Microseismicity of non-linear fluid-rock interactions: from stimulations of geothermic reservoirs to hydraulic fracturing of shales

Characterization of fluid-transport properties of rocks is one of the most important, yet one of most challenging goals of reservoir geophysics. There are some fundamental difficulties related to using active seismic methods for estimating the fluid mobility and the rock permeability. However, it is very attractive to have a possibility of exploring rock hydraulic properties using seismic methods because of their large penetration range combined with their potentially high resolution. Microseismic monitoring of borehole fluid injections is exactly the tool providing us with such a possibility. Borehole fluid injections are typical for stimulation and development of hydrocarbon or geothermal reservoirs. Production of shale gas and heavy oil as well as CO2 sequestration are examples of relatively new applications of this technology. The fact that fluid injection causes seismicity has been well-established for several decades. Current on going research is aimed at quantifying and control of this process. It becomes clear that understanding and monitoring of fluid-induced seismicity is necessary to hydraulically characterize reservoir systems and to assess results of their stimulations. However, a fluid-induced seismicity can be caused by a wide range of processes. Here we show that a linear pore pressure relaxation corresponding to the low frequency limit of the slow wave on the one hand, and a hydraulic fracturing on the other hand are two asymptotic end members of a set of non-linear diffusional phenomena responsible for seismicity triggering. To account for the whole range of processes we propose a rather general non-linear diffusional equation describing the pore pressure evolution. This equation takes into account a possibly strong enhancement of the medium permeability. Both, linear pore pressure relaxation and a hydraulic fracturing, can be obtained then as special limiting cases of pore pressure diffusion. The hydraulic fracturing can be then considered as a weak shock wave. We demonstrate corresponding seismicity signatures on different case studies and show possibilities of microseismic monitoring for hydraulic characterization of rocks  and for assessments of seismic risk related to reservoir developments.