实际状态下的页岩气表观渗透率计算新模型

摘要

In all the existing apparent permeability calculation models used for shale gas,the effect of surface diffusion of adsorbed gas is neglected and shale gas is assumed to be in an ideal state,so the calculation results may not reflect the real situations.In this paper,a new apparent permeability model suitable for the shale gas of real state was developed by seepage mechanics method.In this model,the influential factors (e.g.the viscous flow and Knudsen diffusion of free gas and the surface diffusion of adsorbed gas) are taken into account to present the real flowing situations of shale gas in nanopores.Then,the accuracy of this new apparent permeability model was verified by comparing its calculation results with the experimental data.Finally,all factors influencing the apparent permeability of shale gas were analyzed in this new model.It is shown that the apparent permeability of shale gas is most affected by pressure and pore radius,and less by relative molecular mass and blockage coefficient.Langmuir maximum adsorption capacity,Langmuir pressure and isosteric adsorption heat mainly affect the fraction of permeability contributed by surface diffusion.Besides,the effects of each factor on apparent permeability and permeability fraction under low pressure are different from those under high pressure.Under low pressure,the effects of temperature and pore radius on apparent permeability are more obvious,and the effects of temperature,pore radius,Langmuir maximum adsorption capacity,Langmuir pressure and isosteric adsorption heat on the fraction of permeability are also more obvious.Finally,when the pressure is low and the pore radius is small,the surface diffusion is dominant.When the pressure is high and the pore radius is large,the viscous flow is dominant.In the case of small pore radius or low pressure,the surface diffusion shall not be neglected.%现有的页岩气表观渗透率计算模型均假设页岩气为理想状态,未考虑吸附气表面扩散等的影响,因而有可能与实际状态下的结果存在着差异.为此,针对实际状态下页岩气在纳米孔隙中的渗流情况,考虑了游离气的黏滞流、Knudsen扩散以及吸附气的表面扩散等影响因素,通过渗流力学方法,推导出了一种适用于实际状态的页岩气表观渗透率计算新模型;通过与实验测量结果的对比,验证了新模型的准确性,并利用所建模型分析了影响页岩气表观渗透率的各种因素.研究结果表明:①压力和孔隙半径对页岩气表观渗透率的影响最大,相对分子质量及阻塞系数对其的影响较小,Langmuir最大吸附量、Langmuir压力以及等量吸附热主要影响表面扩散渗透率比重;②在低压和高压条件下,各因素对表观渗透率及各渗透率比重的影响趋势存在着差异,低压下温度及孔隙半径对表观渗透率的影响更明显,同时温度、孔隙半径、Langmuir最大吸附量、Langmuir压力、等量吸附热等因素对各渗透率比重的影响也更明显;③压力较小、孔隙半径较小时,表面扩散占主要地位,压力较大、孔隙半径较大时,黏滞流占主要地位,小孔隙半径或低压条件下,表面扩散现象不可忽略.