Direct observation of reactive flow in a single fracture

We carried out a laboratory experiment to examine the relationship between local rate of dissolution and local aperture during flow of a slightly acidic aqueous solution through a rough fracture in Carrara marble under a confining pressure of 0.2 MPa. Fracture surfaces were digitized in three dimensions before the fluid flow tests and after the tests.,Digital reconstruction of the aperture then allowed numerical simulation of flow patterns, and digital comparison of surfaces before and after dissolution allowed mapping of patterns of dissolution. We observed that bath mean aperture and fracture permeability decreased as a result of dissolution. Despite the low confining pressure, the experiments thus simulate dissolution in deeply buried formations, in contrast to the gaping and karst formation that occur under vanishingly low confining pressure in the shallow crust. We observed that the growth of new pathways for flow changed from stable to unstable as length scale increased. At the millimeter scale the fracture aperture evolved in stable fashion from a strongly heterogeneous arrangement of tortuous flow channels to a smoother topography, while at the scale of the full rock (50 mm), the aperture developed a single, broad flow channel. The scale dependence of the dissolution pattern may be the result of changes with scale in extent of reaction (i.e., the Damkohler number) and in the relative importance of diffusion (the Peclet number). Finally, we also see evidence of a negative relationship between local fluid flux and local rate of dissolution in some locations in the fracture.