Geothermal Reservoir Deformation Monitoring Based on Coda Wave Interferometry

Abstract

Monitoring of geothermal reservoir deformation is essential for the normal development of the Enhanced geothermal system (EGS). Coda wave interferometry (CWI) with ambient noise is regarded as an effective and low-cost monitoring technique and draws more and more attentions. But the connection between the obtained CWI measurements and the undergoing physical changes of deep reservoir is still not so clear. In this study, we take Rittershoffen geothermal system (France) as a case study and conduct a series of forward simulations regarding the propagation of scattered wavefield through the deformed model considering acoustic-elastic effect based on Code_ASTER (mechanical loading) and SPECFEM2D (wave propagation). The simulations are based on a two dimensional numerical model with a scale of 12km (width)×20km (height), in which the upper reservoir model contains 8 layers to mimic Rittershoffen geothermal reservoir, the lower sub model with multiple circular inclusions is set to scatter the waves emitted from point source at bottom and produce scattered wavefield; two seismic stations are located at the top of the model. The model is first verified by reproducing the seasonal variation of relative wave velocity changes obtained from ambient noise cross-correlation functions (ANCCF) induced by the underground water table elevation changes. Based on the validated model, we study the effect of in-situ reservoir deformation on CWI measurements by modelling the hydraulic pressure increases on an open hole and the aseismic slip of an embedded fault which is based on the case of hydraulic injection of GRT-1 well, Rittershoffen. The result indicates the induced small reservoir deformation in both situations can be detected by CWI measurements, which helps us to have a better understanding about the connection between the obtained CWI measurements and the undergoing deformation of deep geothermal reservoir.