Gas-Solid Coupling Evolution Pattern Of Terrestrial Low-Rank Coal Under Thermal Action

The study of gas-solid coupling evolution of coal under thermal action is important for explaining the kinetic evolution process and control mechanism of reservoir properties.This paper takes the continental low-rank coal of the IIIA coal group in the Huolinhe Depression of Erlian Basin as the research object,and the thermal simulation in gold tube for hydrogen generation experiment was carried out,then the gas composition,yield and hydrocarbon isotope characteristics during pyrolysis were analyzed.The experiments of X-ray diffraction,Fourier infrared spectroscopy and ¹³C-NMR revealed the molecular structure evolution mechanism of low-rank coal during pyrolysis;and the pore evolution characteristics during the pyrolysis progress was explained according to the mercury intrusion porosimetry,low-temperature liquid nitrogen and CO₂ adsorption experiments,then the coupling relationship between the maceral-gas-pore of coal in the progress of evolution under thermal action was discussed.The main research findings maintain:?1?the gas yield of CH₄?C₂-C₅?C₆₊and non-hydrocarbon gases and the key hydrocarbon generation kinetic parameters of different samples in the pyrolysis progress was obtained,and the characteristic of three-stage between hydrocarbon generation and coal rank evolution in the pyrolysis progress was revealed.This paper points out that:the carbon isotope of alkane gas tends to be lighter in the effect of molecular bond energy,and then it becomes heavier with the function group being further removed,as well as the carbon isotope of CO₂ is firstly heavier and lighter later under the influence of carbon isotope exchange reaction.?2?Under thermal action,the evolution of coal molecular structure shows that the spacing of carbon network decreases gradually,the stacking degree increases,the number of aromatic layers increases,the ductility decreases firstly and increases later,the coal crystal nucleus gradually develops flat,and the coal structure gradually becomes dense.In the early stage of pyrolysis(R_o,max<0.7%),as a large number of aliphatic structures such as methyl,methylene,quaternary carbon,and oxygen-bonded carbon and oxygen-containing functional groups,hydroxyl groups gradually fall off,CO₂ and hydrocarbons are formed in large quantities;and in the middle stage of pyrolysis(0.7%<R_o,max<1.3%),the content of some oxygen-containing functional groups and a small amount of aliphatic structure increase slightly due to the cracking of macromolecular substances;then in the late stage of pyrolysis(R_o,max>1.3%),the aliphatic structures further decrease and with the strengthening of aromatization,the bridgehead carbon content and the aromatic carbon rate increase.?3?In the early stage of pyrolysis(R_o,max<0.7%),the resinite in the maceral of coal decomposes firstly;and in the middle stage of pyrolysis(0.7%<R_o,max<1.3%),the pore content of eu-ulminite and humodetrinite increases;then in the late stage of pyrolysis,the pore content of humodetrinite in the coal increases rapidly,and the form of the inertinite does not change in pyrolysis.?4?In the early stage of pyrolysis,the volume of different types of pore becomes smaller under the influence of temperature,pressure and pyrolysis of liquid hydrocarbon,and in the middle stage of pyrolysis,the content of pore increase and the volume of different types of pore gradually increases because of the pyrolysis of liquid hydrocarbon and resolution of matrix;then in the late stage of pyrolysis,the micropores content within 2nm continues to increase,the pores content of 2¹00nm decreases under the influence of molecular ordering and the content of pore more than 100nm was basically unchanged.These findings preliminary revealed the gas-solid coupling evolution law of continental low-rank coals in hydrocarbon generation,having important reference value for the exploration of coalbed methane resources.