3D analysis of geometry and flow changes in a limestone fracture during dissolution

摘要

The effects of reactive transport on fracture geometry and fluid flow were investigated through an integrated experimental and modelling approach. A fractured limestone sample (90% calcite) was injected with an acidic CO_2-rich solution over a period of 55h to induce carbonate dissolution. The changes in fracture geometry and related parameters are reported for six data sets obtained from synchrotron X-ray micro-tomography experiments. A series of algorithms was used to extract the aperture and fracture walls from 3D images and allowed quantification of the geometry changes with an optical resolution of 4.91μm. In addition, measurement of fluid chemistry, hydraulic tests and computation of Navier-Stokes flow constrained the characterisation of the dissolution process. The effects of reactive transport on fracture geometry and fluid flow were then discussed. The presence of silicates in the rocks led to heterogeneous dissolution at the micro-scale, despite dissolution appearing to be quite homogeneous at sample-scale. No formation of preferential flow pathways was noticed, although heterogeneous dissolution at the micro-scale led to fracture walls and aperture decorrelation, and to modification of the flow velocity profiles in the fracture.