Experimental Studies Of N₂ Injection In Coalbed Gas Reservoirs

Coalbed methane is an unconventional natural gas,which is widely distributed in China and has huge reserves.At the present stage,the main way of coalbed methane exploitation is natural depressurization.However,the natural pressure reduction mining has the characteristics of long production cycle,low gas production and poor project economy.Injecting non hydrocarbon gas into reservoirs to supplement formation energy and improve coalbed methane recovery has become a very good way of mining and has considerable application prospects.Therefore,it is of great practical significance to study the injection of N₂ to improve the recovery of coalbed methane.In this paper,the mineral composition,porosity,permeability and other physical parameters of the coal and rock samples are analyzed by experiments.On the basis of fully research and analysis of the physical characteristics of coal and rock samples,the adsorptio n and desorption experiments of coal and rock samples have been carried out by the gas adsorption desorption tester.The adsorption characteristics and rules of CH₄ for coal and rock samples at different temperatures and pressures were studied.Meanwhile,the adsorption and desorption laws of CH₄ and N₂ under the same conditions were compared and analyzed.The experimental device was designed and built by ourselves,and the simulated natural step-down mining and simulated injection N₂ mining experiment were carried out respectively.Under the same temperature conditions,the two ways of natural pressure reduction mining and injection N₂ mining are compared and analyzed from three aspects:gas production rate,gas production rate and recovery rate.The experimental results show that the main composition of inorganic minerals in coal and rock samples is quartz,the porosity range is 5%⁹%,and the permeability range is0.1×10^-3μm²⁰.5×10^-3μm²,and porosity and permeability are all low.Adsorption and desorption experiments show that pressure and temperature influence the adsorption and desorption of CH₄ in coal and rock.Under different saturation pressures,the ultimate recovery rate of CH₄ is between 64%and 72%,and the recovery rate is low.A large number of CH₄still have no desorption.Through the simulation of constant speed and constant pressure injection of N₂ for coalbed methane production,it is found that within a certain range,the higher the rate of N₂ injection,the higher the production of CH₄.Meanwhile,with the increase of N₂ injection rate,the stable production period of CH₄ mining is prolonged.Meanwhile,with the increase of N₂ injection rate,the stable production period of CH₄ mining is prolonged.In a certain range,the higher the injection of N2 pressure,the higher the production of CH₄,and with the increase of the injection of N₂ pressure,the stable production period of the CH₄ mining is prolonged.Compared with the natural step-down mining,the two mining methods can increase the recovery rate of CH₄ is about 5%to 23%.