Présentations Colloques

Oral Presentation
Session 8.07: Hydrogeophysics: innovative non-invasive technologies for groundwater resources exploitation and management
Philippe Pezard Perroud
Seismic amplitude anomalies revealed by the monitoring of underground CO2 injection
Multiparameter geophysical monitoring of shallow depth gas injection experiments in a saline aquifer have been conducted over a 4 years period at the Maguelone site, situated on a Mediterranean lido, near Montpellier (France). The aim of these experiments concerns the behavior with time of the water-CO2 mixture in the context of long-term CO2 underground storage as a possible solution to reduce atmospheric concentration. At Maguelone, the gas is injected into two clastic reservoirs located at depth 8-9m and 13-16m. These reservoirs are interbedded in a clay-rich sedimentary sequence, and saturated with saline fluids. Subsurface biogenic gas production also takes place in this environment. The experimental setup includes both surface and downhole, electrical and seismic, baseline and injection-time, monitoring in order to follow possible changes of formation physical parameters (porosity, density, conductivity, velocity, reflectivity), together with In situ temperature, pressure as well as pH and chemical composition of the saturating fluids.**This presentation is focused on the seismic measurements results, obtained using an impact source at the surface, and recorded by a combination of 20m vertical and 40m surface antennas. The baseline monitoring reveals that natural changes of the site seismic response can occur due to changes in the surface climatic conditions on the one hand, but also due to induced biogenic gas release on the other hand, triggered by the seismic impact source. Nevertheless, after correction of these effects, a clear seismic response, linked with the gas injection experiments, can be retrieved. Due to the thin thicknesses of the reservoirs, time delays and their associated velocity changes cannot be demonstrated. However, significant transmitted and reflected seismic amplitude modifications, synchronous with the gas injections, can be established. A physical model is presented to link these amplitude anomalies with the involved physical parameters (saturation, porosity, velocity, density). As a result of these investigations, we emphasize the usefulness of an adequately designed seismic surveying method as a tool for underground gas storage and induced hydrological interactions monitoring.