Effective gradients in porous media due to susceptibility differences
Effective gradients in porous media due to susceptibility differences
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DOI：
10.1006/jmre.1998.1364
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发表时间：
1998-04-01
影响因子：
2.2
通讯作者：
Hürlimann, MD
中科院分区：
文献类型：
--
作者：
Hürlimann, MD
在多孔介质中，固相和填充孔隙空间的流体之间的磁化率差异导致孔隙空间内部的场不均匀性。在许多情况下，流体相中的自旋通过这些内部不均匀性的扩散控制了NMR信号的横向衰减速率。在无序的多孔介质，如沉积岩，详细评估这一过程是在实践中是不可能的，因为字段的不均匀性不仅取决于磁化率的差异，但也对孔隙几何形状的细节。本文用有效梯度的概念分析了内梯度扩散的主要特征。有效梯度与失相长度上保持的不均匀性有关，失相长度是自旋在失相之前扩散的典型长度。对于CPMG序列，弛豫速率对回波间隔的依赖性可以通过有效梯度的分布来描述到一阶。有人认为，对于一个给定的磁化率差，有一个最大值，这些有效的梯度，g（最大值），仅取决于扩散系数，拉莫尔频率，和磁化率差。这种分析适用于水饱和的沉积岩的情况。从一组核磁共振测量和大量的磁化率测量的汇编，我们得出结论，碳酸盐岩的有效梯度通常小于目前的核磁共振测井工具的梯度，而在许多砂岩，内部梯度可以媲美或大于工具梯度。(C)北京：科学出版社.
In porous media, magnetic susceptibility differences between the solid phase and the fluid filling the pore space lead to field inhomogeneities inside the pore space. In many cases, diffusion of the spins in the fluid phase through these internal inhomogeneities controls the transverse decay rate of the NMR signal. In disordered porous media such as sedimentary rocks, a detailed evaluation of this process is in practice not possible because the field inhomogeneities depend not only on the susceptibility difference but also on the details of the pore geometry. In this report, the major features of diffusion in internal gradients are analyzed with the concept of effective gradients. Effective gradients are related to the held inhomogeneities over the dephasing length, the typical length over which the spins diffuse before they dephase. For the CPMG sequence, the dependence of relaxation rate on echo spacing can be described to first order by a distribution of effective gradients. It is argued that for a given susceptibility difference, there is a maximum value for these effective gradients, g(max) that depends on only the diffusion coefficient, the Larmor frequency, and the susceptibility difference. This analysis is applied to the case of water-saturated sedimentary rocks. From a set of NMR measurements and a compilation of a large number of susceptibility measurements, we conclude that the effective gradients in carbonates are typically smaller than gradients of current NMR well logging tools, whereas in many sandstones, internal gradients can be comparable to or larger than tool gradients. (C) 1998 Academic Press.