Experimental Study On Carbon Dioxide Displacement Methane In Three Different-Ranked Coalbeds

Carbon dioxide is one of the main greenhouse gases which create global warming.One of the effective ways to solve the problem of carbon dioxide which bring disaster to the human environment is the carbon dioxide sequestration in coal seams.Geological sequestration of carbon dioxide in coal seams is regarded as an economical approach because of the mutual benefit of greenhouse gas sequestration and recovery of coalbed methane by replacement.China's coal resources and rank are very rich,however,the result of carbon dioxide displacement methane varies with the rank of coals which because of the different ranked coals have different metamorphic and structural.Therefore,according to the process of carbon dioxide displacement methane,firstly,we study and summarize the mechanism of gas's adsorption,permeability and diffusivity in the process of replacement recovery.Secondly,we study the primary pores and cracks structure characteristics of three different coals.Finally,an experiment about the adsorption,diffusivity and permeability of carbon dioxide and coalbed methane replacement in three different-ranked coals is carried out with coal specimens of large size(10Omm×100mm × 200mm).The main results are generalized as follows:(1)Studying on the CT experiment for the three coals find that the primary pores and cracks structure characteristics have obvious differences,macroporous porosity,percolation probability and macroporous specific surface area of weakly caking coal is larger than meager-lean coal,while the macroporous porosity,percolation probability and macroporous specific surface area porosity of meager-lean coal is larger than coking coal.The results show that the weakly caking coal permeability is larger than meager-lean coal,meager-lean coal permeability is larger than coking coal.(2)Studying on the carbon dioxide permeability for the three coals find that volume stress,permeation pressure and different-ranked coals all have significant effect on carbon dioxide permeability.For the same coal specimen,the carbon dioxide permeability decreases with the increasing of volume stress and increases with the increasing of permeation pressure.Meanwhile,for the different coal specimen,carbon dioxide permeability presents a trend of high-low-high variation along with the rising of coal rank under the same stress conditions,namely weakly caking coal permeability is larger than meager-lean coal,meager-lean coal permeability is larger than coking coal.The results show that the effect of injecting carbon dioxide to coalbeds can be promoted when the in-situ stress is low,the permeation pressure is high and the pore structure characteristics is developing well.(3)Studying on the adsorption and diffusivity characteristic for the three coals find that different-ranked coals and gas type both have significant effect on adsorption and diffusivity under the same experimental condition.For the same coal specimen,the adsorptive and diffusion capacity for different gas is different,which shows that carbon dioxide storage amount is about two times of methane,diffusion time and diffusion rate of carbon dioxide also higher than methane.Meanwhile,for the same gas,the adsorptive and diffusion capacity for different ranked coal is also different.The adsorptive capacity and the diffusion time increase with the increasing of coal rank from low to high.(4)Studying on the coalbed methane replacement by carbon dioxide for the three coals find that carbon dioxide storage amount is always larger than methane output amount under the same test condition for the coal specimen.Meanwhile,the effect of carbon dioxide storage and methane output in the initial experimental stage is better than later stage.For the different coal specimen,the carbon dioxide storage amount of weakly caking coal is larger than meager-lean coal and the carbon dioxide storage amount of meager-lean coal is larger than coking coal,while methane output amount of meager-lean coal is larger than coking coal and methane output amount of coking coal is larger than weakly caking coal during the CO2-ECBM experiment duration of 10h.Carbon dioxide storage rate and methane output rate are different among the three coal specimens,which shows that the effect of carbon dioxide storage and methane output have differences.The effect of carbon dioxide storage increases with the increasing of coal rank from low to high,while the effect of methane output decreases with the increasing of coal rank from low to high.The results show that the effect of carbon dioxide storage in high-ranked coalbeds is better,while the effect of methane output in low-ranked coalbeds is better.(5)Through comprehensively studying the carbon dioxide storage rate,the methane output rate and the replacement ratio,the following conclusions can be made:the CO2-ECBM's law of the three coal specimens is basically consistent,while the efficiency of CO2-ECBM presents a trend of high-low-high variation along with the rising of coal rank during the CO2-ECBM experiment duration of 10h,namely the effects of methane replacement by carbon dioxide in high and low ranked coals are better than in medium-ranked coals.The results show that carbon dioxide is suitable for geological storage in high and low ranked coalbeds.