Study on microscopic pore structure types of Jurassic sandstone and water abundance in Jinjie Coal Mine

The water abundance of sandstone is one of the important indexes for the risk assessment of roof water inrush. It is of great significance to study the microscopic pore structure of sandstone for understanding its relationship with the water abundance of aquifer.In this paper,taking the Jurassic sandstone of 3-1 coal roof in Jinjie Coal Mine as an example. The microscopic pore structure of sandstone was studied by thin section identification and mercury porosimeter method. Finally,the relationship between the microscopic pore structure and water abundance of sandstone in the study area was analyzed by comparing the unit water inflow of borehole. The results show that the clay content(25.5% on average) in Yan’an formation of Jurassic is much higher than that in Zhiluo formation(4% on average).According to the shape of capillary pressure curve and displacement pressure,the pore structure of sandstone can be divided into three types:low displacement pressure type Ⅰ,medium displacement pressure type Ⅱ and high displacement pressure type Ⅲ. The low displacement pressure type Ⅰ sandstone is mainly composed of intergranular and intercrystal pores which is larger than 10 μm. If the pores in the study area larger than 10 meters were regarded as effective pores,the ratio of effective pore volume would reach 61.68%. The confined water head of the aquifer is about 66.30 m,with a certain pressure head,and a large number of gravity water in the pores moves freely under a certain confined water head,which is an important reasons of mediumwater abundance. In the medium displacement pressure type Ⅱ sandstone are dominated by intragranular and intergranular pores between 0.1 μm and 10.0 μm,The pore volume of this type accounts for 51.44%. In the high displacement pressure type Ⅲ sandstone,the intergranular pores are mainly less than 0.1 μm,The pore volume of this class accounts for 45.32%. In the medium displacement pressure type Ⅱ and in the high displacement pressure type Ⅲ,the effective pore volume are only 21.92%,which are weak water abundance.