Experimental fault sealing: shear band permeability dependency on cataclastic fault gouge characteristics

Abstract An experimental rock mechanics facility was used to assess the finite permeability of shear bands artificially induced in cylindrical core plugs of high porosity sandstone under triaxially compressive stress states of varying magnitude. A method of estimating shear band permeability from axial transient pulse-decay measurements is presented, based on certain assumptions including a linear thickness decrease with resolved normal stress across the fault. Computed permeability reductions range from 2.5 to 3.5 orders of magnitude, with cataclastic sealing exhibiting an order of magnitude increase with increasing normal stress and angular shear strain imposed on the gouge zone. A laser particle sizing technique was applied to the experimentally generated fault gouge samples to appraise any differences in their distributions down to 0.1 μm particle size. By defining the peak (relative fineness) and width of distribution (closeness of grading) of each, the Rosin-Rammler exponential function was able to quantify ‘cataclastic intensity’ via a ‘coefficient of variation’ statistical parameter. An empirical relation demonstrated between fault seal and degree of cataclasis is rationalized through consideration of the Kozeny-Carman relation, which analyses permeability in terms of its constituent parameters of tortuosity, porosity and surface area.