The Role of gas desorption on gas outbursts in underground mining of coal

Open Access
Author:
Zhi, Sheng
Graduate Program:
Energy and Mineral Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 15, 2015
Committee Members:
  • Derek Elsworth, Thesis Advisor
Keywords:
  • Gas outburst
  • Permeability evolution
  • Gas desorption
  • Energetic failure
Abstract:
Violent gas outbursts are one of the most severe hazards in underground mining. When outbursts occur, a large amount of coal and gas is suddenly and violently ejected into the roadway and working area with the possibility of serious hazard and injury. Recent studies have shown that the physical behavior responsible for the energetic failure of coal is entirely consistent with coal viewed as a dual porosity–dual permeability–dual stiffness continuum where strength is proportional to effective stresses, and where effective stresses are controlled by both the pore pressure and varying stress field. Gas desorption driven by overstress is highlighted in this study as the key factor responsible for the increase in pore pressure close to the working face, and implicated together with elevated stress level, permeability evolution and drainage conditions in the triggering of outbursts. In this work, we incorporate the likely mass rates of desorption driven by an increase in abutment stress and mediated by permeability evolution to define the rates and distributions of gas pressure changes. The changing pattern of pressure redistribution is identified, and parametric studies are then performed to investigate all the key factors that influence the redistribution of pore pressure with respect to the deformation of the coal seam. Permeability evolution in the overstressed zone is determined by the evolution of porosity, which is attributed to both the change in effective stresses in the abutment and sorption-induced strain. Considering the weakening effects of desorption-induced pressure increase, energetic failure may be triggered from the pillar as defined by the Mohr-Coulomb failure criterion. According to this analysis, the pore pressure adjacent to the mining face may be lowered by drainage in a measurable way to reduce the likelihood of an outburst. This model is capable of predicting the potential risk ahead of the working face during mining and can be adapted to different conditions in terms of varying mechanical factors, coal properties and mining methods.