“Resolving density variations in chalk percolated by CO2 saturated brine using X-ray microtomography at multiple energies”
Time-resolved X-ray microtomography combined with a flow cell setup allows studying dynamic pore scale processes like the formation of heterogeneous dissolution patterns in chalk upon injection of CO2 acidified brine. This is relevant to judging the suitability and long term safety of potential geological carbon storage sites in the North Sea region.
The limitation of absorption based X-ray imaging is that it only provides information on local electron density of the imaged materials and chalk is a very fine grained material. Consequently, the observed image intensities are always a mixture of varying contributions of solid calcium carbonate and seawater with varying CO2 concentration, i.e., standard absorption imaging can only provide a qualitative representation of the evolving microstructure (Fig. 1A). In the REDEVA project we explore the potential of adding an additional dimension of information to the reconstructed water-gas-solid system by using microtomography at multiple X-ray energies. This provides an energy dispersive footprint for every voxel with a unique solution to its density and the elemental composition of the materials mixed within (Fig. 1B).
The key challenge of REDEVA is to provide a quantitative reconstruction from incomplete and imperfect datasets. A flow cell setup imaged with micrometer resolution only allows for field of view tomography, i.e., the datasets are incomplete. Furthermore, synchrotrons need to reinject electrons permanently to maintain a constant energy flux. Variations in energy density cause temperature fluctuations in the imaging setup. At micrometer resolution the consequences are visible as non-linear time-dependent artifacts that are only insufficiently addressed by traditional sinogram normalization and correction techniques. REDEVA addresses these artifacts with a self-optimizing dynamic flatfield correction (Fig. 2).