基于分形理论的煤岩孔裂隙结构特征与渗透特性研究

【作者】 王斌；

【导师】 李波波；

【作者基本信息】 贵州大学 ， 矿业工程， 2021， 硕士

【摘要】 石油、天然气、煤炭是世界上主要的能源资源,随着经济和人口逐年增长不仅造成了能源的大规模消耗,而且加剧了环境污染。煤层中通常含有大量的甲烷,可以以煤层气的形式被回收,开采非常规储层条件下的煤层气已成为重要的研究课题。渗透率是表征煤层气开采效率的重要参数,开采过程中渗透率变化受孔裂隙结构、孔隙压力、温度以及滑脱效应等的影响。因此,结合分形理论研究煤岩孔裂隙结构特征,在应力、温度综合作用下揭示煤岩变形与渗透性演化机理对矿井灾害防治和煤层气注气开采具有重要意义。本文煤岩样品取自山西晋城赵庄矿3~#煤层,利用HCA型高压容量法吸附装置和含瓦斯煤三轴渗流装置,进行了不同温度下等温吸附试验及不同温度、应力的三轴渗流试验。借助分形理论定量表征了温度引起的煤岩孔裂隙扩展和滑脱因子变化情况,构建了温度作用下考虑过剩吸附量修正的分形Langmuir模型。考虑应力作用下毛细管分形特征,建立裂隙体积应力敏感性模型,进一步了构建考虑温度、煤基质内部膨胀变形、气体压力变化的煤岩渗透率模型。主要取得以下成果:1)进行了煤岩等温吸附试验和低温液氮吸附试验,建立了不同温度下考虑煤岩非均质性及过剩吸附的分形吸附模型,探讨了煤岩吸附的不均匀性。随分形维数D的增大,煤岩吸附气体能力、速率均增强。并利用新建分形吸附模型计算煤岩吸附变形量,进一步构建考虑滑脱效应影响的煤岩分形渗透率模型,为大力推进煤层气抽采提供理论依据。2)开展了不同温度、应力下的三轴渗流试验,基于分形理论表征温度引起的煤岩孔裂隙扩展和滑脱因子变化情况,进一步考虑压缩变形及滑脱效应对煤岩渗透率的影响,建立应力与温度综合作用的煤岩分形渗透率模型,可以较好表征复杂煤储层的气体渗流规律。3)考虑应力作用下毛细管分形特征,建立了裂隙体积应力敏感性模型,并在此基础上建立考虑煤基质内部膨胀变形、温度及气体压力变化的煤岩渗透率模型,基于所建立的模型分析吸附解吸、压缩系数、滑脱效应与煤岩渗透率的协同关系,揭示应力和温度综合作用下煤岩孔裂隙特征变化与渗透性的调控机理,为煤层瓦斯抽采奠定理论基础。更多还原

【Abstract】 Oil,natural gas,and coal are the main energy resources in the world.The annual growth of the economy and population has not only caused large-scale energy consumption,but also aggravated environmental pollution.Coal usually contain a large amount of methane,which can be recovered in the form of coalbed methane.The exploitation of coalbed methane under unconventional reservoir conditions has become an important research topic.Permeability is an important parameter that characterizes the efficiency of coalbed methane recovery.The change of permeability during the mining process is affected by the pore and fissure structure,pore pressure,temperature,and slippage effect.Therefore,combining fractal theory to study the characteristics of coal and rock pore and fissure structure,and revealing the evolution mechanism of coal deformation and permeability under the combined effects of stress and temperature is of great significance for mine disaster prevention and coalbed methane gas injection.The coal samples in this paper were taken from the 3# coal seam of Zhaozhuang Mine,Jincheng,Shanxi.Using the HCA type high pressure capacity method adsorption device and the gas-containing coal triaxial seepage device,the isothermal adsorption test at different temperatures and the triaxial seepage test at different temperatures and stresses were carried out.With the help of fractal theory,the temperature-induced coal pore crack expansion and the change of slippage factor are quantitatively characterized.And a fractal Langmuir model modified by considering the excess adsorption capacity under the effect of temperature is constructed.Considering the fractal characteristics of capillary tube under stress,a fracture volume stress sensitivity model is established.And a coal permeability model considering the expansion and deformation of the coal matrix,temperature and gas pressure changes is further constructed.The main results are as follows:1)Carried out coal isotherm adsorption test and cryogenic liquid nitrogen adsorption test.A fractal adsorption model considering coal heterogeneity and excess adsorption at different temperatures was established.Discussed the inhomogeneity of coal adsorption.With the increase of the fractal dimension D,the gas adsorption capacity and rate of coal are enhanced.The newly-built fractal adsorption model is used to calculate the coal adsorption deformation,and the coal fractal permeability model considering the effect of slippage effect is further constructed.Provide theoretical basis for vigorously promoting coalbed methane extraction.2)Triaxial seepage tests under different temperature and stress conditions were carried out,based on fractal theory to characterize the temperature-induced crack expansion and slippage factor changes in coal.Further considering the influence of compression deformation and slippage effect on coal permeability,a coal fractal permeability model with the combined effect of stress and temperature is established.It can better characterize the gas seepage laws of complex coal reservoirs.3)Considering the fractal characteristics of capillary under stress,a fracture volume stress sensitivity model is established.On this basis,a coal permeability model considering the expansion and deformation of the coal matrix,temperature and gas pressure changes is established.Based on the established model,the synergistic relationship between adsorption and desorption,compression coefficient,slippage effect and coal permeability is analyzed.It reveals the regulation mechanism of coal pore characteristics and permeability under the combined action of stress and temperature,and lays a theoretical foundation for coal seam gas drainage.更多还原