Response properties of geometries of coal penetrating fracture on seepage behavior

Abstract

The fracture surfaces of coal-rock masses formed under mining-induced stress generally exhibit complex geometries, and the fracture geometry is one of the primary factors affecting the seepage characteristics of coal-rock penetrating fracture. This paper investigates the seepage characteristics of 5 groups of coal penetrating fracture (CPF) with different joint roughness coefficients (JRCs). Based on 3D morphology scanner tests and hydraulic coupling tests, a characterization method of effective geometric parameters in fracture surfaces under various confining pressures was improved, and a relationship between effective geometric parameters and the confining pressure is established. The results indicate that the nonlinear flow behavior in a CPF primarily includes three types: non-Newtonian fluid seepage under high confining pressure and low JRC, non-Darcy seepage under low confining pressure and high JRC, and the whole process of seepage characteristics between these two conditions. Among them, non-Newtonian fluid seepage is caused by significant fracture expansion, while non-Darcy seepage can be attributed to turbulence effects. During the seepage process, the geometric parameters with different JRC fracture samples all exhibit exponential changes with the increase of confining pressure. In addition, under high confining pressure, the effective contact ratio, effective fracture aperture, and void deviation ratio with high JRC fracture samples under high confining pressure increase by 93.5%, 67.4%, and 24.9%, respectively, compared with those of low JRC fracture samples. According to the variation of geometric parameters in a CPF with external stress, a seepage model considering geometric parameters in a CPF is proposed. By introducing the root mean square error (RMSE) and coefficient of determination (R2) to evaluate the error and goodness of fit between model curves and experimental data, it is found that the theoretical curves of model in this paper have the best matching with the experimental data. The average values of RMSE and R2 for model in this paper are 0.002 and 0.70, respectively, which are better than models in the existing literature.

1. Introduction

The excavation of underground spaces inevitably leads to stress concentration in the surrounding rock, which in turn results in the development of fractures within coal-rock masses, especially for longwall face where mine water hazards exist. In the process of coal mining, the presence of water has two main impacts. On the one hand, it reduces the bearing capacity of coal masses, leading to accidents such as rib spalling and floor heave [1,2]. On the other hand, it changes the failure modes of coal masses and the roughness of fracture surfaces [3]. This results in changes in seepage pathways and permeability characteristics, ultimately accelerating the inflow of water and gas from the roof and surrounding rocks into the longwall face [4]. Furthermore, for low-permeability coal seams, the characteristic of gas migration within coal seam after hydraulic fracturing was widely interested [5,6]. Therefore, understanding the flow characteristics of fluids within fractures of coal-rock masses is crucial for various technological applications, including gas permeability enhancement, mine water prevention and control, and carbon dioxide sequestration.

The fracture network within coal masses is typically formed by the random arrangement and combination of single fractures [7]. Therefore, the investigation of the fluid–solid coupling characteristics in a coal penetrating fracture (CPF) is crucial for understanding the mechanical behavior and seepage characteristics of coal fracture networks. Since its proposal in 1856, Darcy law has been widely used for the quantitative characterization of linear seepage laws under laminar flow conditions [8]. For fracture coal-rock masses, when fracture surfaces with different joint roughness coefficient (JRC) are equivalent to smoothly parallel plates with different spacings, and the fluid between fracture surfaces satisfies Darcy seepage conditions, the cubic law can be derived [9]. However, the fracture surfaces of coal-rock masses generated under mining-induced stress generally exhibit complex geometries, and the voids between fracture surfaces are highly variable. For a CPF with wide voids, the effect of fracture roughness on a linear flow of fluids is small under low confining pressure. However, as the confining pressure increases, the void between fracture surfaces continues to decrease, leading to meandering of fluid seepage channels as well as changes in the seepage velocity, which induces a nonlinear fluid flow. At this point, using the cube law to calculate the fluid flow rete inevitably overestimates its test results [10]. Additionally, the high-speed seepage process within voids can also induce nonlinear fluid flow behavior [11,12].