Jérôme Azzola will defend his PhD thesis entitled “Unconventional geophysical monitoring of deep geothermal reservoirs” on Thursday, May 14, 2020.
The monitoring of deep reservoirs is a major issue for the development of renewable energies based on the exploitation of subsurface resources. This applies to the exploitation of resources stored in Enhanced Geothermal Systems (EGS). In the case of such reservoirs, exploitation is based on the circulation of fluids in the upper crust. These circulations take place on large scales, through extended fracture networks. In this context, the challenge is to provide an accurate imaging of the subsurface at the various phases of the project, i.e. from reservoir exploration to resource exploitation, through well drilling and system stimulation. The future development of such projects relies on our ability to develop monitoring methods capable of monitoring, at a lower cost, the key physical properties of the rock mass and the ongoing deformation of the reservoir. This includes the detection of potential aseismic displacements and/or possible nucleation phases of major seismic events.
One emerging technique for continuous and cost-effective geophysical monitoring of geological structures such as EGS is the correlation of ambient seismic noise. Specifically, this involves applying coda wave interferometry (CWI) methods to the waveforms extracted from the cross-correlation of ambient noise. When the distribution of noise sources satisfies the assumptions underlying the study, such methods can be used to follow minute changes in the medium. This is made possible by studying the evolution of the late part of the recordings, called the “coda” in reference to the Latin word for “tail”.
However, a clear link between the physical processes involved in the evolution of the medium and the changes quantified by CWI has not yet been clearly described. More generally, the interpretation of CWI measurements is difficult in terms of perturbation: the measurements encompass all the sensitivities of the diffuse wave field toward the different physical processes involved in the evolution of the system. CWI methods, and in particular ambient noise interferometry, could enable to monitor the medium continuously, but the signals currently lack precise physical interpretation. Considering these issues, the approach we are developing contributes to the interpretation of the interferometric measurements and opens new perspectives for the application of such approaches to the monitoring of deep geothermal reservoirs.
Our approach is based on the development of a numerical scheme which enables to study the signature on the diffuse wave field – and thus on CWI measurements – of the elastic deformation of the medium. The formalism that we propose is based on the use of two discrete modelling codes, the combination of which makes it possible to model the wave propagation in a complex diffusive medium during its elastic deformation.
– At the laboratory scale, where loading paths applied to simplified systems are easily constrained, the development of such a numerical approach allows the analysis of the processes underlying CWI measurements. In particular, the aim is to test the approximations necessary to retrieve numerically the results of representative laboratory experiments, and then to study the partitioning between the different modelled processes.
– Given this newly elaborated analog model, we develop a numerical model simulating the propagation of diffuse waves through a reservoir during its deformation. By calibrating our model on the case study of the Rittershoffen geothermal reservoir (France), we test the sensitivity of interferometry measurements to deformation scenarios characteristic of deep reservoir operations.
Our measurements bring new perspectives for the interpretation of CWI measurements, in particular by discerning the contribution of different processes mobilized during the elastic deformation of the system. The interferometry technique has the potential to detect and monitor deformations related to the stimulation or operation of deep reservoirs, on different spatial scales and over different time periods. Our results open new perspectives to monitor reservoir deformations using ambient noise based techniques, whether natural or anthropogenic. Our analysis provides quantitative arguments for optimizing the monitoring of deep reservoirs using these interferometric techniques.