Physical simulation of hydrodynamic conditions in high rank coalbed methane reservoir formation

In order to select highly productive and enriched areas of high rank coalbed methane reservoirs,based on hydrologic geology as one of the main factors controlling coalbed methane(CBM) reservoir formations,the effect of hydrodynamic forces controlling CBM reservoir formations was studied by a physical simulation experiment in which we used CBM reservoir simulation facilities.The hydrodynamic conditions of high coal rank reservoirs in the Qinshui basin were analyzed.Our experiment shows the following results:under strong hydrodynamic alternating action,δC1 of coalbed methane reservoir changed from the start at -2.95%～-3.66%,and the lightening process occurred in phases;the CH4 volume reduced from 96.35% to 12.42%;the CO2 volume decreased from 0.75% in sample 1 to 0.68% in sample 2,then rose to 1.13% in sample 3;the N2 volume changed from 2.9% in sample 1 to 86.45% in sample 3.On one hand,these changes show the complexity of CBM reservoir formation;on the other hand,they indicate that strong hydrodynamic actions have an unfavorable impact on CBM reservoir formation.It was found that the gas volume and hydrodynamic intensity were negatively correlated and low hydrodynamic flow conditions might result in highly productive and enriched areas of high rank CBM.

Structure and production fluid flow pattern of post-fracturing high-rank coal reservoir in Southern Qinshui Basin

Field geological work, field engineering monitoring, laboratory experiments and numerical simulation were used to study the development characteristics of pore-fracture system and hydraulic fracture of No.3 coal reservoir in Southern Qinshui Basin. Flow patterns of methane and water in pore-fracture system and hydraulic fracture were discussed by using limit method and average method. Based on the structure model and flow pattern of post-fracturing high-rank coal reservoir, flow patterns of methane and water were established. Results show that seepage pattern of methane in pore-fracture system is linked with pore diameter, fracture width, coal bed pressure and flow velocity. While in hydraulic fracture, it is controlled by fracture height, pressure and flow velocity. Seepage pattern of water in pore-fracture system is linked with pore diameter, fracture width and flow velocity. While in hydraulic fracture, it is controlled by fracture height and flow velocity. Pores and fractures in different sizes are linked up by ultramicroscopic fissures, micro-fissures and hydraulic fracture. In post-fracturing high-rank coal reservoir, methane has level-three flow and gets through triple medium to the wellbore; and water passes mainly through double medium to the wellbore which is level-two flow.

Research on Permeability of Multiphase Medium of Middle to High-Rank Coals

The permeability of coal of middle to high ranks were tested using He,CH 4 and H 2O in single phase medium and using CH 4 and H 2O in double phase medium. The relation between adsorption and permeability of those media was discussed, and the seepage flow characteristics of methane-water medium in coals were analyzed. The result shows that the coalbed methane resource of high-rank coal reservoirs in China is still recoverable.

Influence of depressurization rate on gas production capacity of high-rank coal in the south of Qinshui Basin, China

A desorption simulation experiment with the condition of simulated strata was designed. The experiment, under different depressurizing rates and the same fluid saturation, was conducted on the sample from 3# coal of Daning coal mine in Jincheng, Shanxi Province. The gas production rate and pressure change at both ends of the sample were studied systematically, and the mechanisms of some phenomena in the experiment were discussed. The experimental results show that, whether at fast or slow depressurizing rate, the methane adsorbed to high-rank coal can effectively desorb and the desorption efficiency can reach above 90%. There is an obvious inflection point on the gas yield curve during the desorption process and it appears after the pressure on the lump of coal reduces below the desorption pressure. The desorption of methane from high-rank coal is mainly driven by differential pressure, and high pressure difference is conducive to fast desorption. In the scenario of fast depressurization, the desorption inflection appears earlier and the gas production rate in the stage of rapid desorption is higher. It is experimentally concluded that the originally recognized strategy of long-term slow CBM production is doubtful and the economic benefit of CBM exploitation from high-rank coal can be effectively improved by rapid drainage and pressure reduction. The field experiment results in pilot blocks of Fanzhuang and Zhengzhuang show that by increasing the drainage depressurization rate, the peak production of gas well would increase greatly, the time of gas well to reach the economic production shortened, the average time for a gas well to reach expected production reduced by half, and the peak gas production is higher.

Resistivity response to the porosity and permeability of low rank coal

Laojunmiao coal samples from the eastern Junggar basin were studied to understand the relationship between coal resistivity and the physical parameters of coal reservoirs under high temperatures and pressures.Specifically,we analysed the relationship of coal resistivity to porosity and permeability via heating and pressurization experiments.The results indicated that coal resistivity decreases exponentially with increasing pressure.Increasing the temperature decreases the resistivity.The sensitivity of coal resistivity to the confining pressure is worse when the temperature is higher.The resistivity of dry coal samples was linearly related to φ~m.Increasing the temperature decreased the cementation exponent(m).Increasing the confining pressure exponentially decreases the porosity.Decreasing the pressure increases the resistivity and porosity for a constant temperature.Increasing the temperature yields a quadratic relationship between the resistivity and permeability for a constant confining pressure.Based on the Archie formula,we obtained the coupling relationship between coal resistivity and permeability for Laojunmiao coal samples at different temperatures and confining pressures.

鄂尔多斯盆地石炭系本溪组煤岩气地质特征与勘探突破

通过对鄂尔多斯盆地石炭系本溪组煤岩分布、煤岩储层特征、煤质特征、煤岩气特征以及煤岩气资源和富集规律等方面开展系统研究,评价其勘探潜力。研究表明:①煤岩气是有别于煤层气的优质天然气资源,在埋深、气源、储层、含气性、碳同位素组成等方面具有独特特征;②本溪组煤岩分布面积达16×10^(4)km^(2),厚度2~25m,以原生结构的光亮和半亮煤为主,挥发分和灰分含量低,煤质好;③中高阶煤岩TOC值为33.49%~86.11%,平均值为75.16%,演化程度高(Ro为1.2%~2.8%),生气能力强,气体稳定碳同位素值高(δ^(13)C_(1)值为-37.6‰~-16.0‰,δ^(13)C_(2)值为-21.7‰~-14.3‰);④深层煤岩发育气孔、有机质孔和无机矿物孔等基质孔隙,与割理、裂缝共同构成良好储集空间,储层孔隙度为0.54%~10.67%,平均值为5.42%,渗透率为(0.001~14.600)×10^(-3)μm^(2),平均值为2.32×10^(-3)μm^(2);⑤纵向上发育5种煤岩气聚散组合,其中煤岩-泥岩聚气组合与煤岩-灰岩聚气组合最为重要,封闭条件好,录井全烃气测峰值高;⑥构建了广覆式分布的中高阶煤岩持续生气、煤岩基质孔和割理裂缝规模储集、源-储一体赋存、致密岩盖层密闭封堵的煤岩气富集模式,存在煤岩侧向尖灭体、透镜体、低幅度构造、鼻状构造和岩性自封闭5种高效聚气类型。⑦依据煤岩气地质特征评价划分出8个区带,估算埋深超过2000m的煤岩气资源量超过12.33×10^(12)m^(3)。上述认识指导风险勘探部署,两口井实施后分别获得工业气流,推动进一步部署预探井和评价井,获得规模突破,提交超万亿方预测储量和超千亿方探明储量,对中国天然气效益增储和高效开发具有重大意义。

高、低阶煤孔隙结构差异性及其对甲烷吸附特性的影响研究

采集沁水盆地高阶煤、二连盆地低阶煤,采用氮气、二氧化碳吸附法对煤基质纳米级吸附孔隙进行测试,分析高低阶煤的储气特征,从而确定影响高、低阶煤煤层气开发的关键因素。研究结果发现,高、低阶煤的比表面积相似,高阶煤的比表面积均值为215.33 m~2/g,可吸附甲烷49.53 cm~3/g;低阶煤的比表面积均值为212.17 m~2/g,可吸附甲烷48.96 cm~3/g。高、低阶煤储气主要为吸附气,占总储气量的99%以上,高阶煤储气主要是微孔吸附气,占总储气量的99.71%;低阶煤不同孔隙均有贡献,微孔最多,占比47.33%,其次是中孔,占比32.644%,大孔最少,占比19.69%。地层水侵入对低阶煤储气量影响大于高阶煤,平衡水条件下,高阶煤可储气量43.46 cm~3/g,低阶煤可储气量33.89 cm~3/g;受微孔特性影响,高阶煤储层储气量较高,但易储难逸,因此煤层气开采的关键问题是疏导微孔吸附气;受中、大孔及亲水性强影响,低阶煤储气量较低,且易储易散,勘察储气聚集区是低阶煤煤层气开发的首要工作。

Structure of different types of coal metamorphism by HTEM

In order to discuss the effect of tectonic stress on the structural evolution of coal, given the importance attached to High-resolution Transmission Electron Microscopy (HTEM), we investigated several aspects of material structures of high-rank Carboniferous period coal, located in the northern foreland basin of the Dabie orogenic belt in eastern China. High powered crystal lattice images of Bright Fields (BF) and Selected Area Diffraction patterns (SAD) of different types of metamorphism in coal were obtained. The results show that the Basic Structural Units (BSU) become increasingly more compact as a function of rising tem-perature and pressure. Under pressure, the local orientation of molecules is strengthened, the arrangement of BSU speeds up and the degree of order is clearly enhanced.

Volume and Surface Distribution Heterogeneity of Nano-pore in Coal Samples by CO2 and N2 Adsorption Experiments

In this paper,the heterogeneity of adsorption pores in middle and high rank coal samples were analyzed by using low temperature N2 and CO2 adsorption technology and fractal theory.The following results were achieved.1)According to the results of volume and surface fractal dimension,meso-pores can be classified into Mep-1,Mep-2,and Mep-3,respectively.Micro-pore can be classified into Mip-1,Mip-2,and Mip-3,respectively.2)Pore types play an important role in affecting the heterogeneity of meso-pores.The volume heterogeneity(VHY)of Mep-1 is simpler than that of Mep-2 and Mep-3 in type A samples.However,the VHY of Mep-1 becomes gradually larger than that of Mep-2 and Mep-3 from type A to type B and C.The VHY of open pore in the same diameter is higher than that of semi-open or closed pore.Meanwhile,the surface heterogeneity(SHY)of types A and B samples is significantly larger than that of type C,the SHY of semi-open or closed pores is more complicated than that of open pores.3)Coal rank mainly affects the heterogeneity of micro-pores.The heterogeneity of type A is always smaller than that of type B and C.The VHY of Mip-1 is more complicated than that of Mip-2 and Mip-3 in the same samples,and the sensitivity of the VHY of Mip-1 and Mip-2 to the degree of coal rank is smaller than that of Mip-3.Meanwhile,the SHY of Mip-1 and Mip-2 is simpler than that of Mip-3 in the same sample,the SHY of micro-pores remains stable as the pore size decreases,and the affect of coalification level on SHY decreases with the decrease in pore diameter.Full-scale fractal characterization has enabled quantitative characterization of adsorption pore properties and provided useful information with regards to the similarity of pore features in different coal reservoirs.

高煤阶煤样水力压裂前后应力−渗透率试验研究

煤储层水力压裂是提高煤层气井产量的关键技术,水力压裂前后煤储层渗透率的变化反映了煤储层压裂改造效果。采用山西晋城矿区寺河煤矿二叠系山西组3号煤层高煤阶试样,通过柱型煤样水力压裂前后渗透率试验对比分析,测试了4个高煤阶煤样水力压裂前后渗透率分布特征,揭示了水力压裂前后煤样渗透率随应力的变化规律和控制机理。结果表明,在围压和轴压恒定条件下,煤样孔隙压力随注入压力增大逐渐增高;当注入压力大于破裂压力时,煤样发生破裂,煤样的破裂压力随围压的增加呈线性增大的规律。在注入压力相同的情况下,随着围压和轴压的增大,有效应力增高,水力压裂前后煤样渗透率随有效应力的增大呈指数函数关系减小,且压裂后的渗透率要明显大于压裂前的渗透率。采用煤储层渗透率改善率来评价围压下煤样水力压裂增渗效果,4个试验煤样渗透率改善率随有效应力的增高呈指数函数关系增大,但随围压的增大,渗透率改善率提升幅度逐渐降低。煤储层的渗透性主要取决于煤中裂隙发育程度和裂隙开度的大小,裂缝储层的渗透率的大小与裂缝张开度的3次方成正比例关系。水力压裂后裂缝平均长度、裂缝孔隙度和裂缝开度增幅分别为70.81%~253.25%、171.88%~383.02%和20.31%~32.43%,且水力压裂前后煤样裂缝开度均随有效应力的增高而呈负指数函数规律降低。试验结果与实测水力压裂前后煤储层渗透率对比,两者具有较好的一致性,为煤储层水力压裂改造效果评价探索了有效途径。

高阶煤酸化增透效果影响因素及机制研究

为使低透气性高阶煤达到最佳的酸化增透效果,以九里山矿无烟煤为研究对象,利用单因素法研究了酸化效果的主要影响因素,采用正交试验法优选了酸化增透试验参数;使用搭配能谱仪的扫描电子显微镜研究了酸化对煤表面微观孔隙结构及元素分布的影响;通过工业分析并借助X射线衍射仪研究了酸化对煤中矿物组分及含量的影响。结果表明:酸化效果随HCl质量分数的增加呈先增大后减小的变化规律,随HF质量分数的增加单调增加,在总质量分数相同条件下,混合酸液的酸化效果优于单组分酸的酸化效果;酸液对煤中矿物的有效溶解,使煤样表面出现许多溶蚀孔及裂隙,且溶蚀孔之间及孔隙-裂隙之间相互贯通形成大孔隙和裂缝,改善了煤中孔隙的连通性。

超临界CO_(2)对高阶构造煤微观结构的影响机制

为揭示深部煤层注入CO_(2)过程中,超临界CO_(2)(SC-CO_(2))对构造煤微观结构的影响规律,以山西大平矿贫煤和贵州振兴矿无烟煤样品为研究对象,对两类不同破坏程度的高阶煤进行萃取实验,比较了萃取前后煤中孔隙结构和芳香碳网结构参数的差异,探讨了SC-CO_(2)对不同破坏程度的高阶煤微观结构的影响机制。研究表明:(1)随着破坏程度增加,两类高阶煤的孔容、孔比表面积增大,孔隙连通性变好。孔径>4 nm时,构造煤分形维数增大,孔隙结构更加复杂。(2)SC-CO_(2)作用下,两类煤的平均孔径明显增大,孔隙分形维数减小,孔隙结构趋于简单化。而不同破坏程度煤的孔容、比表面积、孔隙连通性呈现复杂的变化,表现为SC-CO_(2)作用后,大平矿的构造煤微孔、介孔段比表面积和孔容显著减小,使得总比表面积和总孔容亦明显减小,孔隙连通性变差;振兴矿构造煤的介孔比表面积和孔容显著增大,使得总比表面积和总孔容增加。(3)SC-CO_(2)萃取作用下,不同破坏程度的高阶煤的碳网层间距d002增大,而延展度La、堆砌度Lc和芳香层数Nc呈减小趋势,且随着破坏程度的增加相应的变化趋于显著,促使不同破坏程度煤的碳网层参数趋于一致。研究认为,SC-CO_(2)与煤岩反应产物溶离煤体引起的扩孔效应,促使煤样平均孔径增大,而产物生成量不同和产物滞留造成的堵孔效应,是引起两类不同破坏程度的高阶煤孔隙差异性变化的主要原因;SC-CO_(2)流体对两类高阶煤的芳香碳网层结构参数的影响与构造破坏作用相反,前者表现为疏松作用,而后者主要为压密作用。

微生物增产煤层气作用机理研究

通过向煤层产出液中添加营养剂激活内源菌群,与高阶煤相互作用,开展了煤层内源菌与高阶煤相互作用机理的研究。分别利用气相色谱仪、元素分析仪、傅里叶变换红外光谱仪等仪器设备对微生物作用后气体组分、煤组分、煤样品官能团、煤样品的微晶结构以及煤样品孔隙度进行测定。实验结果表明:反应过程中产生了氮气、甲烷等生物气体;微生物作用后煤样品中的中C、S元素含量平均降低18.99%、20.17%;N、H元素平均含量升高119.76%、70.84%;煤样中芳烃含量下降,醚基、羟基含量上升;微生物作用后煤样品孔隙度平均增加31.4%。

高阶煤润湿过程中水分运移的低场核磁共振试验

为深入了解煤层注水过程中水分在高阶煤煤体中的运移特征,以焦作中马村矿高阶煤为研究对象,利用低场核磁共振(LNMR)设备,测试煤样在负压浸水试验、等温浸润试验过程中不同浸水时间时的核磁谱图,结合煤样负压浸水过程的质量变化,获得制备饱水煤样所需的试验条件;结合煤样等温浸润过程质量变化,从微观层面定量分析煤样润湿过程中水分进入煤体的分布特征、煤样含水率和含水饱和度的变化规律。结果表明:水分在毛细管力作用下,可以浸润到煤体的最小孔径对应的核磁弛豫时间为0.014 ms;水分按照较大孔→小孔→微孔的路径依次进入到煤体中,按照微孔→小孔→较大孔的路径依次达到饱和状态,试验煤样在相同的润湿时间时,虽然含水率不同,但整体含水饱和度几乎一致,且煤样整体含水饱和度与润湿时间的0.5次方之间存在定量关系。

复杂地质条件下煤层气高效开发实践与认识——以沁水盆地郑庄区块为例

我国煤层气整体构造复杂且有诸多地质相似性,随着近年来逐步向中深部、超低渗、构造复杂区域迈进,总结探索一套复杂区域高效开发手段对盘活国内大批煤层气资源意义重大。以早期低产且地质相对复杂的郑庄区块的成功实践为例,针对区块高阶煤断裂构造复杂、非均质性强、渗透率低、早期工程技术匹配欠缺、早期达产率不足30%等不利因素和问题,通过深化煤层气地质富集规律研究和新技术研发与应用,形成了甜点区优选和高效开发理论,应用二开全通径水平井优快钻井、疏导式增产改造、定量化疏导排采控制等系列技术,大幅度提高单井产量,方案达产率由最初的29.3%提升至98.5%。实践研究认为:(1)及时有效的开发方案调整能够实现复杂低渗的高阶煤煤层气高效开发,为技术持续升级提供了空间与平台。(2)丛式井组滚动运行方式可有效降低产建风险,使整体方案达产率提高20%以上。(3)水平井地质–工程一体化技术的持续创新,是实现达产率整体提升的关键。

高剪切调浆强化低阶煤泥浮选及其作用机理

低阶煤泥浮选过程中,由于其表面疏水性差,烃类油捕收剂能否顺利吸附在其表面,对最终的浮选效果具有重要影响。为强化烃类油捕收剂在低阶煤表面的吸附,采用理论计算、分子模拟和试验分析方法研究了高剪切调浆对低阶煤泥浮选的影响规律和强化机理。通过吸附量测试和理论计算研究了正十二烷油滴在煤颗粒表面的吸附规律;借助分子动力学模拟方法构建了正十二烷-水-低阶煤模拟体系,探索了正十二烷与煤表面的初始距离对最终吸附结果的影响;通过开展高剪切调浆浮选试验研究了叶轮转速对浮选精煤产率和灰分的影响规律。结果表明,高剪切调浆可显著提高捕收剂正十二烷在低阶煤表面的吸附量,当叶轮转速超2000 r/min时,吸附量增幅逐渐降低,最终吸附量趋于稳定;正十二烷油滴与煤颗粒之间存在一个能垒,越过能垒后,总势能迅速降低;初始状态下正十二烷油滴与低阶煤表面的距离对最终模拟平衡结果有重要影响,初始距离较近时,油滴可在煤表面吸附并铺展,而初始距离较远时,则无法吸附;高剪切调浆可显著改善低阶煤的浮选效果,调浆叶轮转速为2000 r时,浮选精煤产率增加了近13%,而灰分基本保持不变。

Physical characteristics of high-rank coal reservoirs in different coal-body structures and the mechanism of coalbed methane production

The physical characteristics of coal reservoirs are important for evaluating the potential for gas desorption,diffusion,and seepage during coalbed methane(CBM)production,and influence the performance of CBM wells.Based on data from mercury injection experiments,low-temperature liquid nitrogen adsorption,isothermal adsorption,initial velocity tests of methane diffusion,and gas natural desorption data from a CBM field,herein the physical characteristics of reservoirs of high-rank coals with different coal-body structures are described,including porosity,adsorption/desorption,diffusion,and seepage.Geometric models are constructed for these reservoirs.The modes of diffusion are discussed and a comprehensive diffusion-seepage model is constructed.The following conclusions were obtained.First,the pore distribution of tectonically deformed coal is different from that of normal coal.Compared to normal coal,all types of pore,including micropores(<10 nm),transitional pores(10-100run),mesopores(100-1000 nm),and macropores(>1000 nm),are more abundant in tectonically deformed coal,especially mesopores and macropores.The increase in pore abundance is greater with increasing tectonic deformation of coal;in addition,the pore connectivity is altered.These are the key factors causing differences in other reservoir physical characteristics,such as adsorption/desorption and diffusion in coals with different coal-body structures.Second,normal and cataclastic coals mainly contain micropores.The lack of macropores and its bad connectivity limit gas desorption and diffusion during the early stage of CBM production.However,the good connectivity of micropores is favorable for gas desorption and diffusion in later gas production stage.Thus,because of the slow decline in the rate of gas desorption,long-term gas production can easily be obtained from these reservoirs.Third,under natural conditions the adsorption/desorption properties of granulated and mylonitized coal are good,and the diffusion ability is also enhanced.However,for in situ reservoir conditions,the high dependence of reservoir permeability on stress results in a weak seepage of gas;thus,desorption and diffusion is limited.Fourth,during gas production,the pore range in which transitional diffusion takes place always increases,but that for Fick diffusion decreases.This is a reason for the reduction in diffusion capacity,in which micropores and transitional pores are the primary factors limiting gas diffusion.Finally,the proposed comprehensive model of CBM production under in situ reservoir conditions elucidates the key factors limiting gas production,which is helpful for selection of reservoir stimulation methods.

Jumpy evolution of EPR response of Chinese high-rank coals and its implication to geochemistry

Based upon a series of the coal graphite samples collected from China, the jumpy evolution of the EPR response of the high rank coals was found and its geochemical mechanism was further inquired. The results showed that the makingup, which is activated notably at about 6.0% maximum vitrinite reflectance, might play a principal role in the development of the macromolecular basic structural units in the coals being in the middle to late stage of high rank coalification and that the homologisation of the chemical bonds in the macro molecular structure might provide a geochemical microenvironment for the realization of the makingup.

Triple Medium Physical Model of Post Fracturing High-Rank Coal Reservoir in Southern Qinshui Basin

In this paper, influences on the reservoir permeability, the reservoir architecture and the fluid flow pattern caused by hydraulic fracturing are analyzed. Based on the structure and production fluid flow model of post fracturing high-rank coal reservoir, Warren-Root Model is improved. A new physical model that is more suitable for post fracturing high-rank coal reservoir is established. The results show that the width, the flow conductivity and the permeability of hydraulic fractures are much larger than natural fractures in coal bed reservoir. Hydraulic fracture changes the flow pattern of gas and flow channel to wellbore, thus should be treated as an independent medium. Warrant-Root Model has some limitations and can’t give a comprehensive interpretation of seepage mechanism in post fracturing high-rank coal reservoir. Modified Warrant-Root Model simplifies coal bed reservoir to an ideal system with hydraulic fracture, orthogonal macroscopic fracture and cuboid matrix. Hydraulic fracture is double wing, vertical and symmetric to wellbore. Coal bed reservoir is divided into cuboids by hydraulic fracture and further by macroscopic fractures. Flow behaviors in coal bed reservoir are simplified to three step flows of gas and two step flows of water. The swap mode of methane between coal matrix and macroscopic fractures is pseudo steady fluid channeling. The flow behaviors of methane to wellbore no longer follow Darcy’s Law and are mainly affected by inertia force. The flow pattern of water follows Darcy’s Law. The new physical model is more suitable for post fracturing high-rank coal reservoir.

Experimental Simulation on Dynamic Variation of the Permeability of High-Rank Coal Reservoirs

In terms of the coal reservoir permeability of effective stress, coal matrix shrinkage and gas slippage,we conduct the tests of gas permeability under constant confining pressure and effective stress, as well as illustrate the cumulating method of permeability increment caused by the effects of gas slippage and coal matrix shrinkage.The results show that under the constant confining pressure, gas slippage affecting coal permeability changes to effective stress affecting it mainly. The change point increases with the increase of the confining pressure. The gas slippage effect leads to high permeability under low confining pressure, but coal matrix expansion results in the low value as confining and gas pressures increase. Combined with the drainage process of coalbed methane(CBM)well, the permeability is divided into four change stages based on the above analysis about the three effects, which can improve the change regulation understanding. Four stages are the downward phase under effective stress,the conversion phase of effective stress-coal matrix contraction effect(mainly based on effective stress), the rising stage of the effective stress-coal matrix contraction effect(mainly based on coal matrix contraction effect) and the rising phase of coal matrix contraction-slippage effect(mainly based on slippage effect). Permeability of coal reservoir during the process of drainage and production goes through four stages.