Nanopores and Apparent Permeability of Gas Flow in Mudrocks (Shales and Siltstone)
Nanopores and Apparent Permeability of Gas Flow in Mudrocks (Shales and Siltstone)
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DOI：
10.2118/09-08-16-da
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发表时间：
2009-08-01
影响因子：
--
通讯作者：
Javadpour, F.
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文献类型：
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作者：
Javadpour, F.
产气泥岩系统在北美动荡的能源行业中发挥着重要作用，不久将在欧洲发挥同样重要的作用。泥岩由非常细的颗粒组成，它们的孔隙非常小，在纳米级。由于达西渗透率非常低，这些地层的天然气产量远远高于预期。本文首次利用原子力显微镜（AFM）获得了纳米孔的图像。纳米孔中的气体流动不能简单地用达西方程来描述。固体基质中的克努森扩散和滑移流等过程将气体流动行为与达西型流动分开。我们提出了一种基于努森扩散和滑移流的泥岩纳米孔隙中气体流动的公式。通过比较这种新的气体流动公式和可压缩气体的达西流动，我们引入了一个表观渗透率项，该项包括纳米孔隙中流动的复杂性，并且它采用达西方程的形式，以便它可以很容易地在油藏模拟器中实现。结果表明，随着孔隙尺寸减小到小于100 nm，表观渗透率与达西渗透率的比值急剧增大。此外，努森扩散的流量的贡献增加，孔隙变得更小。与达西渗透率不同，达西渗透率仅是岩石的特征，气体在泥岩的纳米孔中的渗透取决于岩石、气体类型和操作条件。
Gas-producing mudrock systems are playing an important role in the volatile energy industry in North America and will soon play an equally important role in Europe. Mudrocks are composed of very fine grained particles, and their pores are very small, at the scale of nanometers. Gas production from these strata is much greater than what is anticipated given their very low Darcy permeability. In this paper, images of nanopores obtained by Atomic Force Microscopy (AFM) are presented for the first time. Gas flow in nanopores cannot be described simply by the Darcy equation. Processes such as Knudsen diffusion and slip flow at the solid matrix separate gas flow behaviour from Darcy-type flow. We present a formulation for gas flow in the nanopores of mudrocks based on Knudsen diffusion and slip flow. By comparing this new gas flow formulation and Darcy flow for compressible gas, we introduce an apparent permeability term that includes the complexity of flow in nanopores, and it takes the form of the Darcy equation so that it can easily be implemented in reservoir simulators. Results show that the ratio of apparent permeability to Darcy permeability increases sharply as pore sizes reduce to smaller than 100 nm. Also, Knudsen diffusion's contributions to flow increase as pores become smaller. Unlike Darcy permeability, which is a characteristic of the rock only, permeation of gas in nanopores of mudrocks; depends on rock, gas type and operating conditions.