Haitao Zhang

Coal-mine underground reservoirs play pivotal roles in water purification through water-rock interactions; however, underlying mechanisms associated with water-rock interactions remain unclear to date. To address this issue, this study considered the underground reservoirs of the Daliuta coal mine, Shendong to conduct the analysis. Through onsite sampling and laboratory simulation experiments, along with hydrochemical-diagram-based analysis, multivariate statistical analysis, and hydrogeochemical simulations, the sources and evolutions of major ions associated with the water of the underground reservoirs were elucidated. Results revealed that the suspended-solid content, turbidity, electrical conductivity, and total dissolved solid concentration in the water at the outlet of the underground reservoirs decreased significantly compared to corresponding values at the inlet. Alteration in the hydrochemical type of water was observed from the Cl·HCO₃-Na·Ca, Cl-Na·Ca·Mg, and Cl·SO₄-Na types at the inlet of the reservoirs to SO₄·Cl-Na at the outlet of the reservoirs. Further, the primary hydrogeochemical processes carried out along the water flow path in the underground reservoirs included mineral dissolution (including halite, silicate, calcite, gypsum, and anhydrite dissolution), pyrite oxidation, cation exchange, and clay-mineral adsorption. Water-rock interactions in the underground reservoirs can be categorized into three stages. The first stage was dominated by dissolution and oxidation, resulting in decreased water pH and overall increased concentrations of total dissolved solids along with major ions in the water. The second stage was dominated by clay-mineral adsorption and cation exchange, resulting in increased water pH and overall concentrations of Na⁺ and K⁺ as well as decreased concentrations of Ca²⁺, Mg²⁺, and total dissolved solids in the water body. The third stage was characterized by weak water-rock interactions, wherein the concentrations of total dissolved solids along with major ions, as well as the pH of the water stabilized. Thus, our research findings provide important theoretical references for understanding water-purification mechanisms and water-storage-duration optimization in coal-mine underground reservoirs.

Constructing underground reservoirs in coal mines can effectively improve the recycling of mine water. Water-rock interactions within underground reservoirs have been demonstrated to improve water quality; however, the mechanisms underlying these water-rock interactions remain unclear, hindering the widespread applications of underground reservoirs. Thus, this study focused on the underground reservoir of the Shendong Daliuta coal mine. Through on-site sampling tests and single-mineral leaching experiments, combined with X-ray diffraction, X-ray fluorescence spectrometry, and scanning electron microscopy, the water quality characteristics of the inlet and outlet water samples from the coal mine underground reservoir were analyzed. Moreover, the physical and chemical properties of the collapsed rocks in the reservoir were investigated, with the aim of clarifying the mechanism underlying the water-rock interactions in coal mine underground reservoirs. The results revealed a significant self-purification effect of the coal mine underground reservoir. Compared with the inlet water sample, the outlet water sample featured substantially reduced amounts of solid-suspended substances, turbidity, total dissolved solids, and electrical conductivity, with the average removal rates of Fe and Mn approaching 98.73 and 92.12%, respectively. Along the flow direction of the inlet and outlet water of the coal mine underground reservoir, the concentrations of Na⁺ and Cl^- presented an increasing trend, whereas the concentrations of Ca²⁺, Mg²⁺, and HCO₃ ^- presented a decreasing trend. The concentration of K⁺ changed insignificantly, while the concentration of SO₄ ^2- fluctuated unstably. The collapsed rocks in the Daliuta coal mine underground reservoir primarily comprised mudstone and sandstone with mineral components including quartz, orthoclase, albite, illite, kaolinite, glauconite, calcite, and pyrite. Among these, kaolinite exhibited the strongest adsorption capacity for Na⁺, Ca²⁺, and Mg²⁺ present in the mine water, while glauconite demonstrated the strongest dissolution capacity for Mg²⁺. Illite presented the strongest dissolution capacity for K⁺, while albite presented the strongest dissolution capacity for Na⁺. The water-rock interactions within the coal mine underground reservoir primarily included dissolution and adsorption processes, wherein mudstone and fine sandstone both played dominant roles in the adsorption of Ca²⁺, as well as in the dissolution of K⁺, Na⁺, and Mg²⁺. In particular, mudstone exhibited a stronger adsorption capacity than fine sandstone, whereas fine sandstone presented a stronger dissolution capacity than mudstone. Thus, our results offer theoretical guidance for understanding water quality purification mechanisms in coal mine underground reservoirs.

Karst collapse columns (KCCs) are channels where groundwater and gas gather, and they pose a great threat to mining safety. In this study, a mechanical model and criterion for the roof collapse of KCCs were established, and simulations were performed to analyze the formation mechanism of KCCs in the Huainan coalfield of Northern China. The results showed that the roof collapse and upward development of KCCs were facilitated by increasing the cave radius of the KCC basement and decreasing the groundwater pressure, single-layer thickness of the roof strata, lateral pressure coefficient of the rock mass, and cohesion and internal friction angle between fractures. The KCC formation process in the Huainan coalfield could be divided into four stages: (I) early collapse, (II) middle-early rapid collapse, (III) middle-late slow collapse, and (IV) late filling compaction. The simulation results were generally consistent with actual KCCs observed in the Huainan coalfield, which verified the theoretical analysis. The results of this study provide an important reference for the formation mechanism and evolution of KCCs in Northern China.

Mining activities considerably affect groundwater quality and resources. It is crucial to investigate water chemistry and hydrochemical processes under coal mining settings for water environment protection and sustainable utilization of groundwater resources. In this study, 36 groundwater samples from the Cenozoic pore aquifer in the Gubei coal mine in Huainan (Anhui Province, China) were analyzed using hydrochemical, multivariate statistical methods, and inverse geochemical modeling. Results revealed a hydrochemical boundary around 100 m below the ground surface, forming two flow zones participating in different hydrological cycles. In the shallow (circulation) zone above the boundary, the main hydrochemical processes are silicate dissolution and cation exchange, followed by evaporite dissolution. In this zone, the Na⁺ + K⁺, Cl⁻, and SO₄²⁻ concentrations are low and stable, forming low-salinity water (total dissolved solids [TDS] < 1 g/L) dominated by Na*Ca*Mg–HCO₃ and Na*Ca–HCO₃ types. At greater burial depths, the pH, TDS, Na⁺ + K⁺, Cl⁻, and SO₄²⁻ concentrations in the groundwater gradually increase, while the HCO₃⁻ concentration gradually decreases, which is mainly due to the enhanced evaporite dissolution and decarbonation in the deep (circulation) zone below the hydrochemical boundary. In this zone, the Na⁺ + K⁺, Cl⁻, and SO₄²⁻ concentrations are high and variable, forming high-salinity water (TDS > 1 g/L) dominated by Na–Cl and Na–Cl*SO₄ types. A hydrochemical evolution model of the Cenozoic aquifer was finally established, which advances our understanding of the evolutionary processes of groundwater chemistry under mining drainage.

This study focus on saturation evaluation of Chang 8 tight sandstone reservoir in Longdong West area of Ordos Basin, China. An improved saturation calculation method was proposed based on the equivalent rock capillary bundle theory. Firstly, according to characteristics of reservoir pore structure and rock conductivity, the conductive space of reservoir rock is equivalent to the parallel conductive of micro capillary bundle representing the micro pores and the coarse capillary bundle representing the macro pores. Then, the variable cementation index(m) saturation model was deduced by using Poiseuille flow equation and Darcy's law. During the calculation of model parameters, the T₂ spectrum data of nuclear magnetic resonance (NMR) was used to calculate the equivalent radius of reservoir micro pores and macro pores, which ensured the ability of model popularization and application. Finally, the proposed saturation calculation method is applied to reservoir evaluation of the study area, and compared with the classical Archie saturation model. The application effect shows that the calculated saturation from the proposed variable m model is much closer to the sealed coring data than that from classical Archie model, and the average relative error of saturation calculated by the variable m model is within 7%, which proves that the proposed saturation calculation method is applicable and effective.

With the reduction of oil and gas reserves and the increase of mining difficulty in Northern China, the carbonate rocks in Southern North China Basin are becoming a significant exploration target for carbonate reservoirs. However, the development characteristics, formation stages, formation environments and mechanisms of the carbonate reservoirs in Southern North China Basin are still unclear, which caused the failures of many oil and gas exploration wells. This study focused on addressing this unsolved issue from the Ordovician carbonate paleokarst in the Huai-Fu Basin, which is located in the southeast of Southern North China Basin and one of the key areas for oil and gas exploration. Based on petrology, mineralogy and geochemical data, pore types, distribution characteristics, and formation stages of the Ordovician paleokarst were analyzed. Then, in attempt to define the origins of porosity development, the formation environments and mechanisms were illustrated. The results of this study showed that pore types of the Ordovician carbonates in the Huai-Fu Basin are mainly composed of intragranular pores, intercrystalline (intergranular) pores, dissolution pores (vugs), fractures, channels, and caves, which are usually in fault and fold zones and paleoweathering crust. Furthermore, five stages and five formation environments of the Ordovician paleokarst were identified. Syngenetic karst, eogenetic karst, and paleoweathering crust karst were all developed in a relatively open near-surface environment, and their formations are mainly related to meteoric water dissolution. Mesogenetic karst was developed in a closed buried environment, and its formation is mainly related to the diagenesis of organic matters and thermochemical sulfate reduction in the Permian-Carboniferous strata. Hydrothermal (water) karst was developed in a deep-buried and high-temperature environment, where hydrothermal fluids (waters) migrated upward through structures such as faults and fractures to dissolve carbonate rocks and simultaneously deposited hydrothermal minerals and calcites. Lastly, a paleokarst evolution model, combined with the related porosity evolution processes, nicely revealed the Ordovician carbonate reservoir development. This study provides insights and guidance for further oil and gas exploration in the Southern North China Basin, and also advances our understanding of the genesis of carbonate paleokarst around the world.