Methane Adsorption/Desorption And Diffusion Behaviors In Coals Of Different Coal Structures And Their Controlling Mechanisms

Original coals destroyed by geological stresses occur series of changes in terms of pore structures,adsorption/desorption,diffusion and permeable ability,which can affect gas accumulation and migration in coalbed methane（CBM）development.This thesis comprehensively uses the knowledge of coal geology,coalbed gas development geology and logging theory and method of coal field to study the logging combination characteristics and prediction models of different coal structure,and divided the spatial distribution law of different coal structures in study area as well as evaluated the gas content characters and simulation analysis of well distribution in structural coal area;Meanwhile it is analyzed the distribution characteristics of pore structures with different coal deformation degrees,and recognize the variations of pore structure of deformed coal and its effects on the adsorption/desorption characteristics of methane;comparing the the gas diffusion differences in different coal structures,and the mechanism of adsorption/desorption and diffusion in coals of different coal structures are revealed.The following research results have been achieved:（1）As the increased deformation of coals,acoustic transit time（AC）,compensated neutron（CNL）,natural gamma（GR）and caliper（CAL）show an upward trend,but response values of density（DEN）and lateral resistivity（RD）decrease.Different coal structures can response similar values in well logs,which lead to the multiple interpretations for coal structures.The BP neural network model for identifying coal structure with multiple logging parameters is revealed.BP neural network model was proposed to find coal structures.The proposed BP model was trained and then it was used to identify the nonlinear relationships between well logs and coal structures by extracting key information from logging data sets.（2）The variations of pore structures on coal structures were revealed.Based on the experimental results and analysis,a suitable pore size classification scheme was proposed,including ultramicropores（<2nm）,micropores（2-10nm）,mesopores（10-100nm）and macropores（>100nm）.The results show that pore volume of coals are predominately provided by mesopores（50-100 nm）and macropores（>100nm）,and the contributions to SSAs are mainly ultramicropores（<2nm）,micropres（2-10nm）and mesopores（10-50nm）.With the increase of coal deformation intensity,the proportion of pore size of 50-300nm decreased,showing more macropores and mesopores were deformed to the smaller pores（<50nm）.Micropores（2-10nm）in granulated and mylonitic coals obviously increased.The predominant ultramicropore in all four kinds of coal structures are distributed at 0.45-0.6 nm and 0.80-1.0 nm.（3）The adsorption of CH₄,N₂ and CO₂ increase as the coal deformation degree increases,following the order:mylonitic coal>granulated coal>cataclastic coal>intact coal.Fractal dimensions show that tectonic coals characterize the rougher pore surface（higher D₁）and more homogeneous pore structures（lower D₂）,which lead to the higher adsorption capacity.The development of micropores is positively with Langmuir V_L,but compared to ultramicropore SSAs,the contribution to adsorption capacity of micropores and mesopores can be negligible.（4）The diffusion coefficient of four coal structures decrease as the de-pressurization processes,and diffusion coefficient of tectonic coals is higher than intact coal in the whole diffusion stages.At pressure zone of higher than 5 MPa,1.5-5MPa and lower than 1.5 MPa,gas diffusion mainly occurred in macropores,mesopres and micropores,respectively,corresponding to Fick’s,transitional and Knudsen diffusion.（5）The variations of adsorption potential increased with the increased coal deformation intensity,and the smaller the adsorption space volume is,the larger adsorption potential will be,illustrating adsorption in micropores is higher than mesopores and macropores.Tectonic coals have the higher reduced rate of cumulative surface free energy,which shows that tectonic damages promote the adsorption potential and surface free energy in coals.The differences of adsorption/desorption,diffusion and pore structures characteristics of intact and deformed coals and mechanisms of their kinetic variations based on isosteric heat of adsorption were investigated.Langmuir V_L in coal was dependent on temperature and coal structures.From 25℃to 45℃,Langmuir V_L in coals decreased with the increased temperatures.Adsorption heat in intact coals was higher than deformed coals,illustrating interaction actions between gas molecules and coal atoms were more intense in intact coal,which led to the hard desorption of gas molecules from intact coal surfaces.The high initial diffusion coefficient in deformed coals is related to the low isosteric heat of adsorption.（6）As the burial depth increased,the heterogeneity of coal structures decreased.The burial depth of intact coal usually is higher than 1000m,and it is found that cataclastic coal was mainly developed in study area,which is concentrated in the central fault belt A2 and east fault belt A3.Carbon isotope of methane shows the methane in stidy area is related to thermogenic gas.Measured gas content of wells in granulated coal located in shallow area usually lower than 12 cm³/g,but that in cataclastic coal zone with deeper depth can even reach above 20 cm³/g.Gas saturation varies with burial depth,and the gas saturation in cataclastic coal zone is higher than granulated coal.The numerical simulation shows that the primary coal structure area has low gas content,low resource abundance and good permeability.The relatively high gas bearing area should be selected during well layout,and the well spacing should be appropriately expanded,so that the single well can control more geological reserves and produce more gas;For the areas with large destruction of coal structures with high gas content,large reserve abundance and poor permeability,it is suggested to reduce the well spacing and increase the degree of reservoir reconstruction,so as to improve the initial production and final recovery of single well.The variation law of deformed coal structures reservoir were built.