页岩储层纳米孔气体传输耦合模型新研究

Coupling Model for Nanopore Gas Transport in Shale Reservoirs

页岩气在纳米孔隙的传输过程中受多种因素影响,包括孔隙尺寸和压力、孔隙壁面粗糙度、孔隙力学反应、吸附-诱导膨胀反应以及权重因子等。因此需要综合考虑以上因素以及吸附气分子在孔隙中所占空间对气体流动影响的条件下,厘清页岩气的不同运移机制(表面扩散、滑脱流、Knudsen扩散和黏性流动)在不同孔隙尺寸和压力下对纳米孔中总气体流量的贡献率。首先,对页岩气的不同运移方式进行了物理描述及数学表征,然后,在考虑孔隙壁面粗糙度、孔隙力学反应、吸附-诱导膨胀反应和权重因子等因素的条件下,建立页岩气在储层纳米孔中的气体传输耦合数学模型,模型可靠性通过格子Boltzmann方法计算结果验证。研究结果表明,当孔径小于10 nm时,纳米孔的总流量主要由表面扩散流量组成,孔径越小,表面扩散流量越大;当孔径为40～250 nm和低压条件下,滑脱流和Knudsen扩散对气体传输影响较大;当孔径大于10μm时,纳米孔的总流量主要为黏性流量。

Shale gas is affected by many factors during nanopore transport,including pore size and pressure,pore wall surface roughness,pore mechanics reaction,adsorption-induced expansion reaction,and weighting factors.Therefore,the effects that these factors and the space occupied by the adsorbed gas molecules in the pores have on the gas flow must be considered.This is necessary to clarify the contribution to the total gas flow in the nanopores resulting from different migration mechanisms of shale gas(surface diffusion,slip flow,Knudsen diffusion,and viscous flow)based on different pore sizes and pressures.First,physical descriptions and mathematical characterizations of different migration mechanisms of shale gas are provided.A mathematical gas transport coupling model for shale gas is then developed that considers pore wall surface roughness,pore mechanics reaction,adsorption-induced expansion reaction,and weighting factors.The reliability of the model is verified by the lattice Boltzmann method.The results show that when the pore diameter is less than 10 nm,the total flow in the nanopores mainly consists of surface diffusion flux.In addition,the smaller the pore size,the greater is the surface diffusion flux.When the pore diameter is 40~250 nm at low pressure,the slip flow and Knudsen diffusion have a considerable effect on gas transport.When the pore diameter is longer than 10μm,the total flow in the nanopores is primarily viscous.