Sole injection of steam or solvent into heterogeneous reservoirs usually yields an inefficient recovery performance. The SOS- FR (Steam-Over-Solvent Injection in Fractured Reservoirs) 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): Steam is injected at low temperatures to heat the matrix and condition the oil for subsequent solvent injection; Phase (2): solvent injection to dilute matrix oil by diffusion and enhance gravity drainage recovery rate, and; Phase (3): low temperature (around the boiling point of solvent) steam injection to retrieve the solvent diffused into matrix.

Previously, we focused on the effectiveness of Phases (1) and (2) of the method (Al-Bahlani and Babadagli 2009a-b) and observed very positive responses at the field scale (Al-Bahlani and Babadagli 2010) with high ultimate recoveries. The efficiency of this process, however, is purely determined by the amount of solvent retrieved at the end of the process. This paper, therefore, focuses on Phase (3), which is done mainly for solvent retrieval.

About twelve static core experiments were performed on water and oil-wet sandstones. After saturating the rock samples with different heavy-oils, they were immersed into different liquid solvents—hexane, heptane, decane, and diluent oil. Once the ultimate recovery was achieved by diffusion and gravity drainage [Phase (2)], the samples were exposed to different temperature hot-water [Phase (3)] and the amount of solvent retrieved was measured through volumetric and weight measurements, as well as refractometer readings. The retrieval of solvent diffused into matrix was mainly due to 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 were determined and the efficiencies were analyzed for different parameters including rock wettability, solvent type, solvent-soaking period, different combinations of steam-solvent cycle, and temperature applied.

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