Objective Hydraulic fracturing of coal seam floors represents an important technical approach to mitigating damage to the floor rock masses through proactive pressure relief and to improving seepage pathways. However, there is a lack of systematic, quantitative insights into the evolutionary characteristics of floor fracture networks, as well as their impacts on the stress redistribution and rock stability of coal seam floors, during hydraulic fracturing.
Methods This study aims to address the challenge that is prone to cause severe damage to the floor rock masses. First, a coal seam floor was considered a saturated porous medium containing multi-scale fractures based on power-law fracture networks and the fluid-solid coupling theory. Then, by incorporating three statistical parameters, i.e., the power-law exponent of fractures, maximum fracture length, and fracture length ratio, this study established a fluid-solid-fracture coupling model for the hydraulic fracturing of coal seam floors for the unified characterization of primary and hydraulic-fracturing-induced fractures. This model facilitates the quantitative and comprehensive analysis of the evolution characteristics of floor fracture networks. Subsequently, the model’s capabilities to characterize fluid migration and stress redistribution under the control of fracture networks were verified using data from laboratory seepage experiments and on-site engineering of fracture media. Finally, this model was applied to the coal seam floor of mining face 1221(3)W in a coal mine within the Huainan mining area, Anhui Province to analyze the spatiotemporal evolution characteristics of stress along the central line of the floor under the presence of confined aquifers or injection boundary conditions. These characteristics, combined with time-varying curves of the axial stress and acoustic emission R-value at the monitoring points for floor failure, provided a reference for the stress evolution and failure criteria in the numerical model.
Results and Conclusions The calculation results of mining face 1221(3)W derived using the proposed coupling model indicate that the statistical characteristics of fractures exerted significantly different impacts on the floor stress. Specifically, the maximum stress applied to the rock masses of the coal seam floor increased by approximately 24.91% as the power-law exponent of fractures decreased to 1.3 from 1.7, increased by approximately 43.71% when the maximum fracture length increased from 0.012 m to 0.020 m, and increased by approximately 10.85% as the fracture length ratio increased from 0.002 to 0.010. Therefore, the maximum fracture length was identified as the most sensitive parameter for controlling the stress concentration and stability of the coal seam floor, followed by the power-law exponent of fractures, with the fracture length ratio producing relatively weak impacts. Additionally, the results reveal that a small number of long, highly penetrating fractures in coal seam floors play a predominant role in stress concentration and the expansion of hydraulically conductive fracture zones. Therefore, the identification and control of these fractures should be highlighted in the hydraulic fracturing design of coal seam floors. Overall, the results of this study provide a reference for the stability assessment of coal seam floors and the prevention and control of their water inrush risk in deep coal mines.
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