ABSTRACT:

The Sichuan Basin of southwest China is the host to an increasing number of earthquakes potentially linked to hydraulic fracturing of gas shales. The current major target reservoirs are in the lower Silurian Longmaxi formation. We report laboratory shear experiments on simulated Longmaxi shale gouges at high pressure and high temperature (HPHT) that probe the underlying mechanisms controlling injection-induced earthquakes. The frictional strength and stability properties of shale gouges were obtained via velocity-stepping experiments at a confining pressure of 60 MPa, pore fluid pressures spanning 30-55 MPa, and temperatures spanning 30-300°C. We observe a coupled effect of temperature and pore fluid pressure on the shale fault stability. Due to different depositional environments, the mineral compositions of the Longmaxi shales are highly variable. At in-situ conditions, most shale gouges are frictionally strong and exhibit velocity-strengthening response, indicative of fault aseismic slip, except for ~5% of shale gouges showing velocity-weakening behavior and potentially promoting seismic slip. Considering the small proportion of unstable shale gouges and the dense distribution of earthquakes around the injection sites, we highlight the importance of aseismic fault slip that may reactivate adjacent and distal unstable faults and trigger the observed seismicity.

1. Introduction

The Sichuan Basin of southwest China possesses rich shale gas resources with broad prospects for development. However, the basin is the host of an increasing number of induced earthquakes potentially linked to fluid injection for hydraulic fracturing (Lei et al., 2017; Meng et al., 2019). In the Changning-Weiyuan national shale gas demonstration area located in the southern edge of the basin (Fig. 1a), recorded seismicity has rapidly increased from 2009 to 2019 (Fig. 1b). The drilling of horizontal wells in the Changning and Weiyuan areas was initiated in 2011, but systematic hydraulic fracturing for shale gas exploitation in those horizontal wells only started in 2014. The target reservoirs are located in the lower Silurian Longmaxi formation shales at depths of 2-3 km. This dramatic increase in earthquake rate correlates with an increased intensity in hydraulic fracturing. Moreover, the 2017 Mw 4.7 and 2018 ML 5.7 induced earthquakes in the Changning area resulted in serious damage to nearby buildings and structures (Lei et al., 2017), constituting a threat to the safety of local residents. Therefore, understanding the physics of injection-induced seismicity is vital to mitigate its occurrence.

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