- freely available
Energies 2012, 5(2), 518-530; https://doi.org/10.3390/en5020518
2. Experimental Section
2.1. Experimental Apparatus
2.2. Preparation of the Methane Hydrate Samples
2.3. Depressurization Dissociation
3. Results and Discussion
|Run 1||Run 2||Run 3||Run 4||Run 5||Run 6|
|Initial pressure (MPa)||12.8||13.5||15.2||13.7||13.7||13.8|
|Inner temperature (°C)||9.1||9.1||9.0||9.0||8.9||9.0|
|Equilibrium pressure (MPa)||6.3||6.3||6.3||6.3||6.3||6.3|
|Hydrate saturation (%)||35.5||23.3||15.6||25.3||27.0||25.7|
|Water saturation (%)||12.3||21.8||28.2||31.5||29.2||30.5|
|Production pressure (MPa)||6.0||6.0||6.0||5.0||5.5||6.0|
- Phase 1: free gas release process. With depressurization, the free gas is released at first, then temperatures and resistances decline slightly, while the gas production increases sharply. About 40% of the cumulative gas is produced in this section.
- Phase 2: rapid hydrate dissociation process. When the pressure drops rapidly to 6 MPa, the gas production rate increases suddenly, while temperatures and resistances simultaneously decrease sharply. The drop of the temperature accompanies with that gas production rate increases suddenly and the curve of the cumulative gas production rises sharply. Then, temperatures increase gradually because of the heat transferred from the water bath. Meanwhile, the resistances drop continuously, and this indicates that the hydrate keeps dissociating. About 40% of cumulative gas is produced in this phase.
- Phase 3: slow hydrate dissociation process. Gas production is unconspicuous in this phase. Temperatures and resistances display few changes, and the curve of the cumulative gas production tends to flatten. About 20% of cumulative gas is produced in this section, and the phase lasts for a long time.
|Run 1||Run 2||Run 3|
|Gas production in Phase 1 (%)||40||60||70|
|Gas production in Phase 2 (%)||40||30||25|
|Gas production in Phase 3 (%)||20||10||5|
- The experimental process of hydrate dissociation induced by depressurization can be divided into three phases. The first one is the free gas release phase. In this phase, the gas production increases sharply, temperature and resistance decrease slightly as depressurization occurs. The second one is the rapid hydrate rapid dissociation phase. In this phase temperature and resistance decrease sharply at the beginning, then temperature recovers gradually because of the heat transfer, while resistance goes down gradually, and the production rate is large. The third one is the slow hydrate dissociation phase. In this phase, temperature and resistance experience few changes, but the gas production increases slowly.
- The initial hydrate saturation can affect the amount of hydrate dissociation in the different phases. It shows that as the saturation increases, the proportion of hydrate dissociating during the rapid dissociation phase will decrease, thus, it allows more hydrates to dissociate in the third stage with slower rate and the temperature recovers slowly as well.
- The dissociation rate increases by lowering the dissociation pressure, and this tendency is more apparent when the dissociation pressure approaches the equilibrium. At a lower dissociation pressure, more heat in the core can be applied for dissociation. Moreover, the dissociation pressure can also affect the temperature-drop during the hydrate dissociation process. Furthermore, the decrease of dissociation pressure may negatively affect the secondary hydrate formation.
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