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NMR oil well logging: Diffusional coupling and internal gradients in porous media.

机译：NMR油测井：多孔介质中的扩散耦合和内部梯度。

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The default assumptions used for interpreting Nuclear Magnetic Resonance (NMR) measurements with reservoir rocks fail for many sandstone and carbonate formations. This study provides quantitative understanding of the mechanisms governing NMR relaxation of formation fluids for two important cases in which default assumptions are not valid. The first is diffusional coupling between micro and macropore, the second is susceptibility-induced magnetic field inhomogeneities. Understanding of governing mechanisms can aid in better estimation of formation properties such as pore size distribution and irreducible water saturation.; The assumption of direct correspondence between relaxation time and pore size distribution of a rock fails if fluid in different sized pores is coupled by diffusion. Pore scale simulations of relaxation in coupled micro and macropores are done to analyze the effect of governing parameters such as surface relaxivity, pore geometry and fluid diffusivity. A new coupling parameter (alpha) is introduced which quantifies the extent of coupling by comparing the rate of relaxation in a coupled pore to the rate of diffusional transport. Depending on alpha, the pores can communicate through total, intermediate or decoupled regimes of coupling. This work also develops a new technique for accurate estimation of irreducible saturation, an approach that is applicable in all coupling regimes. The theory is validated for representative cases of pore coupling in sandstone and carbonate formations.; Another assumption used in NMR formation evaluation is that the magnetic field distribution in the pores corresponds to the externally applied field. However, strong field inhomogeneities can be induced in presence of paramagnetic minerals such as iron on pore surfaces of sedimentary rocks. A generalized relaxation theory is proposed which identifies three asymptotic relaxation regimes of motionally averaging, localization and free diffusion. The relaxation characteristics of the asymptotic regimes such as T 1/T2 ratio and echo spacing dependence are quantitatively illustrated by random walk simulations and experiments with paramagnetic particles of several sizes. The theory can aid in better interpretation of diffusion measurements in porous media as well as imaging experiments in Magnetic Resonance Imaging (MRI).
机译：用于解释储层岩石的核磁共振（NMR）测量的默认假设对于许多砂岩和碳酸盐岩地层均无效。这项研究为默认假设无效的两种重要情况提供了对控制地层NMR弛豫机理的定量理解。第一个是微孔和大孔之间的扩散耦合，第二个是磁化率诱导的磁场不均匀性。了解控制机制有助于更好地估计地层性质，例如孔径分布和不可还原的水饱和度。如果不同尺寸孔隙中的流体通过扩散耦合，则弛豫时间与孔隙尺寸分布之间直接对应的假设将失败。进行了耦合的微孔和大孔中弛豫的孔尺度模拟，以分析控制参数（如表面弛豫性，孔几何形状和流体扩散率）的影响。引入了新的耦合参数（alpha），该参数通过将耦合孔中的​​松弛速率与扩散传输速率进行比较来量化耦合程度。取决于α，孔可以通过全部，中间或解耦的偶联方式连通。这项工作还开发了一种新技术，用于精确估计不可约饱和度，该方法适用于所有耦合方式。该理论已被验证用于砂岩和碳酸盐岩地层中孔隙耦合的典型案例。 NMR形成评估中使用的另一个假设是，孔中的磁场分布对应于外部施加的磁场。但是，在沉积岩石的孔隙表面上存在顺磁性矿物（如铁）时，可能会引起强场不均匀性。提出了广义松弛理论，该理论确定了运动平均，局部化和自由扩散的三种渐近松弛形式。通过随机游走模拟和几种尺寸的顺磁性粒子的实验，定量地说明了渐近状态的松弛特性，例如T 1 / T2比和回声间距依赖性。该理论有助于更好地解释多孔介质中的扩散测量结果，以及磁共振成像（MRI）中的成像实验。

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