Mechanism And Experimental Study On Coal-bed Methane Displacement By Carbon Dioxide

China is a big coal industry country with very rich coal-bed methane resources, however ,coal-bed methane reservoirs are three low-permeability reservoir, which is different from the U.S coal-bed geological conditions, coal-bed methane exploitation can not fully follow the U.S. approach. The traditional coalbed methane exploitation methods are lower productivity,longer recovering period and poor economic results.The direct consequences of low recovery ratio of coal-bed methane are gas disaster-prone,inefficient mining , resources waste and environmental pollution.based on low permeability characteistic of most coal-bed and corrent ststus of low recovery ratio of coal-bed methane in china ,the corbon dioxide enhanced coal-bed methane technology has been suggested for the purpose of enhancing coal-bed mehtane recovery ratio. carbon dioxide displacement coal-bed methane is considered a better prospect coal-bed methaneexploitation technology.In this paper, mainly based on existing technologies of carbon dioxide sequestration in coal-bed and enhanced coal-bed methane production by gas injection,according to the theories of coal-bed methane geology,rock mechanics,solid mechanics,porous medium,flow mechanics in porous medium,coal-bed methane and carbon dioxide laboratory permeability tests and carbon dioxide displacement coal-bed methane tests.doing the following work: Through the understanding of coal structure,the characteistic of coal-bed methane reservoir and the mechanisms of storage,migration and production of coal-bed methane were introduced.carbon dioxide adsorption quantity is higher than coal-bed methane,analysis the mechanism and applicability of carbon dioxide displacement coal-bed methane.under the coal-bed methane and carbon dioxide permeability tests.the results show that: In the experiment, permeability of the coal specimen was measured firstly and it is found that the permeability of the specimen is different for the two gases. The permeability for carbon dioxide is larger than that for methane two magnitudes at least under the tested condition. There exists a negative exponent relationship between the permeability and applied body stress on the specimen.Improving the coal-bed methane concntration and flow rate ,the technology of carbon dioxide enhanced coal-bed methane can substantially increase the methane recovery and mining efficiency. we present our experimental study results of carbon dioxide sequestration and methane displacement by carbon dioxide in coal specimens of large size, which factually simulated the process of the green house gas sequestration and methane displacement by it in coal bed. In the experiment, permeability of the coal specimen was measured firstly and it is found that the permeability of the specimen is different for the two gases. The permeability for carbon dioxide is larger than that for methane two magnitudes at least under the tested condition. There exists a negative exponent relationship between the permeability and applied body stress on the specimen. Under the simulated stress condition in the experiment,17.47 to 28.0 units of carbon dioxide can be stored in per unit of the specimen, and the displacement ratio of carbon dioxide to methane is as large as 7.03 to 13.91. The process of injection, adsorption and desorption, displacement, and output of gases can be proceed smoothly under the given pressures, the percentage of methane in the production gas can be amounted to 20 to 50% at the early stage and still be maintained at the level of 10 to 16% even at the last stage during the experiment process. It is concluded that the production amount and the percentage of methane are determined by complex factors including contained methane and the development of pores and fissures in the coal bed, the displacement style and applied stress during the project implementation,et al. During the process of carbon dioxide injection and gas displacement,coal body swelling can happen with comprehensive effects of gas adsorption and desorption, and the deformation of coal framework. In the experiment,it is concluded that gas desorption effect resulted from temperature increase played much role on the specimen swelling than shrink effect of the coal framework.the results showed that the main mechanism of carbon dioxide displacement of coal-bed methane:(1)At the time of coal-bed methane and carbon dioxide in the competitive adsorption, due to the adsorption of carbon dioxide is much higher than on the adsorption of coal-bed methane,ingected carbon dioxide can retention in coal, and replacement of large number of coal-bed methane;(2) The process of carbon dioxide injection, because of gas seepage and migration, reduces the partial pressure of adsorption methane, allowing its continued desorption; increased methane production. This study on non-coal layer carbon dioxide geological storage and the replacement drive for coal seam gas are of great theoretical significance and application value.