Three-scale multiphysics finite element framework (FE³) modelling fault reactivation

Abstract

Fluid injection or production in petroleum reservoirs affects the reservoir stresses such that it can even sometime reactivate dormant faults in the vicinity. In the particular case of deep carbonate reservoirs, faults can also be chemically active and chemical dissolution of the fault core can transform an otherwise impermeable barrier to a flow channel. Due to the scale separation between the fault and the reservoir, the implementation of highly non-linear multiphysics processes for the fault, needed for such phenomenon, is not compatible with simpler poromechanics controlling the reservoir behaviour. This contribution presents a three-scale finite element framework using the REDBACK simulator to account for those multiphysics couplings in faults during fluid production. This approach links the reservoir (km) scale – implementing poromechanics both for the fault interface and its surrounding reservoir – with the fault at the meso-scale (m) – implementing a THMC reactivation model – and the micro-scale (μm) – implementing a hydro-chemical model on meshed μCT-scan images. This model can explain the permeability increase during fault reactivation and successfully replicate fault activation, slip evolution and deactivation features, predicted by common fault reactivation models, yet with continuous transitions between the sequences. The multiscale coupling allows to resolve the heterogeneous propagation of the fault slip which can be uncorrelated from the initial highest slip tendency location. We also demonstrate the advantage of dynamically upscaled laws compared to empirical ones as we show that a hydraulically imperceptible alteration of the microstructure's geometry can lead to different durations of the reactivation event at the macro-scale.