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We present a novel experimental approach devised to test the hydro-mechanical behaviour of different structural elements of carbonate fault rocks during experimental re-activation. Experimentally faulted core plugs were subject to triaxial tests under water saturated conditions simulating depletion processes in reservoirs. Different fault zone structural elements were created by shearing initially intact travertine blocks (nominal size: 240 × 110 × 150 mm) to a maximum displacement of 20 and 120 mm under different normal stresses. Meso-and microstructural features of these sample and the thickness to displacement ratio characteristics of their deformation zones allowed to classify them as experimentally created damage zones (displacement of 20 mm) and fault cores (displacement of 120 mm). Following direct shear testing, cylindrical plugs with diameter of 38 mm were drilled across the slip surface to be re-activated in a conventional triaxial configuration monitoring the permeability and frictional behaviour of the samples as a function of applied stress. All re-activation experiments on faulted plugs showed consistent frictional response consisting of an initial fast hardening followed by apparent yield up to a friction coefficient of approximately 0.6 attained at around 2 mm of displacement. Permeability in the re-activation experiments shows exponential decay with increasing mean effective stress. The rate of permeability decline with mean effective stress is higher in the fault core plugs than in the simulated damage zone ones. It can be concluded that the presence of gouge in un-cemented carbonate faults results in their sealing character and that leakage cannot be achieved by renewed movement on the fault plane alone, at least not within the range of slip measureable with our apparatus (i.e. approximately 7 mm of cumulative displacement). Additionally, it is shown that under sub seismic slip rates re-activated carbonate faults remain strong and no frictional weakening was observed during re-activation. [ABSTRACT FROM AUTHOR]

Abstract: To determine the ultrafine nano-scale grains in a fault gouge; the Chelungpu fault gouge was sampled and analyzed. It was separated into different particle size ranges and was analyzed by synchrotron X-ray diffraction and transmission electron microscopy. The minerals of gouge are predominantly composed of quartz, plagioclase, smectite, illite, chlorite, and kaolinite. The mineral association of <100nm particles are quartz, smectite, and illite. However, only smectite and illite are included except quartz in the 1-to-25nm fractions. We propose that quartz is the index mineral associated with co-seismic fracture and the possible limit grain size is ~25nm on fracturing. The smectite and illite nano-particles may be associated with weathering process of gouge at shallow or surface conditions. [Copyright &y& Elsevier]

When faults form and slip they produce granular wear material that accommodates most of the strain. We investigate the effect of grain size and grain size distribution on slip dynamics by using sheared elliptical acrylic discs as a fault gouge proxy. Physical experiments on three different grain size assemblages are performed in a spring slider apparatus. Our results indicate that below a grain diameter threshold of 1/10 of the shear zone width, increasing grain size corresponds to faster slip velocities during slip events. In experiments with more than one grain size, the relative proportions of grain sizes and effective grain sizes control the bulk behavior. We show that shear zones with grains that are larger than 1/10 of the shear zone width show deformation dynamics that are independent of the small grain fraction. In shear zones where the grain sizes are below this threshold a high abundance of small grains promotes slower and smaller slip events. • Grain size and grain size distribution affect slip dynamics in simple shear. • For grains with a diameter <1/10 of the shear zone width, larger grains lead to larger and faster stick-slip events. • The fraction of small grains has no impact on slip dynamics when grains larger than 1/10 of the shear zone width are present. • The grain size abundance controls their contribution to the bulk slip behavior of the system. [ABSTRACT FROM AUTHOR]