Storage coefficient modeling and capacity evaluation of coal mine underground reservoirs considering gangue deformation and goaf structure

Coal mine underground reservoirs help address the severe water imbalance in ecologically fragile mining regions of western China, but evaluating their storage capacity remains challenging due to the coupled effects of gangue deformation, saturation, and goaf geometry. This study investigates the deformation and void evolution of fragmented gangue with varying lithologies, particle sizes, and water contents through an independent-developed testing system and theoretical model. A planar micro-unit model and a three-dimensional spatial structure model are proposed to quantify the storage coefficient and total reservoir capacity of underground water storage structures. These models incorporate the effects of stratified lithologies, saturation-induced softening, and spatially distributed stress conditions. The methodology is applied to the underground reservoir in Chahasu coal mine, and the results show that under increasing stress, storage coefficients decline exponentially, with pronounced differences between single- and double-lithology structures. The storage coefficient in the spatial model demonstrate greater resilience to stress concentration compared to planar models, and further analysis identifies critical thresholds in roof fracture distances and stress-recovery times affecting long-term storage performance. This research provides a comprehensive framework for evaluating underground reservoir storage potential, offering theoretical support and engineering guidance for the sustainable utilization of mine water.