| The low permeability,high gas content and strong adsorption capacity of most coalbed in our country severely restrict the efficiency of methane extraction and negatively affect the mine safety production,how to improve the coalbed permeability and enhance the efficiency of CBM recovery is one of the hotspots research contents in the field of gas disaster manage in coal mine.Liquid CO2 as a fluid with low temperature,low viscosity and easy seepage,phase-transition pressurization,and strong adsorption potential energy features,injecting it into coalbed will change the permeability in the process of coal microstructure damaged under the action of liquid CO2 cold impact and phase-transition pressure,and then the gaseous CO2 will enhance CH4 recovery in the coalbed.However,as an extremely unstable fluid,liquid CO2 has different seepage evolution law and efficiency characteristic in the respective migration range with a liquid,gas and liquid mixture or gas phase states when it is injected into the coalbed.Therefore,this paper adopts the combined methods of theoretical analysis,experimental test,numerical simulation and field test,the evolution law of pore fracture structure and permeability increasing characteristics of coal damaged by liquid CO2 are quantitatively characterized,the efficiency of CO2 with a gaseous seepage state to displace CH4 in the coal are analyzed.Then,the multi-field coupling seepage characteristics and their relations of liquid CO2 injection to enhance CH4 recovery in the coalbed are studied.Finally,a field application test of liquid CO2 injection to enhance CBM recovery is carried out and the process effect is verified.The main results are as follows:(1)The damage effect of coal microstructure caused by the frost-heaving fore and phase-transition force in the process of liquid CO2 injected into coalbed are analyzed.Adopting the methods of LP-N2-Ad and MIP to study the evolution law of coal pore structure in the full aperture section that damaged by liquid CO2 injection into coal,resulting the liquid stress damage effect induced the secondary development of coal pore structure,the proportion of total pores increased and the fractal characteristics are obvious,in addition,a large number of adsorption pores are transformed into seepage pore structure,it indicates the gas seepage capability in the coal microstructure is increased.(2)Using X-ray CT scanning technology,with the help of image processing,three-dimensional visualization and algorithm function modules by Avizo system software,the theoretical model of coal fracture network distribution and percolation is constructed,and the variation law of fracture network and permeability enhancement of coal damaged by liquid CO2 injection are quantitatively analyzed.The results show that under the damaged effects by liquid CO2 injection,the total volume of coal pillar(Vt),the volume of fracture network(V0)and the proportion of fracture network(μ’0)are significantly increased,resulting the stress damaged effect caused the coal is deformed and the characteristics of fracture network regeneration and spatial distribution are obvious.Furthermore,the single value function coverage region in the percolation model becomes larger with coal block treated by liquid CO2,it shows that the liquid CO2 injected into coalbed can significantly improve the coal permeability within the range of CO2 seepage as a liquid state.(3)A system platform is conducted to test the efficiency characteristics of CO2 injection to displace CH4 recovery in the coal under different pressure and temperature conditions.The test results shown that as the displacement pressure is constant,resulting the low-temperature circumstance has a distinct delaying effect on the efficiency of CO2 injection to displace CH4,However,with the increase of temperature,the displacement efficiency increases gradually.Additionally,as the displacement temperature is constant and the displacement pressure is appropriately increased,the efficiency of CO2 injection to displace CH4 is improved,meanwhile the displacement time is reduced,displaying an obviously efficiency for a given period time.According to the efficiency and replacement ratio of CO2 injection to displace CH4 recovery in the coal,with the increase of displacement pressure and temperature,ηmax increased from 56.02%%to 92.87%,μmax changed from 1.57:1 to 1:2.5.Comparing with the recovery of CH4 per unit volume,the amount of CO2 usage is reduced and the efficiency is significantly enhanced.The mechanism of CO2 injection to displace CH4 is obtained,which divided into the high-efficiency displacement stage of CO2 gas flow carrying effect and CO2/CH4 mixed gas competitive adsorption effect,and the low-efficiency displacement stage of residual CH4 diluted by the high-concentration CO2 gas molecules.(4)A thermal-fluid-solid coupled numerical solution model of CH4 displacement by liquid CO2 injection in coalbed is established.Through the numerical analysis,the evolution laws and multi-phases seepage process of temperature field,pressure field,gas migration field in the process of liquid CO2 injection to displace CH4 recovery in the coalbed are studied.It is determined that the range of CO2 seepage as a liquid state is within 5 m and the range of CO2 seepage as a gaseous state is up to 15 m based on the geometric model.According to the variation law of CO2/CH4 concentration with time in the simulated extraction process,the critical time node of CH4 efficiency attenuation of liquid CO2 displacement coalbed is obtained.(5)The in-situ application test of liquid CO2 injection to displace CH4 recovery in coalbed is carried out.Analyzing the corresponding relationship between the parameters such as pressure,temperature and flow and the CO2 seepage rate in the coalbed during liquid CO2 injected into coalbed.The effective influence radius of CO2 seepage as a liquid state or gas state in the coalbed is defined,and the high-efficiency of liquid CO2 injection to enhance CH4 recovery is verified. |
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