Seismic to acoustic characterization of geomechanical and microstructural properties of a vadose zone heterogeneous limestone formation

Heterogeneities control rock properties, especially hydraulic 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 play coupled roles and introduces difficulties in the characterization of the whole formation.

In order to constrain these coupled effects, we use seismic to acoustic data to characterize a multi-scale double porosity network and to understand the corresponding flow and mechanical properties of a shallow aquifer reservoir. The study focusses 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. It is composed by an exceptional well (20 m-depth, 4 m-diameter) surrounded by 8 cored boreholes.

Two seismic refraction profiles crossing the O-ZNS site were carried out to determine P-wave velocities. The profiles delineated three main geological units: (i) a clayey soil (0-2 m), (ii) a weathered and karstified limestone layer (2-7 m), and (iii) massive limestone down to the underlying Molasse du Gâtinais layer at a depth of 25 m. In consistence with the lithological log, a thin layer of more massive limestone is highlighted around 5 m-depth. In addition, we also observed that the increase in P-wave velocity slows down after 15 m. This effect is consistent with the increasing fracture density and karst development observed on the direct log imagery and on the well 3D scan. In the massive thin limestone layer of 5 m-depth, the interpreted relative crack density is low, around 0.08. However, in the last layer from 15 to 20m-depth, the relative crack density is much more important, even so discrepant, with maximal values around 0.4.

In parallel to large scale field investigation, mechanical tests and elastic wave velocities have been measured on representative core samples. A strong discrepancy is observed, whatever the property. For example, at 16 m-depth, P-wave velocities are distributed from 3,650 to 5,700 m.s^-1 and the corresponding mechanical parameter of crack density ranges from 0 to 0.5. In addition, extreme values of crack density, above 1 are observed around 19 m-depth. These large discrepancies and crack density values are consistent with mechanical behavior and microstructure observation made directly on core samples, even though some samples are more porous than cracked and the distinction need to be kept. Samples are then classified through image processing in three categories: the porous ones, the cracked ones, and the mixed ones allowing to discuss and organize the heterogeneity distribution of the O-ZNS.

To complete the study, an intermediate characterization is running on metric blocs sampled at different depth from the O-ZNS well. Their analyses include 3D external scans at high resolution (as for the surface of O-ZNS well) and P-wave velocity measurements at intermediate frequencies. These blocs are then iterativelly, cut into smaller blocs and re-characterized in order to obtain the distribution of heterogeneity size and characteristics with depth targeting the determination of a REV (Relative Elementary Volume) for future modeling developments.