Time-resolved 3D characterisation of flow and dissolution patterns in a single rough-walled fracture

Chapter 43International audienceAn application of X-ray computed microtomography (XCMT) for 3D measurement of fracture geometry is presented. The study demonstrates the ability of XCMT to non-invasively measure the fracture walls and aperture during the course of a reactive flow experiment. The method allows estimation of both the local and global scale dissolution kinetics of a fractured limestone sample percolated by acidic water. The measured fracture geometry was then used as an input for flow modelling, in order to compare the hydraulic aperture calculated by numerical simulation with different evaluations of the aperture: hydraulic aperture measured from pressure drop during the flow experiment, mechanical aperture measured with XCMT, and chemical aperture deduced from calcium removal in the sample. The effects of reactive transport on geometry and fluid flow are discussed. Dissolution appears heterogeneous at both the small scale due to the presence of insoluble clays in the rock, and at larger scales with the formation of preferential flow pathways. These heterogeneous dissolution patterns are not predictable simply by the identification of the areas of higher fluid velocity, where transport of the chemical reaction products (i.e. rate of aperture increase) is presumed to be higher

Study of dolomite dissolution at various temperatures – Evidence for the formation of nanocrystalline secondary phases at dolomite surface and influence on dolomite interactions with other minerals

International audienceIn most clay-rock geological formation studied for the storage of nuclear waste,pore water compositions are expected to be at equilibrium with carbonate minerals, which are always included in predictive models for pore water composition calculations [1]. Among the carbonates known to be present, dolomite may be problematic in the pore water composition calculation because its solubility spans a large range of values as a function of its crystallinity in thermodynamic databases. In addition, the composition of dolomite minerals observed in clay-rock formations such as Callovian-Oxfordian or Opalinus clay formation differs from this of a pure dolomite: the Ca/Mg stoichiometry is not ideal, and the minerals contain minor amounts of Fe and traces of many other elements [2]. To understand the influence of secondary phases precipitation during dolomite dissolution on pore water chemistry, the dissolution of monocrystals of dolomite were investigated at 25 °C and at 80 °C in a pH range 3 to 8 for various time periods (30 minutes to 21 days) in sealed PTFE reactors. Solution analyses evidenced a stoichiometric release of Ca and Mg in solution during dolomite dissolution. Scanning Electron Microscopy (SEM), Raman and X-Ray Diffraction (XRD) analyses did not evidence secondary Mg-bearing minerals precipitation, but revealed the formation of Fe-bearing particles on the dolomite surface. Morphological characterizations performed with Small-angle X-ray scattering (SAXS)evidenced that the precipitation occurs along a specific crystallographic plane of the dolomite monocrystal. Thus, the precipitated nanoparticles clustered on specific surface sites, and are made of Fe-rich phases poorly crystallized (carbonates, oxides and hydroxides)

Study of dolomite dissolution at various temperatures – Evidence for the formation of nanocrystalline secondary phases at dolomite surface and influence on dolomite interactions with other minerals

International audienceIn most clay-rock geological formation studied for the storage of nuclear waste,pore water compositions are expected to be at equilibrium with carbonate minerals, which are always included in predictive models for pore water composition calculations [1]. Among the carbonates known to be present, dolomite may be problematic in the pore water composition calculation because its solubility spans a large range of values as a function of its crystallinity in thermodynamic databases. In addition, the composition of dolomite minerals observed in clay-rock formations such as Callovian-Oxfordian or Opalinus clay formation differs from this of a pure dolomite: the Ca/Mg stoichiometry is not ideal, and the minerals contain minor amounts of Fe and traces of many other elements [2]. To understand the influence of secondary phases precipitation during dolomite dissolution on pore water chemistry, the dissolution of monocrystals of dolomite were investigated at 25 °C and at 80 °C in a pH range 3 to 8 for various time periods (30 minutes to 21 days) in sealed PTFE reactors. Solution analyses evidenced a stoichiometric release of Ca and Mg in solution during dolomite dissolution. Scanning Electron Microscopy (SEM), Raman and X-Ray Diffraction (XRD) analyses did not evidence secondary Mg-bearing minerals precipitation, but revealed the formation of Fe-bearing particles on the dolomite surface. Morphological characterizations performed with Small-angle X-ray scattering (SAXS)evidenced that the precipitation occurs along a specific crystallographic plane of the dolomite monocrystal. Thus, the precipitated nanoparticles clustered on specific surface sites, and are made of Fe-rich phases poorly crystallized (carbonates, oxides and hydroxides)

On the kinetic control of hydro-geochemical reactions during the diagenetic processes in sedimentary basins

Madé Benoît. On the kinetic control of hydro-geochemical reactions during the diagenetic processes in sedimentary basins. In: Transferts dans les systèmes sédimentaires : de l'échelle du pore à celle du bassin. Réunion spécialisée SGF-TRABAS/CNRS, Paris 27-28 septembre 1999. Résumés. Strasbourg : Institut de Géologie – Université Louis-Pasteur, 1999. pp. 93-97. (Sciences Géologiques. Mémoire, 99

On the kinetic control of hydro-geochemical reactions during the diagenetic processes in sedimentary basins

Madé Benoît. On the kinetic control of hydro-geochemical reactions during the diagenetic processes in sedimentary basins. In: Transferts dans les systèmes sédimentaires : de l'échelle du pore à celle du bassin. Réunion spécialisée SGF-TRABAS/CNRS, Paris 27-28 septembre 1999. Résumés. Strasbourg : Institut de Géologie – Université Louis-Pasteur, 1999. pp. 93-97. (Sciences Géologiques. Mémoire, 99

Significance of clay dehydration on overpressures in the Alwyn area, Viking Graben, North Sea

Bruel Dominique, Madé Benoît. Significance of clay dehydration on overpressures in the Alwyn area, Viking Graben, North Sea. In: Transferts dans les systèmes sédimentaires : de l'échelle du pore à celle du bassin. Réunion spécialisée SGF-TRABAS/CNRS, Paris 27-28 septembre 1999. Résumés. Strasbourg : Institut de Géologie – Université Louis-Pasteur, 1999. pp. 33-36. (Sciences Géologiques. Mémoire, 99

Impact of coating development on the hydraulic and transport properties in argillaceous limestone fracture

Results are reported for an acidic water flow-through experiment conducted in a fractured argillaceous limestone sample (73% carbonates). The change in fracture geometry and related parameters is reported for six data sets obtained from synchrotron X-ray microtomography experiments. High-resolution three-dimensional images of the sample allowed quantification of the changes in fracture morphology at a spatial resolution of 6 ;μm. Mineral mass loss and permeability changes in the sample were also determined. Several physico-chemical phenomena were identified during the experiment. Initial smooth fracture surfaces evolved rapidly toward rough surfaces with uneven clay coverage due to the preferential dissolution of carbonate minerals compared to clay minerals whose dissolution rate is about 106 slower. A microporous clay coating progressively developed at the fluid-rock interface during heterogeneous dissolution of the fracture, while the global dissolution rate of the fracture walls exponentially decreased. The increase in surface roughness and the presumed reorganization of clays caused a progressive reduction in permeability. During the last flow-through stage, a large decrease in sample permeability was attributed to the large removal of clay particles; this process was responsible for a dramatic collapse of the fracture walls near the sample inlet and led to the development of preferential flow pathways. The development of the clay coating also acted as a barrier to flow and mass transfer between calcite grains and bulk solution and affected transport processes within the fractur