## 1 Introduction  
China is one of the countries with the most severe gas disasters in the world [1]. According to the statistics, from 2013 to 2017, there were 1945 coal mine accidents in China, of which 218 were caused by gas, accounting for 11.21% of the total [2]. Coal is a porous medium with abundant internal pores [3]. To recognize the gas flow law in coal, many theories have been developed, e.g., [PERSON]'s permeation, Fick's diffusion, the diffusion-permeation mixture, etc. [4; 5; 6; 7]. Based on the above theories, researchers found that the flow law of gas in coal is mainly determined by the gas pressure and the permeability of the coal seam; the permeability is significantly influenced by the stress paths and the integrity of coal [8; 9]. Thus, it is necessary to investigate the permeability evolution of different integrity (undamaged, fractured, post-strength) states of coal under various stress paths (loading, cyclic loading, unloading). Previous studies on the permeability changes with different stress paths and different integrity states of coal are mainly divided into four aspects. First, the permeability change rules of initial undamaged coal under loading conditions were studied. In the mining process, the original stress distribution is destroyed by the mining activities and forms the stress concentration zone, resulting in the change of permeability. The permeability of initial undamaged coal is intensely sensitive to the effective stress and decreases with the increase of the effective stress, and it is also affected by the swelling stress of coal [10; 11; 12]. There is an exponential relationship between the permeability and the effective stress, and the permeability anisotropy of undamaged coal under effective stress shows a dynamic development change law [13; 14]. Second, the permeability change rules of initial undamaged coal under cyclic loading and unloading conditions were studied. With the advancing of the working face in the mining process, the overlying coal and rock are subjected to repeated loading and unloading stress. The permeability is significantly influenced by the repeated mining cycles. When the stress exceeds a specific value, the permeability increases with the increases of loading and unloading cycles. Conversely, the permeability decreases with the cyclic loading and unloading cycles [15; 16; 17]. Other parameters relevant to permeability were investigated to further understand the permeability evolution with cyclic loading and unloading stress, for instance the recovery rate, residual deformation, acoustic emission (AE) characteristics, and energy dissipation [18; 19]. Third, the permeability change rules of fractured (prefabricated fractures) coal under loading conditions were studied. In the stress concentration zone, some coal is damaged, but since the stress changes complexly in front of the working face, the fractured coal is still in the stress loading stage. An increase in axial stress results in the expansion of the fracture and a decrease of the permeability [20], and the anisotropy of permeability in fractured coal is more evident than in intact coal [21]. Fourth, the permeability change rules of fractured coal (prefabricated fractures) under cyclic loading and unloading conditions were studied. In the mining process, some coal has reached the peak strength and damage, and the fractured coal is loaded and unloaded periodically under the repeated action of mining pressure and supporting resistance; even secondary damage occurs. The permeability of fractured coal can be modified by the evolution of the fracture network and is sensitive to the effective stress and stress paths [22; 23]. The results of the literature review showed that most of the studies were focused on the permeability evolution in the loading stage, cyclic loading, and unloading stage of initial undamaged or fractured (prefabricated fractures) coal.