| Abstract |
The state of stress and its implications for the shearing mechanisms on fault planes during hydraulic injections are crucial issues for the general functioning of HDR systems. It is especially true for hydraulic stimulation experiments, aimed at enhancing the permeability and connectivity of natural fracture network: they tend to induce the shearing of joints, which is controlled by the local stress tensor. In this study we present some results about the shearing mechanisms of microseismic events occurred during the 2000 and 2003 stimulation tests at Soultz-sous-ForÍts and their significance in terms of stress. About 7200 microearthquakes have been located in 2000 and 3600 in 2003, from a surface seismological network. In both cases, several hundreds of double-couple focal mechanisms have been automatically determined with the FPFIT program (Reasenberg and Oppenheimer, 1985), using first-motion polarities. Results indicate a majority of normal-faulting movements with a more or less pronounced strike-slip component. Quasi-pure strike-slip events also occur, especially in the deeper part of the reservoir. Although we found a double-couple solution for all events, we tried to observe and quantify the proportion of non-double-couple component in the seismic moment tensor for several microseisms from the 2003 data. The study shows that there is a higher proportion of tensional opening for the events in the vicinity of the injection well than in the far reservoir. We used the method of Rivera and Cisternas (1990) to perform the inversion of the deviatoric part of the stress tensor from P-wave polarities. We applied this method to differents sets of the 2000 data, taken from the shallower and deeper parts of the reservoir. Results show a stable, horizontal, NE-SW-oriented trend of the minimum stress, but a rotation of the maximum stress from a subvertical direction (top of the reservoir) to a subhorizontal one (bottom of the reservoir), which implies a change from a normal-faulting regime to a strike-slip regime with depth, in agreement with our fault-plane solutions. Finally we applied the stress tensor to the nodal planes of several events: we have been able to determine their fault plane and to obtain a 3D image of the fracture network, based on real data. |