Elmar Rothert

Characterization of fluid transport properties of reservoirs using induced microseismicity

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 the low-frequency anisotropic, second-type Biots wave (i.e., slow wave). Further, we generalize SBRC for 3-D mapping of the permeability tensor of heterogeneous reservoirs and aquifers. For this we apply an approach similar to the geometric optics approximation. We derive an equation 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.

Characterization of hydraulic properties of rocks using probability of fluid-induced microearthquakes

The use of borehole fluid injections is typical for exploration and development of hydrocarbon or geothermal reservoirs. Such injections often induce small-magnitude earthquakes. The nature of processes leading to triggering of such microseismicity is still not completely understood. Here, we consider induced microseismicity, using as examples two case studies of geothermal reservoirs in crystalline rocks and one case study of a tight-gas sandstone reservoir. In all three cases, we found that the probability of induced earthquakes occurring is very well described by the relaxation law of pressure perturbation in fluids filling the pore space in rocks. This strongly supports the hypothesis of seismicity triggered by pore pressure. Moreover, this opens additional possibilities of using passive seismic monitoring to characterize hydraulic properties of rocks on the reservoir scale with high precision.

Characterization of fluid transport properties of the Hot Dry Rock reservoir Soultz-2000 using induced microseismicity

Hydraulic stimulation is a procedure for increasing the permeability of a reservoir. At the geothermal site of Soultz, France, such experiments have been carried out since 1993 at different depths. During the Soultz-2000 hydraulic stimulation, about 7200 seismic events were located using a borehole and free surface seismic network. We analyse the spatio-temporal distribution and density of the events to estimate the large-scale permeability of the medium. We assume that the main triggering mechanism is a pore-pressure diffusion process. Based on this idea, we apply different, already developed, methods for the Soultz-2000 hydraulic stimulation. We obtain two independent scalar permeability estimations, a permeability tensor and a heterogeneous reconstruction of the hydraulic diffusivity. The results agree very well with independent in situ tests.

Dynamics of Microseismicity by Hydraulic Fracturing - An Approach to Interpretation

Several basic dynamic processes related to propagation of hydraulic fracturing modify the effective stress in rocks and, therefore, are relevant for triggering of microseismicity. For instance, these are the creation of the new fracture volume, fracturing fluid loss and its infiltration into reservoir rocks as well as diffusion of the injection pressure into the surrounding rocks and inside the fracture. Here, using real data we show that some of these processes can be seen from features of the spatio-temporal distributions of the induced microseismicity. Especially, the back front of the induced seismicity starting to propagate after termination of the fluid injection seems to be characteristic and informative for the aims of reservoir engineering.