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The seepage characteristics of groundwater in the overlying fissures are the key factors to be considered in the later maintenance of coal mining and goaf, especially in the underground reservoir construction area of coal mines. It is also an important part of coal mine underground reservoir. In shallow groundwater-rich areas or under extreme summer rainfall conditions, underground reservoirs form vertical recharge, and analyzing the seepage characteristics of groundwater in overlying fissures provides a scientific basis for the safe operation of coal mine underground reservoirs and the protection of groundwater resources. In this study, the solid-liquid coupling similarity model test was used to obtain the development and stability of the overlying fissures in the mining area and the characteristics of groundwater seepage after the aquifer was connected. The analysis showed that the large abscission fissures and micro-fissures that extend far and wide are mostly water storage. , the vertical fissures running through multiple rock layers have strong water conductivity, and the hydraulic connection between the separation layer fissures is mainly formed by the vertical fissures in the two sides. Under the condition of vertical recharge, groundwater first infiltrates along the vertical fissures in an unsaturated manner, and is gradually saturated regionally from the upper and lower overlying fissures, finally forming a stable saturated infiltration form. On this basis, a mathematical model of groundwater seepage in saturated seepage state is established, and the numerical method is used to solve it. It is verified with similar simulation experiments that vertical fractures are the main water conduction channels, and their water conduction can account for up to 97%. At the same time, the migration speed of groundwater in vertical fractures is also many orders of magnitude higher than that in abscission fractures. Finally, through the sensitivity analysis, it is concluded that the vertical fracture seepage is positively correlated with the fracture development degree and the total water inflow, and negatively correlated with the rock permeability. The migration time of groundwater in overlying fissures is negatively correlated with the degree of fissure development, rock permeability and total water inflow.

Nano-particles possess desirable attributes such as small particle size, excellent injectivity, and migration performance, making them highly compatible and adaptable for addressing the water flooding requirements of the low-permeability oil reservoir. When selecting an oil displacement agent for enhancing water flooding and improving oil recovery, factors such as injectivity and migration need to be carefully considered. In this study, through a comprehensive analysis of the mechanism and technical characteristics of nano-particle oil displacement agents, the plugging and profile control mechanisms recognized by the mainstream of nano-particles are elucidated. By examining various elements including outcrop fractures, natural micro-fractures, artificial support fractures, and dynamic monitoring data, a reevaluation of the dominant channel scale governing water drive in low permeability reservoirs is conducted, thereby defining the target entities for profile control and flooding operations. Drawing upon Darcy’s percolation law and leveraging enhanced oil recovery techniques based on the classical Kozeny equation, a profile control and flooding mechanism is proposed that focuses on increasing the specific surface area of polymer particles while simultaneously reducing reservoir permeability. This innovative approach establishes a novel matching method between nano-polymer particles and the diverse media found within the reservoir. Lastly, the application of nanoparticle flooding technology in Changqing Oilfield is presented, highlighting its practical implementation and benefits.

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