Steam injection in heavy-oil containing naturally fractured reservoir aims at heating matrix to reduce the viscosity and enhance gravity drainage. This technique, however, is not feasible in deep reservoirs. Hydrocarbon solvent injection is also impractical due to low gravity of oil, heterogeneity, and retrieval of solvent diffused into matrix. A hybrid application of these two techniques was tested for deep reservoir conditions earlier (Steam Over Solvent Injection in Fractured Reservoirs Method -SOS-FR) and proved success if applied suitably. The cost of hydrocarbon solvent and greenhouse gas concerns, however, entail investigations on other techniques and materials.

The use of CO2 in this type of process as solvent was considered and tested in this paper. Several issues are highly critical in this process. Like other hydrocarbon solvents used under non-isothermal conditions, the recovery process is highly sensitive to pressure and temperature as they determine the miscibility level. Also important is the capability of CO2 to extract matrix oil. Our earlier studies with light oil showed that heavier ends can be extracted if enough time is allowed for CO2 to interact with matrix oil. The same needs to be investigated for heavy-oils. Another dilemma was inverse proportionality of CO2 solubility with temperature. Steam (or heating) is inevitable to condition oil and decrease its viscosity before CO2 injection but temperature should be critically adjusted not to sacrifice CO2 solubility of oil.

To clarify all these points and determine optimal application conditions (duration of each cycles and CO2 soaking time), we conducted a series of experiments by soaking core samples saturated with heavy oil into steam first followed by CO2. In the third cycle, steam (or hot water) was injected again to produce upgraded oil in the matrix. The experiments were performed under static conditions (soaking sandpacks and sandstone samples into steam or CO2 chambers) at different temperatures and pressures to determine optimal application conditions for mutual goals; heavy oil recovery and CO2 storage in the matrix. Finally, the results were compared to those of experiments with hydrocarbon solvents from technical point of view including storage benefits of CO2.

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