Crack Features and Shear-Wave Splitting Associated with Fracture Extension during Hydraulic Stimulation of the Geothermal Reservoir in Soultz-sous-Forêts

Abstract

-The recent tomography results obtained within the scope of the Enhanced Geothermal System (EGS) European Soultz project led us to revisit the meso-fracturing properties of Soultz test site. In this paper, we develop a novel approach coupling effective medium modeling and shear-wave splitting to characterize the evolution of crack properties throughout the hydraulic stimulation process. The stimulation experiment performed in 2000 consisted of 3 successive injection steps spanning over 6 days. An accurate 4-D tomographic image was first carried out based upon the travel-times measured for the induced seismicity [Calò M., Dorbath C., Cornet F.H., Cuenot N. (2011) Large-scale aseismic motion identified through 4-D P-wave tomography, Geophys. J. Int. 186, 1295Int. 186, -1314. The current study shows how to take advantage of the resulting compressional wave (Calò et al., 2011) and shear-wave velocity models. These are given as input data to an anisotropic effective medium model and converted into crack properties. In short, the effective medium model aims to estimate the impact of cracks on velocities. It refers to a crack-free matrix and 2 families of penny-shaped cracks with orientations in agreement with the main observed geological features: North-South strike and dip of 65°East and 65°West [Genter A., Traineau H. (1996) Analysis of macroscopic fractures in granite in the HDR geothermal well EPS-1, Soultz-sousForêts, France, J. Vol. Geoth. Res. 72, 121-141], respectively. The resulting output data are the spatial distributions of crack features (lengths and apertures) within the 3-D geological formation. We point out that a flow rate increase results in a crack shortening in the area imaged by both compressional and shear waves, especially in the upper part of the reservoir. Conversely, the crack length, estimated during continuous injection rate phases, is higher than during the increasing injection rate phases. A possible explanation for this is that cracks remain large because the system has time to relax. We also calculate the extension and opening rates during all hydraulic stimulation sets. While the opening rate is unchanged, the extension rate varies depending on the stimulation phase. It is also shown to be higher around and above the open-hole section than below. This can indicate a potential upward path that makes fluid percolation easier within the granite formation, this path being induced by the temperature gradient. We also compare the evolution of crack extension during injection with shear-wave splitting. Split shear waves were recorded at 2 stations during hydraulic stimulation and processed in terms of splitting parameters. The fast shear-wave polarization remains constant and parallel to the maximum horizontal stress orientation while the amplitude of splitting varies with time. We observe a good agreement between travel-time differences and crack extension rates during the first 4 days of the stimulation experiment. Afterwards, these two parameters depart from each other. This stud...

Temporal Variations of Near‐Surface Anisotropy Induced by Hydraulic Fracturing at a Shale Play Site in Southwest China

“…Polarization seismic anisotropy is often measured with two parameters—the fast polarization direction ϕ f and delay time δt between the fast and slow directions—from the splitting of nearly vertical‐incidence S waves, such as the local S waves and teleseismic SKS waves (e.g., Adelinet et al, ; Silver & Chan, ; Teanby et al, ; Wuestefeld et al, ). The fast polarization direction is usually parallel to the orientation of the vertical cracks, and the delay time is related to the crack density.…”

“…On the other hand, using human‐controlled processes, such as hydraulic fracturing of hydrocarbon reservoirs, might be a better way to quantify the relationship between temporal variations of seismic anisotropy and changes of subsurface fractures. Some studies used S wave splitting of microseismic events to characterize the evolution of crack properties throughout the hydraulic stimulation process (Adelinet et al, ; Baird et al, ). Riahi et al () applied a three‐component beamforming algorithm to ambient noise recorded at receivers above underground gas storage in the Paris Basin and found an increase in the strength of seismic anisotropy that is likely to be caused by changes in pore pressure in the reservoir.…”

Comparing Theoretical With Measured Seismic Anisotropy in the Central Taupo Volcanic Zone

“…Many shear wave splitting studies in geothermal fields have been carried out elsewhere, e.g. at the Krafla geothermal Field (Iceland; Rial et al, 2005); modelling of effective medium and shear-wave splitting at Soultz-sous-Forêts (France; Adelinet et al, 2015); Larderello-Travale geothermal field (Italy; Piccinini and Saccorotti, 2018; the Geysers Erten et al, 2001;Evans et al, 1995) and Coso (Lees & Wu, 2000;Lou & Rial, 1997) geothermal fields in California (USA).…”

“…The density of quartz is 2660 kg/m 3 and the density of pyrite is 5013 kg/m 3 , (Babuska & Cara, 1991). The body wave velocities are estimated from Figure 1 The default values for the CD and AR are chosen according to the likelihood of the values deep below the ground surface (Adelinet et al, 2015;Crampin, 1994Crampin, , 1999Crampin & Leary, 1999), and the range of values are extended to obtain a reasonable size dataset for imaging the variation of shear wave anisotropy with the rock parameters. The crack densities used in the rock models contain the estimated values in Crampin and Leary (1999) but are in a wider range, from 0 to 0.5.…”

“…The crack densities used in the rock models contain the estimated values in Crampin and Leary (1999) but are in a wider range, from 0 to 0.5. The set range of AR is from 0.00001 to 0.001, similar to (Adelinet et al, 2015) and also Alongside the material properties effect (Tables 3.1 and 3.2) on shear wave splitting, directional dependence is also studied. This is done by evaluating a range of synthetic earthquake back azimuths (baz) and incidence angles (inc) passing through a single dominant fracture family (225/080).…”