纳米孔隙中页岩气扩散模拟实验和数学模型分析

Simulation experiment and mathematical model analysis for shale gas diffusion in nano-scale pores

深层页岩气在纳米孔隙中的扩散行为分为体相扩散(Fick和Knudsen扩散)和表面扩散。为了定量评价温度、压力等对扩散系数的影响,揭示深层页岩气的保存机理,以南方鄂西秭归茅坪地区寒武系牛蹄塘组页岩为实验对象,在不同温压条件下,通过等压扩散实验对纳米孔隙甲烷扩散进行实验模拟。结果表明:(1)扩散系数DF随压力增大而减小(当压力大于30 MPa时,DF趋于平稳),随温度升高而增大;(2)在高温高压环境下,DF受压力影响更大,总体趋于减小。随后,定量考虑了温度、压力、孔隙及岩性特征对各种扩散行为的影响,建立了数学模型。该模型与模拟实验结果相似,可以相互验证:(1)温度升高促使分子动能增大,导致体相和表面扩散系数都增大,而压力增大虽然会使Fick扩散和表面扩散作用稍微加强,但会显著限制Knudsen扩散并最终导致总扩散作用降低;(2)孔径增大加强了体相扩散作用,削弱了表面扩散作用。最后,结合具体研究区块,认为深层高压环境有利于页岩纳米孔隙气藏的保存,而地层抬升释放压力的过程是页岩气散失的主要阶段。

Gas diffusion in nano-scale porous media of deeply burried shale includes bulk diffusion(Fick and Knudsen diffusions)and surface diffusion.To reveal the migration mechanism of this process,the influence of temperature and pressure on diffusion coefficient needs to be quantitatively evaluated.A case study was made with the Cambrian Niutitang Formation in the Maoping area,Zigui,western Hubei,South China.Gas diffusion was simulated by isobaric diffusion experiments under different temperature and pressure conditions.The results indicated that:(1)The diffusion coefficient DF decreases with increasing of pressure(when the pressure is higher than 30 MPa,DF tends to be constant),and increases with increasing of temperature;(2)In the high temperaturepressure setting,DF is affected significantly by pressure and generally tends to decrease.Moreover,the impacts of temperature,pressure,porosity and lithology were quantitatively calculated,and a mathematical model of gas diffusion was established,which had comparable results with simulation experiment.The following conclusions were thus drawn:(1)Higher temperature will cause stronger molecular kinetic energy,resulting in increasing bulk and surface diffusion coefficients,while higher pressure will slightly strengthen the Fick and surface diffusions,but significantly limit the Knudsen diffusion,and cause lower total diffusion coefficient;(2)Larger pore size leads to stronger bulk diffusion,but weaker surface diffusion.Finally,according to the studies of a specific research block,high pressure setting is conducive to the preservation of nano-scale porous gas reservoir in shale,while the uplift accompanied by pressure release is the main stage of shale gas loss.