In recent years, the hydrogeology research of low-permeability rocks has received unprecedented attention and development, which is closely related to the storage of high-level radioactive waste and the increase of the proportion of nuclear energy in the world energy structure. The central problem of research is groundwater flow and solute (radioactive substance) migration in low permeability rocks, including field research, indoor test, large-scale field test, parameter calculation, numerical simulation and so on. These rocks include crystalline rocks, clay and evaporite. At present, North America and European countries have a high degree of research. This situation is also reflected in the papers of the International Geological Congress in recent years 10. At the 30th International Geological Congress, most papers came from the United States, Canada, France, Belgium, Spain and Denmark.
American scholars have studied the paleohydrogeological conditions of liquid phase migration in sedimentary basins and formed some valuable mineral resources, and discussed the formation and evolution of ground pressure in sedimentary basins and the spatial change relationship between low permeability K and rock structure in heterogeneous rocks. They also measured the hydraulic characteristics of rocks with low conductivity on the spot, and even set up test stations in tuff areas. Of course, this kind of test is more difficult. At present, the latest progress in studying the hydraulic characteristics of fractured rock mass is mainly to establish a three-dimensional flow model by combining geological and geophysical methods. In Germany, the observation test of water flow line in low permeability clay layer was carried out.
As for the hydrogeological problems of low-permeability crystalline rocks that need further study, Chin-Fu Tsang of the United States pointed out that there are five kinds: ① test methods. Because the flow rate of fissure water is several orders of magnitude larger than that of rock matrix, conventional pumping test cannot be adopted. Usually, water injection test and pressure drop test are adopted, and the technology of measuring low flow velocity is developed. It is still necessary to study the relationship between the mechanics of rock fractures (opening or closing) and the pressure of injected water, including effectively measuring the hydraulic conductivity of fractures. ② Field test and analysis technology. The permeability coefficient of low permeability fractured media changes greatly in space, so it is necessary to study the method of carefully analyzing and reviewing field well test data. ③ Body movement process. In low permeability media, besides some known action processes, there should be four additional influencing factors: first, the gap change of a single fracture; Second, there is channel water flow or concentrated water flow; Thirdly, the diffusion of tracer in matrix is a slow process; Fourthly, the thermal-water-mechanical coupling action leads to the opening or closing of cracks, thus changing the permeability coefficient. Although these processes are understood to varying degrees, the technical treatment in model research is still developing. ④ Simulation technology. Generally, fracture network model and stochastic continuous model are adopted. How to use stochastic model to understand field experiments, modify parameters and simulate physical phenomena is still a problem. ⑤ Forecast model strategy. Hydrological effects related to solute flow and migration in geological structures, including dispersion and its proportional relationship, matrix diffusion, river flow and density-driven flow, are still unclear. Scientific issues related to the prediction model include how to obtain appropriate boundary conditions and long-term planning (temporal and spatial changes) of related systems; How to master the heterogeneity of geological system; How to develop the conceptual model of the system and how to combine the conceptual models consistent with the valid data is under study, and there is still a lack of experience in forecasting the situation of 104a.