Characterization of near-surface nuclear magnetic resonance relaxation measurements for groundwater applications [electronic resource]
Keywords: Geophysics

NMR measurements have been utilized for decades in the oil and gas industry to characterize hydrocarbon reservoirs. Recent efforts have aimed to extend NMR techniques to near-surface application using logging tools or non-invasive surface-based instruments to characterize groundwater aquifers. Quantifying aquifer properties such as porosity, pore-size, and permeability from NMR measurements, however, requires appropriate rock physics models that relate the measured NMR relaxation response to the underlying properties of interest. Conventional rock physics models have been primarily developed and tested for NMR applications in oil and gas applications, but applications of NMR in the near-surface differ in two significant respects: (1) in the near surface, unconsolidated sediments rather than consolidated rock are commonly encountered; and (2) surface-based NMR instruments record a fundamentally different relaxation signal than NMR logging tools. This dissertation presents laboratory and numerical studies to illuminate how these differences will influence the NMR measurement and how existing rock physics models must be revised to improve the interpretation of NMR data for groundwater applications. First, two complementary studies demonstrate that the presence of well-connected pores in unconsolidated sediments can lead to a process known as diffusional coupling, which can complicate the conventional model used to relate the NMR relaxation time distribution to an underlying distribution of pore environments. Laboratory measurements and theoretical modeling of relaxation in synthetic silica gels show that diffusional coupling tends to obscure an underlying bimodal pore-size distribution, leading to underestimation of the size of the largest pores and also underestimation of the fractional porosity comprised of small pores. These experiments further demonstrate that surface geochemistry can act to control the extent to which coupling occurs. A second study extends upon these results, considering NMR relaxation and the effect of diffusional coupling in unconsolidated heterogeneous sediments. It is shown that at a more fundamental level, the extent of coupling is primarily controlled by the relative magnitude of two key length scales: one describing the scale of molecular diffusion, and the other describing the scale of heterogeneity. These studies provide a basis for predicting and accounting for the effects of diffusional coupling in unconsolidated sediments. A second group of studies investigates how the signal measured by surface NMR instruments differs from that measured by logging tools. The first study uses laboratory and numerical experiment to systematically characterize the differences between the decay time measured by surface NMR (T2*) and that measured by logging tools (T2). It is shown that in weakly magnetic sediments, T2*, like T2, can convey useful information about pore size and permeability, but this parameter yields more limited information in sediments with higher magnetic susceptibility. A final study investigates how the shape of the signal measured by surface NMR can deviate from a commonly assumed exponential form. It is shown that they decay is most likely to be non-exponential when pores are large and the magnetic field is highly inhomogeneous; in such cases, an invalid assumption of exponential decay can lead to significant overestimation of water content. By advancing our understanding of near-surface NMR relaxation measurements, this research will enable more effective use and interpretation of NMR data for groundwater application.