2014 年 9 巻 2 号 p. JFST0012
Water flooding is one of the most important processes in oil production. Water is injected into an oil reservoir to maintain the reservoir's pressure and sweep some of the oil toward the production well. In an actual oil reservoir, in situ water, or connate water, is usually present and significantly impacts the water flooding process. We developed a visualization scheme utilizing a microfocused X-ray CT scanner to three-dimensionally observe the effects of connate water during water flooding at the pore scale. The water phase was injected upward into packed glass beads containing an oil phase both with and without connate water, and the process was scanned every minute until steady state was reached. Three-dimensional images were then constructed from X-ray CT data to clearly show the phenomena. Connate water significantly reduces oil recovery. In the porous medium without connate water, water flooding was able to produce approximately 1.5 times more oil than in the medium with connate water because of steadier interface movement. When injected water came in contact with connate water, the displacement front suddenly expanded into the pores containing connate water, thereby creating a jump-like movement leading to fingering. The pressure gradient between the inlet and outlet forced water to select the shortest route to the outlet. Water reached the outlet earlier when connate water exists. Short-circuiting due to fingering leads to formation of entrapped oil pockets and oil recovery yields.