Abstract:The combination of energy source and geohazardous potential of natural gas hydrate has raised the need to understand the processes related to hydrate dissociation within the sediment. In this article, several existing methane hydrate dissociation apparatuses are listed and their sample size capabilities given. A new design for line dissociation tests by combined electrical heating and pressure reduction from a miniature wellbore is presented. The 180-mm-diameter × 225-mm-length hydrate-bearing soil samples can… Show more
“…Three clear trends can be observed from the figure; (1) during initial cooling , in which the temperature gradient is from the sample and outwards (i.e., the confining liquid is cooler than the sample), where a temperature difference of 3.5 was observed, (2) at the final stage of hydrate formation , during which the temperature gradient turns direction, as the sample reaches a stable - low - temperature while the outer cell is affected by the room temperature (even if great thermal isolation efforts are taken), where a temperature difference of was observed, and (3) at hydrate formation initiation , in which the sample temperature spikes as the result of an exothermic reaction to the hydrate formation, where a temperature difference of was observed. Similar temperature difference has been previously reported in the context of thermal gradients even inside the MHBS sample 35 . Figure 2 ( a ) Temperatures measured by the four thermometers in the system and ( b ) the gas consumption during the hydrate formation process.…”
Section: Accurate Temperature Measurementssupporting
Over recent years, there has been a growing interest in producing methane gas from hydrate-bearing sands (MHBS) located below the permafrost in arctic regions and offshore within continental margins. Geotechnical stability of production wellbores is one of the significant challenges during the gas extraction process. The vast majority of geotechnical investigations of MHBS have been conducted on laboratory-formed samples due to the complex procedure of undisturbed sample extraction. One of the most commonly used hydrate laboratory-formation methods is the excess-gas method. This work investigates fundamental aspects in the excess-gas formation of MHBS that are affecting the geotechnical interpretation and modeling. The work finds that (1) the measured temperature in the experimental system may be quite different from the in-sample temperature, and can reach 4 $$^\circ$$
∘
C difference during thermodynamic processes. This potential difference must be considered in investigation of hydrate formation or dissociation, (2) various calculation approaches may yield different hydrate saturation values of up to tens of percentages difference in high hydrate saturations. The calculation formulas are specified together with the fundamental difference between them, (3) the water mixture method during the sample assembling is critical for homogeneous MHBS laboratory formation, in which a maximum initial water content threshold of 9.1 to 1.3 % are obtained for a minimal fraction size of 0.01 to 0.8 mm, respectively, (4) the hydrate formation duration may influence the MHBS properties, and should be rigorously estimated according to the real-time gas consumption convergence. The outcomes of this work may contribute to the integration of data sets derived from various experiments for the study of MHBS mechanical behavior.
“…Three clear trends can be observed from the figure; (1) during initial cooling , in which the temperature gradient is from the sample and outwards (i.e., the confining liquid is cooler than the sample), where a temperature difference of 3.5 was observed, (2) at the final stage of hydrate formation , during which the temperature gradient turns direction, as the sample reaches a stable - low - temperature while the outer cell is affected by the room temperature (even if great thermal isolation efforts are taken), where a temperature difference of was observed, and (3) at hydrate formation initiation , in which the sample temperature spikes as the result of an exothermic reaction to the hydrate formation, where a temperature difference of was observed. Similar temperature difference has been previously reported in the context of thermal gradients even inside the MHBS sample 35 . Figure 2 ( a ) Temperatures measured by the four thermometers in the system and ( b ) the gas consumption during the hydrate formation process.…”
Section: Accurate Temperature Measurementssupporting
Over recent years, there has been a growing interest in producing methane gas from hydrate-bearing sands (MHBS) located below the permafrost in arctic regions and offshore within continental margins. Geotechnical stability of production wellbores is one of the significant challenges during the gas extraction process. The vast majority of geotechnical investigations of MHBS have been conducted on laboratory-formed samples due to the complex procedure of undisturbed sample extraction. One of the most commonly used hydrate laboratory-formation methods is the excess-gas method. This work investigates fundamental aspects in the excess-gas formation of MHBS that are affecting the geotechnical interpretation and modeling. The work finds that (1) the measured temperature in the experimental system may be quite different from the in-sample temperature, and can reach 4 $$^\circ$$
∘
C difference during thermodynamic processes. This potential difference must be considered in investigation of hydrate formation or dissociation, (2) various calculation approaches may yield different hydrate saturation values of up to tens of percentages difference in high hydrate saturations. The calculation formulas are specified together with the fundamental difference between them, (3) the water mixture method during the sample assembling is critical for homogeneous MHBS laboratory formation, in which a maximum initial water content threshold of 9.1 to 1.3 % are obtained for a minimal fraction size of 0.01 to 0.8 mm, respectively, (4) the hydrate formation duration may influence the MHBS properties, and should be rigorously estimated according to the real-time gas consumption convergence. The outcomes of this work may contribute to the integration of data sets derived from various experiments for the study of MHBS mechanical behavior.
“…20,21 Falser et al conducted line dissociation tests of MH-saturated soil by combined electrical heating and pressure reduction. 22 The effect of the pore size of the sediments on the dissociation of MH was studied. It was found that the hydrate equilibrium condition was determined by the effect of the pore size on the water activity and that, during the initial stage of hydrate dissociation, the recovery rate of methane was strongly dependent upon the pore size.…”
Depressurization has been considered an economic and practicable method for natural gas hydrate (NGH) exploitation. To obtain the kinetic data of methane hydrate (MH) dissociation under different backpressures, MH dissociation by depressurization in a porous medium was investigated in situ using magnetic resonance imaging (MRI). MH was dissociated under backpressures that were varied from 2.8 to 2.2 MPa, and the hydrate saturation variation during dissociation was analyzed. One experimental case was carried out with constant backpressure, and four cases of variable backpressure depressurization experiments were carried out. The radial dissociation pattern during depressurization was confirmed. During hydrate dissociation, free water was observed to move toward the outlet of the vessel and decreased the water saturation after the hydrate totally dissociated in the field of view (FOV). The MRI data provided excellent information on the spatial distribution of water in the porous media during hydrate dissociation.
“…Earlier experimental work on hydrate-bearing sand at the National University of Singapore (NUS) had been focusing on developing and testing production schemes of gas from NGH [29][30][31][32][33] . This led on to analysis of fraccability of hydrate-bearing sand, motivated by gas production and the need to understand this complex material property.…”
Section: Application Of Hydraulic Fracturing In Hydrate-bearing Sandmentioning
In this paper, a method is proposed to determine the fracture toughness of brittle and heterogeneous material through hydraulic fracturing. The intent of the proposed method is to deploy it on materials whose are not easily determined by standard test. One example of such a material is hydrate-bearing sand, which is unstable under atmospheric pressure and temperature. The pressure-time record, injection flow rate and total volume injection are necessary information to determine the fracture toughness of the material using the classical linear elastic fracture mechanics approach. To verify the method, frozen sand is used as a substitute material and its fracture toughness found using the standard test. Frozen sand has similar composition as natural hydrate-bearing sediment except for the absence of methane gas and high pressure. Then, this method is applied on the hydrate-bearing sand.Keywords hydraulic fracturing · fracture toughness · brittle materials · frozen sand · hydrate-bearing sand
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