Monitoring and modelling of gas dynamics in multi-level longwall top coal caving of ultra-thick coal seams, Part II: Numerical modelling Guangyao Si a, ⁎, Ji-Quan Shi a , Sevket Durucan a , Anna Korre a , Jerneja Lazar b , Sergej Jamnikar b , Simon Zavšek b a Department of Earth Science and Engineering, Royal School of Mines, Imperial College, London SW7 2AZ, United Kingdom b Coal Mine Velenje, Partizanska 78, Velenje, Slovenia abstract article info Article history: Received 23 January 2015 Received in revised form 16 April 2015 Accepted 19 April 2015 Available online 25 April 2015 Keywords: Coupled modelling Gas emission modelling Longwall top coal caving Permeability Gas emission source The longwall top coal caving method, which enables the most productive exploitation of thick/ultra-thick coal seams, may result in a distinct geomechanical response of strata and associated gas emission patterns around longwall layouts. A two-way sequential coupling of a geomechanical and a reservoir simulator for the modelling of gas emissions around a longwall top coal caving (LTCC) panels was developed building on the understanding established from the analysis of in-situ gas pressure and concentration measurements carried out at Coal Mine Velenje in Slovenia. Model findings have shown that the modelling method implemented can reproduce the dy- namic changes of stresses and gas pressure around a LTCC face and predict the total gas emissions and mixed gas concentrations accurately. It was found that, in LTCC panels, although the rate of gas emission from mined coal depends highly on the coal face advance, floor coal and roof goaf act as a constant and steady gas source account- ing for a considerable part of the overall gas emission. Research has shown that, at first and/or second mining levels of multi-level LTCC mining, a notable stress relief and pore pressure drop induced by fracturing of the mined and roof coal can be experienced within 40 m ahead of the face-line. In the floor coal, on the other hand, the pore pressure change was found to extend to 20 m below the mining horizon. Model results have clear- ly shown the permeability enhancement and gas mobilisation zones around the LTCC panel, which can be the tar- get zones for gas drainage boreholes. © 2015 Published by Elsevier B.V. 1. Introduction Gas emissions, which occur along with coal extraction activities, have long been recognised as one of the main hazards affecting produc- tion and safety in coal mining. Recent advances in computational methods have provided cost-effective solutions to investigate the mechanisms of gas emissions induced by coal mining. Geomechanical response of the strata to coal extraction and the associated gas flow around a longwall coal face can be realistically reproduced by geome- chanical and fluid flow simulators. Over the past 40 years significant progress has been made in model- ling gas emissions from longwall panels and the surround strata. Kidybinski (1973) used finite element method to study stress redis- tribution and fracturing within the roof rocks of a longwall face. Researchers at Nottingham University conducted numerical model- ling studies on gas emissions in longwall mining (Keen, 1977; O'Shaughnessy, 1980). Durucan (1981) combined numerical modelling and laboratory investigations into stress-permeability relationship of coals and derived models for permeability distribution and associated gas flow around an advancing longwall coal face. Based on these studies Ren and Edwards (2000) developed a three-dimensional methane flow model using computational fluid dynamics, with permeability values ranging from 10 -14 to 10 -8 m 2 assigned to different regions in the flow model. However, in most of these early studies, the mechanical im- pact of coal extraction on gas flow was neglected or over simplified. More recently, Whittles et al. (2006) presented a methodology to derive dynamic permeability changes in coal measure rocks from the results of geomechanical modelling of a longwall face for fluid flow sim- ulation. A one-way explicit coupling approach was developed by Esterhuizen and Karacan (2005) to investigate gas migration from sur- rounding rocks towards longwall working faces. In their models, a geomechanical simulator was used to simulate the continuous advance of a longwall face and provide dynamic permeabilities for gas emission modelling in a reservoir simulator. The advance of a longwall face was modelled as a moving boundary problem and restart models were run sequentially representing different mining steps and corresponding strata responses. Based on this approach, the performance of in-seam degasification boreholes and goaf gas ventholes was evaluated by the authors (Karacan et al., 2007a,b). International Journal of Coal Geology 144–145 (2015) 58–70 ⁎ Corresponding author. Tel.: +44 20 7594 7382; fax: +44 207594 7444. E-mail address: g.si11@imperial.ac.uk (G. Si). http://dx.doi.org/10.1016/j.coal.2015.04.009 0166-5162/© 2015 Published by Elsevier B.V. Contents lists available at ScienceDirect International Journal of Coal Geology journal homepage: www.elsevier.com/locate/ijcoalgeo