How to quantitatively investigate the effect of fracture roughness on gas flow: A new fully coupled interdisciplinary model

Abstract

In recent years, due to the increasing demand for cleaner energy sources, coal bed methane (CBM) has garnered substantial attention for its potential as an efficient energy resource. Extensive research has been conducted to explore various factors integral to the CBM extraction process. However, the roughness of the coal bed fracture system, a crucial conduit for gas transportation, significantly influences its efficacy. Additionally, the roughness of fracture surfaces has been inadequately addressed in the comprehensive analysis of multi-process coupling in CBM mining. Currently, the lack of progress in exploring this topic is that existing studies have neglected the influence of fracture surface roughness and the two distinct timescales in the fracture and coal matrix. In this paper, we propose a new multi-field coupling model that utilizes the finite element method. This model can account for the impact of fracture surface roughness, coupled multi-field effects, gas flow in fractures, and gas diffusion in the coal matrix. This study aims to quantify the relationship between fracture surface roughness, porous media, production parameters, and diffusion times. Existing models lack the capability to quantitatively analyze the extent of fracture roughness. To achieve this, we will apply a quantitative approach that considers the aforementioned factors. The model results demonstrate that the proposed model in this paper performs better than the current classical model in exploring gas flow, fracture surface roughness, and fracture–matrix interactions in porous media. Furthermore, the permeability of the fracture is greatly influenced by the roughness of its surface. A smoother fracture surface results in higher permeability. The interdisciplinary model introduced in this study is versatile, making it applicable to CBM mining projects under diverse geological conditions. Interested practitioners and readers can easily modify the initial values of boundary conditions in the finite element simulation. This adjustment facilitates a comprehensive analysis of both fracture roughness and the gas production rate of the project.