Study on the time law of water-rock interaction in coal mine groundwater reservoir

The retention time of mine water in the coal mine underground reservoir will affect the ion concentration of effluent, but the time law of water-rock interaction during operation of the reservoir is still unclear. In order to reveal the occurrence type and intensity of water-rock interaction in different operation stages of coal mine underground reservoir, two groups of static simulation tests of water-rock interaction were designed based on the test and analysis on-site samples, namely the fine sandstone-mine water (S-K) and mudstone-mine water (N-K). The types of water-rock interaction were analyzed from two aspects: the change of rock composition and the change of main ions in water before and after the test. Meanwhile, the time law of water-rock interaction was analyzed and discussed with the Q-type cluster analysis. The change in mineral composition of rock shows that the dissolution of albite, orthoclase and other minerals and the precipitation of kaolinite, quartz and other minerals occurred in the process of water-rock interaction. The ion ratio analysis and statistical analysis show that the cation exchange reaction, the dissolution of minerals (such as silicate and evaporate) and the precipitation of calcium carbonate minerals also occur in the process of water-rock interaction. In addition, the Q-type cluster analysis shows that the water samples of static simulation test can be divided into three categories according to the time series, representing the early stage of water-rock interaction (0-0.5 days), the middle stage (1-10 days or 1-8 days) and the later stage (12-47 days or 10-47 days). In accordance with the analysis of ion source, it is comprehensively judged that the cation exchange reaction is the main reaction in the early stage of water-rock interaction in this test, with the ion change mainly manifested in the sharp increase of Na⁺ concentration and the sharp decrease of Ca²⁺ concentration. In the middle stage, cation exchange reaction and mineral dissolution-precipitation are dominated, with the ion changes mainly manifested in the increase of Na⁺, K⁺ and Cl⁻ concentration, the decrease of Ca²⁺, HCO₃⁻ and Mg²⁺ concentration, the increase of SO₄ ²⁻ concentration in S-K group, and the decrease of SO₄ ²⁻concentration in N-K group. In the later stage, the water-rock interaction is weak in a dynamic equilibrium state, without any obvious change in the concentration of each ion. Moreover, the rock properties have a certain influence on ion changes. Generally, the research results could provide a theoretical reference for the purification mechanism of mine water in different operating periods of coal mine underground reservoirs.