The mechanical behaviors and seepage properties of sandy mudstone have been widely studied in the field of radioactive waste disposal, shale gas exploration, cap-rock behavior of hydrocarbon reservoirs, carbon geo-sequestration, underground coal gasification, and coal mining (Gale et al, 2007; Fu et al., 2015; Rezaeyan et al., 2015; Wilson et al., 2021). With increasing depth, the transitional deformation behavior from brittleness to ductility and complex transport characteristics occurred in the rock (Bishop, 1967; Renner et al., 2000; Nygård et al., 2006; Wu et al., 2021). At the macroscopic scale, brittle rock is characterized by a macroscopic fracture with much localized deformation, whereas ductile rock is distinguished by uniformly distributed deformation (Evans et al., 1990). Furthermore, the brittle-ductile transition deformation behavior can be obtained with a mixed microscopic transfiguration mechanism (Evans et al., 1990), while the permeability change varies with the mining activity. An abundant work focused on this field and outstanding findings were reported.
Existing studies in this area mainly focused on the destruction properties, which includes the bearing capacity and fracturing patterns (Petley, 1999; Renner et al., 2000; Boulin et al., 2013; Rezaeyan et al., 2015; Wu et al., 2021). Meanwhile, the stress depended brittle or ductile rock was also a hot issue, and it was evaluated using triaxial compression strengths and residual strengths (Rutter, 1986; Wang et al., 2019), as well as Young's modulus and Poisson's ratios during the linear elastic deformation stage (Rickman et al., 2008; Bai and Wierzbicki, 2010; Memon et al., 2020), of rocks under various stress situations. The two key parameters of internal friction angle and cohesion derived using the Mohr-Coulomb failure criterion may be used to define it for brittle rocks(Hucka and Das, 1974; Singh et al., 2011; Nooraiepour et al., 2017). The smaller disparity between the maximum strength and residual strength, or the lower Young's modulus and internal friction angle of rocks, on the other hand, indicates a higher degree of ductility (Evans et al., 1990; Brantut et al., 2011; Wu et al., 2021).
The seepage property influences the mechanical behavior in mudstone. The permeability evolution of mudstone caused by different deformation modes, such as elastic deformation, brittle failure, and ductile deformation, is extensively investigated (Djeran-Maigre et al., 2000; Zhang and Rothfuchs, 2008; Zhang, 2016; Wu et al., 2021). The permeability is observed to drop fast during the early loading stage and later rise, and the permeability turning point associates well with the alteration in volumetric strain known as dilatancy (Schulze et al., 2001). Therefore, the permeability of mudstone in brittle regime increases considerably as dilatancy and failure develop, while the permeability of ductile mudstone keeps constant at the strain hardening stage (Popp et al., 2001; Alkan, 2009). Additionally, the dynamic response of permeability is reported by the pattern of localized or distributed deformation determined by the effective stress (De Paola et al., 2009). Wu et al., (2021) conducted a series of experiments, composed of the conventional triaxial compression tests, the hydrostatic loading-unloading tests, and the triaxial loading-unloading tests, on mudstone to study the permeability evolution at various stress conditions. Zhang et al., (2021) characterized the permeability variation of the fractured sandy mudstone by considering the impact of hydro-mechanical coupling, fracture morphology, and mineral composition. In addition, the permeability models are usually developed on different empirical functions, such as exponential functions (Seidle et al., 1992; Li et al., 2013; Tan et al., 2019), cubic functions (Gangi, 1978; Kwon et al., 2001) and logarithmic functions (Kranzz et al., 1979; Walsh, 1981). To predict the coal permeability at the yielding and post-failure stage, Xue et al., (2015) developed a permeability model considering the damage process. To define the variation in coal permeability when effective stress changes from the elastic deformation stage to the post-peak stage, Chen et al. (2016) introduced an effective stress-dependent exponential permeability model. To investigate the permeability behavior with stress varies throughout the unloading process, Zhang et al., (2017) put forward an analytical permeability model that included the influence of damage evolution.
The change in mudstone permeability and mechanical properties caused by mining activity is complex, it would be of great significance to investigate the mechanical behavior and transport properties of mudstone subjected to the in-site stress impact. In this study, the triaxial compression-seepage experiments were performed, and the relationship of damage behavior and seepage characteristics and the influence of deformation on permeability response at brittle or ductile state were analyzed. An improved permeability model was developed, and the permeability evolution of mudstone was discussed.