A Geomaterials Approach to Fault-Zone Characterisation

Abstract

Observations demonstrate that some faults appear to be single planes where frictional concepts may be appropriate to assess evolution, stability and properties. Other fault examples show finite-thickness zones of either homogenised fault-rock, or spatially-ordered fault-related components – together these zones might be called a fault core. Such fault zones need to be acknowledged in fluid flow simulations, or in stability assessment, so there is a need to understand what phenomena control the spatial arrangement of fault-rock characteristics, and how those property distributions are expressed in seismic images or in fluid flow simulations. Geomaterials research (experiments, numerical simulation, and observations of natural examples) has been developing important new understanding about the processes that operate during the creation and evolution of shear zones/bands. Lab experiments using uniform material, with full-volume pre-, syn- and post-deformation observations, show that shearing processes often operate to create a finite-thickness zone within which states of stress and strain are far from uniform, and bear little or no relationship to the far-field state. Within the zone, the deformation becomes organised into distinct (often lozenge-shaped) regions where volumetric strains are dilative or compactant, with varying amounts of shear. These outcomes are comparable to the results of numerical simulations, which additionally reveal the variability of local stress states. Smaller-scale natural shears seem to be well explained by the processes identified in lab and simulation. Large-scale faults are compatible with these concepts, but outcrops are rarely/never of sufficient size and quality to allow a demonstration of the direct applicability (length-scales of lozenges exceed outcrop limits). Synthetic seismic models, based on strain states from the numerical methods, would be interpreted as showing multi-stranded faults, where no discontinuities exist. The understanding gained at lab-scale allows the calculation of deformation-caused poro-perm changes, which, when used in reservoir flow models, show the role of a fault zone in terms of flow performance. The standard approaches (transmissibility modifier of cell boundaries) lead to flow performance far from that predicted using the property arrangements derived from the geomechanical approach. A next-generation strategy has been developed for including geomechanical-derived properties in reservoir models.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015