Study On Thermal Damage And Evaluation Of Overburden Rock In Underground Coal Gasification

The high temperature environment of underground coal gasification?UCG?will damage the roof rock,affect the stability of roof overburden,and restrict the safety of gasification process.Based on the thermal damage of overburden rock in the process of underground coal gasification,sandstone was used as the research object to conduct high temperature tests at different temperatures?25??1300??.The physical properties,mechanical strength,mineral composition,microstructure and thermal stability of sandstone were analyzed by various test methods.The non-isothermal reaction kinetics equation based on mineral thermodynamics was considered.The evolution mechanism of thermal damage of sandstone is expounded,and the methods and suggestions for evaluating thermal damage of rock were put forward with physical and mechanical parameters as damage variables.The main achievements are as follows.?1?The change of physical and mechanical properties of sandstone can be divided into three ranges:At 25??400?,the color of sandstone is dark red-brown.The chroma slightly increases with the increase of temperature,but other physical and mechanical parameters do not change significantly.At 400??1000?,the color of sandstone is bright red-brown,the pores and cracks develop,and the thermal damage develops rapidly.Chroma and porosity increase significantly,hardness,capacitance,dielectric constant,thermal conductivity,P-wave velocity,tensile strength decrease significantly.At 1000??1300?,the sandstone melts,the color turns to olive green,stomatal structural develop,gloss and magnetic susceptibility increase sharply,porosity decreased rapidly,thermal conductivity,hardness,capacitance and dielectric constant all increase,and tensile strength increases greatly.?2?During the heating process,a series of physical and chemical reactions occur in sandstone,including evaporation of bound water and crystalline water?200??,evaporation of structural water and quartz phase transition?600??,decomposition of cement?800??,and mineral melting?1100??.The non-isothermal thermal reaction kinetics of sandstone can be explained by first order?F1?and second order?F2?chemical reactions.The reaction around 600?and 800?require higher activation energy,which are 411.69 k J·mol^-1 and 429.37 k J·mol^-1,respectively.The thermal damage mechanism of sandstone mainly includes mineral dehydration and decomposition,crystal expansion and partial melting.The change of physical and mechanical properties of sandstone after high temperature is closely related to this.?3?Chroma,hardness,thermal conductivity,dielectric constant,capacitance,porosity,and P-wave velocity have a high linear correlation with tensile strength,and the damage evolution equation has good regularity,which can be used as an index to evaluate the strength of sandstone.The chroma method,hardness method,dielectric constant method,capacitance method,nuclear magnetic resonance method,and ultrasonic detection method can be used for rapid non-destructive testing of rocks to evaluate rock strength.In the process of damage evolution,the development of pores and cracks has an important influence on the stability of roof overburden in the process of underground coal gasification and should be paid enough attention.