On Thursday, May 14, 2020, Jérôme Azzola successfully defended his PhD on unconventional geophysical monitoring of deep geothermal reservoirs.
Our ability to accurately monitor deep reservoirs is a major issue for the development of renewable energies, including in Enhanced Geothermal Systems (EGS). A major challenge in EGS is having an accurate and up-to-date image of the subsurface throughout the project lifespan: from reservoir exploration to resource exploitation, through to well drilling and system stimulation. Future development of these types of sites relies on low-cost monitoring methods that can tell us about the physical properties and ongoing deformation in the reservoir, key factors that can help us identify potential aseismic displacements and nucleation phases for major seismic events.
Dr. Azzola’s work focusses on how correlation of ambient seismic noise – using coda wave interferometry (CWI) – can be developed as a continuous and cost-effective tool capable of monitoring minute changes in the reservoir. However, until now, our understanding of the physical processes that result in changes to a signal’s coda has not yet been clearly described. Indeed, while CWI methods – particularly ambient noice interferometry – make it possible to monitor a reservoir continuously, these signals currently lack precise physical interpretation.
Dr. Azzola developed of a numerical scheme capable of studying the signature of elastic deformation of a medium on diffuse wave fields, and thus CWI measurements. This scheme is based on the use of two discrete modelling codes, the combination of which makes it possible to model seismic wave propagation in a complex diffusive medium during elastic deformation.
By starting in the laboratory, where the elastic deformation of simplified systems is easily constrained, the development of such a numerical approach allowed Dr. Azzola to analyse the processes underlying CWI measurements. Using these analogue experiments, he was then able to develop a more complex numerical model that simulates the propagation of diffuse waves through a reservoir during deformation. Finally, by applying this model to the Rittershoffen geothermal reservoir, Dr. Azzola was able to test the sensitivity of interferometry measurements to deformation scenarios typical of deep reservoirs.
His PhD work brings highlights new frontiers for the interpretation of CWI measurements, in particular by discerning the contribution of different processes active during the elastic deformation of the system. This interferometry technique has the potential to detect and monitor deformation related to the stimulation or operation of deep reservoirs, on different spatial scales and over different time periods. Further this work shows that we can monitor reservoir deformation using ambient noise-based techniques, whether natural or anthropogenic, thus removing the need for active seismic sources.
Congratulations Dr. Azzola!