Simulation Study On Water-Rock-Gas Reaction In Closed Coal Mines

Closed gassy coal mines still have many residual gases such as CH₄,CO₂,and N₂ etc.These gases may bring gas inrush to ground and cause groundwater pollution problems.This paper presents experiment and numerical simulation study on water-rock（coal）-gas reaction in closed coal mines,and to reveal the mechanism of mine water evolution and mineral phase changes in three phase interaction.The main findings are as follows:（1）Closed highly gassy coal mines in China are mainly located in Heilongjiang,Shanxi,Hunan,Jiangxi,Sichuan,Yunnan,Guizhou,and Chongqing provinces.The chemical characteristics of highly gassy coal mine water usually have acid mine water,high salinity and high sulphate content.The main hydrochemical types of highly gassy coal mines are HCO₃·SO^4-Na,HCO₃^-Na,SO₄^-Ca·Mg,Cl·SO₄^-Na,Cl·HCO₃^-Na,Cl·HCO₃·SO^4-Na and SO₄^-Ca·Mg etc.SO₄^-Ca·Mg type is the most common one.（2）The results of the experiments of water-rock（coal）-gas reaction in closed coal mines show that with the increasing reaction time,taking CO₂/N₂ as gas phase in the water-rock reaction,pH value increases gradually,Eh is negatively correlated with pH and decreases gradually,the concentration of HCO₃^-,SO₄^2-,Ca²⁺,Mg²⁺ and SiO₂ increases with a different extent,the concentration of Fe decreases gradually and the TDS of solution increases.Taking CO₂ as gas phase in the water-coal reaction,the content of trace elements such as Cr、As、Cd、Al、Fe、Pb and TOC increases gradually,and the pollution of heavy metals and organic may aggravate.This paper presents the dissolution and precipitation of main minerals in the process of three phase reaction via SEM,XRD,XRF and other methods.The kaolinite and quartz mainly have dissolution reaction.Calcite has dissolution reaction and accompanies by the regeneration of carbonate minerals.Pyrite produces hematite,siderite and chlorite and other new minerals on the surface.（3）Numerical simulation was taken by PHREEQC to address water-rock-gas reaction equilibrium in closed coal mines at different temperatures,pressures and water chemistry.The results show that with the riase of the temperature,the pH value decreases a little,the concentration of CO₂,HCO₃^-and Ca²⁺ decreases gradually and calcite dissolves obviously.With the raise of CO₂ pressure,the pH decreases gradually,the concentration of CO₂,HCO₃^-,Ca²⁺,Mg²⁺ increases gradually and calcite dissolves obviously.The water chemistry and mineral dissolution & precipitation have no changes at different CH₄ pressure.With the raise of pH,the concentration of CO₂ increases slightly at first and then decreases,HCO₃^-increases gradually,Ca²⁺and Mg²⁺ decreases gradually,the dissolution of calcite and kaolinite decreases gradually and the precipitation increases gradually,quartz precipitation reduces gradually and occurs micro-dissolve,With the raise of Fe³⁺ concentration,the pH decreases rapidly,the concentration of HCO₃^-decreases gradually,the concentration of Ca²⁺,Fe²⁺,SO₄^2-increase gradually,and the dissolution of kaolinite,calcite and pyrite increase.The addition of sulfate increases dissolution of minerals and calcite is the most obvious among them.