Because of low efficiencies and the high cost of the individual injection of steam and solvent for heavy-oil recovery, their hybrid applications have gained significant attention recently. Although numerous laboratory studies exist and there are a considerable number of field projects for sandstone environments, fractured carbonates lack technologies to drain matrix oil efficiently. An alternative-method injection of solvent and steam was proposed and tested earlier (Babadagli and Al-Bahlani 2008). This process applies steam initially to condition the matrix oil for succeeding solvent injection and steam reinjection to retrieve solvent in the matrix and to recover additional upgraded oil. The present study uses carbon dioxide (CO2) as a solvent and compares it with hydrocarbon solvents in this type of application. To clarify the physics of the process and to test the applicability of the method for different reservoir and injection conditions, we conducted a series of experiments by first injecting steam, followed by CO2 injection. In the third cycle, steam was injected again to produce upgraded oil in the matrix. The experiments were performed under static conditions (soaking sand and carbonate samples in steam or CO2 chambers) at different temperatures and pressures. CO2 is shown to be a reasonable alternative for hydrocarbon solvents in such a process in terms of cost and benefits by reducing the solvent expenses, keeping the oil-production levels, and disposing of a greenhouse gas.

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