The Study About The Displacement Of Coal Bed Methane By Carbon Dioxied In Different States

Coal bed methane, commonly known as gas, is the companion when thecoal bed formed. The main components, is same to the conventional natural gas.As we all know, the existence of gas is a threat to the workers’ and propertysafety of coal. When gas accumulation reaches a certain concentration, a littlefire can cause the explosion, and the explosion can bring a disaster. But the gashazard is always a major problem to us. In addition, as the main components ofthe gas the same as the conventional natural gas, and they are rich in the coal, ifwe can make the full use of it, the shortage of resources will be greatlyimproved. So, it is meaningful to the reality and economics to turn waste of coalgas into treasure.In view of the above situation, this paper has describe experiment on thepermeability of the coal, desorption with different temperature, carbon dioxideflooding in different statement, permeability determination before and after thedisplacement by Helium. Study the permeability of the coal samples systemly.The main research contents and results are as follows: 1) Using the Helium to test the whole process of permeability from elasticand plasticity stage to the fracture failure. In our country, most of the coal arelow permeability, there are some requirement to the permeability if we want todrive the gas. So, before we begin our experiment, we should test thepermeability. And the test result shows that the permeability didn’t changedduring the elastic stage, but when the coal came into the plastic stage, thestructure of the coal, gap occur to irregular deformation, all these had lead thesignificant changes of the permeability. When the stress stronger on the axial,the coal had come into strain hardening stage, during the stage, the coaloccurred strain, the strain were large and uniform. The fracture passage openedand permeability turn up largely in a short time.2) According to the statement which said that the temperature is higher, theadsorption capacity of coal gas is worse, conduct the coal with water bathheating which is full of gas to resolution and set the temperature respectively on40℃、60℃、80℃. We can come to conclusion that the higher the temperature,the faster and more the gas came out. Compare the efficiency of thedisplacement in these three temperature, we can conclude that keep thetemperature state, desorption capacity are little difference. Because in this stage,even though the high temperature can make desorption easier, but at the sametime the high temperature can lead the coal expansion and the channel turn toclose. In the warming up phase as the temperature changed, the coal expansion is not uniform, new fracture came out, thus the adsorption capacity are moredifferent.3) The coal adsorption methane saturation, using natural carbon dioxide,and we can find that the volume of the methane is letter than carbon dioxide.Coal can adsorb more carbon dioxide than methane. Setting pore pressure7Mpaand8Mpa respectively. The higher pore pressure, the less the time it takes andthe higher the efficiency. In terms of penetration, original permeability is0.0125mD and after the displacement it turn to0.0067mD, so using naturalcarbon dioxide can lead the permeability reduce.4) Subcritical carbon dioxide displace the methane, as the temperature ofcarbon dioxide is31.26, the critical pressure is72.9atm,if we set the Porepressure7Mpa and the temperature40℃、60℃、80℃respectively, we can findthat The lower the temperature the lower the efficiency and longer the time itwaste, and they had little difference between60℃、80℃. In terms ofpenetration, original permeability is0.01496mD and after the displacement itturn to0.01126mD, the penetration has little change. So Subcritical carbondioxide displace can make the Permeability improved, but the function is notobviously.5) Supercritical carbon dioxide displace the methane, set the Porepressure8Mpa and the temperature40℃、60℃、80℃respectively, the law issame to the Subcritical carbon dioxide displace the methane in the displacementvelocity and displacement efficiency,but the former is better than the latter. In terms of penetration, original permeability is0.01033mD and after thedisplacement it turn to0.01824mD, after displacement, the permeabilitychange greatly, so we can conclude that supercritical carbon dioxide displacethe methane can make coal seam permeability increased significantly.