Permeability is a key parameter for characterizing an unconventional tight-rock reservoir and predicting hydrocarbon production from the reservoir. Because of the technical difficulty in accurately measuring extremely low tight-rock permeability, the permeability measurements have been commonly made in laboratory under hydrostatic (or isotropic stress) conditions. For unconventional reservoirs, stress distributions, however, are generally anisotropic. This paper presents a theoretical and experimental study on stress-dependent tight-rock permeability under anisotropic stress conditions. The theoretical development is based on the concept of effective stress and expresses rock permeability as an exponential function of the effective stress defined in this paper. The laboratory permeability data obtained under anisotropic stress conditions support the usefulness of the theoretical model for the stress-dependent permeability and demonstrate the importance of stress anisotropy in determining rock permeability. Considering that the effective stress is the only independent variable for determining the relationship between rock permeability and stress, we further propose a workflow to convert tight-rock permeability data collected under hydrostatic stress conditions to those for anisotropic stress conditions that are more realistic for field applications. A theoretical relation for upscaling a parameter characterizing the impact of stress anisotropy on local-scale permeability is also discussed as the first step to address this important upscaling issue.

中文翻译：

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各向异性应力条件下与应力相关的致密岩渗透率

渗透率是表征非常规致密岩油藏和预测油藏油气产量的关键参数。由于精确测量极低致密岩石渗透率的技术难度，渗透率测量通常在实验室在静水（或各向同性应力）条件下进行。然而，对于非常规油藏，应力分布通常是各向异性的。本文提出了各向异性应力条件下与应力相关的致密岩石渗透率的理论和实验研究。理论发展基于有效应力的概念，并将岩石渗透率表示为本文定义的有效应力的指数函数。在各向异性应力条件下获得的实验室渗透率数据支持了应力相关渗透率理论模型的有用性，并证明了应力各向异性在确定岩石渗透率中的重要性。考虑到有效应力是确定岩石渗透率与应力之间关系的唯一自变量，我们进一步提出了一种工作流程，将在静水应力条件下收集的致密岩石渗透率数据转换为更适合现场应用的各向异性应力条件下的渗透率数据。还讨论了升级表征应力各向异性对局部尺度渗透率影响的参数的理论关系，作为解决这一重要升级问题的第一步。