Fluid-induced microseismicity in pre-stressed rock masses

Abstract

We model microseismicity triggered by fluid injection on the basis of the theory of poroelasticity accounting for the external stress field. Consideration of the fully coupled poroelastic field equations enables us to apply a Coulomb failure criterion using pore fluid pressure and stress tensor as well as the coefficient of friction. The poroelastic fields are calculated with the finite-element method simulating fluid injection with constant injection rate into a 2-D domain. The influence of diffusivity, injection rate and stress field on the occurrence of microseismicity is analysed and compared to simulations based on pore fluid pressure diffusion only. We show that an anisotropic initial stress field causes elongated microseismic clouds. These clouds are indistinguishable from those generated in poroelastic solids under isotropic stress but exhibiting anisotropic hydraulic diffusivity. This similarity shows that microseismicity distributions dependent on both, the hydraulic properties and the coupling of pore fluid pressure to the stress field. In particular, neglecting the influence of the external stress field may lead to overestimation of the anisotropy of diffusivity tensor components. Furthermore, the results of our numerical simulations are strongly sensitive to changes of fluid injection rate.