Experimental Study on Seepage Characteristics of Jurassic Weakly Cemented Sandstone under Water-Rock Interaction

Qin Liu,Yajun Sun,Jian Li

doi:10.1155/2020/8543687

Abstract

As unconventional water-bearing rocks, the hydraulic conductivity of weakly cemented sandstones can increase by several orders of magnitude during drainages or pumping tests, posing great challenges to water prevention and control of coal mines in northwestern China. In this study, seepage experiments on Jurassic weakly cemented sandstones were performed and the hydraulic conductivity during the seepage process was analyzed. Combined with laboratory test and theoretical analysis methods, the relation between the permeability and micropore structures was studied, and the permeability evolution mechanism of the weakly cemented sandstones was eventually clarified. According to the experimental results, the seepage process can be divided into the saturated seepage stage, the stable seepage stage, and the seepage mutation stage. The hydraulic conductivity increases as the porosity and the mercury extrusion rate increase, but there is no obvious correlation between them that can be identified. In contrast, there is a linear positive correlation between the hydraulic conductivity and the average pore-throat radius. The variation trend of the pore-throat ratio can be used as the main reference indicator for judging whether the seepage mutation occurs in weakly cemented sandstone. Based on the correlation analysis of micropore structures and the hydraulic conductivity, a seepage model of straight capillary was constructed and the theoretical permeability equations of stable seepage stage and seepage mutation stage were proposed. It is concluded that the specific permeability of weakly cemented sandstones is directly proportional to porosity and the square of the average pore-throat radius. A theoretical equation to calculate the specific permeability during the latter two stages was also presented in this paper. Theoretical calculation results are roughly consistent with actual values obtained in the experiments.

Highlights

Jurassic and Cretaceous weakly cemented strata are widespread in western China due to the special diagenetic environment [1, 2]
The variation curve of hydraulic conductivity of sandstone samples with time is given based on Equation (1), see Figure 6 and the S2-S5 in the Supplementary Material
The seepage process of weakly cemented sandstones can be divided into the saturated seepage stage, the stable seepage stage, and the seepage mutation stage

Summary

Introduction

Jurassic and Cretaceous weakly cemented strata are widespread in western China due to the special diagenetic environment [1, 2]. The weakly cemented strata are dominated by sandstones, which are characterized by poor cementation, susceptibility to weathering, high porosity, and low strength [3] They are significantly different from normal sandstones and soft rocks in terms of cementation, mechanical, and hydraulic properties. As pumping tests of Dananhu No 5 coal mine carried on in the Hami mining area of Xinjiang, the hydraulic conductivity of weakly cemented sandstones continued to increase [4]. During a large-scale drainage project in Yili No 1 coal mine, it took initially three months for the water level of observation wells about 200 m away from the evacuation center began to fall This time interval decreased to 10 minutes at last. The III-1 aquifer of weakly cemented sandstones in Hami mining area is identified as Jurassic Xishanyao formation [18] (Figure 1). The hydraulic conductivity of the aquifer increased again by an order of magnitude, reaching 4.33 m/d [4]

Methods

Results

Discussion

Conclusion