactant-Polymer process. This article gives the target formulation parameters which arise from the mechanisms at play (viscous drive and imbibition in oil-wet matrix), and are realistically achieved with industrial surfactants.