Three-dimensional distribution of geomechanically based fault properties and their consequence for poroperm and overpressure: a synthetic fault model study

Fault sealing properties are often represented using a simplistic, essentially planar, method in which variations from background permeability are provided via transmissibility multipliers applied to the cells along the fault plane. Transmissibility multipliers can be useful as they are typically used in fluid flow simulations. But they are only one way of representing the changed petrophysical properties and they need to represent the realistic deformation distribution in and around the fault zone, which is almost always complex. Geomechanical simulations of faulting which specifically incorporate poroplastic behaviour can predict deformation-induced permanent volume increase/loss. The associated pore system changes can also be predicted leading to more appropriate fault rock porosities and permeabilities. This work shows how the dilation/compaction and resulting altered porosity and permeability, together with the stress redistribution produced by fault zone geomechanical simulations, are used in a 3D fault zone model. It then evaluates their flow consequences using fluid flow simulations. A synthetic faulted 3D grid model is created containing a listric normal fault with variation in throw. The surrounding sedimentary rocks are assigned uniform generic pre-deformation petrophysical properties (e.g. porosity, perm NTG). The 3D geomodel is then assigned the cell by cell changed porosity and permeability values calculated from the geomechanically derived dilational and compactional strains. Fluid flow simulations using this geomodel show that inclusion of geomechanical derived parameters significantly affects the enhanced/degraded flow around fault zones and the consequent redistribution of pressure regimes.