Vol 48, No 10 (2017)

Different spatial structures that arise due to rotation around simple bonds without violating the integrity of the molecule (without breaking chemical bonds) are called conformations. ¹H nuclear magnetic resonance (NMR) study of the spatial structure of calcium gluconate in the aqueous solution has been carried out. It was shown that molecules of calcium gluconate exist in the form of two conformations: zigzag 1-P and cyclic ₃G⁺. The results of homonuclear 2D ¹H NMR spectroscopy indicate the predominantly zigzag conformation. It was found that the intermolecular hydrogen bonds are formed and the spatial structure of molecules changes at the increase in the solution concentration. The observed concentration behavior of the conformation of calcium gluconate is also associated with the presence of the intramolecular hydrogen bonds –O(C4)H··O(C2).

This paper proposes a new method to determine the permeability of tight sandstone using characteristic parameters of the nuclear magnetic resonance (NMR) transverse relaxation time (T₂) distribution. First, the Swanson parameters (T_s) and Capillary–Parachor parameters (T_cp) are calculated as the percolation characteristic parameters (T_c) of NMR T₂ distribution. The logarithmic mean (T_lm), arithmetic mean (T_am), and harmonic mean (T_hm) are calculated as the pore structure characteristic parameters (T_m) of NMR T₂ distribution. T_x, the transverse relaxation time when the value of Y-axis is x% in the normalized accumulated T₂ distribution curve accumulated from long relaxation time part to short relaxation time part, is selected as a characteristic parameter of pore size distribution. Second, different T_c, T_m, T_x, and NMR porosity (T_por) values are selected to establish single-, double-, three-, and four- parameter models for estimating permeability. An analysis of the relationships between calculated permeabilities of different models and measured permeability in tight sandstone rocks indicated that the four-parameter model based on T_cp, T₄₀, T_am, and T_por was the best model. Moreover, this model was superior to the calibrated Timur model and the calibrated SDR model for calculating permeability in tight sandstone reservoirs.

Norm smoothing is commonly used in nuclear magnetic resonance (NMR) T₂ inversion and the choice of a suitable regularization parameter is a key step for obtaining a satisfactory inversion result, which is usually achieved by repeating T₂ inversion multiple times. However, a greater number of inversions result in a slower speed for the inversion process. In this paper, we propose a rapid norm smoothing T₂ inversion method achieved using a new selection method for the regularization parameter. First, the singular value decomposition (SVD) method is used to calculate singular values of the kernel matrix to compress the echo train data. Subsequently, a suitable regularization parameter is calculated based on the signal-to-noise ratio (SNR) of the echo train and the maximum singular value of the kernel matrix, which avoids the repetitions of the T₂ inversion. Finally, a rapid T₂ inversion is obtained using the Butler–Reeds–Dawson (BRD) method. Numerical simulation and logging data inversion results show that the new method can rapidly provide reasonable T₂ spectra for data with different SNRs and is insensitive to the amount of the compressed data.

Self-calibrating GRAPPA operator gridding (SC-GROG) is a method by which non-Cartesian (NC) data in magnetic resonance imaging (MRI) are shifted to the Cartesian k-space grid locations using the parallel imaging concept of GRAPPA operator. However, gridding with SC-GROG becomes computationally expensive and leads to longer reconstruction time when mapping a large number of NC samples in MRI data to the nearest Cartesian grid locations. This work aims to accelerate the SC-GROG for radial acquisitions in MRI, using massively parallel architecture of graphics processing units (GPUs). For this purpose, a novel implementation of GPU-accelerated SC-GROG is presented, which exploits the inherent parallelism in gridding operations. The proposed method employs the look-up-table (LUT)-based optimized kernels of compute unified device architecture (CUDA), to pre-calculate all the possible combinations of 2D-gridding weight sets and uses appropriate weight sets to shift the NC signals from multi-channel receiver coils at the nearest Cartesian grid locations. In the proposed method, LUTs are implemented to avoid the race condition among the CUDA kernel threads while shifting various NC points to the same Cartesian grid location. Several experiments using 24-channel simulated phantom and (12 and 30 channel) in vivo data sets are performed to evaluate the efficacy of the proposed method in terms of computation time and reconstruction accuracy. The results show that the GPU-based implementation of SC-GROG can significantly improve the image reconstruction efficiency, typically achieving 6× to 30× speed-up (including transfer time between CPU and GPU memory) without compromising the quality of image reconstruction.

The present work involves a comprehensive study to provide a theoretical model of the internal magnetic field gradients, present in paramagnetic shale pores, to explain the main relaxation features observed by nuclear magnetic resonance transversal relaxation measurements. In the systematic analysis process of relaxation data it is necessary to know up to what extent the magnetic field gradients are generated by the logging tool and/or arise internally in the rock due to their paramagnetic impurities content. The physical model to explain the relaxation features is based on the calculation of field gradients in a planar pore with and without relaxatives walls. The results reproduce the features of the relaxation parameters in pores due to paramagnetic and tortuous walls. The mechanism that drives the relaxation process is governed by anomalous diffusion within micro-pores. These relaxation processes arise from the interactions between the protons, belonging to the liquid molecules and the pore walls, whose structure is characterized by both large tortuosity and abundance of paramagnetic impurities, giving rise to local strong time dependent magnetic field gradients. The theoretical results are compared with those obtained experimentally to validate the relaxation model. The experimental data were gathered from a sample belonging to the “Vaca Muerta” formation of the Neuquén basin, Argentina.