A mathematical model of gas flow during coal outburst initiation

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جزئیات بیشتر

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۲۰۲۱

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scopus – master journals – JCR

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۴٫۲۷۶ در سال ۲۰۲۰

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A mathematical model of gas flow during coal outburst initiation


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A mathematical model of gas flow during coal outburst initiation

Abstract

A proposed concept of outburst initiation examines the release of a large amount of gas  from coal  seams resulted from disintegrating thermodynamically unstable coal  organic matter (COM).  A coal  microstruc- ture is assumed to getting unstable due to shear component appearance triggered by  mining operations and tectonic activities considered as  the primary factor while COM disintegration under the impact of weak electric fields can  be  defined as  a  secondary one. The  energy of  elastic deformations stored in the coal  microstructure activates chemical reactions to tilt the energy balance in  a ‘‘coal–gas” system. Based on  this concept a mathematical model of a gas  flow  in  the coal  where porosity and permeability are  changed due to chemical reactions has been developed. Using this model we  calculated gas  pressure changes in the pores initiated by gas  release near the working face  till  satisfying force and energy criteria of outburst. The  simulation results demonstrated forming overpressure zone in the area of intensive gas release with enhanced porosity and permeability. The calculated outburst parameters are  well combined with those evaluated by  field measurements.

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  1. Introduction

The development of coal  deposits is associated with deepening the underground excavations where numerous coal  and gas  out- bursts occur thus causing miners’ injuries and deaths accompanied by significant economic damage as well  [۱–۳].

The  outburst concept prevailing in mining sciences for  the last six  decades regards rapid changes of  stresses near the boundary of a gas-bearing rock  as  the main cause of this gas  dynamic phe- nomenon. The  best known theory of V. Khodot describes the out- burst mechanism as  a  mutual interaction of  numerous  factors including rock  pressure, cracking, gas  desorption, increasing coal permeability, changing physical and mechanical coal  properties, emerging zones with a high gas  pressure gradient, and converting the elastic energy of coal  to  the dynamic energy of moving parti- cles  and fragments [4,5].  Later  updates and modifications of this theory were presented, particularly, in  studies by  Petrosyan et al. [6–۸].

Recent relevant  studies  have  employed more  sophisticated models. The  concept, outlined in  study by  Jin et al., is based on  a set  of  nonlinear partial differential equations with the structure forms, material properties, load  patterns, and abutment pressure as  the factors having much impact on  outburst initiation [9].  The

model, presented in  study by  Chen  et al.,  describes the outburst mechanism according to the mass and momentum conservation principles of  the breakdown section in  the  ‘‘coal–rock” system; and it  proposes necessary conditions for  extrusion and dump of coal  and gas  from the frontage tunneling work [10].

Last  years a  series of numerical models coupling stresses and gas flow  have been developed. The scale and computation stability of the dynamic  system  ‘‘coal–gas” were investigated by  Fan  et al. [11]. The stress simulation program FLAC3D and the gas simulation program SIMED II were employed in study by Li and Saghafi to cal- culate a  gas  flow  under typical stress regimes in  gassy mines in Australia and China   [۱۲].  A theoretical gas-solid coupled model developed in  COMSOL Multiphysics software took into account the influence of  ground stress, gas  pressure, and mining depth [13].  A model, presented by Xue et al., described outburst initiation in  roadway excavation coupling coal  deformation, pore pressure, principal stress vector redistribution, and yield and tensile failure zones [14].   Three stages of  outburst  evolution discriminated in study by  Choi  and Wold M can  be  listed asfollows: pre-initiation with quasi-static deformation behavior, initiation as  the moment of  sudden conversion of  this behavior into dynamic, and post- initiation with intensive gas  and coal  fragment ejection [15].  This stage of outburst was  simulated by  using Ansys  Flotran program based on  the relations of fluid  and gas  mechanics applied to mov- ing  gas-solid mixture [16].  The role  of gas  desorption triggered by effective stresses ahead of the working face  during the process of initiating outburst  and changing porosity and permeability was examined in details in study by Zhi and Elsworth [17].

Statistical analysis as  the alternative approach is  applied for deriving the criteria for evaluation to assess outburst hazard under various mining and geological conditions. Based  on the analysis of 90 outburst occurrences in Donbas and Vorkuta coal basins (former USSR) the criterion proposed by  Feit  related the change of  gas internal energy with the stress, bulk density, compressive strength, and seam inclination [18].  The  latest studies have employed vari- ous  methods including cluster and time series analysis to  identify the trends and anomalies in  outbursts occurred in  Hexi  colliery (China), a pattern recognition to  evaluate the risk  class  based on the set  of attributes characterizing safety conditions at the mine, clustering theory  and  multi-objective  classification to  optimize outburst prevention measures at Jinzhushan Tuzhu Mine  (China), ANOVA and contingency table analysis of the major factors causing outbursts, coupled sampling with computer modeling, and the geophysical approach combining the  relations of  geomechanics with the method of acoustic emissions [19–۲۴].

The  experimental studies revealed the critical importance to enhancing porosity and permeability in  outburst initiation depending on stress, desorption, and coal  destruction. Particularly, the results of desorption tests on  coal  specimens  from West Cliff colliery (Australia) showed much lower permeability values in comparison with the values obtained while testing permeability on  the same samples due to  the impact of gas  pressure and rock stress [25].  The  study carried out  on  the samples from Songzao coalfield (China) has  identified slow flow  rates in  the intact coal and the increase of the gas  flow  rate in the damaged coal  propor- tional to  the square of a  pressure gradient under a  low  effective stress [26].

The  review of  the models and methods to  predict outburst allows drawing the following conclusions.

The   vast  majority  of  the  models applying a  ‘‘mechanistic” approach identified mechanically induced desorption as  the only reason to  cause the gas  release and proper attention was  not  paid to physicochemical transformations in coal  under various impacts resulted in critical changes of pressure, porosity, and permeability. Formerly, this approach played a positive role  in understanding the driven forces of outbursts and preventing the accidents in  mines. Nevertheless, a  large number of  outbursts occurred in  the coal seams considered not  to  be  hazardous by ‘‘mechanistic” models.

To overcome insufficient reliability of these models in terms of outburst  predictions various statistical methods were employed and put into mining practice thus giving possibility to  establish correlations between outburst hazard and major mining and geo- logical factors. However, the  applications of  statistical methods are  site-limited and do  not  allow studying the mechanism of out- burst initiation and evolution.

Therefore, the existing theories and models should be  updated and the description of  physicochemical factors having influence on critical coal  properties is recommended to be added. This study aims to develop a numerical model of a gas flow  in the coal  during outburst initiation where physical-chemical transformations in the ‘‘coal–gas” system  are  taken into proper account.

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