| Extraction of coal bed methane(CBM)can lower the risk level of coal mine gas hazards and reduce the greenhouse effect and obtain natural gas resources.Therefore the extraction of coalbed methane has drawn the concern of the world.China has a technically recoverable CBM resources of up to 11×1012 m3.Yunnan Province is abundant in low rank coal CBM resources and has a great CBM development potential.But the low rank coalbed methane resources research and the exploration work is relative low.In recent years,with the development of the coalbed methane exploration in China,the exploration and development of coalbed methane resources in Yunnan Province has been paid attention.Therefore,it is of great theoretical and practical significance to study the basic theory of coalbed methane in Yunnan province.Coalbed methane is largely stored in coal matrix pores in an adsorbed state and the transport of methane in coal seams includes three stages,desorption,diffusion and seepage.Adsorption-diffusion behavior is one of the key factors that influences the resource evaluation and development potential of coalbed methane.At present,the adsorption and diffusion properties and control mechanism of low coal rank coal are not clear On this issue,this article take the typical low rank coals collected from southeastern Yunnan Province as examples,combining the basic theories of coal petrology,coalbed methane geology and seepage mechanics,Through the experimental test,theoretical research and numerical simulation and other methods,the characteristics of coal petrology and quality,coal matrix porosity,adsorption-diffusion behavior and gas production of low rank coal seam were studied and the adsorption-diffusion law and its controlling mechanism were addressed.The results obtained are as follows.(1)the macro-lithotype of the low rank coal samples in the research area are largely dull coals with grain-like and shell-like fractures and have a low bulk specific gravity.The loose coal samples tends to stain hands.The no.2 Shicaohe sample contains well-developed cleats with mineral fillings and is relatively hard,containing Brown porphyritic pyrite and lens.Shanxincun Coalmine coal samples have a relatively high moisture content,a low bulk specific gravity and a loose texture.Coal samples from Puyang Coalmine Wenshan Prefecture is rather hard with some pyrite grans on the fractures.The coal samples have a high moisture content,approximately 20.24%~57.26%,a low ash yield of 2.39%~20.46%,volatile content on dry ash free basis is approximately 50%,fixed carbon content on dry ash free basis is 23.37%~47.98%,the reflectance of humic group is approximately 0.3%.The XRD analysis of low coal rank coal samples in southeastern Yunnan,shows that the coal samples contain some montmorillonite,kaolinite,quartz,calcite,gypsum,pyrite,and much amorphous polymer.The occurrence of swelling clay mineral in coal samples,indicates that there is a possibility of reservoir damage in the construction of coal bed methane well,and anti-swelling agent should be used in drilling.(2)The pore size distribution of the coal samples were measured using low temperature nitrogen adsorption method and high pressure mercury injection method respectively.The morphology of the coal matrix pores were imaged using an Atomic Force Microscope and the comparison between the matric pore morphology for the low rank coal samples from the study area,a medium rank bituminous coal sample from Xin'an coalmine Henan Province and a high rank anthracite coal sample from Sihe coalmine Shanxi Province was performed.The results show that,the porosity of the coal in the low coal rank is dominated by the plateau open porosity,and the semipermeal pore is contained.The degree of stomatal development in low rank coal is significantly higher than middle and high rank coal,and the average coal specific surface is 1.32468m2/g.the pore specific surface of mesopore takes up the highest percentage,more than 60%.Although a small amount of micropores is measured,it has a great contribution to the total specific surface of the coals.The mean pore diameter is 27.2523 nm,falling into the range of mesopore diameter;coal matrix pore is mainly composed of mesopores in this area,the pore volume of mesopores accounts for over 80% of the total volume and macropore and micropore contribute to the other portion of the total pore volume.The mercury injection measurement shows that in low rank coal sample,the macropore and mesopore is more developed than micropore while their connectivity is poor;lignite samples in the study also have a certain ability of adsorption,containing "ink bottle" pore structure and semi-closed adsorption pore;the lignite samples have a high porosity with an average porosity of 20.7913%,and the average of total pore area is 10.005m2/g.Through the observation using an atomic force microscope,we found the lignite coal samples and bituminous coal sample from Xin'an coalmine Henan Province has a number of mesopores,besides,the anthracite coal sample in Jincheng Sihe mine,matrix pore is macropore.In the same resolution,the pore number of the lignite is more than anthracite,so we believe that in the anthracite,there is more ultramicropore and micropore beyond the atomic force microscope observation range.(3)Isothermal adsorption experiments for the typical low rank coal samples from in southeastern Yunnan were carried out using high pressure volumetric method and the influences of temperature and moisture on adsorption were elucidated.Experiment result shows,the methane adsorption capacity is much lower for the low rank coal samples than for the medium and high rank coals.Under the condition of water equilibrium and 25? ~ 27?,the Langmuir volume of lignite coal sample in southeastern Yunnan is 2.15 m3/t~4.61 m3/t,the Langmuir pressure is 3.17 MPa~7.67 MPa.And for the medium rank coal sample in Xishan coal mining area under the same condition,the Langmuir volume is 25.37 m3/t and the Langmuir pressure is 1.91 MPa.Under the condition of temperature approximation,the adsorption capacity is higher for dry coal sample than for the equilibrium water samples.Experiment results show that,with the rise of temperature,the adsorption capacity of equilibrium water samples rises slightly,there are maybe two reasons: 1)with the temperature increment,the water loss rate increases,the free space moisture content increases,and then the actual water content of coal matrix decreases,the sites occupied by moisture molecules decrease and therefore the methane adsorption capacity increases slightly;2)there may be an experimental error,because the temperature is not a constant value during the process of experiments,it fluctuates over time.The temperature fluctuation will affect the adsorption experimental results,low rank coal sample is more sensitive to the temperature fluctuation than the middle rank coal.So the lower the adsorption capacity is,the more obvious the temperature fluctuation is,and the greater the experimental result error is.The Langmuir pressure is relatively high in southeastern Yunnan and it is higher for the low rank coals than for the medium rank coal,indicating that in the process of water drainage,the gas in low rank coal can be relatively easily desorbed.With the increase of temperature,the Langmuir pressure of low rank coal rises,which indicates that the methane can be desorbed easily,and the adsorption capacity increases.The influence of temperature variation on the adsorption capacity is relatively small,and it has a great impact on Langmuir pressure.(4)The calculation model of diffusion coefficient of low-grade coal gas is established,and a single-pore model describing the initial adsorption and diffusion behavior of low-order coal is proposed.Based on the real-time data record in isothermal adsorption experiments,the methane diffusion coefficient for the low rank coal samples were estimated using a unipore diffusion modelling and the differences in diffusion behaviors for different coal rank samples were investigated;and the measured data and fitted data were compared to reveal the effects of temperature,moisture and adsorption capacity on the diffusion behavior of methane The results shows that the diffusion coefficient of methane is much greater in low rank coal than in medium rank coal.The diffusion coefficient of a typical low rank coal sample in southeastern Yunnan is 4.4E-13~2.63E-12 m2/s.And for the coal particles with a size of 0.20mm~0.25 mm in diameter,the adsorption time constant is 320.76s~1917.85 s,the bigger the grain size is,the more complex the diffusion path is,the greater the adsorption time constant is.The methane adsorption capacity of low rank coal is related to the diffusion coefficient,the higher the adsorption capacity is,and the higher the diffusion coefficient is.The unipore diffusion model can characterize the methane diffusion behavior in low rank coals in the study area and for a medium rank coal sample,this model can describe the initial stage of methane diffusion and will overestimate the diffusion behavior,that is,as the time goes,the diffusion coefficient value estimated using the unipore model is higher than the actual value,the actual diffusion coefficient decreases over time.The results of diffusion simulation can be easily affected by the variation of temperature and this effect is very obvious for the coal samples with a low adsorption capacity.Both the temperature and moisture affect the diffusion coefficient of low rank coal.The coal samples have a low adsorption capacity,the influence of temperature and moisture on diffusion is more obvious.The higher the temperature is,the higher the diffusion coefficient is.The presence of moisture hinders the methane diffusion,reduces the diffusion coefficient of low rank coal.This diffusion modelling indicates that,for low coal rank coals,the diffusion coefficient based on a unipore model is higher for dry samples than for the equilibrium water samples,which indicates that the effect of moisture on the diffusion coefficient is stronger than that of temperature.(5)Based on the numerical simulation of gas production characteristics in low coal rank gas wells,the effects of gas saturation,coal thickness,drainage control area,temperature,permeability and shear compression ratio on gas production potential of coalbed methane wells were analyzed,and reveals the rapid diffusion efficiency and desorption characteristics of low coal rank coalbed methane...For adsorption-saturated CBM reservoir,the gas production is sensitive to the area of the depletion.The gas rate,cumulative gas and water production rate are apparently higher for a gas well with a large depletion area than for a gas well with a small depletion area,while the peak gas rates are not obviously different for the two type wells.The peak gas rate is slightly higher for the gas well with a large draining area than for a gas well with a small drainage area.However the gas rate following the peak value is slightly lower for the former than for the latter.For the adsorption saturated coal seam,the production law of CBM wells is not sensitive to liquid level drop rate.The gas rate and cumulative gas production are slightly higher for a gas well with a high drop rate of liquid level than for a gas well with a low drop rate of liquid level,while the water production rate is slightly lower for the former than for the latter.At the same formation pressure,if the adsorption saturation is high,then the gas rate and cumulative gas production will be relatively high and water rate will be relatively low.However,for an unsaturated CBM reservoir,even with the similar gas content,due to a relatively long term drainage,the time to a peak gas rate will be later and the peak value of gas production rate will be lower,which is not conducive to gas depletion.It is necessary to take reservoir enhancement measures for an under saturation CBM reservoir.For a coal seam with under-saturated gas adsorption,the effect of coal seam thickness on gas production is significant.The thick coal seam is conducive to gas production.The effect of temperature on the gas and water production varies with the gas saturation of the coal seam.The peak time of gas production rate with gas adsorption saturation of less than 1 is generally later than that of coal seam with adsorption saturation of 1,and the water production rate of the former is much higher than the latter.The gas and water production in coal seam with gas saturation below 1 is more significantly affected by temperature.Permeability has a significant effect on the law of gas production in low-rank coal seams.The gas production rate is sensitive to the cleat compression coefficient.If the cleat compression coefficient is low,the peak of gas production rate will appear early,and the accumulative gas production is high in the same time,which is beneficial to a good economic benefit.A cleat compression coefficient will induce a high water production rate.The effect of desorption time constant on production is not significant for the low-rank coal usually with a high permeability and a high diffusion coefficient is conducive to getting a better gas production.For under-saturated low rank coal seam,fracturing could significantly enhance the depletion effect and achieve a satisfactory economic benefits. |
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