Porosity-Permeability Relationships in Clay-bearing Fault Gouge

Abstract

Abstract We have performed a series of coupled deformation and permeability experiments on "synthetic" fault gouge to investigate the influence of "shaliness" on fluid flow properties. Biminerallic gouge layers were sheared between steel sliders under triaxial compression and permeability measured continuously using the pore pressure oscillation technique. Permeability was observed to range over six orders of magnitude depending on clay content and sliding. Measured porosity variations, post-test laser particle sizing and direct observation of thin-section microstructures allows development of a geometrical model for permeability evolution in fault gouge containing varying amounts of clay. Introduction Faults can serve as both seals to hydrocarbon accumulation and conduits for secondary hydrocarbon migration. Evaluation of hydrocarbon entrapment and production patterns in faulted reservoirs requires understanding of the nature and properties of the fault rock material. Fault rock properties impact fluid movement regardless of timescale. In basin analysis, hydrocarbon migration over geologic time is dominated by the capillary properties of the fault zone, however fault rock permeability controls fluid flow on the development/production timescale. Fault rock properties are conventionally incorporated in flow simulation using transmissibility multipliers, which adjust the effective permeability between model cell centers based on estimates of fault zone permeability and thickness1. Fault transmissibility is a key parameter on the production timescale as it impacts well planning, well test interpretation and reservoir simulation of potential development scenarios in faulted reservoirs. Fault rock permeability is a function of the microfabric developed within the fault zone, which in turn reflects the deformation mechanism(s) operative under different conditions of faulting2. Grain scale deformation in siliciclastic hydrocarbon reservoirs is primarily via cataclasis, diffusive mass transfer or a combination of these processes, depending upon interaction between lithology (mineralogy, texture) and environment (stress level, temperature). Fault zones are typically highly heterogeneous and variable over short distances. Fault seal analysis thus involves assessment of the distribution of fault rock types over individual faults, and estimation of their associated properties3.