| Title | Stress Heterogeneity in the Granite of the Soultz EGS Reservoir Inferred from Analysis of Wellbore Failure |
|---|---|
| Authors | Benoît Valley, Keith F. Evans |
| Year | 2010 |
| Conference | World Geothermal Congress |
| Keywords | Borehole failure, Stress characterisation, Stress heterogeneities, Rhine Graben, Enhanced Geothermal System, Soultz-sous-Forêt |
| Abstract | Stress heterogeneity is thought to play an important role in many aspects of crustal mechanics, including the influencing of the space-time distribution and scaling of earthquakes. As such, it is likely to have some influence on the reaction of rock masses to massive fluid injection such as at EGS sites. Unfortunately, too little is known about the magnitudes of spatial variations of stress in the crust, primarily because heterogeneity is so difficult to measure. In this paper we describe variations in stress orientation indicated by wellbore failure in two boreholes of the Soultz-sous-Forêts EGS site and attempt to explain them. A novel feature of these data is that the holes are separated by only 20 m between 1400 m and 2400 m where DITFs cover 55% of the borehole lengths, thus allowing the lateral coherence of the variations to be examined. The variations are seen to occur at all scales, from relatively abrupt changes over a couple of metres, to gradual variations over scales of several hundred metres. Deviations in SHmax orientation from the mean in excess of 90° occasionally occur. The variations follow a power law scaling with an index close to -2.0, indicating self-affine behaviour where variations appear progressively “rougher” at shorter scales. Two large long-wavelength (>100 m) variations seen at 2.0 and 4.7 km correlate with the two most prominent fracture zones penetrated by the wells. The lowermost stress perturbation extends over several hundred metres and is characterised by SHmax rotations of up to 90° and changes in the mode of failure from compression (breakouts) to tension (DITFs). The uppermost stress perturbation occurs where the wells are separated by only 20 m, and is only partly correlated between wells, indicating high spatial gradients in stress about the perturbing fracture zone. Short wavelength variations in orientation occur more or less continuously and are usually associated with natural fractures. Little coherence is seen for short-wavelength variations between wells. Almost all fractures associated with stress perturbations are “critically stressed” under the linear stress characterisation of the site. E-DITFs are often seen to form parallel to natural fractures, which typically dip at 50 – 70°. The parallel geometry suggests partial relaxation of shear stress from near critical level on the associated natural fractures. |