Integrating the multi-scale elastic velocities for interpreting and predicting the Dogger carbonate geothermal reservoir

The Dogger geothermal reservoir of the Paris Basin is one of the most actively exploited carbonate aquifers in France and serves as a key target for sustainable district-heating systems. It consists mainly of Middle Jurassic carbonates deposited on a ramp, where sedimentary facies and diagenetic overprinting produce strong spatial heterogeneity. Understanding the heterogeneous petrophysical distributions is essential for predicting fluid circulation and designing subsequent geothermal operation plan. However, petrophysical interpretations derived from geophysical methods remain scale-dependent: laboratory acoustic measurements, well logs, and seismic data have different resolutions, making it challenging to reconcile acoustic signatures and to map heterogeneity consistently across scales.

To address these challenges, we conducted ultrasonic transmission experiments on core fragments taken from the SEIF-01 geothermal well in Melun area to determine Vp and Vs across key facies types. We further measured pressure-dependent Vp-Vs variations on cylindrical plugs, to better understand how the sedimentary microstructures and crack closure control the seismic velocity. Using seismic rock velocity models, we interpret the influence of pore shape (measured by pore aspect ratio) and fluid on the seismic velocity, to provide a quantitative link between micro-scale pore geometry and macroscopic elastic properties.

Finally, we will integrate the laboratory results with in-situ sonic logs and 2D seismic reflection data to bridge acoustic observations across scales. This multi-scale integration provides new insights into the internal heterogeneity of the Dogger reservoir and improves the interpretation of geophysical datasets for geothermal development. Our results highlight the potential of combining ultrasonic experiments, well logs, and seismic data, to better constrain reservoir properties and support more reliable geothermal resource assessment in heterogeneous carbonate systems.