Permeability of oolitic limestones from the Upper Rhine Graben

The Grande Oolithe is an oolitic limestone from the Middle Jurassic, present at various depths within the Upper Rhine Graben (Alsace, France). It has been identified as a prospective target for geothermal energy extraction. A comprehensive evaluation of the geothermal potential of this formation hinges on a detailed understanding of its mechanical and physical properties, in particular permeability. Previous studies on porous carbonates highlighted the diversity and the microstructural complexity of this rock type. Permeability could be strongly influenced in particular by the degree of cementation and the proportion of macro and micropores in limestones, which often have a dual porosity structure. To identify the parameters controlling fluid flow in the Grande Oolithe, we initiated a systematic study to map its permeability over the entire Upper Rhine Graben and quantify its possible variations with pressure.

Cylindrical samples were prepared from 18 blocks collected from several outcrops in Alsace. Porosity measured on 90 samples span from 4 to 26% for the different blocks, while permeability was found to range from 10⁻¹⁵ to 10⁻¹⁸ m². Our preliminary microstructural analysis and X-ray Computed Tomography data revealed a high degree of cementation in most of our samples and that the pore space is dominated by micropores, mostly of submicron sizes. For high-pressure experiments, we targeted so far the high-porosity/high permeability end-members, from Bouxwiller (GO) and Gueberschwihr (GU), with respective porosity of 25 and 20%. Both limestones are made of 99% calcite. Conventional triaxial experiments were performed at room temperature on water-saturated samples, in drained conditions with a constant pore pressure of 10 MPa and at effective pressures up to 100 MPa. The experiments were performed at a constant strain rate of 10^-5 s^-1 and permeability was measured using steady-state flow technique at different stages of deformation.

Under hydrostatic compression, permeability was found to decrease moderately in both GO and GU during the poroelastic stage and then more significantly beyond the onset of pore-collapse. The total permeability decrease was more pronounced in GO than in GU. At an effective pressure of 100 MPa, inelastic compaction resulted in a permeability reduction of a factor 15 in GO and a factor 4 in GU, while respective porosity reduction was 7.8% and 2.5%. Under triaxial compression, the permeability measured in samples deformed at various effective pressures showed somehow similar variations, in qualitative agreement with previous studies on permeability in porous carbonates under triaxial compression.