Experimental and Numerical Assessment of Chemical EOR in Oil-Wet Naturally-Fractured Reservoirs

Abstract

Abstract Among various ways to extend the lifetime of mature fields, chemical EOR processes have been subject of renewed interest in the recent years. Oil-wet fractured reservoirs represent a real challenge for chemical EOR as the matrix medium does not spontaneously imbibe the aqueous solvent of chemical additives. However, a wide variety of surfactants can now be considered for EOR, among which products that alter the matrix wettability. The present paper deals with that recovery strategy and compares it with other strategies based on viscous drive enhancement. Comparison is based on the physical and numerical interpretation of original representative experiments. The kinetics of spontaneous imbibition of chemical solutions in oil-wet limestone plugs and mini-plugs has been quantified thanks to X-ray CT-scanning and NMR measurements. Despite the small size of samples and the slowness of experiments, accurate recovery curves could be inferred from in-situ fluid saturation measurements. Scale effects were found quite consistent between mini-plugs and plugs. During a second experimental step, representative drive conditions of a fractured reservoir were imposed between the end-faces of a plug, in order to account for the possibly-significant contribution of fracture viscous drive to matrix oil recovery. These experiments were carefully analyzed and their numerical modeling was initiated with a simulation software that takes into account the multiple effects of surfactant presence on rock-fluids systems, including rock wettability modification and water-oil interfacial tension reduction. Model capability to reproduce experimental results is quite satisfactory provided that the evolution of capillary pressures in the presence of additives is sufficiently characterized. A few sensitivity studies underline also the necessity of disposing of a completely-documented physico-chemical data set to minimize modeling uncertainties in view of reliable recovery kinetics optimization. In summary, the present paper provides an original quantitative description of chemical EOR process in neutral to oil-wet fractured reservoirs. This work calls for further development in order to specify the conditions on chemical additives and recovery process implementation that satisfy economic viability.