Study On The Evolution Mechanism And Formation Mechanism Of Closed Pores In Tectonically Deformation Coal

The pore structure characteristics of pore volume,specific surface area,pore size distribution are important physical properties in coal,which is of great significance in the development of coalbed methane and gas outburst prediction.Currently,the occurrence state of methane in coal mainly include adsorbed state,free state and dissolved state,however,some other scholars think that part of methane present solid solution state or closed in the crevice,the character and structure of the pores in coal still exist many arguments.Based on this,we quantitatively investigate the characteristics of the pore volume,specific surface area and pore size distribution of open pore,closed pore and total pore,and at the same time we in-depth study of the pore structure evolution mechanism coal in different metamorphism and deformation coal.Finally we discuss the formation mechanism of the closed pore in coal combined with the observation of atomic force microscopy(AFM),the conclusion as follows:(1)The pore volume and specific surface area of the open pore in coal show a high-low-high change trend with the increasing of metamorphism degree,and reach a trough around Ro,ran=2.5%.Coal deformation promotes the increase of pore volume,especially influent the most to the micropore volume.In general,the specific surface area of the coal porosity increases with the increase of the deformation degree,but it does not show a strict positive correlation.Fractal dimension can be used as a parameter to characterize the capacity of porous adsorption,and it has a positive relationship with specific surface area and pore volume,conversely,a negative relationship with average pore size range.(2)In middle and high metamorphic coal the total pore volume of SAXS is increased with the increase of metamorphism,and the change of pore volume is slightly different from that of open pores,the reason is that the total pore contains a certain amount of closed pores.It reflects a shift and reciprocal transformation relationship between open pore and closed pore.Open pores are blocked to form closed pores,and the walls of closed pore are broken and connected with each other and then form closed pores.(3)We found that the volume of the closed pore increases first and then decreases with increasing metamorphism degree,the peak is at about Ro,ran=2.5%.The structure function makes the change of 2-50 nm pores in two ways: On the one hand,the coal deformation leads to the 10-50 nm pores break into 5-10 nm,which mainly appeared in brittle deformed coal;On the other hand,micropores and 2-5nm pores are connected and form 5-10 nm pores,the changes of open pores and closed pores are a dynamic process,however the open pore volume increases faster than closed pores in strong deformation coal,which is the reason that the closed pore volume ratio decreases.(4)We found that a certain amount of closed pores existed in coal by AFM,the metamorphism resulting in the closed pores compact and deformation causes pore more broken or even crumple structure in coal,the cause of closed pore forming main has two kinds: one is local matrix of non-uniform shrinkage,another is pore throats decreasing and pore local fault caused by tectonic stress.