Watching Flow Inside Rock

Flow through porous substances has been a major interest in fluid dynamics for the last hundred years because rocks are porous. For most of that period, we’ve used Darcy’s law to calculate how a fluid flows through pores in a rock. (Incidentally, it can also be used for determining the perfect length of time for dunking a cookie in milk.) Often, however, there is more than one fluid in a pore – for example, both a liquid and a gas could be trapped there. In that case, researchers made a few assumptions that allowed them to extend Darcy’s law for these multiphase situations. For a long time, that was the best anyone could do because it was impossible to observe what’s actually happening in the pores inside an actual rock.

Recently, however, scientists have begun observing these multiphase flows inside sandstone pores using x-ray imaging. They’re only able to take an image every 45 seconds or so, but even that is frequent enough to show that the flow is surprisingly unsteady. An example image is shown above. The colored areas show pores filled with nitrogen inside the rock. Brine is also being injected into the rock but not being shown. The colors indicate how connected the nitrogen-filled pores are to other pores nearby. Red areas are highly connected; blue have moderate connections; and green areas are smaller and have fewer connections. The network connections inside the rock change relatively rapidly, even with steady-state injection conditions. That varying connectivity implies that some of the injection energy is going into shifting interfaces around rather than actually moving the fluids through the pores. More work will be needed to unravel what’s really happening inside the porous network, but the results have far-reaching implications for understanding groundwater filtration, fossil fuel extraction, and, in the future, the possibility of carbon sequestration. (Image credit: C. Reynolds et al., source; submitted by Simon D.)

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