| In this dissertation,middle and high rank coal samples in the multi-coal seam development area of the western Guizhou and eastern Yunnan are taken as research object.Double nine coal samples were collected from the Tucheng and Laochang area,respectively.Then pore distribution and heterogeneity of pore-fracture system were detailed characterized by combing scanning electron microscopy,high pressure mercury intrusive,low temperature liquid nitrogen adsorption,carbon dioxide adsorption,high pressure isothermal adsorption with laser Raman tests.After that,stress sensitivity of pore and fracture were studied by overlaying nuclear magnetic and permeability measuring technology,and stress-strain of adsorption pore,seepage pore and fracture were analyzed,respectively.A dynamic permeability model was established by using the variable volume compressibility.Meanwhile,dynamic variation of adsorbed and bulk methane content were studied by a serial of physical simulation experiments.Based on the above results,numerical simulation technology was used to study multi-layer production process of typical CBM well,so dynamic variarion of porosity-permeability and its effect on multi-layer combined production of CBM can be obtained.The detailed conclusions are as follows.(1)All 18 samples were classified into three types based on the pore type and connectivity of adsorption pore.Type A is represented by middle rank coal samples,in which adsorption pore are dominated by semi-open pores,and dominant diameter is from 10 to 50 nm.Type B and C samples are represented by high rank coal samples,type B are dominated by ink-bottle-shaped and slit-plated pore,and dominant diameter is from 2 to 10 nm.Adsorption pore morphology of type C varies greatly,which is transition stage of the first two types.Volume heterogeneity of adsorption pore in type A is the strongest among the three types,while its surface area heterogeneity is weaker than that of type B and C.With increasing coal rank,micro-pore distribution gradually changed from ‘multimodal peak' of type A to the ‘triplet peak' of type C and ‘double peak' of type B.There is a good consistency between volume heterogeneity and surface heterogeneity of micro-pore.Different from adsorption pore,seepage pore of all the samples are mainly dominated by semi-open pore.Among them,seepage pore are the most developed in type A,and volume and surface heterogeneity of seepage pore in this type of samples was stronger than that of other two types.(2)There exists difference in porosity and permeability variation between middle and high rank coal samples.Pore and fracture volume decrease exponentially with increasing confining stress,and stress sensitivity of seepage pore and fracture is much higher than that of adsorption pore for the same sample.Adsorption pore gradually develop with increase of the coal rank.Stress sensitivity of high rank coal samples is lower than that of middle rank ones,and stress sensitivity of seepage pore is higher than that of adsorption pore.Heterogeneity variation of adsorption pore is greater than that of seepage pore and fracture under the effect of confining pressure.The effect of confining stress on heterogeneity of pore-fracture system has a two-stage periods.At the initial stage,heterogeneity of a pore-fracture system is more complicated.And it moves to the stable stage that the heterogeneity tends to be stable when confining pressure is higher than 9 MPa.(3)Migration progress between adsorbed and bulk methane is different.During adsorption process,amount of adsorbed,bulk methane and methane injecting pressure satisfy the Langmuir equation and linear relationship,respectively.Under same methane injecting pressure,amount of adsorbed methane increased linearly in the early stage and increased logarithmically in the later stage.The amount of bulk methane increased linearly with the injecting time,and saturation time is much shorter than that of adsorbed methane.For the one-stop desorption,desorption of adsorbed methane can be divided into early rapid decline stage and later slow desorption stage.Meanwhile,seepage pore of middle rank coal samples are developed,and initial bulk methane content is high,which results in variation rate of bulk methane higher than that of high rank ones during methane desorption.In addition,depressurization gradient has a negative correlation with methane desorption amount.That is,the gradual depressurization process can significantly improve the desorption of adsorbed methane in coal samples.(4)In the process of combined drainage,the multi-layer interference occurs in the early stage,and gradually weakens to disappear in the later stage.Porosity,permeability and reservoir pressure are the main parameters that affect the drainage of combined production.It is indicates that pore and permeability variation were one of factors that affect process of drainage,depressurization and gas production.The improved permeability model shows that higher compressibility make permeability decrease larger,then reservoir pressure funnel is difficult to expand,so it would inhibit the maximum and average gas production rate of this coal seam.In addition,rapid decrease of permeability leads to the increase of output water and pressure drop range of adjacent seam,which results in increase of maximum and average gas production rates of adjacent seams.Meanwhile,permeability variation has little effect on the combined production capacity when volume compressibility is lower than invalid critical value.Existing sample indicates that this critical compressibility mostly corresponds to compressibility of adsorption pore.Therefore,dynamic permeability model with variable compressibility,which only includes seepage pore and fracture compression space,can more accurately represents the variation of porosity and permeability caused by stress sensitivity during CBM drainage.There are 105 figures,16 tables and 296 references in this dissertation. |
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