Experimental simulation on mineral and hydrochemical evolution in response to water–coal–gas interaction of closed gassy coal mines

Abstract

One ubiquitous environmental issue with the abandoned gassy coal mine is the potential of changes in water quality due to water–rock–gas action caused by high residual gas content. Therefore, a better understanding of how to accurately characterize the new multiphase system in the closed gassy coal mine is required. The gas in closed coal mine mainly includes CH $$_{4}$$ , CO $$_{2,}$$ N $$_{2}$$ , H $$_{2}$$ S, CO, etc., existing in the adsorbed, dissolved and free phases, which also leads to complex gas–water–rock interactions in the underground flooded pool. The recovery of the water level caused by the closure of the coal mine varied dramatically in hydrogeochemical conditions as well as gas flow performance in the flooded mine. Based on the nationwide investigation and case studies of high residual coal mine gas in abandoned coal mines in China, some sealed gassy coal mines have been experiencing the CO $$_{2}$$ inrush problems. Modeling experiments of mine water–rock (coal)–gas(CO $$_{2}$$ /N $$_{2}$$ ) reaction were conducted in closed coal mines. The results show that with the increasing reaction time, pH value increases gradually, and the relationship between Eh and pH is negative, by taking CO $$_{2}$$ /N $$_{2}$$ as gas phases in the water–rock reaction. The concentration of HCO $$_{3}^{-}$$ , SO $$_{4}^{2-}$$ , Ca $$^{2+}$$ , Mg $$^{2+, }$$ and SiO $$_{2}$$ increases with a different magnitude, and the level of total Fe decreases gradually, while the TDS of the solution increases. The reaction between typical minerals in coal and high SO $$_{4}^{2-}$$ and Fe $$^{3+}$$ mine water is simulated. The chemical composition of mine water and coal mineral compositions are modified in order to obtain the influence of particular reactions. The results show that in the mine water–rock–gas reaction with CO $$_{2}$$ as gas phase, the pH value of the solution increases, and the Eh decreases. The dissolution of mineral phases leads to an increasing trend in the content of HCO $$_{3}^{-}$$ , SO $$_{4}^{2-}$$ , Ca $$^{2+}$$ , Mg $$^{2+}$$ , and soluble SiO $$_{2}$$ in the water. The concentration of the total Fe in the solution gradually decreases, and the mineralization of the solution gradually increases. Each mineral has individual distinct dissolution reaction rate. The dissolution of kaolinite promotes the dissolution of quartz, while calcite dissolves along with the regeneration of new carbonate minerals. Pyrite dissolves accompanied by chlorite formation. For the N $$_{2}$$ gas phase, changes in hydrochemistry and mineral phases are mainly controlled by water–rock interaction.