Study On Pore Change And Adsorption Characteristics Of Liquid Nitrogen Fracturing Coal And Engineering Scale Fracturing Law

As an associated resource of coal,the efficient utilization of coalbed methane can not only reduce mine disasters,but also reduce pollution and protect the environment.Coal seams in China generally have the characteristics of low permeability and permeability.How to improve the efficiency of coalbed methane extraction has gradually become the industry consensus and academic research hotspot.In this thesis,Yunnan Buzhaoba low-rank lignite,Anhui Xutuan medium-rank bituminous coal and Shanxi Yangquan high-rank anthracite were taken as the research objects to explore the pore structure and adsorption characteristics of coal liquid nitrogen fracturing and the law of engineering-scale fracturing.Firstly,the influence mechanism of liquid nitrogen cracking on different temperature gradients and different coal ranks is analyzed.Secondly,the pore changes and adsorption characteristics of coal before and after liquid nitrogen fracturing were quantitatively characterized by mercury intrusion experiment and methane adsorption experiment.With the help of coal adsorption gas tester and mercury intrusion tester,methane adsorption experiments of coal samples with different temperature gradients and different coal ranks were carried out,and the adsorption characteristics of liquid nitrogen fracturing on different temperature gradients and different coal ranks were revealed.Finally,the thermodynamic model of liquid nitrogen fracturing coal seam is constructed,and the numerical simulation of liquid nitrogen fracturing coal seam with different ground temperature,different borehole spacing and different borehole diameter is established.The main research results are as follows :(1)Through mercury injection experiment and methane adsorption experiment,the evolution characteristics and adsorption characteristics of coal pore structure before and after liquid nitrogen cracking were studied.The results showed that liquid nitrogen cracking changed the pore structure of coal and reduced the adsorption capacity.The specific performance is that the pore size ratio of micropores,small pores and mesopores of coal decreases after liquid nitrogen cracking,the pore size ratio of large pores increases,the fractal dimension decreases,the adsorption equilibrium pressure increases and the adsorption capacity decreases.The porosity and permeability of coal increase,so as to achieve the effect of increasing permeability of coal.(2)The adsorption characteristics of liquid nitrogen cracking in coal with different temperature gradients were studied.The results showed that the greater the temperature gradient of coal,the greater the thermal stress generated after liquid nitrogen cracking,the damage of pore structure and the less the adsorption capacity.The greater the influence range of liquid nitrogen fracturing on the temperature field and stress field of coal seam,the higher the permeability of coal seam.With the increase of coal temperature gradient,the adsorption capacity of coal to methane decreases after liquid nitrogen fracturing.(3)The adsorption characteristics of liquid nitrogen fracturing of different rank coals were studied.The results show that after liquid nitrogen fracturing of different rank coals,the adsorption capacity of coal decreases,the adsorption potential decreases,the adsorption space decreases,and the adsorption site of coal to methane also decreases.The relationship between the adsorption amount of methane and the metamorphic degree of coal before and after liquid nitrogen fracturing satisfies the ’ U ’ shape relationship,that is,the adsorption amount of methane decreases first and then increases with the increase of the metamorphic degree of coal.(4)Through the engineering scale numerical simulation of liquid nitrogen fracturing,the research shows that the drilling spacing gradually increases,the liquid nitrogen fracturing effect gradually weakens,and the fracturing range gradually increases;the larger the borehole diameter,the better the effect and range of liquid nitrogen fracturing.However,in the face of on-site construction,the increase of borehole diameter and borehole spacing will bring a sharp increase in cost.Therefore,in view of the different conditions of coal seams,reasonable selection of borehole diameter and borehole spacing can improve the effect of liquid nitrogen fracturing,thus saving costs.This thesis has 34 figures,2 tables and 91 references.