Microseismic signatures of non-linear pore-fluid pressure diffusion

Most fractured porous rocks show a strong dependence of their fluid transport properties on the effective pressure and thus on the pore-fluid pressure. We investigate the case where the hydraulic transport properties are exponentially dependent on pore-fluid pressure. As a consequence, the process of pore-fluid pressure diffusion is governed by a non-linear diffusion equation. Solutions of this equation that describe fluid injection experiments are analysed and the associated microseismic signatures are explored. In particular, it is shown that this process of non-linear pore-fluid pressure diffusion allows us to define two signatures. One which is the triggering front corresponding to the triggering of microseismic events without changing the fluid transport properties significantly. The other signature is the so-called fracturing domain. It triggers the majority of microseismic events and alters the fluid transport properties significantly. Evidence for the microseismic signature of a fracturing domain at the Fenton Hill hydrofrac experiment is given.