TDR was introduced to solve the problem of how to measure hydrate saturation accurately. Then a series of un-drained triaxial tests were carried out on methane hydrate-bearing sediments under various conditions with effective confining pressures at 1, 2 and 4 MPa, average hydrate saturations at 15.71, 35.7 and 56.49% and strain rate at 0.8%/min. The results indicate that the shear strength increases with the increases of effective confining pressure and hydrate saturation, but the maximum failure time decreases with the increasing effective confining pressures. According to Mohr-Coulomb failure criterion, the shear strength of methane hydrate-bearing sediments was analyzed. It can be found that the internal friction angles are not sensitive to hydrate saturation, but the cohesion shows a high hydrate saturation dependency.
The simulate experiments of gas production from methane hydrates reservoirs was proceeded with an experimental apparatus. Especially, TDR technique was applied to represent the change of hydrate saturation in real time during gas hydrate formation and dissociation. In this paper, we discussed and explained material transformation during hydrate formation and dissociation. The hydrates form and grow on the top of the sediments where the sediments and gas connect firstly. During hydrates dissociation by depressurization, the temperatures and hydrate saturation presented variously in different locations of sediments, which shows that hydrates dissociate earlier on the surface and outer layer of the sediments than those of in inner. The regulation of hydrates dissociation is consistent with the law of decomposition kinetics. Furthermore, we investigated the depressurizing range influence on hydrate dissociation process.
In this paper, we focused on the determination of phase equilibrium conditions of hydrates formed in the pore water and porous media from South China Sea. High pressure differential scanning calorimetry (HP DSC), a relatively new thermo-analytical technique was applied to this research. During the study, nitrogen hydrates and methane hydrates phase equilibrium conditions were determined in thewater-hydrates- gas (W-H-G) system. Then a series of experiments were carried on using core sample drilled from South China Sea to determine the phase equilibrium properties in pore water and porous media systems. It show that the hydrates phase equilibrium point is about 2K lower than in pure water from pressure range 10 to 30 MPa, due to the thermodynamic inhibition effect of brine solution.
Dry water (DW) is a powder composed of hydrophobic silica surrounding tiny water droplets. It’s recently demonstrated to be an effective medium for storage methane in a hydrated form. But the recyclability of DW is not so clear at present. Here we used a dedicated pressure cell to form four dry water methane hydrates samples (DW-MH), corresponding to four cycle-types of hydrate formation and dissociation, to investigate the reuse of dry water. The results show that part of water escape from DW and become “free water” after DW-MH is dissociated. The free water is hard to form hydrate during the next cycle of hydrate formation. As a result, the gas storage capacity of DW-MH decreases dramatically as the using times of DW increases.
It is necessary to know gas hydrate saturation in evaluating gas hydrate volume. So far, there are only few models to estimate gas hydrate saturation. These models are often empirical or derived from observed data. Therefore, we need to confirm their legitimacy and get an approach to use the correct model. For the first time, we combine Ultrasonic Wave and Time Domain Reflectometry to study the relationship between gas hydrate saturation and acoustic parameters, which gets fine results. Subsequently, it attempts to compare the calculated data by time-average equation, Wood’s equation, Lee’s weighted equation and BGTL with the observed data in experiments. The results show the experimental method is very effective. It suggests that the Lee’s weighted equation and BGTL are more applicable in our experiments and various sediments.
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