Abstract
Faults can be introduced as discontinuities in subsurface models or more explicitly given an internal structure (central core, fractured zones) or as an upscaled homogeneous layer (Fredman et al. 2007). Concepts such as shale gouge ratio (SGR) tune the mechanical and petrophysical properties of the fault (Bjørnarå et al. 2021). The detailed topology of the fault and its surroundings will however be very dependent on the history of deformation that has taken place (e.g., Fig. 1). This in turn will depend on the local and basin-scale stress field, as well as the properties of the different lithologies, including their anisotropy. In fact, the same rock stratum can have differing properties on either side of a fault, or near the fault or deformation bands (A. Varella, pers. comm.). Also, the full 3D structure of the fault has an effect on its activation, which might be different than apparent on a 2D surface cut, as on a visible outcrop (A. Zanella, pers. comm.). In this paper, we review the modelling approaches performed in the last few years at SINTEF, looking at a very simplified geometry with two horizontal sandstone layers at different depths, connected by a bounding fault (Fig. 2). An attempt is made to give structure to the fault by including a process zone (Vilarrasa et al. 2016). The simplest way is to upscale fault core and process zone into one homogeneous layer, assigning it an intact permeability between that of the sandstone layers and the assumed uniform shale elsewhere. SINTEF's in-house Modified Discrete Element tool explores fault reactivation and associated fracturing in the process zone. Coupled with the MRST reservoir simulator, it allows for calculating permeability changes along the fault for different stress hysteresis scenarios, related to injection in a reservoir, with or without prior depletion (Rongved & Cerasi, 2019). An upscaling is proposed using the explicit finite volume program FLAC3D, where the fault is treated as a homogeneous zone (albeit with given thickness). It is weaker than surrounding shaley formations and given similarly low intact permeability. Upon failure, the permeability is either increased by several orders of magnitude or left unchanged. The finite volume simulations also investigate the role of the process zone width on failure extent, together with the introduction of a layered structure, containing a central core (Xie et al. 2023). Finally, healing of fractures is looked at thanks to shale caprock creep and weakening when exposed to CO2-brine. The COMSOL Multiphysics software is used to simulate creep vs. fracture opening pressure.
