Friction-Stability-Permeability Relationship of Longmaxi Shale Fractures from the Southern Sichuan Basin, Southwest China

Abstract

ABSTRACT: Hydraulic fracturing induced seismicity has been frequently reported in the southern Sichuan Basin of southwest China, but the relationship between induced earthquakes and frictional stability-permeability of natural fractures in this area has not been well understood yet. Hydraulic fracturing process may change the friction-permeability relationship of fracture surface and thus has an important influence on the stability of fractures/faults. In this paper, we employed the friction -permeability coupling experiments on natural shale fractures, recovered from the 3700 m depth reservoir in the southern Sichuan Basin, to measure the friction coefficient and permeability change and clarify the friction-permeability-stability relationship of natural shale fractures. We carried out constant velocity shearing experiments, velocity step shearing experiments, constant velocity shear-flow experiments, and velocity step shear-flow experiments. In the experiments, the confining pressure is kept at 3 MPa, the constant velocity is set at μm/s and the velocity step are set at 10-1-10-1-10 μm/s. Results show that the fractures are frictional strong with the friction coefficient is 0.6 ~ 0.75. The friction coefficient of the same fracture in the pure shearing experiments is lower than that in the shear-flow experiments. The results of the velocity step experiments show that (a − b) >0 under all conditions, which is manifested as velocity strengthening, indicating aseismic slips possibly caused by the rich phyllosilicate content in the shale samples. We believe that the stability of the fracture is affected by the combination of factors such as the mineral composition, roughness of the fracture surface and the pressure of the injected fluid. 1. INTRODUCTION Seismic activity induced by fluid injection has attracted more and more attention, especially in hydraulic fracturing of shale gas exploitation (Igonin et al., 2021; Schultz et al., 2020; Grigoli et al., 2017; Elsworth et al., 2016; Bao & Eaton, 2016; Ellsworth, 2013). Sichuan Basin is rich in shale gas resources. Since 2009, the initial exploration has gradually entered the stage of large-scale exploitation while hydraulic fracturing is the main stimulation technology (Zou et al., 2016; Liang et al., 2012). However, the complex geological conditions of the Sichuan Basin make natural faults and fractures extensively developed in this area (Charvet, 2013). The characteristics of natural fractures in the deeper Longmaxi shale and peripheral faults have an important impact on the safe and efficient exploitation of shale gas (Atkinson et al., 2020; Wang & Shen, 2020; Zhou et al., 2012). With the increasing concerns of induced seismicity in recent years, it is necessary to study the friction-stability-permeability of natural fracture surfaces (Fang et al., 2017; Zhang et al., 2017; Ikari et al., 2011; Crawford et al., 2008; Yasuhara et al., 2004) for Longmaxi shale (Kohli & Zoback, 2013).