Gas hydrate in-situ formation and dissociation in clayey-silt sediments: An investigation by low-field NMR

Abstract

The hydrate reservoir in the Shenhu Area of the South China Sea is a typical clayey-silt porous media with high clay mineral content and poor cementation, in which gas hydrate formation and dissociation characteristics are unclear. In this study, the CO2 hydrate saturation, growth rate, and permeability were studied in sandstone, artificial samples, and clayey-silt sediments using a custom-built measurement apparatus based on the low-field NMR technique. Results show that the T2 spectra amplitudes decrease with the hydrate formation and increase with the dissociation process. For the artificial samples and Shenhu sediments, the CO2 hydrate occupies larger pores first and the homogeneity of the sandstone pores becomes poor. Meanwhile, compared with the clayey-silt sediments, CO2 hydrate is easier to form and with higher hydrate saturation for the sandstone and artificial samples. In hydrate dissociation process, there exists a protection mechanism, i.e. the dissociation near the center of hydrates grain is suppressed when gas pressure drops suddenly and quickly. For permeability of those samples, it decreased with hydrate forms, and increases with hydrate dissociation. Meanwhile, with the same hydrate saturation, permeability is higher in hydrate formation than in dissociation.

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

“…Given the advances in modern technology, many non-destructive detection techniques (e.g., X-ray computed tomography (CT), magnetic resonance imaging (MRI), Cryogenic scanning electron microscopy (Cryo-SEM), 13 C solid-state NMR, and NMR T 2 ) are used to acquire in situ quantitative and real-time characterization of gas hydrate formation process [47,50,51,[68][69][70][107][108][109][110][111][112][113][114][115][116]. This study compares their application characteristics in the fields of resolution, phase recognition, scanning time, advantages, and drawbacks.…”

“…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 ρ value is anticipated to vary (Table 4) when the mineralogy and morphology in sediments change at the water-solid interface [68,92,93]. In addition, the ρ value of hydrate-bearing sediments is assumed to be constant in the process of hydrate formation [51,86]. Since the water relaxation at the water-sand interface mainly occurs at the sand particle surface while the proton spin is relaxed dominantly by intra-dipole-dipole interaction at the water-hydrate interface, the ρ value must be changed by the variations of the fluid-solid interface mineralogy and morphology in the hydrate formation process [53,68,70,79].…”

“…Although there have been a lot of excellent studies on the gas hydrate formation in sediments using NMR T 2 method, a few of them combined NMR T 2 with other methods to characterize the formation process of gas hydrate and the pore structure in sediments in a more effective way [30,45,[50][51][52][53]56,[66][67][68][69][70]74,[147][148][149]. The combined use of these techniques can provide more information about the gas hydrate formation in sediments than any single technique.…”

“…The NMR T 2 method is used to determine the petrophysical characteristics of conventional and unconventional porous media in oil and gas fields, such as porosity, pore geometry, pore connectivity, permeability, and even the fluid saturation in reservoirs [37][38][39][40][41][42][43][44]. 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].…”