HR: 1340h
AN: MR33A-0162    [Abstracts]
TI: Statistics of Rock Criticality and Fluid Injection Induced Seismicity.
AU: * Shapiro, S A
EM: shapiro@geophysik.fu-berlin.de
AF: Fachrichtung Geophysik, Freie Universitaet Berlin, Malteserstrasse 74-100, Build.D., Berlin, 12249 Germany
AU: Rothert, E
EM: rothert@geophysik.fu-berlin.de
AF: Fachrichtung Geophysik, Freie Universitaet Berlin, Malteserstrasse 74-100, Build.D., Berlin, 12249 Germany
AB: We estimate the strength of fractured rocks in-situ by analyzing fluid-induced microseismicity. Such an analysis is able to provide us with a completely new feature of rocks, the statistics of their strength. The strength corresponds to the value of critical pressure in the pore space that must be exceeded in order to activate pre-existing fractures, i.e., to trigger earthquakes. We assume that during hydraulic injection experiments in boreholes, microseismicity is mainly triggered by a diffusive process of pore pressure perturbation. An analytical solution can be applied to find time-dependent pore pressure perturbations in rocks caused by fluid injections. Characteristics of the spatio-temporal evolution of microseismic clouds can be then used to estimate minimum and maximum pressures necessary to trigger earthquakes. Moreover, we present a method for the reconstruction of the full spectrum of the rock strength, i.e. we show how to estimate the probability density function of the critical pressure. We verify the approach using numerical data and apply it to real data of injection-induced microseismicity from the German deep drilling site (KTB), Hot-Dry-Rock tests in crystalline rocks and a hydro-fracturing experiment in sediments. We find that quite low critical pressures, in the range of 1-1000 kPa, are characterizing the strength of pre-existing cracks for all data sets analysed. The critical pressures is found to be quasi uniformly distributed within three to four orders of magnitude. This indicates multi scale character of instabilities in fractured rocks. However, lower and upper limits of the strength probability density functions are quite sharp. This still requires a better physical understanding from the point of view of fault mechanics.
DE: 3265 Stochastic processes (3235, 4468, 4475, 7857)
DE: 5102 Acoustic properties
DE: 5114 Permeability and porosity
DE: 7209 Earthquake dynamics (1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005