NG007-0007
Gravity-driven Reactive Miscible Fluid Mixing in a Fracture

Wednesday, 16 December 2020
Poster
Zhenyu Xu1, Hongfan Cao2, Seonkyoo Yoon2, Peter K. Kang3 and Laura J Pyrak-Nolte4, (1)Purdue University, Department of Physics and Astronomy, West Lafayette, IN, United States, (2)University of Minnesota Twin Cities, Department of Earth and Environmental Sciences, Minneapolis, MN, United States, (3)University of Minnesota, Department of Earth and Environmental Sciences, Minneapolis, MN, United States, (4)Purdue University, Department of Physics and Astronomy; Department of Earth, Atomospheric and Planetary Sciences; Lyles School of Civil Engineering, West Lafayette, IN, United States
Abstract:
As a fluid flows through fractures in rock, the fluid may mix with rock formation fluids or geochemically interact with minerals along the fracture surface. These interactions lead to changes in the fluid properties, specifically species concentrations and density. Here we examine, experimentally and numerically, gravity-driven reactive mixing in a fracture to examine the role of density in the mixing of two fluids in a fracture and how this mixing affects the spatial distribution of precipitates.

Experiments were performed on transparent uniform aperture fractures (1, 2 and 4 mm) made from poly-carbonate plates, each measuring 100 mm x 100 mm by 12.7 m, for a range of fracture orientations (0o – 90o in 15o increments). After initial saturation with a less dense fluid, two fluids, with a density contrast of r1/ r2 ~ 1.07, were simultaneously injected through two inlets. The denser fluid was composed of Na2CO3, NaCl and Bromocresol green, appearing blue. The less dense fluid was colorless Na2CO3 solution. Experiments were also performed to induce CaCO3 precipitation in a fracture from the invasion of 1M CaCl2 and 0.3M Na2CO3 solutions. A custom-built digital imaging system was used to record images during fluid invasion.

The experimental results show that the density contrast between the fluids confines the less dense fluid to a narrow runlet, thus influencing the mixing lines and the spatial distribution of precipitates. Orientation of the fracture affects the shape of the runlet which reaches a minimum width when the fracture is parallel to gravity. Initial 3D numerical simulations show that the runlet geometry is affected by the formation of 3D vortices that change in size and shape with changes in fluid velocity. The formation of vortices impacts the ability of the fluids to mix and in the experiments causes mineral precipitation in vertical fractures to be confined to the region of the runlet. The presence of gravity-induced instabilities in laminar regimes has the potential to affect design and operation of subsurface operations for contaminant clean-up in fractured rock.

This work was supported by the former Center for Nanoscale Controls on Geologic CO₂ (NCGC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-AC02-05CH11231

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