Permeability evolution of the thick Cretaceous aquifer and the high-level bed separation water accumulation model during coal mining

Bed separation water, as a source of sudden water inrush induced by mining, is characterized by large instantaneous discharge, periodic occurrence, and subtle warning signs, making it highly hazardous and extremely challenging to prevent and control. Taking the Zhaoxian Coal Mine in the Yonglong Mining Area of the Huanglong Coalfield as the study area, a long-term multi-well pumping test was conducted on the Cretaceous aquifer. Hydraulic tomography inversion technology, based on the Simultaneous Sequential Linear Estimation (SimSLE) algorithm, was used to analyze the permeability evolution of the aquifer during mining. Finally, a mining-induced permeability evolution and bed separation water accumulation model, incorporating vacuum negative pressure effects, was established through groundwater dynamics and numerical simulation methods. The results indicate that: ①The permeability of the Cretaceous aquifer in the overlying strata exhibits a trend of initially increasing and then decreasing as mining progresses. Within the goaf, the permeability coefficient of the Cretaceous aquifer ranges from 0.115 to 1.96 m/d, indicating a permeability increase of 23 to 392 times. In the mining influence area, the permeability coefficient of the Cretaceous aquifer ranges from 0.005 to 0.333 m/d, showing a permeability increase of 1 to 67 times. In the horizontal plane, as the working face advances, the permeability in the front of the working face undergoes a sequential and progressive increase. ②Based on the conceptual model of the convergence point in a semi-infinite aquifer, a theoretical model of bed separation water accumulation under vacuum negative pressure was derived. A "circular island model" for classic steady-state conditions was developed using COMSOL numerical simulation software. The numerical results showed minimal deviation from the theoretical model, indicating that the bed separation water accumulation model established using COMSOL is reasonable and reliable. ③When the permeability of individual blocks evolves sequentially, the bed separation water accumulation rate increases only slightly. However, once all blocks evolve, the accumulation rate rises significantly from 14.09 m³/h to 98.95 m³/h, an increase of 84.86 m³/h. Additionally, the water inflow rate under absolute vacuum is 2.5 m³/h higher than under standard atmospheric pressure. The proposed aquifer permeability evolution-bed separation water accumulation model provides a research framework for predicting and analyzing the accumulation rate and evolution of high-level bed separation water.