Numerical simulation of seismicity due to fluid injection in a brittle poroelastic medium

Abstract

It has recently been shown that rather small perturbations in effective stress due to fluid injection or withdrawal may trigger microseismi c events. Such events, typically in the magnitude range below magnitude 4 ML, have similar characteristics to normal tectonic earthquakes, with double-couple focal mechanisms implying a dominant shear motion at the source. In this paper we examine the nature of this mechanical fluid—rock interaction for the case of fluid injection in a hydrocarbon reservoir. Our model is based on the combination of a model of seismicity in dry rocks and a model of pore fluid pressure diffusion. The former involves a finite difference approximation of the equation of motion, and the latter follows a lattice Boltzmann approach. They are coupled via the concept of effective stress, applied both to the Mohr—Coulomb rupture criterion and to the volumetric elastic deformations, which in turn perturb the pore pressure. The simplifying assumptions are that the fluid and solid phases have the same bulk moduli and that strain compatibility conditions are not included. 2-D plane-strain simulations of fluid injections in an anisotropically pre-stressed elastic brittle medium illustrate the capabilities of the model: spontaneous inception and growth of shear fractures, in both the near and far fields, and explicit calculation of their seismic radiation, which feeds back into pore pressure perturbations. They also show that, as expected, the amount and rate of stress drop during the brittle rupture control the size and spatial density of the resulting fractures. Our model should prove to be relevant at least in two cases: as a simulation tool in the investigation of correlations between injection and producer wells which are thought to be of geomechanical origin in certain oil fields, and as a forward model in the inversion of focal mechanisms of pore-pressure-induced shear events.