Microscopic pore structure types of sandstone and its effects on aquifer water abundance:Taking in Ningtiaota coal mine as an example

The microscopic pore structure characteristics of sandstone determine the water yield property and permeability of sandstone,and the prediction of water yield property of sandstone aquifer is an important problem that must be studied and solved firstly. Take Ningtiaota coal mine in Northern Shaaxi Jurassic coalfield as an example,this paper collected 7 typical sandstone samples of different lithology which include 3 blocks of Zhiluo Formation and 4 blocks of Yan’an Formation,and studied the microscopic pore structure characteristics of Zhiluo and Yan’ an Formation with different lithology using experimental testing techniques that include thin section,cast thin section,high pressure mercury injection,nuclear magnetic resonance (NMR) and MATLAB image analysis. The distribution of pore throat radius was obtained by high pressure mercury injection and NMR. According to the pore throat size,distribution and condition of the continuity,the sandstone microscopic pore structure was divided into Type Ⅰ,Ⅱ,Ⅲ. Comparing with the unit water inflow of borehole pumping test results,a certain relationship between the microscopic pore structure and aquifer water abundance was revealed. The results show that TypeⅠof pore structure has higher proportion of coarse and medium-grained sandstone,smaller displacement pressure,later position of T2 spectrum peak than that of Type Ⅱ,Ⅲ of pore structure. The distribution of pore radius obtained by high pressure mercury injection is basically consistent with that by nuclear magnetic resonance. Under the same stratigraphic age and lithology,the stronger the weathering degree is,the stronger the water abundance is. Coarse and medium-grained sandstone is stronger water abundance than siltstone under the same stratigraphic age and different lithology conditions. Under the same lithology conditions in different strata ages,the microscopic pore structure of Zhiluo Formation (the sandstone microscopic pore is Type Ⅰ) is better than Yan’an formation (the sandstone microscopic pore is Type Ⅱ),and the water abundance of Zhiluo formation is stronger than Yan’an formation. It provides a new idea for the characteristics and scientific prediction of water abundance of sandstone aquifer,and a technical support for mine water control and water resources protection.