In the past decade, in-situ heavy oil recovery processes have been proposed and successfully applied to field operations. In conventional ways like cyclic steam stimulation (CSS) and steam assisted gravity drainage (SAGD), viscosity of heavy oil is reduced by steam injection. However, since the cost of steam generation remains high, solvent injection has been considered to decrease steam-oil ratio (SOR). In many cases, solvent means relatively heavy hydrocarbon gas components ranging from C3 to C6. In this paper, we discuss the effect of heavy gas solvents on viscosity reduction.

Our target reservoir is the Yufutsu oil field, Hokkaido, Japan. Its produced oil is highly viscous and we have to consider an enhanced oil recovery process. Viscosity and gravity of the Yufutsu fluid are 846 cP and 11°API respectively at stock tank condition. Since gas injection is one reasonable choice to reduce oil viscosity, we conducted swelling tests and viscosity measurements using two different gas samples. The first gas sample is a light gas mixture which is a main product of the facility and pipelined to the markets. The second one is a heavy gas mixture which is a subsidiary product of the manufacturing processes, and contains C2, C3 and C4 components significantly. According to our swelling tests, the heavy gas mixture achieved greater swelling factor and viscosity reduction than the other. In terms of viscosity reduction, the heavy gas achieved 94%, while the light gas achieved 88%.

Additionally, we conducted simulation studies to examine the effect of heavy gas solvent on viscosity reduction. We generated 2-dimentional preliminary heavy oil reservoir models. The results of simulation studies indicate that the injection of heavy gas solvent reduces oil viscosity, and increases oil recovery more than the light gas case.

In conclusion, according to our experiments and simulation analyses, the heavy gas solvent shows more oil viscosity reduction than the light gas solvent. This fact implies the heavy gas components, C2, C3 and C4, play important roles in oil viscosity reduction, and will help to decide an optimal composition of solvents in heavy oil production.

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