Grain Size and Fluid Effects on the Frictional Behavior of Simulated Carbonate Fault Gouge

Abstract

Carbonate rocks exhibit a span of clast and grain size in fault zones. However, the effect of grain size distribution on the frictional behavior of carbonate faults and its implications for natural and induced seismicity remain poorly understood. This study elucidates the effect of grain size distribution on the frictional behavior of Lower Carboniferous (Dinantian) simulated fault gouge, relevant to geothermal energy exploitation in Northwest Europe. We tested samples in a rotary shear apparatus under air dry and drained conditions using deionized (DI) water or geothermal brine. In each experiment, we stepped the normal stress from 1 to 10 (Formula presented.) and back, in 1 (Formula presented.) intervals. At each interval, we performed a slide-hold-slide procedure with 10 (Formula presented.) slide time, 10-100-1000 (Formula presented.) hold times, and shear velocity of 20 (Formula presented.). Our results demonstrated that the presence of fluids in carbonate gouges increased the creep coefficient of internal friction, healing, and relaxation (absolute values and rates), consistent with the effects of pressure solution and corroborated by the CNS microphysical modeling. Additionally, frictional sliding facilitated by water films along grain boundaries controlled the peak and steady-state coefficients of friction. Healing was independent of the initial grain size distribution and occurred in the PSZ. Relaxation occurred both in the PSZ and bulk gouge and was governed by PSZ thickness, which positively correlated with the initial mean grain size.