Investigation Of The Adsorption/desorption,induced Strain And Permeability In Different Rank Coal N₂-ECBM Process

Currently,there are about 5000 non-producing gas wells or low producing wells in China,most of them are low permeability,low water pressure and low gas pressure.The N₂-enhanced coalbed methane(N₂–ECBM)recovery technology is one of the specially developed for the low producing reservoirs.In this thesis,twelve different rank coals from Shanxi Province of China were selected as the research objects.Based on the problems of N₂ injection promoting CH₄ desorption,coal matrix shrinkage strain and pore permeability change,the characteristics of strain,desorption percentage and permeability of coal samples with different combined concentration of CH₄+N₂,different gas content/different saturation,with/without nitrogen injection desorption(atmospheric pressure/step-by-step depressurization desorption)were studied via experimental simulation.Then,N₂ induced methane desorption shrinkage strain model,pore fracture evolution model during nitrogen injection and sealing(N₂–IS)and three-stage model of nitrogen promoting coalbed methane production were established.Moreover,the thesis reveals the mechanism of N₂–IS improves permeability,and evaluates the permeability enhancement effect of N₂–ECBM in different coal rank reservoirs.The results show that during CH₄+N₂ injection,the volume strain was negative-exponentially related to the N₂ fraction in the injected gas,and the desorption shrinkage strain rate was negatively related to the N₂ concentration.The N₂ fraction exceeds 50%in the CH₄/N₂ mixture can inhibit methane adsorption and promote methane desorption.The methane production of different pore sizes and different phase states is not synchronous,and the methane production rate of step-by-step depressurization desorption is 11.62%higher than that of atmospheric desorption(0.1MPa).After N₂–IS,the cumulative methane production percentage increases with the increase of methane saturation,gas content and nitrogen injection pressure.When the nitrogen injection pressure reaches more than 3 times of the reservoir pressure,N₂–ECBM has the best effect.The order of cumulative production percentage during N₂–ECBM is high-rank coal<low-rank coal<medium-rank coal.Nitrogen injection successively"flushing"the free methane in the fracture,displaces the free methane in the seepage pore,and reduces the methane partial pressure to promotes the desorption of adsorbed methane in the adsorption pore.Druing N₂–IS,the permeability recovery is mainly caused by the increase of fracture aperture caused by coal matrix shrinkage,and the permeability recovery rate is high-rank coal>medium-rank coal>low-rank coal.Nitrogen injection has the effect of"dredging and extended"on fractures connectivity and aperture,and"enlarge"on pores volume with pore diameter greater than 10nm.The research results of this thesis reveal the mechanism of N₂-IS increasing the permeability,which expected to provide a theoretical basis for CBM recovery of coal reservoirs with low permeability,low water pressure and low gas pressure.There are 88 figures,48 tables and 222 references in this thesis.