Viscous Oil Recovery and In Situ Deasphalting in Fractured Reservoirs: Part 1. The Effect of Solvent Injection Rate

Abstract

Steam-based EOR methods for viscous oil recovery from fractured reservoirs have significant challenges in both cost and energy efficiency. In response, solvent-based methods have been of interest because of their low energy intensity, low greenhouse gas emissions, and no fresh water consumption. Injection strategies for viscous oil recovery by solvent include liquid extraction and vapor oil gravity drainage. Understanding the mechanisms in each phase is of great value for the successful application and optimization of solvent EOR processes. The work presented here studies the effect of solvent injection rate on viscous oil recovery by liquid extraction with n-butane in vertically placed sandstone cores with an artificial fracture. The oil production rate, ultimate recovery, and in situ deasphalting in different sections of the core are analyzed. The oil production rate increased with solvent injection rate until it leveled off as the injection rate exceeded a critical value. The ultimate recovery factor is nearly the same for all solvent injection rates below the critical value. However, it is significantly reduced at higher injection rates. A conceptual model based on convective mass transfer is proposed and the effect of mechanical dispersion is discussed. In situ deasphalting was observed in all cases. The cause of the unexpected changes in production rate was attributed to severe asphaltene deposition and remobilization in the fractured permeable rock. In such a medium, solvent injection rate seems to show an optimal value for maximizing oil production rate, ultimate recovery factor, and solvent efficiency.