The Modeling of the Viscous Drive in Oil-wet Naturally Fractured Reservoirs

Abstract

Recent experiments have shown that gels can promulgate the viscous drive recovery mechanism in fractured oil-wet matrix cores. The large viscosity of gels can increase the pressure gradient in the fractured network, generating higher local pressures in the fractures that can overcome the capillary entrance pressure of the oil-wet matrix. When that occurs, the water leaking-off from the gel invades the matrix and displaces the oil to the fractures, promoting the viscous drive as the dominant recovery mechanism. The goal of this research is to comprehend whether the viscous drive recovery mechanism could enhance the oil recovery in oil-wet NFR under realistic conditions and furthermore to determine the range of applicability. The laboratory tests executed by Brattekås et al. (2013), where the gel was injected at high velocity in the fracture provoking matrix invasion, allowed generating a conceptual model of the process that was adapted in a numerical reservoir simulator to reproduce the experiments. As a step forward, the lab-scale simulations are scaled up to field conditions to analyze the phenomenon in real reservoirs. Multiple simulations were performed to evaluate the influence of different parameters such as the gel viscosity, the velocity of injection and the matrix block size on the recovery rate and the ultimate recovery factor. The simulation results demonstrated on the one hand that in smaller matrix blocks higher values of velocity-viscosity are needed to initiate the process, and additionally the capillary end-effect significantly affects oil production. On the other hand, in the larger matrix blocks the recovery time is considerably longer and could be unrealistic for practical applications. The dimensionless criterion controlling this process is derived as the viscous-capillary ratio relating the differential pressure along the fracture to the capillary pressure of the oil-wet matrix. To provoke the matrix invasion by the viscous drive, this dimensionless number should overcome a critical value which was estimated for this study. The novelty of this work has a significant contribution for the EOR implementation in oil-wet NFR. It is demonstrated that the viscous drive recovery mechanism can have a strong contribution on oil recovery.