Application of Low-Field Nuclear Magnetic Resonance (LFNMR) in Characterizing the Dissociation of Gas Hydrate in a Porous Media

Abstract

Clear understandings of the dissociation behaviors and characteristics of a gas hydrate are essential for the exploitation of hydrate-bearing reservoirs. To study the dissociation characteristics of a gas hydrate in porous media, measurements using the low-field nuclear magnetic resonance (LFNMR) method on three different sand samples are conducted in this work. The changes of pore radius and phase saturation during the dissociation are determined based on the transverse relaxation time (T 2) spectra. The fractal dimension is employed to depict the distributions of pore spaces occupied by water in the media, which shows a decreasing function with the decreasing hydrate saturation. Predictions of water permeability during the hydrate dissociation are comparably conducted using a fractal model and the widely used Schlumberger-Doll Research model. The fractal model performs better in showing the influences of pore-structure factors on the water permeability. The changes of specific reaction surface areas during the dissociation are estimated and preliminarily analyzed based on the results of LFNMR measurements. Results in this work may provide valuable insights for the related researches including the mechanical, acoustic, and electrical properties of the hydrate-bearing sediments.

Advances in Characterizing Gas Hydrate Formation in Sediments with NMR Transverse Relaxation Time

“…NMR T 2 test provides a fast, easy, and non-destructive way of detecting hydrogenbearing fluids, which is widely used to monitor the dynamic behavior of gas hydrate formation and pore structure in sediments [30,[45][46][47][48][49][50][51][52][53]56,66,[68][69][70]. Figure 4 shows the schematic diagram of the NMR T 2 method applied to quantitative characterization of gas hydrate formation in sediments.…”

“…The gas hydrate showing a short T 2 cannot be identified because complete relaxation occurs before the first echo is detected [73]. Therefore, it is assumed that the NMR signals of hydrate and gas can be ignored by the LF-NMR [49,52,[54][55][56]66,69,70,74]. That is, the effective signals in the NMR T 2 measurement of gas hydrates are all from the water.…”

“…The permeability of the hydrate formation process can be calculated by the T 2 spectrum. In oilfield NMR loggings, the permeability of sediments is obtained by the Schlumberger Doll Research model [56,70,105]. The permeability k can be calculated by:…”

“…Kuang et al [48] determinated that the permeability of the hydrate-bearing sediments was predicted under different hydrate saturation conditions by T 2 spectrum. Zhang et al [56] used Schlumberger Doll Research model to predict the permeability in the hydrate dissociation based on the NMR T 2 spectra and found that the water permeability is not sensitive to the changes in pore structure characteristics in the hydrate dissociation.…”

“…Therefore, the NMR T 2 method is also widely used for monitoring the pore structure and the formation process of gas hydrate in sediments [45][46][47][48][49][50][51][52][53]. NMR T 2 is an effective method for microscopic detection of hydrate-bearing sediments with considerable advantages (e.g., fast response, high resolution, and no damage) in the quantitative analysis of gas hydrate in sediments by low-field NMR (LF-NMR) [54][55][56]. For instance, Minagawa et al [57] calculated the pore-size distribution of hydrate-bearing sediments using the transverse relaxation time (T 2 ).…”