Fault Mechanics and Fluid Flow

Abstract

In reservoirs and petroleum systems, both geoscientists and engineers think of faults as having petrophysical properties, which means that, for the purposes of fluid flow, faults are considered to be volumes – that is, there is a thickness to the fault zone (uniform or not). In contrast, the mechanical analysis of faults is typically expressed from the perspective of discontinuities, which means that faults are treated like a surface that has cohesive/frictional properties. Thus, there is a profound difference in the model adopted for faults, depending on the purpose. Is it not preferable to adopt compatible models for both/all purposes? A conjecture posed here is that the separate models arise because few work in both disciplines at the same time or are familiar with alternate mechanical models for faults. So most interpretations involve just thefrictional conceptual idea. Here, we present a summary of experimental and numerical studies, along with outcrop observations, that underpin a mechanical model for faults that is completely compatible with the type of model adopted for fluid flows. This alternate mechanical model is referenced to geomaterials, rather than to continua, and thus acknowledges the mean stress dependence, volumetric strains, and localisation behaviour. The deformation of geomaterials, within a localised shear zone (fault), is rich in complexity. Strains accumulate in lozenge-like regions of higher and lower strain, with shear strains and volumetric strains varying on short length-scales, and revealing both dilation and compaction. During the development of the overall shear zone, local stress states exhibit extreme variability, and have no relationship to any far-field state. If we assume that the petrophysical properties of the fault-zone components are related to the strain or stress state, locally (a plausible notion), then the flow properties of the fault zone properties are also heterogeneous but in ways that are organised. The mechanical model outlined here leads to a very different analysis of the idea of fault stability, or ideas about how fluid flow might be influenced by active faulting.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016