Experimental And Numerical Investigation On Enhancing Coalbed Methane Recovery By Injecting Heated CO₂

The methane occurrence in china is abundant.There are 36.8 trillion cubic meters of coal bed methane?CBM?resources in coal fields buried within a depth of 2000 m,ranking China the third in the world in terms of resource size.However,due to the low pressure of coal bed methane,low permeability of coal and a high degree of metamorphism of coal,the development of coal bed methane is always ideal.China has been seeking for the high-efficiency technology to explore coal bed methane and has listed the coal bed methane development into the 13 th Five Year Plan.The technology of Enhanced Coal Bed Methane recovery?ECBM?is put forward as a possible method to not only increase coal bed methane production,but also store the greenhouse gas?CO₂?into the coal seam,which can achieve the economic and environmental benefits at the same time.In order to further improve the yield-increasing effect of ECBM,researchers put forward the technique of enhancing coal bed methane by supercritical CO₂,and the study results show that it can increase the production of coal bed methane significantly.However,since the specificity of some coal seam formulation,the CO₂ cannot reach the critical point and become supercritical CO₂.Based on the impact of temperature on the porosity of coal and the adsorption characteristic of gas,the research idea of enhancing coal bed methane recovery by high temperature CO₂ is presented.Therefore,this research is conducted by using the self-developed device to investigate the influence of different injecting pressure and temperature on change of coal permeability?20-70??3-8MPa?,the alteration of absorption properties of gas?30-70??1-6MPa?,the displacement result and the change of mechanics characteristic of coal?28-60??2-4MPa?during high-temperature CO₂-ECBM.The results are shown as below.?1?At the same temperature,the correlation of coal permeability and effective stress is negative exponential.When the effective stress is same,the permeability of coal descends linearity with the increase of temperature.Taking both the influence of temperature and effective stress into account,the relationship of temperature,effective and permeability can be denoted by the quantitative equation.?2?Under different effective stress and tempearture?1-6 MPa and 20-70 °C?the permeability of CO₂ in weakly caking coal samples??25×50mm?is higher than CH₄ and CO₂ is more likely to transport in the coal.?3?The increase in temperature can result in the decrease of the amount of adsorbed gas.This is because that the temperature increment can lead to the activity of gas,which makes it hard for gas to form adsorbed layer.At the same time,the interior structure of coal can change because of the increase of temperature.The amount of micropore in coal decreases and it is also the reason why the amount of adsorbed gas declines.?4?No matter what the condition is,the affinity of coal on CO₂ is more than CH₄,coal can absorb more than three times of CO₂ than CH₄.?5?the pressure and temperature of injected CO₂ are the main factors which have impact on output rate of CH₄ and CO₂ storage,higher injection pressure and temperature can lead to more CH₄ output and CO₂ storage and more rapid rate of CH₄ output and CO₂ storage,the increase of CO₂ pressure,from 2MPa to 4MPa,can result in the increase of CH₄ output rate,from 6.7% to 17.4%,and CO₂ storage?78.6%⁹9.7%?,the replacement ratio can increase by 8.5%⁸2.0%;the increase of injecting temperature?28? to 60 ??can lead to the increase of CH₄ output rate?40.0%⁴3.8%?and CO₂ storage?23.8%³8.4%?,but reduce the replacement ratio,which is from 8.4% to 20.2%.?6?The increase of injection pressure and temperature can cause greater deformation of coal samples,the axial strain of ?50×100mm weakly caking coal samples can increase by 98.1% and 104.7%.When the temperature of injected gas is 28 °C,the permeability of coal will become 63.0% and 28.7% of the initial permeability after the injecting emperiments under 2MPa and 4MPa.When the temperature of injected gas increases to 45 °C,the figures turn to be 71.1% and 32.6%.When the temperature of injected gas further increases to 60 °C,the figures become 80.2% and 35.7%.?7?The multi-fields coupling numerical simulation was conducted in suit under different injected pressure and temperature.It is fond that,by improving the temperature and pressure of injectd CO₂,the output of CH₄ can be increased and there will be more CO₂ stored in the coal seam.