Gas Seepage And Diffusion Patterns Of High-pressure Gas And Water Two-phase Displacement In Pore Fractures Of Gas-bearing Coal Bodies

The primary hazardous source used in the extraction of coalbed methane is gas.Gas extraction must be done safely and effectively because it is a clean energy source that people may use.At present,the commonly used gas permeability improvement measures mainly include water flooding and gas injection flooding technology,but it still can not solve the problems of difficult gas extraction in low permeability coal seam,resulting in water lock effect and gas overflow reflux.In addition,in recent years,some scholars have proposed the combination of water flooding and gas flooding into coal seams to improve the efficiency of gas displacement in coal seams.However,relevant theories and researches are still immature,and there is no further study on the effect of gas displacement under different conditions and multiple injection of high pressure gas and water into coal seams.In this thesis,we aim to establish a three-phase flow and seepage model and a high-pressure gas-water combination replacement device to study the characteristics of gas desorption and dispersion under different replacement methods and replacement pressures.The main research contents are described as follows:(1)By studying the microscopic mechanism of gas desorption and transport by different media and the corresponding seepage calculation model,we obtained the theoretical basis of high-pressure gas-water combination for gas replacement and created the corresponding three-phase flow seepage model.(2)Based on the three-phase flow and seepage model,the dynamic characteristics of gas seepage diffusion,coal seam pore pressure and the influence range of replacement under different injection methods,different injection pressures and borehole diameters were systematically studied.Through comprehensive analysis of the research results,it can be concluded that the second cycle high-pressure water injection and gas injection alternately inject the best effect of replacement,the gas volume fraction compared to the original coal seam reduced by 11.1% to 54.8%,and the replacement influence range reached 97.9m,both of which achieved the ideal effect.In addition,the increase of replacement pressure and the larger borehole diameter will also be beneficial to the gas desorption and replacement.(3)The gas replacement experiments were carried out under different injection pressures,different envelope pressures and different injection methods using an inhouse built high-pressure gas-water mixture gas replacement system.From the results,the increase of the repelling pressure will not only increase the gas repelling rate,but also make it easier for the gas to break through the coal sample in the gas injection stage,and the gas flow rate will increase rapidly;The increase of the surrounding pressure will hinder the desorption and repulsion of gas,and reduce the flow of repelled gas;It can be seen from the combination of the five different injection methods that the secondary cycle high-pressure water injection gas injection has the best displacement effect,and will further displace the gas that is more difficult to desorption in the nanoscale micro-porous cracks.This thesis has 52 figures,3 tables,and 101 references.