Petrophysical characterization of multi-scale heterogeneities of a vadose zone within continental limestone formation

Heterogeneities control rock properties, especially hydraulic, reactivity, and geophysical properties. Complex systems typically include multiple porosities at embedded scales, from the micro/meso cracks and pores to geological macro-fractures and karsts. This complex network plays interdependent roles and introduces difficulties in the characterization of the whole formation, especially with difficulties extrapolating laboratory interpretation to field investigation. A general and comprehensive workflow of the upscaling processes needs to be developed.

Our study focuses on the platform “Observatoire des transferts dans la Zone Non-Saturée” (O-ZNS, Orléans, France), an artificial excavation in the karstified and fractured limestone formation of Beauce aquifer. The observatory is composed of an exceptional well (20 m-depth, 4 m-diameter) surrounded by 8 cored boreholes. During the development of this platform, we excavated metric heterogeneous blocks to investigate different properties at an intermediate scale between laboratory and field. Our aim is to characterize these samples thanks to direct (micro)structure observation and quantification compared to petrophysical property measurements (transport and acoustic properties) to define their representative elementary volume (REV). The though protocol is a repetitive process including series of 3D scans and petrophysical measurements followed by cutting blocks in smaller and smaller parts. In complement, 3D scans made from photogrammetric reconstruction of the blocks provides digital models at various scales that help us in locating the geophysical heterogeneities with respect to the observed geological features at various scales.

For the biggest blocks, the carrying out of this protocol is challenging and tremendous. The key methodologies still need to be confronted to discussion.

We started the investigation on some intermediate blocks (around 30 to 50 cm-size). We highlight the possibility of measuring acoustic at different frequencies even on the rough surface of the block. Permeability can be obtained with air-measurements using a tiny-perm apparatus. Porosity is more challenging. As a first approximation, we estimate it assuming a certain grain density, and by measuring its dry weight and volume. Finally, 3D scans allow us to describe the embedded structure of the block, and to count and measure pores and cracks sizes. The first series of results are coherent, and we will be able to go a step-down and tentatively obtain the REV for all these properties. The final objective would be to define the link between the REV of the different explored properties, considering the heterogeneities.