A laboratory study of NMR relaxation times in unconsolidated heterogeneous sediments
A laboratory study of NMR relaxation times in unconsolidated heterogeneous sediments
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DOI：
10.1190/1.3581094
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发表时间：
2011-07-01
期刊：
影响因子：
3.3
通讯作者：
Knight, Rosemary
中科院分区：
文献类型：
--
作者：
Grunewald, Elliot;Knight, Rosemary
核磁共振（NMR）弛豫时间测量可以提供关于水饱和介质的物理化学性质的关键信息，并经常用于表征地质材料。在松散的沉积物中，当扩散的水分子在连接良好的孔隙空间内耦合非均质区域的弛豫响应时，测量到的弛豫时间与孔隙尺度性质之间的联系可能会变得复杂。受控的实验室实验使我们能够研究是什么因素控制了松散沉积物中扩散耦合的程度，以及在不同条件下弛豫时间分布所传达的信息。一系列沉积物样品表现出石英和赤铁矿双峰矿物学形式的非均质性，混合了不同的矿物浓度和粒度。核磁共振弛豫测量和这些混合物的几何分析证明了控制弛豫响应的两个临界长度尺度的重要性：扩散长度l(D)，描述了水分子在核磁共振测量期间扩散的距离，以及分离长度l(S)，描述了非均质性发生的尺度。对于低赤铁矿浓度和粗粒度样品的l(S)>l(D)条件，耦合较弱，双峰弛豫时间分布独立反映了非均质混合物中两种矿物组分的弛豫特性。对于条件l(S)
Nuclear magnetic resonance (NMR) relaxation-time measurements can provide critical information about the physiochemical properties of water-saturated media and are used often to characterize geologic materials. In unconsolidated sediments, the link between measured relaxation times and pore-scale properties can be complicated when diffusing water molecules couple the relaxation response of heterogeneous regions within a well-connected pore space. Controlled laboratory experiments have allowed us to investigate what factors control the extent of diffusional coupling in unconsolidated sediments and what information is conveyed by the relaxation-time distribution under varied conditions. A range of sediment samples exhibiting heterogeneity in the form of a bimodal mineralogy of quartz and hematite were mixed with varied mineral concentration and grain size. NMR relaxation measurements and geometric analysis of these mixtures demonstrate the importance of two critical length scales controlling the relaxation response: the diffusion length l(D), describing the distance a water molecule diffuses during the NMR measurement, and the separation length l(S), describing the scale at which heterogeneity occurs. For the condition of l(S)>l(D), which prevails for samples with low hematite concentrations and coarser grain size, coupling is weak and the bimodal relaxation-time distribution independently reflects the relaxation properties of the two mineral constituents in the heterogeneous mixtures. For the condition of l(S)