Hydrogeochemical evolution law of Taiyuan Formation limestone water under coal seam floor caused by the influence of regional grouting

The main threat source of coal mining in the lower formation of Huaibei Coalfield is limestone water from the coal seam floor (including the Carboniferous Taiyuan Formation and the Ordovician limestone water), and some well flooding accidents occur from time to time. For the control of water disaster in the limestone floor, at present, the ground directional drilling high-pressure grouting technology is mostly used for carrying out the regional reinforcement and reconstruction of the third layer of thin limestone (referred to as “three ashes”) in the Taiyuan Formation of the coal seam floor. Therefore, it seals the karst cracks of the three ashes and blocks the vertical water channel passing through the three ashes. However, with the implementation of large-scale high-pressure grouting project, the groundwater flow field in the injected aquifer will be disturbed. Because of the coupling effect of a large amount of slurry water, the groundwater chemical field will inevitably be seriously disturbed. Taking the limestone aquifer of the Taiyuan Formation (hereinafter referred to as “Taihui water”) in the Taoyuan Coalmine of the Huaibei Coalfield as the research object, based on the constant component monitoring data statistics, Piper diagram, ion combination ratio, and saturation index analysis, this paper studies the hydrogeochemical evolution law of the Taihui water before grouting, during grouting and after grouting (i.e. “three periods before, during and after regional grouting”). The results show that ① in the three periods, Na⁺+K⁺, HCO₃ ⁻, Cl⁻ in the Taihui water first decrease and then increase. Ca²⁺, SO₄ ²⁻ and pH values first increase and then decrease, while Mg²⁺, TDS decrease as a whole. In the grouting control area, Ca²⁺ and SO₄ ²⁻ are relatively high and alkalinity is relatively high, while Na⁺+K⁺ and HCO₃ ⁻ are relatively low. Before grouting, SO₄·Cl−Na and SO₄·Cl−Na·Ca (mixed type) are the main types. Since regional grouting, SO₄·Cl−Na·Ca and SO₄·Cl−Na are the main types in the Taihui water, and the proportion of mixed water increases. ② During grouting, the dissolution of carbonate and sulfate, the desalfation and the cation exchange in the Taihui water are weakened, the dissolution of calcite and dolomite is enhanced, and the mixing effect of the precipitates water of cement slurry is obvious. After grouting, the dissolution of carbonate and sulfate, desulphation and cation exchange are gradually enhanced, the dissolution of calcite and dolomite is gradually weakened, and the mixing effect of precipitates water is weakened. One year after the completion of grouting, the hydrochemical effect is gradually recovered toward the state before grouting, but it has not yet recovered to the state before grouting. It can be seen that the regional grouting treatment has a relatively obvious impact on the disturbance of the chemical field of the Taihui water. ③ The saturation index of dolomite and calcite in the Taihui water is large, and most of them are in precipitation state. The saturation index of gypsum and rock salt is small, and they mostly exist in the state of dissolution. In the three periods, the saturation indexes of calcite, dolomite and gypsum in the Taihui water increase first and then decrease, in which dolomite is more obvious, while the rock salt saturation index does not change significantly. The saturation index of most minerals has a strong positive correlation with pH value, while the saturation index of gypsum has a strong positive correlation with TDS. ④ Due to the mixing of the water from the grouting slurry with high pH value, the original ion balance is broken, which leads to the reduction of the correlation of the conventional indicators of the Taihui water since the regional grouting, and the correlation is lower after the grouting than that during the grouting.