Numerical modelling and rescaled range analysis on spontaneous combustion under surface methane drainage in a Chinese coal mine

Abstract

In China, surface boreholes have been used frequently to drain methane/gas emitting from overlying layers and longwall mining gob areas in underground gassy coal mines. In this work, a numerical modeling was established using COMSOL ^TM to study the influence of surface drainage borehole on coal spontaneous combustion and Rescaled Range Analysis (R/S analysis) was employed to investigate the chaos characteristic of Nitrogen (N₂)/Oxygen (O₂) drained from the gob. The simulation results show that there is a circular “dissipation zone” around the drainage borehole all the time and an elliptic “spontaneous combustion zone” when the borehole locates in deep gob. It is also found that the advancement of drainage borehole has little influence on spontaneous combustion zone on the intake side of the gob but it can tremendously enlarge the width of “spontaneous combustion zone” in the middle gob and reduce the depth of self-heating zone near the return side. The R/S analysis indicates that the influence of surface borehole on spontaneous combustion can be divided into two stages: the safety drainage stage (Hurst index > 0.85) and the spontaneous combustion initiating stage (Hurst index ≤ 0.85). It can be concluded that the gas drainage from gob through surface borehole can tremendously intervene spontaneous combustion "three zones" in gob. In addition, the length-fixed R/S analysis on N₂/ O₂ series from surface boreholes can effectively reflect coal spontaneous combustion condition in gob.