Nuclear magnetic resonance (NMR) measurements have been attractive options for wettability characterization of reservoir rocks as they are sensitive to the type of fluid in contact with the grain surface. Several NMR-based wettability indices are documented in previous publications. Most of these methods require extensive calibration or involve complex inversion algorithms, which makes them computationally expensive and complicates their applicability in mixed-wet multimodal rocks. In this paper, we introduce a new NMR-based wettability index and verify its reliability in pore-scale and core-scale domains using numerical simulations and experimental measurements, respectively. This new index requires calibration at fully water-saturated water-wet and fully hydrocarbon-saturated hydrocarbon-wet states and can be applied to mixed-wet rocks at any fluid saturation level.

This new NMR-based wettability index is a function of the transverse magnetization (T₂) of mixed-wet rocks, the bulk relaxivity and saturation of each fluid, and the T₂ distributions for fully water-wet and hydrocarbon-wet samples of the same rock type. The reliability of the new index was first tested in the pore-scale domain. For this part, we selected several pore-scale microcomputed tomography (CT) images of carbonate and sandstone rocks. We used a previously developed finite volume simulator to model the T₂ responses in these images at fully water-wet and fully hydrocarbon-wet wettability states. Then we generated synthetic partially saturated mixed-wet samples and simulated T₂ responses in these synthetic images. We used the simulated T₂ results for determining their NMR-based wettability index and verified its applicability in the pore-scale domain.

Next, we tested the reliability of the new NMR-based wettability index in the core-scale domain using NMR measurements in four Texas Cream (TC) rock samples, obtained from the Edwards formation. We altered the wettability of the cores to be water-, hydrocarbon-, and mixed-wet by injecting brine, a naphthenic acid in decane solution, or an anionic surfactant solution. We quantified the wettability of these samples using the Amott-Harvey (AH) index and contact angle measurements. Next, we measured the T₂ distribution of these samples at different fluid saturation levels. Finally, we quantified the wettability values of these core samples using the new NMR-based index and compared them to those obtained from the AH index and contact angle measurements. We documented successful verification of the proposed method on samples with wettability ranging from –0.90 to 0.98 and from –0.6 to 0.5 (independently quantified using the AH method) in the pore- and core-scale domains, respectively. Results demonstrated that the new NMR-based wettability index reliably estimates the wettability of mixed-wet rocks in a wide range of wettability states. The new wettability index can potentially improve the speed and reliability of NMR-based wettability characterization and is promising for log-scale wettability assessment in mixed-wet rocks.

对于储层岩石的润湿性表征，核磁共振（NMR）测量是有吸引力的选择，因为它们对与谷物表面接触的流体类型敏感。在以前的出版物中记录了几种基于NMR的润湿性指数。这些方法大多数都需要进行广泛的校准或涉及复杂的反演算法，这使它们在计算上变得昂贵，并使它们在混合湿法多峰岩石中的适用性变得复杂。在本文中，我们介绍了一种新的基于NMR的可湿性指数，并分别使用数值模拟和实验测量来验证其在孔尺度和岩心尺度域中的可靠性。

这种新的基于NMR的可湿性指数是混合湿岩石的横向磁化强度（T ₂），每种流体的整体弛豫性和饱和度，以及完全湿润和碳氢化合物湿润样品T ₂分布的函数。相同的岩石类型。新指标的可靠性首先在孔尺度域中进行了测试。对于这一部分，我们选择了碳酸盐岩和砂岩岩石的几个孔尺度微计算机断层扫描（CT）图像。我们使用先前开发的有限体积模拟器对这些图像在完全水湿和完全烃湿润的状态下的T ₂响应进行建模。然后我们生成了合成的部分饱和混合湿样品并模拟了T ₂这些合成图像中的响应。我们使用模拟的T ₂结果确定其基于NMR的润湿性指数，并验证了其在孔尺度域中的适用性。

接下来，我们使用从爱德华兹地层获得的四个德克萨斯奶油（TC）岩石样品中的NMR测量，在岩心尺度域中测试了新的基于NMR的润湿性指数的可靠性。通过注入盐水，癸烷溶液中的环烷酸或阴离子表面活性剂溶液，我们将岩心的润湿性更改为水，烃和混合湿。我们使用Amott-Harvey（AH）指数和接触角测量对这些样品的润湿性进行了量化。接下来，我们测量了T ₂这些样品在不同流体饱和度水平下的分布。最后，我们使用新的基于NMR的指数对这些岩心样品的润湿性值进行了量化，并将其与从AH指数和接触角测量获得的值进行了比较。我们记录了在孔隙度和核心尺度域中分别在–0.90至0.98和–0.6至0.5（使用AH方法独立定量）范围内的润湿性的样品上成功验证了该方法的有效性。结果表明，新的基于NMR的可湿性指数能够可靠地估计各种润湿状态下混合湿岩石的可湿性。新的可湿性指数可以潜在地提高基于NMR的可湿性表征的速度和可靠性，并且有望用于混合湿岩石的对数尺度可湿性评估。