Application of primary and secondary recovery processes in naturally fractured carbonate reservoirs containing heavy-oil/bitumen usually results in no or low recovery. Hence, an enhanced oil recovery (EOR) method is typically needed at early stages. Changing the characteristics of rock/fluid system, such as wettability and interfacial tension, in such challenging reservoirs becomes critically essential in this attempt. Recovery from the Grosmont carbonate unit is further complicated by the immobility of bitumen as well as extreme reservoir heterogeneity. This specific reservoir requires either pre-heating or solvent dilution before any further EOR attempts.

This study investigates the applicability of the modified version of the previously proposed SOS-FR (steam-over-solvent injection in fractured reservoirs) method to recover bitumen from the Grosmont carbonate unit. Using the suggested methodology, three phases were applied on a total of 13 preserved core samples from this unit. In Phase-1, the cores taken from three different formations of the Grosmont unit were soaked into a liquid solvent (heptane or distillate) at ambient conditions. Depending on the quality of the bitumen, a certain amount of recovery was obtained. The original form of the SOS-FR method was based on steam (or hot-water) injection at the temperature near the boiling point of the solvent as the following phase to retrieve the solvent. In Phase-2, instead of this "thermodynamic" approach, we tested an "interfacial" approach and soaked the samples into water with chemicals additions. Wettability modifiers tested include high pH solution, surfactants and ionic liquids, which were screened in our earlier work (SPE 170034) and selected as the most useful wettability alteration chemicals for oil-wet carbonates. One of the key benefits of the ionic liquids is that they are environmentally friendly and can be customized for particular rock/fluid system while pH solutions are economically attractive. Phase-2 was applied at room temperature and at 65°C. Finally, Phase-3 was applied by increasing temperature to the bubble point of the solvent to mimic hot-water injection with chemicals.

Each phase was analyzed in terms of ultimate oil recovery (and wettability alteration), time to reach this amount, the most suitable wettability alteration chemicals, and soaking times needed. The results revealed that the solvent phase not only affects the bitumen properties, but also changes Grosmont rock characteristics significantly. Wettability alteration of the fractured Grosmont reservoir was observed to be highly critical, not only for additional oil recovery but also for solvent retrieval. Specific high pH solution was observed to efficiently alter wettability and recover considerable amount of bitumen/solvent mixture fairly quick showing a potential as a low cost chemical to recover bitumen. The results and discussion on the optimization of the process in terms of the solvent soaking period, wettability alteration chemicals, and the effect of temperature will be useful for further field EOR attempts.

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