Research on Gas Reservoir Space Characteristics in the Goaf of Xinzhuangzi Closed Coal Mine, Huainan Mining Area

China has numerous closed coal mines with abundant residual resources. The combined effect of multiple factors has driven research on gas resources in the goafs of these mines. Elucidating the characteristics of goaf gas reservoir space is crucial for analyzing gas distribution and extraction. Therefore, investigating gas reservoir space characteristics under coal seam group conditions is vital. This study uses the Xinzhuangzi closed coal mine as a case study and presents methods for calculating goaf space under both individual coal seam and coal seam group conditions. The volumes of the caving, fissure, and floor failure zones are determined. The results reveal that 14 coal seams have been mined at the Xinzhuangzi coal mine, with a total mined area of 4583.04 × 104 m2 across all seams. Under individual coal seam assumptions, the caving, fissure, and floor failure zones have volumes of 455.98 × 106, 1648.40 × 106, and 614.65 × 106 m3, respectively, with a total goaf volume of 2719.03 × 106 m3. Under coal seam group conditions, the caving, fissure, and floor failure zones have volumes of 438.22 × 106, 871.24 × 106, and 154.90 × 106 m3, respectively, with a total goaf volume of 1464.36 × 106 m3. The caving, fissure, floor failure zones and total volumes under coal seam group conditions are 96.11%, 52.85%, 25.20%, and 53.86% of the corresponding volumes calculated under individual coal seam assumptions. This indicates that the coal seam group goaf volumes are neither a simply the sum nor a proportional reduction in the individual coal seam goaf volumes. This study provides a concise approach for investigating goaf gas reservoir space under coal seam group conditions.