Numerical Simulation on Mesoscale Mechanism of Seepage in Coal Fractures by Fluid-Sloid Coupling Method

Kai Si,Yaheng Zhu,Yan Zhao,Ruidong Peng,Zhendong Cui,Leilei Zhao,Jianyong Zhang,Rihong Cao

doi:10.1155/2021/5562149

Abstract

Trying to reveal the mechanism of gas seepage in coal is of significance to both safe mining and methane exploitation. A series of FEM numerical models were built up and studied so as to explore the mesoscale mechanism of seepage in coal fractures. The proposed mesoscale FEM model is a cube with micron fractures along three orthogonal directions. The distribution of velocity and pressure under fluid-solid coupling was obtained, and furthermore, the seepage flow flux and an equivalent permeability of the whole model were calculated. The influences of fracture width, outlet velocity, and in situ stress level on seepage were investigated. The numerical results show that nonlinear Darcy seepage occurs during low velocity zone. The permeability is increased linearly with the increasing of facture width and outlet velocity. A certain change of lateral coefficient of in situ stress also affects seepage. The permeability is increased sharply once deviating the isotropic spherical stress state, but it is no longer changed obviously after the lateral coefficient has been increased or decreased more than 20%. The mesoscale seepage mechanism in coal fractures has been preliminarily revealed by considering fluid-solid coupling effect, and the key factors influencing fluid seepage in coal fractures were demonstrated. The proposed methods and results will be helpful to the further study of seepage behaviour in coal with more complex structures.

Highlights

In the process of coal mining, there is a kind of unconventional natural gas stored in coal seams, commonly known as coalbed methane (CBM)
The flow flux increases with the increasing of fracture width
It can be seen that the equivalent permeability increases almost linearly with the increasing of fracture width, which indicates fluid seepage becomes easier for larger fractures

Summary

Introduction

In the process of coal mining, there is a kind of unconventional natural gas stored in coal seams, commonly known as coalbed methane (CBM). This means that the influence of interaction between fluid and solid matrix must be added in the process of various simulation studies After recognizing this problem, most scholars have fully considered the influence of fluidsolid coupling on the seepage of CBM and other fluids and have achieved certain research results [15,16,17,18]. Shi et al [25] conducted a large number of experimental studies and theoretical model derivation on the direction of coal matrix desorption-induced deformation and permeability changes under effective stress and tested coal under different uniaxial strain, displacement control, and pore pressure. The study of fluidsolid coupling mechanism between fracture and solid matrix is helpful to better understand the seepage law of CBM in coal. Some mesoscale FEM models will be built to represent the characteristics of fluidsolid coupling, and the seepage law of CBM along microfractures will be investigated by changing different geometry features and boundary conditions

Theoretical Equations and

Results and Discussion

D Displacement 2 E Displacement 3

Conclusions