Heavy-Oil Recovery in Naturally Fractured Reservoirs with Varying Wettability by Steam Solvent Co-Injection

Abstract

Abstract In heavy-oil recovery, although steam injection has no alternative in many circumstances, it may not be an efficient process under certain reservoir conditions. These conditions include deep reservoirs, where steam injection may turn out to be ineffective hot-water flooding due to excessive heat loss, and oil-wet fractured carbonates, where steam channels through fractures without effectively sweeping the matrix oil. Solutions for heavy oil recovery in consolidated/unconsolidated sandstones have been proposed and some of them are currently in the commercial phase, including steamflooding and its different versions. A more challenging case is heavy-oil fractured carbonates where the recovery is usually limited only to matrix oil drainage gravity due to unfavourable wettability or thermal expansion if heat is introduced during the process. Wettability alteration is usually thought to occur at elevated temperatures which are difficult to achieve in deep reservoirs. Thus, improvement of matrix oil recovery requires different methodologies. We propose a new approach to improve steam/hot-water injection effectiveness and efficiency for this type of reservoir. Static imbibition experiments were run on Berea sandstone and carbonate cores with different wettabilities and for different oil viscosities ranging between 200 cp and 14,000 cP. For wettability alteration, cores were either aged or treated by a wettability altering agent. The experiments were conducted initially in imbibition cells in a 90 oC oven to mimic the matrixfracture interaction in steam condensation zones. Due to its high boiling point, heptane was selected as the solvent and the core samples were exposed alternately to high temperature imbibition and solvent diffusion. The main ideas behind this process were to enhance capillary and gravity interaction by reducing viscosity (heat and solvent effect) and altering wettability (solvent effect). The results showed that further reduction in oil saturation due to s solvent diffusion process preceded by hot water is remarkably fast and the ultimate recovery is high. The magnitude of recovery depends on wettability and the amount of water existing in the core. It was also observed that solvent retrieval is a very fast process and may increase to 85–90% depending on core type, wettability, and saturation history. Introduction Carbonate reservoirs introduce great challenges due to their complex fabric nature (low matrix permeability, poor effective porosity, fractures) and unfavorable wettability. These challenges are further displayed when combined with increased depth and low grade oil (low API and high viscosity). A huge amount of oil is contained in such reservoirs without any technological breakthrough for improving the recovery efficiently.