Nuclear Magnetic Resonance Transverse Surface Relaxivity in Quartzitic Sands Containing Gas Hydrate

Abstract

The proton nuclear magnetic resonance (NMR) technique is becoming increasingly popular in various assessments of physical properties in hydrate-bearing sediments, and NMR transverse relaxation time T 2 spectra are normally converted into pore size distributions by using the NMR transverse surface relaxivity ρ 2 . However, how the NMR ρ 2 value evolves during hydrate dissociation or formation remains elusive largely due to the lack of experimental data. Thus, combined measurements of lowfield NMR and X-ray computed tomography (CT) are performed to quantify the value of NMR ρ 2 in quartzitic sands with different xenon hydrate saturations. Effects of hydrate saturation and pore habits on the value of NMR ρ 2 are analyzed, and theoretical models are correspondingly proposed. These experimental data and theoretical models are extended to NMR-based predictions of hydraulic permeabilities and water retention curves, and suggestions for the predictions are given. Results show that the value of NMR ρ 2 increases first and then decreases due to the presence of xenon hydrate in pores of quartzitic sands, and the water−xenon−hydrate interface is inferred to relax water molecules more quickly than the water−quartzitic−sand interface. The value of NMR ρ 2 changes when gas hydrate is of grain-coating, pore-filling, or graintouching habit, and predictions of hydraulic permeabilities and water retention curves based on NMR T 2 spectra need to be modified under this condition. However, patchy hydrate widespread in natural coarse-grained sediments has little effect on the value of NMR ρ 2 , and there is no need of significant modifications for hydraulic permeability and water retention curve predictions. This study has a great potential to further hydrate related NMR applications both in artificial and natural environments.

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.…”

“…Even though the surface properties of minerals in sediments are different, an equivalent ρ is commonly applied to convert T 2 spectrum to pore sizes based on Equation (13). 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].…”

“…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]. It suggests that further studies are required to understand the water-hydrate interface properties and ρ in hydrate-bearing sediments in the formation process of gas hydrate [94].…”

“…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.…”