Measurements of relaxation time and diffusion coefficient by nuclear magnetic resonance are well-established techniques to study molecular motions in fluids. Diffusion measurements sense the translational diffusion coefficients of the molecules, whereas relaxation times measured at low magnetic fields probe predominantly the rotational diffusion of the molecules. Many complex fluids are composed of a mixture of molecules with a wide distribution of sizes and chemical properties. This results in correspondingly wide distributions of measured diffusion coefficients and relaxation times. To first order, these distributions are determined by the distribution of molecular sizes. Here we show that additional information can be obtained on the chemical composition by measuring two-dimensional diffusion-relaxation distribution functions, a quantity that depends also on the shape and chemical interactions of molecules. We illustrate this with experimental results of diffusion-relaxation distribution functions on a series of hydrocarbon mixtures. For oils without significant amounts of asphaltenes, the diffusion-relaxation distribution functions follow a power-law behavior with an exponent that depends on the relative abundance of saturates and aromatics. Oils with asphaltene deviate from this trend, as asphaltene molecules act as relaxation contrast agent for other molecules without affecting their diffusion coefficient significantly. In waxy oils below the wax appearance temperature a gel forms. This is reflected in the measured diffusion-relaxation distribution functions, where the restrictions due to the gel network reduce the diffusion coefficients without affecting the relaxation rates significantly.

Nuclear magnetic resonance relaxation measurements of bulk fluids provide a sensitive probe of the dynamics of molecular motion. Dissolved oxygen can interfere with this technique as its paramagnetic nature leads to a reduction of the paramagnetic relaxation times of the fluids. We studied this effect for the relaxation properties of crude oils that are in general characterized by a distribution of relaxation times. The samples were stock tank oils that have been exposed to air. We compared T~ and ir relaxation time distributions and their correlation functions of the initial (oxygenated) samples with those from the deoxygenated samples. Oxygen was removed from the oils with a freeze-thaw technique. As expected, the effect of oxygen is most apparent in oils with long relaxation times. In these oils the effect of oxygen can be described by an additional relaxation rate 1/T~~ to the transverse and longitudinal relaxation rates that is sample dependent but does not vary within the relaxation time distribution of the oil. Values of 1/T~~ for different crude oils were found to be in the range of 2.5 to 8.3 s. For crude oils that have components with relaxation times less than 100 ras, no significant oxygen effect is observed.

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We monitor the fracturing of gas shale using high-pressure methane gas, by studying the changes in gas transport using high-field nuclear magnetic resonance (NMR). This helps us understand the fundamental relation between the newly created pathways and the enhanced gas transport. The ability to make such correlations is challenging, partially because of the difficulty in monitoring the gas transport during the transient fracturing process. Here, we demonstrate a methodology for fracturing gas shale core samples inside a high-pressure, high-field NMR sample tube and studying its effect on gas transport kinetics by tracking the time-dependent NMR signal intensity. The ultralow permeability of shale makes the transient gas transport slow enough to be monitored by a series of NMR signals. The time constant that characterizes the transient process toward equilibrium is directly related to the permeability of core samples. The signatures of permanent fractures in the shale core plugs created by a sudden pressure release are identified by the shorter equilibrium time constant. Although these permanent fractures are not visible to the naked eye, they are ex-situ-verified by microcomputed tomography (microCT). These results demonstrate a NMR methodology to characterize the gas transport and fracturing property of gas shale, promising a better understanding of their relationships.