The Silurian Longmaxi shale formation is a widely distributed gas reservoir in the Sichuan Basin of southwest China. The thick clay rich shale has relatively high organic content, is thermally mature and is a major target for China's shale gas production. In recent years the shale has become the host to an increasing number of induced earthquakes potentially linked to fluid injection related to hydraulic fracturing. These induced or triggered events are linked to the reactivation of faults. This paper explores the frictional and stability properties of Longmaxi shale to provide insights into the causal mechanisms of induced seismicity and their geological and structural controls. Rock samples from a geological section spanning ~150 m were powdered for friction experiments at recreated in-situ stresses and temperatures. Experimental results show that fault frictional and stability properties are strongly controlled by mineral composition. Frictional strength increases with a reduction in phyllosilicate content and exerts a significant control on seismicity. Two of the ten samples exhibited velocity weakening behavior under in-situ conditions, potentially identifying compositions that contribute to unstable slip. These findings have important implications in understanding the frequency and occurrence of induced earthquakes triggered as a result of hydraulic fracturing.

1. Introduction

The rapid growing earthquakes induced by shale gas hydraulic fracturing have aroused wide concerns in recent years (Bao and Eaton, 2016; Ellsworth, 2013; Lei et al., 2017). The large-scale fluid injection can generate high pore pressure in the subsurface and may activate the pre-existing faults (Elsworth et al., 2016). Previous researches demonstrated that the fault sliding behavior is governed by the frictional properties of fault gouge (Kohli and Zoback, 2013; Zhang et al., 2019). Thus, a careful examination of the fault gouge friction properties is important for understanding the underlying mechanism of fault activation and mitigating the seismic hazard during shale gas fracturing.

The Silurian Longmaxi shale is a major target for shale exploitation in southwest China. In this work, the Longmaxi shales recovered from the outcrop in Sichuan Basin, southwest China were sieved to simulate the fault gouge. The friction experiments were conducted on these simulated fault gouges at hydrothermal conditions to explore the effect of mineralogy on gouge frictional and stability properties.

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