Investigation of Large-Diameter Borehole for Enhancing Permeability and Gas Extraction in Soft Coal Seam

Abstract

The efficiency of gas extraction from the soft coal seam with ultralow permeability is low. Gas extraction with large-diameter borehole is proposed to deplete gas content for preventing gas outburst disaster in this study. The fractures around the large borehole will enhance the permeability in the damage area to promote gas extraction. We established a damage-stress-seepage coupling model for large-diameter borehole gas extraction in soft coal seam. This mathematical model contains governing equations of gases sorption and transport, coal deformation, and damage, reflecting the coupling responses between gas and coal seam. The model is solved by the finite element method to simulate the gas drainage large-diameter borehole through roadway. Distributions of elastic modulus, damage area, and maximum principal stress in soft coal seam with different borehole diameters including 94 mm, 133 mm, 200 mm, and 300 mm are analyzed. The gas pressure, gas content, and effective extraction area in soft coal seam are discussed. Results show that the shear failure zone appears around the large-diameter borehole, and its permeability rises sharply. This opens up the gas transport channel and is conducive to the rapid extraction. It is confirmed that gas extraction using large-diameter borehole (300 mm) can greatly improve the efficiency of the gas preextraction in soft coal seam by increasing gas extraction rate. These provide a foundation for guiding the operation of gas extraction with large borehole from the soft coal seam in the field.

Effects of Injection Pressure and Duration on Alternate High-Pressure Water-Gas Sequestration of Coalbed Methane

“…Due to the different geological conditions, the permeability of some coal seams is low, which determines the difficulty of methane extraction and drainage [2]. Many researchers put forward a series of enhanced coalbed methane recovery (ECBM) techniques to solve the above problems, such as large-diameter borehole drainage [3], hydraulic punching [4], pulsating shock [5], hydraulic cutting [6] (or ultra-high pressure hydraulic slitting), hydraulic fracturing [7], CO 2 -ECBM [8] and pre-split blasting [9]. The basic mechanism of these techniques is to generate artificial fractures in coal seams, which can improve the permeability of coal seams and thus increase the methane recovery rate.…”

Mining Stress Distribution and Gas Drainage Application of Coal Seam Group under Fault Influence

“…Coal seam drilling gas drainage technology has been studied and applied in gas control [21][22][23]. But in soft coal seams where hydraulic fracturing has poor permeability enhancement effect, large-diameter borehole gas drainage is widely used, and good drainage effect has been achieved [24,25]. However, in deep mining, drilling, and other engineering activities, the mechanical and seepage characteristics of coal and rock will be significantly changed under the influence of cyclic loading or even stratified cyclic loading [26].…”

Construction and Application of Mathematical Model of Stable Fracture Development Zone of Roof

“…The gas drainage effect of different drilling parameters is obtained, while reasonable drilling parameters are gained. Directional drilling can achieve the continuous and stable investigation of gas drainage effects to corresponding vertical horizon and lateral dislocation [34][35][36][37][38][39][40][41], because of the advantages of stable gas drainage effect and long effective drilling distance. In a word, depending on the occurrence of coal seams, the drilling parameters with the best extraction effect in the fracture zone are obtained through investigation, and the key to ensure efficient extraction by directional drilling is to establish a reliable mathematical model and determine the stable fracture development area accurately.…”