| Shale gas with huge reserve and extensive distribution represents a significant portion of unconventional natural gas resources worldwide.Considerable number of researches have been done in how to improve shale gas reservoirs,and currently the most prominent one is depressurizing production.Nevertheless,very rapid decline in gas production and a long term production cycle make it impossible to achieve stable economic trend.Therefore,studies in enhancing shale gas recovery has an important significance in improving energy structure in our country.In this paper,the basic parameters of shale samples,such as mineral composition,organic carbon content?TOC?,vitrinite reflectance Ro,porosity and permeability were carried out in this study,and the physical properties of shale samples were evaluated.On the basis of this,the shale adsorption and desorption experiments were carried out to study the adsorption and desorption characteristics of shale under different temperature,pressure and gas properties.Finally,based on the shale desorption experiment,depressurizing production experiment and CO2 injection experiment were carried out in this study.In order to investigate the potential of improveing shale gas recovery,we compare the depressurizing production which with the pressure decline rate of 0.05 MPa/min with CO2 injection method about the cumulative CH4 production,CH4 production rate and CH4 production recovery.The feasibility of improving the shale gas recovery by CO2 flooding CH4 is discussed.The results show that the shale samples are mainly composed of detrital minerals,clay minerals and self-brittle minerals.The TOC of organic matter is 2.57%5.31%,the vitrinite reflectance Ro is 2.56%2.87%.The porosity range is 3.685%7.671%,the permeability range is 0.0537×10-3?m20.4872×10-3?m2,which means that the shale pore structure is poor.The results also show that the ultimate recovery of depressurizing production experiment is between 55.77%and 59.78%,and there is still a large amount of CH4 gas adsorbed on the shale sample without desorption.It is necessary to take the effective way to remove the residual adsorbed CH4 Gas flooding out to improve shale gas recovery.In the CO2 flooding experiment,CH4 accumulated gas production and CH4 recovery were significantly higher than those of shale depressurization production.The CO2 desorption rate increased by 9.75%26.89%through CO2 displacement.In the injection pressure range,the greater the injection pressure,the higher the accumulated gas production and CH4 recovery.Hence,the CO2injection technology can significantly enhance shale gas recovery. |
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