Abstract Surface relaxivity (ρ) is an intrinsic rock property that makes low-field NMR a valuable tool for determining pore size distribution of geological formations. Its value can vary considerably in different rocks depending on the type and distribution of the rock minerals, rock surface wettability and petrophysical properties. Therefore, laboratory measurement of ρ is required on a case by case basis. However, existing NMR methods are expensive, complex, and most often require supplementary and expensive non-NMR measurements. Rock samples are also required to be well consolidated and cylindrical so that experiments can be run at high pressures – a barrier for measurements of unconsolidated and thinly laminated unconventional rocks. This study proposes a new NMR method termed ‘NMR grain sizing method’, that uses different grain sizes of a given rock to generate pores of corresponding sizes. Using a geometric constant, grain specific surfaces are transformed to pore surfaces, and the governing NMR equation is rearranged so that we have a method that measures ρ from only NMR measurements on rock cuttings. This approach is simple, inexpensive, and does not require supplementary measurements. It offers an added advantage for characterizing thinly laminated formations. A proof of concept is demonstrated for three unconventional rocks (Qusaiba shale, Jafurah carbonate mudrock and Eagle Ford) and two conventional rocks (Berea sandstone and Indiana limestone). The ranges of surface relaxivity are 96.9–134.5 μm/s (Qusaiba shale); 62.07–106 μm/s (Jafurah carbonate mudrock); 55.9–68.72 μm/s (Eagle Ford); 35.51–43.2 μm/s (Berea sandstone); and 33.2–40.54 μm/s (Indiana limestone). All ρ measurements are reproducible with 95% confidence interval. The measured values for Indiana limestone and Berea sandstones were validated with micro CT based pore network models and also compare well with literature data. Measurement of Qusaiba shale and Jafurah carbonate source rocks are apparently reported for the first time here.

中文翻译：

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常规和非常规储层岩屑的新型 NMR 表面弛豫率测量

摘要 表面弛豫率 (ρ) 是一种固有的岩石特性，它使低场 NMR 成为确定地质构造孔径分布的宝贵工具。根据岩石矿物的类型和分布、岩石表面润湿性和岩石物理性质，其值在不同岩石中可能会有很大差异。因此，需要根据具体情况对 ρ 进行实验室测量。然而，现有的 NMR 方法昂贵、复杂，并且通常需要补充且昂贵的非 NMR 测量。岩石样品还需要很好地固结和圆柱形，以便可以在高压下进行实验——这是测量松散和薄层非常规岩石的障碍。这项研究提出了一种新的 NMR 方法，称为“NMR 晶粒尺寸测定法”，它使用给定岩石的不同粒度来生成相应尺寸的孔隙。使用几何常数，颗粒比表面转换为孔隙表面，并重新排列控制 NMR 方程，以便我们有一种方法可以仅从岩屑的 NMR 测量中测量 ρ。这种方法简单、便宜，并且不需要补充测量。它为表征薄层压地层提供了额外的优势。对三种非常规岩石（Qusaiba 页岩、Jafurah 碳酸盐泥岩和 Eagle Ford）和两种常规岩石（Berea 砂岩和印第安纳石灰岩）进行了概念验证。表面弛豫范围为 96.9-134.5 μm/s（曲塞坝页岩）；62.07–106 μm/s（Jafurah 碳酸盐泥岩）；55.9–68.72 μm/s（鹰福特）；35.51–43.2 μm/s（Berea 砂岩）；和 33.2-40。54 μm/s（印第安纳石灰石）。所有 ρ 测量值都具有 95% 的置信区间。印第安纳石灰岩和 Berea 砂岩的测量值通过基于微 CT 的孔隙网络模型进行了验证，并与文献数据进行了很好的比较。对曲塞坝页岩和贾富拉碳酸盐岩烃源岩的测量显然是这里首次报道。