Can faults become barriers for deep fluid circulation? Insights from high‐resolution seismic VSP tomography at the Soultz‐sous‐Forêts geothermal site

Abstract

Vertical Seismic Profile (VSP) surveys are generally used for modelling converted phases of the seismic body waves propagating in the medium allowing the detection of waves interpreted as reflections on steeply dipping reflecting structures such as faults, abrupt lateral changes of lithology, and fractures. At the Enhanced Geothermal System geothermal field of Soultz‐sous‐Forêts the analysis of data recorded during a VSP experiment allowed describing the presence of at least two structures near the wells. Here we show how seismic tomography method can be applied to the VSP data to reconstruct the 3‐D shape of structures in the volume surrounding the geothermal wells. The three‐dimensional P wave velocity model obtained shows positive velocity anomalies associated with the main faults observed by the VSP analysis and negative anomalies in the regions affected by massive hydraulic stimulations performed in the past. This pattern can be explained as a different response of the rock volume to the fluid injections where regions marked by relative pre‐existing high permeability were less affected by the hydraulic stimulations. This difference in permeability produced regions that could work as barriers for fluid diffusion through the reservoir. Comparisons of our high resolved model with the location of the induced seismicity and with another model obtained using seismic noise correlation give evidence of the presence of these structures and may explain the poor connection between the wells GPK4 and GPK2–GPK3 system.

Implications on large-scale flow of the fractured EGS reservoir Soultz inferred from hydraulic data and tracer experiments

“…The results are in agreement with the microseismic inversion of Kohl et al (2006), which indicated a seismically inactive E-W-striking plane that could be either highly permeable or totally sealed. Calò et al (2016) concluded a seismic anomalous zone between the two wells from Vertical Seismic Profiling as well. Barton et al (1995) observed that fractures oriented perpendicular to the maximum horizontal stress have a higher probability to be sealed.…”

“…However, the model is updated and extended in this study by introducing two additional fracture zones as 1) the WNW-ESE-oriented fracture "Separation" between the wells GPK3 and GPK4, and 2) GPK1-FZ2856 fracture intersecting GPK1 in the middle reservoir. The Separation fracture was not drilled but suspected as an anomalous zone of either higher permeability or hydraulic barrier, separating the deeper reservoir into a northern and a southern part (Calò et al, 2016;Kohl et al, 2006;Sausse et al, 2010). The fracture GPK1-FZ2856, identified using Vertical Seismic Profiling (Sausse et al, 2010), is added to allow better adjustment of hydraulic parameters close to the GPK1 well.…”