In this chapter we consider situations of fluid injections with the injection pressure (i.e. the bottom hole pressure or the pressure at the perforation borehole interval) smaller than the absolute value of the minimum principal compressive stress, σ3. Then, usually, permeability enhancement on the reservoir scale is significantly smaller than one order of magnitude. In such a situation the behavior of the seismicity triggering in space and in time is mainly controlled by a linear process of relaxation of stress and pore pressure.

We will start this chapter by considering fluid-injection experiments in crystalline rocks at the German Continental Deep Drilling site KTB (Kontinentale Tiefbohrung). Results of these experiments are helpful for identifying factors controlling fluid-induced seismicity. Then we consider kinematic features of induced seismicity such as its triggering front and its back front. They provide envelopes of clouds of microseismic events in the spatio-temporal domain. We further consider features of the triggering front in a hydraulically anisotropic medium.

We introduce several approaches for the quantitative interpretation of fluid-induced seismicity. These approaches can be applied to characterize hydraulic properties of rocks on the reservoir scale. Thus, they can be useful for constructing models or constraints for reservoir simulations. These approaches are based on the assumption of a linear fluid–rock interaction. Some of these approaches have a heuristic nature. These are the triggering-front-based estimation of hydraulic properties of rocks (in the next chapter we will observe that in some cases of a non-linear pressure diffusion the triggering front can be introduced exactly) and the related eikonal-equation-based approach for characterization of spatially heterogeneous hydraulic diffusivity. We will also consider a dynamic property of induced seismicity: its spatial density. A related issue of statistical properties of the strength of pre-existing defects will be a subject of our consideration too.

Case study: KTB

The German KTB site (see a series of papers with an introductory overview of Emmermann and Lauterjung, 1997) is located in Windischeschenbach (Southeastern Germany, Bavaria).