Coupled Hydro Geomechanical Simulation and Analysis of Fault Reactivation: Influence of Fault Geometry

Abstract

Abstract Fault reactivation is one the unfavourable incidents during drilling and production due to changes in in-situ stresses and reservoir pressure. Only a few studies, in their analysis, have included the effects of fault geometrical properties, in particular length, dip angle and surface geometry, while these are important parameter controlling fault slippage. In this paper, the significant influence of fracture morphology on the mechanical behaviour of rock masses is explained through virtual coupling of FDM/DEM numerical simulations. It will be shown how changes in fracture roughness and inclination angle lead to different failure mechanisms. DEM numerical simulation of synthetic fractures shearing in a bonded particle model was performed to investigate the effects of surface roughness on the creation of damage zones around the pre-existing fractures as a result of broken bonds between particles. Moreover, the effects of the ratio of shear to normal stresses on fracture sliding will be discussed analytically. In the second part of this study, synthetic data from a sandstone reservoir was used for a field-scale FDM modelling to analyse fault reactivation and plastic deformation. Our focus is on shear failures close to the fault plane, which intersects the reservoir rock horizon in a field with normal fault stress regime being dominant. Pressures associated with fluid injection were increased until the fault being reactivated, and this was then analytically confirmed with the Mohr-Coulomb failure criterion. Sensitivity analyses were performed to examine the effects of state of in-situ stress, injection pressure, fault inclination angle, and surface properties on the potential of fault reactivation. The results show that the extent of damage zone along a reactivated fault is highly influenced by fault surface geometry and injection pressure. The higher the inclination angle, the lesser the fault sliding strength. Sensitivity analysis of injection pressure and fault surface properties showed that fault reactivation potential is a function of multiple parameters, but more sensitive to the injection pressure than fault infills properties. It was also observed that the hydrocarbon flow path after fault reactivation is towards the sheared segments of a fault zone, where the permeability was increased. Large displacements were recorded where a large dip angle (here 63º) and a small friction angle (here 10º) were assigned to the fault surface. Distributions of shear strain are clearly heterogeneous along and near fault planes. This suggests that specific fault segments might be prone to experience leakage when others might retain a hydrocarbon column.