Pore Structures Of Various Rank Coals And Their Influences On Adsorption-diffusion Of Coalbed Methane

The adsorption-diffusion property of coal to methane is an important reservoir parameter that affects the gas content and development potential of coalbed methane.The pore structure of coal is an important factor affecting the adsorption-diffusion performance of coalbed methane.Based on the theories and methods of coal geology,coalbed methane geology and physical chemistry,this paper studies the pore distribution characteristics and variation laws of various rank coals and their influence on the adsorption-diffusion performance of methane in coal.Establishing the evolution relationship of pore structure of various rank coals and the change model of gas diffusion coefficient with the change of pressure.Revealing the influence factors and control mechanism of adsorption-diffusion of methane by pore structure.The main achievements of this paper are as follows:(1)The pore change law of various rank coals is revealed.The mercury injection experiment results show that the large pore volume of various rank coals decreases first and then increases with the increase of vitrinite maximum reflectance(R_o,m).The mercury removal efficiency of various rank coals increases first and then decreases with the increase of R_o,m.The adsorption experiments of N₂ and CO₂ at low temperature showed that the volume and specific surface area of mesopores,transition pores and micropores decreased rapidly before R_o,m<1.5%,and increased slowly after R_o,m>1.5%.(2)The pore morphology characteristics of various rank coals are distinguished.The pore shape of coal is mainly cylindrical,fine bottleneck,ink bottle and parallel plate.The content of micropores in low-and medium-rank coals is less and mainly cylindrical pores with one end closed.The content of transition pores and mesopores are relatively high and their connectivity are good,which mainly parallel plate pores with one end open.The high-rank coals mainly contain a large number of micropores with good connectivity,mainly cylindrical pores with two ends opening,and the mesopores and transition pores are mainly parallel plate pores with four sides opening.(3)With the increase of the R_o,m,the adsorption capacity of methane increases,the Langmuir volume of coal increases linearly,and the Langmuir pressure shows decrease first and then increase.The early stage of methane adsorption is mainly controlled by the micropore content of coal.With the increase of adsorption pressure,the influence of transition pore and mesopore on methane adsorption capacity is increasing.The main factors affecting methane adsorption include coal metamorphism,coal molecular structure,coal petrographic composition,temperature and moisture.(4)The diffusion coefficient of various rank coals first decreases and then increases.The pore volume of low-rank coals is relatively large,and methane mainly exists in the form of free state,so its diffusion coefficient is also large.The methane of medium-and high-rank coals mainly exists in adsorption state,and the pore volume of medium-rank coals is relatively small and the desorption-diffusion of methane is relatively slow.The content of adsorbed gas in high-rank coals is the highest.In addition,there are some endogenous fractures in high-rank coals,which is conducive to gas diffusion,so the diffusion coefficient of high-rank coals rises slowly.(5)With the increase of gas pressure,the diffusion coefficient of gas in coal can be fitted in the form of Langmuir equation.The diffusion coefficient of coal has a positive correlation with gas pressure.When the pressure is less than 3Mpa,the diffusion coefficient increases rapidly with the increase of pressure.When the pressure is greater than 3Mpa,the gas diffusion coefficient increases slowly.The adsorption time constant decreases with the increase of pressure and obeys the law of negative exponential function.(6)With the increase of R_o,m,the compressibility coefficient of various rank coals first decreases rapidly and then increases slowly.The low-rank coals have low mechanical strength that is easy to deform under pressure.The mechanical strength of medium-and high-rank coals is high leading to the compressibility changes little under pressure.The slow increase of the compressibility coefficient of high-rank coals is mainly related to micropores and some endogenous fractures.The compressibility of various rank coals is mainly controlled by the metamorphic degree,coal structure and coal petrographic composition.(7)The local gas saturation of No.3 coal seam block in the Qinnan-East block is in the state of saturation and the whole is in the state of undersaturation.On the whole,the gas content,reservoir pressure and gas saturation of the coal seam in the Qinnan-East block increase with an increase in burial depth.The tectonic movement plays a controlling role in the generation and preservation of coalbed methane,and it is an important geological factor affecting the gas bearing property of coal reservoir.