Influence of multiphysics couplings across scales: from digital rock physics to induced fault reactivation

Abstract

This work addresses the problem of pressure equilibration across a seemingly sealing fault, which has been observed during reservoir production. Considering the detrimental industrial consequences of seal failure, predictive occurrence of such events, pointing to a temporarily drastic increase of the fault’s permeability, becomes necessary for safe subsurface operations. Yet, there is currently no consensus concerning the explanation of this complex phenomenon. The dissertation focuses on carbonate reservoirs under relatively high P,T conditions, whereby suggesting that permeability increase stems from chemical dissolution during the fault reacti- vation. The production-enhanced shear-heating of the creeping fault leads to thermal runaway, subsequently activating the chemical reaction. This contribution presents a three-scale numerical framework using the REDBACK simulator to account for 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, evolution and deactivation features, predicted by common fault reactivation models, yet with continuous transitions between phases. The influence of the rock microstructure on fault and reservoir behaviour is quantified in a simulation where a hydraulically imperceptible difference in the microstructure’s geometry results in a different duration of the reactivation event at the macro-scale. We demonstrate the advantage of dynamically upscaled laws compared to empirical laws as we capture permeability hysteresis during dissolution/precipitation of the fault. The alteration of the microstructure also influences mechanical properties and its weakening effect on the homogenised yield stress of 3D printed microstructure samples is accurately predicted by our numerical model. Ultimately, this thesis identifies the potential hazard of fluid production next to a sealing fault under high P,T conditions, which is of great significance as operations are taking place at ever increasing depths. In this regard, we suggest the first model to predict the occurrence and consequences of chemical fault reactivations.