Study On Seepage Law Of N₂ Displacing Coalbed Methane

Injecting N₂ to the coal bed can effectively increase the permeability of coal seam and improve the recovery of coalbed methane,which has a great significance for the development of low permeability coalbed methane in our country.This paper takes coal mine rock of the Big Buddha Temple coal field as the research object: the pore structure of coal rock was measured by NMR;the coal rock adsorption characteristics of N₂,CH₄ and CO₂ were measured by isothermal adsorption experiment;the fracture feature of coal rock before and after adsorption was analyzed by CT scanning technique;the N₂ and four contrasting gases’ displacement characteristics were studied by continuous and intermittent displacement with sand filled tube and displacement equipment;the numerical simulation software was used to verify the experiment of N₂ displacement coalbed methane;the coal rock displacement permeability of N₂ and CO₂ were studied by three axis instrument.The results show that:（1）The porosity of coal rock is poor,it is mainly the adsorption pore,and the coal seam belongs to the tight reservoir.（2）The fracture openness of coal rock decreases after N₂ injection;（3）The adsorption of CO₂,CH₄ and N₂ in coal rock decreases in turn,and the adsorption is negatively correlated with the particle size;（4）With the increase of N₂ displacement pressure,the recovery increases first and then decreases,the displacement efficiency gets worse,the coal permeability gets lower and lower,but still higher than the original permeability;with the increase of CO₂ displacement pressure,the recovery increases all the time,the displacement efficiency changes little,the coal permeability decreases first and then increases,the whole permeability is lower than the original permeability;The law of 80%N₂+20%CO₂ and 90%N₂+10%CO₂ are similar to N₂,and the law of 50%N₂+50%CO₂ is similar to CO₂.The recovery of N₂ constant displacement is better than intermittent displacement;the recovery of CO₂ intermittent displacement is better than constant displacement.（5）Under the condition of multi field coupling,the permeability of coal rock is K（N₂）>K（CO₂）,and the permeability decreases with the increase of displacement pressure.The experimental methods and obtained conclusions in this paper have a reference value for gas injection to improve the recovery of coalbed methane.