Study On The Change Of Pore Structure And Permeability Of Sc-CO₂ Extracted Coal

Injecting CO₂ into the deep coal seam not only has the characteristics of displacement of coalbed methane production,but also can achieve geological storage of greenhouse gases.The coal reservoirs suitable for CO₂-ECBM are generally buried deep,thus,CO₂ often exhibit supercritical state after being injected into the coal seam.Under supercritical conditions,CO₂ can extract small organic molecules in coal,and change the pore fracture structure of coal,which in turn causes the permeability characteristics of coal reservoirs to change.In this paper,five sets of coal samples from Hedong coalfield in the eastern margin of the Ordos Basin were selected,and Sc-CO₂ extraction experiments were carried out under laboratory conditions.Based on mercury intrusion method,nuclear magnetic resonance and permeability experiment,the pore structure and permeability of coal after Sc-CO₂ extraction were analyzed,the general rule of Sc-CO₂ extraction on pore structure and permeability transformation were also analyzed.Coupling the intrinsic relationship between pore fracture modification and permeability change and explain the mechanism of Sc-CO₂extraction coal permeability change.The following understandings are obtained through the research:?1?The pore structure in coal was tested by mercury intrusion method.It was found that after Sc-CO₂ extraction,the total pore volume and total specific surface area of the coal sample increased to different extents;as for the stage pores,The volume of the micropores,mesopores and macroporous pores increase as a whole,and the pore volume of the transition pores increases or decreases;the specific surface area of the micropores,mesopores and macropores increase as a whole,and the specific surface area of the transition pores increases or decreases.After Sc-CO₂extraction,the mercury intrusion hysteresis loops of the five sets of samples were reduced to varying degrees,indicating that the semi-closed pores in the coal are opened and the pore connectivity is improved.At the same time,it is found that the effect of Sc-CO₂ extraction on coal pore structure modification is controlled by coal rank;the transformation of coal pore structure by Sc-CO₂ extraction shows two effects of pore enhancement and expansion,the pore-enhancing effect is remarkable in the micropore and transitional pore stages,and the pore expanding effect is remarkable in the mesoporous and macroporous stages.?2?The coal sample porosity was tested by NMR technology.It was found that the total porosity of coal samples increased with different degrees after Sc-CO₂extraction;in terms of stage porosity,the porosity of micropores,mesopores and macropores increase as a whole.The porosity increases overall and the porosity of the transition pores decreases overall.There is no obvious relationship between the change of total porosity and the coal rank.The change of porosity of transition pores decreases first and then increases with the increase of coal rank,the change of porosity of mesopores increases with the increase of coal rank,the change of porosity of micropores and macropores does not change significantly with the increase of coal rank,and the data of mercury intrusion proves each other,further demonstrating the transformation effect of Sc-CO₂ extraction on pore structure.?3?It is found that the permeability of coal samples increases with different degrees after Sc-CO₂ extraction,and the transition point of the second coalification is the boundary.That is,before the second coalification jump,the increase of permeability decreases slightly with the increase of coal rank;after the second coalification jump,the increase of permeability increases with the increase of coal rank.?4?The relationship between the change of permeability of Sc-CO₂ extraction coal and the pore structure and porosity modification is analyzed.It is found that there is a good positive correlation between the change of permeability of Sc-CO₂extraction coal and the pore volume of macropores or the porosity of mesopores.The Sc-CO₂ extraction mainly controls the permeability by transforming the large mesoporous structure,which proves that the migration of gas in the transition pore is dominated by diffusion,while in the large and medium pores is dominated by seepage,and explains the evolution of Sc-CO₂ extraction coal permeability from the microscopic measurement.