Prevention and control technology for composite hazards of water and H₂S in coal seams underlying strongly water-rich burned rock aquifers

In the Jurassic coalfields of Western China, burned rock is widely distributed, highly fractured, and exhibits strong water richness, serving as a primary aquifer in the region. Composite hazards involving burned rock water, coal seam water, and H₂S gas severely threaten the safety of mine development and coal mining. To address the composite hazard of water and H₂S gas in coal seams underlying burned rock aquifers, this study analyzed the distribution and water-richness characteristics of burned rock in the study area, investigated the origin of coal seam water and H₂S gas, developed prevention and control technology for these composite hazards, and demonstrated the application of detection and treatment techniques. The effectiveness of the prevention measures was evaluated using field monitoring data. The results indicate that Seams 2⁻², 3⁻¹, 4⁻², and 5⁻² in the study area exhibit varying degrees of burning. The burned rock aquifers associated with each seam possess large static reserves and strong water richness, receiving direct recharge from loose layer water and weathered bedrock water. Water from the Seam 2⁻² burned rock aquifer infiltrates the Seam 3⁻¹ coal via vertical fractures in the underlying strata. This water, containing dissolved H₂S, dissolves carbonate minerals within the coal pores, enlarging the pore and fracture spaces and further expanding the water storage capacity of the coal seam. The H₂S gas in the coal seams originates from two sources: conversion of sulfur-bearing organic matter into sulfur-rich kerogen and decomposition of sulfur-bearing hydrocarbons during spontaneous combustion in Seam 2⁻², and sulfate reduction under closed anaerobic conditions within Seam 3⁻¹. The H₂S subsequently escapes from the coal wall with flowing water. Underground directional drilling and borehole transient electromagnetic methods accurately delineated water-rich zones within the coal seams and roof strata, precisely identifying water-rich anomalies within a 30 m radius cylindrical space around the borehole center. Surface borehole grouting effectively sealed the vertical seepage pathways between the Seam 2⁻² burned rock and Seam 3⁻¹ coal. Underground advance small-pipe grouting blocked seepage channels within the coal seam and roof strata. Advance drainage boreholes effectively dewatered the coal seam and roof rock layers. Consequently, water inflow decreased from 182 m³/h to 6.5 m³/h, the hydraulic head dropped from 37 m to 0.8 m, and H₂S concentration reduced from 22 ×10⁻⁶ to 2.8 ×10⁻⁶. This successfully enabled the safe development of the haulage gateway.