An enriched mixed finite element model for the simulation of microseismic and acoustic emissions in fractured porous media with multi-phase flow and thermal coupling

A novel mixed enriched finite element model is developed for coupled non-linear thermo-hydro-mechanical simulation of fractured porous media with three-phase flow and thermal coupling. Simulation of induced acoustic emission (AE) and microseismic emission (ME) due to tensile fracturing and shear slip instability of pre-existing fracture interfaces is carried out and the numerical results of the emitted signals are analysed. The mathematical model is based on the generalized Biot's theory for coupled interaction of solid and fluid phases. A computationally robust non-linear solver is developed to handle the severe non-linearities arising from fluid saturations, relative permeabilities of fluids, constitutive models of interfaces and convective thermal coupling. To model pre-existing natural fractures and faults, discrete fracture propagation and nucleation of cracks (micro-cracking) independently of the original mesh topology, a local Partition-of-Unity (PU) finite element method, namely, the Phantom Node Method (PNM) is implemented. The cohesive fracture modelling scheme is implemented to account for the non-linear behaviour of fracturing and localization, and to rectify the non-physical stress singularity condition at the fracture tip. Effects of different system parameters on fracturing, shear-slip instability and the associated induced AEs and MEs are investigated through various numerical results.