Research On The Characterization And Seepage Mechanism Of Coal Rock Pore Crack Based On CT And NMR Technology

With the gradual transformation of terminal energy demand to high-quality,efficient,and clean energy,the demand for gas grows rapidly.Utilizing coalbed methane can save resources,optimize energy structure,and effectively amend the shortage of conventional gas supply.However,in China,the structure of coal reservoirs is complex,especially with the characteristics of "three of low and one of unevenness",which makes it difficult to extract Therefore,the study of pore structure and seepage characteristics has great practical significance and social value for scientific utilization of coalbed methane and effective prevention of coal mining gas disasters.At present,domestic and foreign scholars have conducted a lot of theoretical and experimental studies,and have obtained abundant research results.However,for high and low-rank coals,using multiple technical means to study pore and fracture structure characteristics and seepage characteristics,and diversified and systematic quantitative characterization research needs to be further deepened.Based on the Scanning Electron Microscope(SEM)technique,this thesis analyzes the types of pore fracture and morphological characteristics of coal.What’s more,by using Nano Measurer software,the pore size characteristics of coal were analyzed and the fracture permeability index of coal was calculated for the quantitative characterization of pore structure characteristics and permeability of coal.Next,Using the low-field nuclear magnetic resonance(NMR)technique perform research on permeability of coal by characterizing the structural characteristics of coal such as pore throat,pore size,porosity,and calculating the permeability of coal by using the(SDR/Coates)model.Then,based on Computed Tomography(CT),the pore and fissure structural characteristics of coal were firstly quantitatively characterized by borrowing Mimics image processing software;then,using Avizo software,a 3D digital coal core was constructed to study the pore structural characteristics and permeability of coal from a microscopic perspective.Finally,based on the three-dimensional digital coal core,a single-phase water flow percolation simulation study was conducted using the absolute permeability model in Avizo software.The results of the study show that:(1)Use SEM technique to obtain the Secondary Electron Images and Backscattered Electron Images of coal,the low-rank coal(Dafosi mine coal)contains the most developed tissue pores,air pores and shear,tensile,and relaxation fractures.In addition,the content of angular pores,fragmentation pores and water loss,and hydrostatic fractures are extremely small;The high-rank coal(Shihe mine coal)contains the most developed tissue pores,angular pores and tensile,shear,and relaxation fractures,and the content of pores,intercrystalline pores,and mold pores are extremely small;A certain amount of mineral composition is distributed on the surface of high and low rank coals.Among them,highrank coals contain clay mineral components(montmorillonite)that swell rapidly when exposed to water.During the process of surface CBM extraction and downhole gas extraction,it is easy to cause the pore fissure blockage,thereby reducing permeability and the affecting effect of surface CBM or downhole gas extraction.(2)Use low-field NMR technique to monitor the water-saturated coal,the micro pores and pore-connected channels are the most developed in the high and low rank coal,while the content of medium-large pores and pore-connected channels is small;Use the permeability model of Avizo to obtain the permeability K and use the low-field NMR technique to obtain the model parameter C,the fitted K and C have a good correlation.It shows that the permeability of low-rank coal is better than that of high-rank coal.(3)Use the Mimics image processing software to analyze the gray value of coal,observe the development and distribution of microscopic pores and fissures of coal.The pores and fissures of low coal rank coal(Dafosi and Xia Shijie mine coal)are relatively more developed than those of high rank coal(Shihe mine coal),the former has more fissures and mineral content,and its distribution is more obvious,while the latter rarely finds large fissures;Use different algorithms of the Avizo software to process CT slice data,the Label Analysis can only count the volume size of pore size,the median algorithm can only count the characteristic parameters of pore size,while the maximum sphere algorithm can count the pore throat radius,pore throat length,pore radius,coordination number,etc.,and it is an important reference value for the research of coal reservoir seepage.(4)Use the absolute permeability simulator of the Avizo to simulate the singlephase water flow percolation,the higher the flow rate of percolation in coal and the lower the pressure of percolation,the more developed the pore fissures of coal and the better the permeability.On the contrary,the smaller the flow rate of percolation and the higher the pressure of percolation,the less developed the pore fissures of coal and the poor permeability.Based on SEM,NMR and CT technology,this thesis studies the structural characteristics of pore-fissure and seepage mechanism of high and low coal rank coal,which can provide important theoretical basis and basic data for the optimization of surface CBM well drainage system and the formulation of coal mine gas extraction technology plan.