Squeezing oil from stone
In many oil wells, especially newly drilled ones, natural subsurface pressure suffices to expel oil from the pores of oil-bearing rock. To squeeze out more oil, companies have long injected water into wells. Despite the effectiveness of that practice, geophysicists don’t know exactly how the oil and pressurized water interact. To investigate, physicist Denis Bartolo of École Normale Supérieure in Lyon, France, and his team built a miniature oil field: an array of interconnected 80-μm-wide channels imprinted in a layer of transparent resin and flooded with silicone oil. The researchers injected dyed water through the structure, varying the flow rate to mimic the range used in real wells. The results reveal that whereas faster water injection moves more oil per unit time, a lower flow rate extracts more oil per unit water volume.
Videos from the experiment reveal four distinct patterns, which are shown in the images; the colors represent the relative thickness of water (yellow) and oil (red) films. At the lowest injection rate (1), water proceeds uniformly through the channels and slowly pushes oil out of the flooded pores. As the flow increases (2), the water forms a thin film above the oil, trapping oil droplets along the channel walls. At a still higher rate (3), the film of water strengthens and sweeps along the oil in a branched pattern. Eventually the flow of water becomes so strong (4) that it pins a layer of oil against the walls.
The results of the experiment, which was conducted in collaboration with the French company Total, could inform guidelines for optimizing oil extraction efforts. (C. Odier et al., Phys. Rev. Lett. 119, 208005, 2017 .)
