Time-dependent frictional healing mechanisms of smectite-rich clay gouge

Abstract

Over the last decades, significant induced seismicity has been experienced in the Groningen gas field in the North-East of the Netherlands (Buijze et al., 2019; Dost et al., 2012). Pre-existing faults that run through the sandstone reservoir into the adjacent formations are reactivated because of changing conditions in the subsurface due to gas production (Van Eijs et al., 2006). This seismic activity is dependent on the strength of the material which is controlled by the healing during an interseismic period. Frictional healing during the seismic cycle has been found to be necessary for repeated earthquake failure of faults (Marone and Saffer 2015). It is therefore essential to understand the mechanisms responsible for fault healing in order to model and predict the behaviour of induced seismicity. A lot of research has been done on calcite, feldspar and quartz-rich gouge material (Karner & Marone, 1998; Niemeijer et al., 2008; Tao & Dang, 2023; Verberne et al., 2014), but the healing mechanisms of clay-rich formations is less understood. Therefore, the aim of this study is to add to the understanding of time-dependent healing mechanisms of clay-rich fault gouge by performing frictional experiments in a hydrothermal ring shear apparatus to investigate the mechanical properties of smectite-rich gouge. Slide-hold-slide experiments were performed in a ring shear in order to study the time-dependent healing behaviour of Opalinus claystone and smectite rich gouge (SWy-2). The results showed contrasting behaviour between the two materials. Opalinus claystone did exhibit minimal frictional healing (Δμ < 0.005 ) in the time-scale of the laboratory experiments of this study (<3 x 104 s after seismic activity), while smectite-rich gouge demonstrated significant healing (Δμ up to 0.05) with frictional strengthening rates of 1.25 to 12.53 x 10-3 log(s-1). This study revealed a positive correlation of the amount and rate of healing with temperature and pore fluid pressure and a negative correlation with the effective normal stress. Higher temperatures, pore fluid pressures and lower effective normal stress facilitate the ease in which water is able to enter the interlayer spaces of the smectite minerals. Water sorption consequently leads to swelling of the material (Zhang 2019), suggesting that sorption is the dominant healing mechanism of clay-rich fault gouge containing smectite. Additionally, this study revealed that the rate-and-state velocity dependence of smectite changes not only with varying conditions but also with the length of the hold time. This means that an increased temperature has a negative effect on the velocity-strengthening behaviour after more than 1 x 102 s after seismic activity, while the pore fluid pressure promotes the velocity-strengthening behaviour in the timescale up to 1 x 104 s. These findings suggest that the presence of smectite in the clay-rich layers adjacent to the Groningen gas field result in reduced frictional healing due to decreasing pore fluid pressure and increasing effective normal stress. However, these conditions also lead to less velocity-strengthening behaviour, thereby increasing the chance of earthquake propagation into the adjacent clay-rich layers of the gas field.