Mechanisms And Experimental Study Of Gas Mixture Enhanced Coalbed Methane Recovery Technology

China is a big coal industrial country with very rich coalbed methane resources. However,the recovery ratio of coalbed methane in China is still at a relatively low level. The direct consequences of low recovery ratio of coalbed methane are gas disaster-prone, inefficient mining,resource waste and environmental pollution.Enhanced coalbed methane recovery ratio has the triple effects of disaster prevention,large-scale use of coalbed methane and greenhouse gas emission reduction.In view of the low permeability characteristic of most coalbed and the current status of low recovery ratio of coalbed methane in China,the N₂-rich gas mixture enhanced coalbed methane(G-ECBM) technology has been suggested for the purpose of enhancing coalbed methane recovery ratio.In this thesis,the G-ECBM technology is studied based on the existing technologies of CO₂ sequestration in coalbed and enhanced coalbed methane production by gas injection, according to the theories of coalbed methane geology,rock mechanics,solid mechanics, porous medium,flow mechanics in porous medium,multiphase flow and the methods of laboratory test and field test.The major contents of this thesis include:1.The characteristic of coalbed methane reservoir and the mechanisms of storage, migration and production of coalbed methane were introduced.In view of the low permeability of most coal and the current status of low recovery ratio of coalbed methane in China,the N₂-rich gas mixture enhanced coalbed methane(G-ECBM) technology was suggested.The mechanism and advantages of G-ECBM technology were analyzed.2.A multiphase flow fluid-solid coupling mathematical model for G-ECBM process was developed,and the numerical solution method was given in detail.A permeability model considering the swelling and shrinkage effects of coal under different constraint conditions was established.3.A method for simultaneously measuring adsorption-strain-permeability and related experimental set-up were developed.The advantages of the method and set-up include:(a) simultaneously measuring adsorption,strain and permeability on one sample,making it more convenient than other methods;(b)adsorption,strain and permeability are measured at the same time,making it better to analyze the relations between these parameters;(c) cylindrical coal sample is used,making the results more valuable and more consistent with the standards of conventional rock mechanics.4.The adsorption,strain and flow tests of single-component gas of CO₂,N₂ and CH₄ were carried out with above method and set up,respectively.The test results showed that:(1) The relationship of the adsorptions of CO₂,N₂ and CH₄ on the same coal sample was:CO₂＞CH₄＞N₂.(2) At constant effective stress,sorption-induced strains of three gases all increased with the increase in pore stress,and the relationship of strains was:CO₂＞CH₄＞N₂. Therefore,compared with the original coal sample saturated by CH₄,coal will swell in CO₂-ECBM and shrink in N₂-ECBM.(3) At constant effective stress,permeabilities of three gases all decreased with the increase in pore stress.The permeability decrease induced by CO₂ was nearly an order of magnitude,which was the most obvious;followed by CH₄;the permeability decrease induced by N₂ was very small.Therefore,compared with the original coal sample saturated by CH₄,coal permeability will decrease remarkably in CO₂-ECBM and increase in N₂-ECBM,which shows the mechanism of permeability-increasing of N₂.(4) The test results above indicated that the proposed method and set-up suitable for simultaneously measuring adsorption,strain and permeability of coal.5.Two different scale underground field tests were carried out.Test results showed that,compared with existing conventional extraction method,G-ECBM technology has great breakthrough in principle and effect,and is technically feasible;gas injection had effect on not only the neighboring production wells but also the a litter far production wells; gas injection enhanced the concentration and flow rate of coalbed methane.6.The industrial test program of underground G-ECBM was proposed,based on the field test results and the actual situation in coal mine.The program includes:construction and layout requirements of wells;drilling sealing process;water and gas separation; monitor and estimation of the test process.7.The theoretical effect,program and feasibility of ground G-ECBM have been studied by contrastive numerical simulation of conventional extraction process and G-ECBM technology.The results showed that,by using G-ECBM technology,coalbed methane recovery ratio can be enhanced substantially,and the time for coalbed methane pre-extraction can be shortened thus improving the efficiency of coal mining.