Efficiency of Steam-Over-Solvent-Injection-in-Fractured-Reservoirs (SOS-FR) Method Considering Oil Recovery and Solvent Retrieval: Core-Scale Experimentation

Abstract

Summary Sole injection of steam or solvent into heterogeneous reservoirs usually yields an inefficient recovery performance. The steam-over-solvent-injection-in-fractured-reservoirs (SOS-FR) method was suggested as a solution to improve the efficiency of heavy-oil/bitumen recovery from fractured carbonates and oil-sands reservoirs after cold production (Al-Bahlani and Babadagli 2008). The method consists of three phases: Phase 1, where steam is injected at low temperatures to heat the matrix and condition the oil for subsequent solvent injection; Phase 2, when solvent-injection dilutes matrix oil by diffusion and enhances gravity-drainage recovery rate; and Phase 3, when low-temperature (at approximately the boiling point of solvent) steam injection retrieves the solvent diffused into matrix. Previous research was focused on the effectiveness of Phases 1 and 2 of the method (Al-Bahlani and Babadagli 2009a, b), and positive responses were reported at the field scale (Al-Bahlani and Babadagli 2010) with high ultimate recoveries. The efficiency of this process, however, is determined purely by the amount of solvent retrieved at the end of the process. This paper, therefore, focuses on Phase 3, which is performed mainly for solvent retrieval (additional oil is also produced during this phase). Twenty-two static core experiments were performed on water- and oil-wet sandstone and limestone. After saturating the rock samples with different heavy oils, they were immersed into different liquid solvents--hexane, heptane, decane, and diluent oil. Diluent oil used in this study is a distillate that was obtained from a heavy-oil-upgrading unit in Canada. Once the ultimate recovery was achieved by diffusion and gravity drainage (Phase 2), the samples were exposed to different temperatures of hot water (Phase 3) and the amount of solvent retrieved was measured through volumetric and weight measurements and refractometer readings. The retrieval of solvent diffused into matrix was caused mainly by two reasons: (1) evaporation of solvent at elevated temperature and (2) imbibition of hot water into rock (if oil-wet samples become more water-wet during Phase 2). The amount of solvent retrieved through these processes was determined, and the efficiencies were analyzed for different parameters, including rock wettability, oil viscosity, solvent type, solvent-soaking period, rock type, boundary conditions, different combinations of steam/solvent cycle, and temperature applied.