低渗致密砂岩纳米-微米尺度孔喉发育,微观非均质性强,渗流机理复杂,现有物理实验方法难以准确评价.采用基于数字岩心理论的微观流动模拟方法来分析低渗致密砂岩中天然气的渗流规律.以川西地区沙溪庙组砂岩样品为基础,利用CT扫描建立目标岩样三维数字岩心,通过孔喉识别、形态简化及喉道调整,构建了包含不同孔喉分布的低渗致密砂岩三维孔隙模型;考虑天然气在纳米-微米孔喉中输运机制的复杂性,建立了耦合Darcy渗流、气体滑脱和Knudsen扩散的流动控制方程,并结合孔隙模型对其进行离散求解.开展流动模拟,评价不同孔隙压力下各输运机制对天然气产出运移的影响.结果表明:低渗致密砂岩中喉道越小、孔隙压力越低,气体流动非线性越强;在孔隙压力高于10 MPa范围,天然气在平均喉道半径大于0.1μm的砂岩孔隙空间中的流动基本满足Darcy渗流;而对于平均喉道半径小于0.1μm的致密砂岩,孔隙压力低于10 MPa后气体滑脱和Knudsen扩散是气体运移中重要机制,对天然气生产的影响不可忽略.%The strong heterogeneity and complicated pore structure of low-permeable tight sandstone with nano-and micro-scale pore-throats could not be accurately covered by using current core testing methods.Micro-scale flowing simulation based on digital core theories was then introduced to study the gas flowing in such stones.Samples taken from the Shaxim-iao Formation in Western Sichuan Basin were scanned with CT to build up three-dimensional (3D) digital cores.Methods such as pore-throat identification,pore topology simplification and throat adjustment,were also resorted to establish 3D pore network models with various throat distribution patterns.Considering the complexities of gas flow in such nano-and micro-scale pore-throats,we also established a multi-mechanism flow equation that takes Darcy flow,gas slippage effect and Knudsen diffusion into consideration,and tried to solve discretely the equation by combining it with the pore network models.Numerical simulations were then carried out to study the influence of various transport mechanisms upon gas migration under different pressures.Results indicate that smaller pore-throat size and lower pore pressure mean a stronger non-linear gas flow.Gas flow was observed to basically follow the Darcy' s law when pore pressure higher than 10MPa and average throat radius larger than 0.1 μm.While in samples with average pore-throat radius less than 0.1 μm and pore pressure less than 10 MPa,gas flow was dominated by gas slippage and Knudsen diffusion,a phenomenon worthy of considering during gas production.
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