Fluid-induced microseismicity in pre-stressed rock masses: Numerical Modelling of poroelastic coupling effects

  • Author:

    M. Schoenball

  • Source:

    Diplomarbeit, Universität Karlsruhe

  • Date: 2009

Zusammenfassung

In this diploma thesis poroelastic coupling effects in connection with the occurrence of microseismic events during fluid injection are studied.
Fluid injection experiments are conducted to increase the hydraulic connectivity between the well bore and the rock formation. Recordings of induced microseismicity are then analysed to gain information of the hydraulic properties, e. g. diffusivity. The standard tool for this analysis is the Seismicity Based Reservoir Characterization. It is based on linear pore fluid pressure diffusion, the influence of the tectonic stress field and poroelastic coupling effects are neglected. However there are indicators that the tectonic stress field has an impact on the spatio-temporal evolution of fluid induced microseismic events.

The physical processes of fluid injection into a stressed, porous medium are described by the theory of linear poroelasticity. One intrinsic characteristic of poroelasticity is the coupling between pore fluid pressure and the stress field. The objective of this thesis is therefore to study poroelastic coupling effects on fluid induced microseismicity. For this purpose I develop a numerical experimental setup to simulate fluid injection into a 2D medium. The coupled poroelastic field equations will be solved using the finite element method, delivering the pore fluid pressure field and the stress tensor. This enables me to apply a Coulomb failure criterion to create microseismic events. 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. Moreover, the numerical setup allows to study the spatio-temporal evolution of the so-called triggering front of microseismic events after fluid injection has been stopped. Due to the deterministic approach used to simulate microseismicity only the behaviour of the triggering front can be studied but not the random occurrence of observed microseismic events observed in real experiments. Experiments with random rock parameters and repeated events are conducted to study this behaviour.

I find that the spatio-temporal evolution of fluid-induced microearthquakes is little sensitive to changes of diffusivity but strongly sensitive to changes of injection rate which is an effect of the implementation of the fluid source. Moreover, I demonstrate that an external stress field with unequal principal stresses causes elongated microseismic clouds. These clouds might by indistinguishable from those generated in poroelastic solids with anisotropic diffusivity for equal principal stresses. This shows that microseismicity distributions are dependent on the coupling of pore fluid pressure and the stress field. In particular, neglecting the influence of the external stress field may lead to overestimation of the anisotropy of diffusivity tensor components.