S U M M A R YDuring the hydraulic-fracturing experiment in the German Continental Deep Drilling Borehole (KTB) in December 1994, microseismic activity was induced. Here we develop a technique for estimating permeability using the spatio-temporal distribution of the fluid-injection-induced seismic emission. The values we have obtained for the KTB experiment ( 0 . 2 5~1 0 -'~ to 1 . 0~1 0 -'~ m2) are in a very good agreement with the previous hydraulic-type permeability estimates from KTB deep-observatory studies. In addition, our estimates of the hydraulic diffusivity support the previously calculated value for the upper crust, which is of the order of 1 m2 s-'. However, this estimate now relates to the depth range 7.5-9 km.

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We systematically describe an approach to estimate the large‐scale permeability of reservoirs using seismic emission (microseismicity) induced by fluid injection. We call this approach seismicity‐based reservoir characterization (SBRC). A simple variant of the approach is based on the hypothesis that the triggering front of hydraulically‐induced microseismicity propagates like a diffusive process (pore pressure relaxation) in an effective homogeneous anisotropic poroelastic fluid‐saturated medium. The permeability tensor of this effective medium is the permeability tensor upscaled to the characteristic size of the seismically active heterogeneous rock volume. We show that in a homogeneous medium the surface of the seismicity triggering front has the same form as the group‐velocity surface of thelow‐frequency anisotropic, second‐type Biots wave describing kinematic aspects of triggering‐front propagation in a way similar to the eikonal equation for seismic wavefronts. In the case of isotropic heterogeneous media, the inversion for the hydraulic properties of rocks follows from a direct application of this equation. In the case of an anisotropic heterogeneous medium, only the magnitude of a global effective permeability tensor can be mapped in a 3‐D spatial domain. We demonstrate the method on several field examples and also test the eikonal equation‐based inversion.

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An important characteristic of seismicity is the distribution of magnitudes of earthquakes. Fluid injection in rocks, aimed to create enhanced geothermal systems (EGS), can sometimes produce significant seismic events (e.g., Majer et al., 2007). This is rarely the case in hydraulic fracturing of hydrocarbon reservoirs. However, in any case the behavior of the seismicity triggering in space and in time is controlled by the process of stress relaxation and pore-pressure perturbation that was initially created at the injection source. This relaxation process can be approximated by pressure diffusion (possibly a nonlinear one) in the pore fluid of rocks (e.g., Shapiro and Dinske, 2009). At some locations the tectonic stress in the Earth's crust is close to a critical stress, causing brittle failure of rocks. Increasing fluid pressure in such a reservoir causes pressure in the connected pore and fracture space of rocks to increase. Such an increase in the pore pressure consequently causes a decrease of the effective normal stress. This leads to sliding along pre-existing, favorably oriented, subcritical cracks.