Evolution of pore-fracture structure and mechanical properties of coal under dry-wet cycle

A water dry-wet (DW) cycle, triggered by water injection into coal seam, affects the pore-fracture structure (PFS) of the coal, thus changing the storage environment of coalbed methane (CBM). In order to study the effect of DW cycle on the PFS, a nuclear magnetic resonance technology (NMR) is employed to measure the PFS after different DW cycles by carrying out online loading experiments with mechanical equipment. Experimental results demonstrate an initial increase followed by stabilization in pore-fracture distribution as DW cycles escalates. The evolution process of the PFS is divided into the development and connectivity stage. The development stage is characterized by the development of individual pore-fracture, the growth rate of adsorption space (AS) is larger than that of seepage space (SS), and the increase of pore-fractures leads to the enhancement of non-homogeneity of SS. The connectivity stage is characterized by the interconnection of pore-fractures, the growth rate of the SS is larger than that of the AS, and the connectivity of pore-fractures to the decrease of the non-homogeneity of the SS. Empirical and generalized models, derived from offline experimental results, eloquently describe the evolution of PFS through DW cycles. The coal matrix becomes more brittle, and the compressibility and stress sensitivity of the SS increase with an increasing number of cycles. A conformable derivative compression model of the SS is established based on online experimental results. The correlations between DW cycle damage and compression model parameters are established by fitting analyses. The mechanism of the influence of water injection into coal seam on the environment of CBM is revealed from a microscopic point of view by establishing the relationship between the DW cycle and the PFS of coal.