Gas hydrate equilibrium dissociation conditions in porous media using two thermodynamic approaches
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Fugacity
Clathrate hydrate
The developed clathrate hydrate has latent-heat over the temp. range of 5〜12℃, and a mixture of clathrate hydrate and aqueous solution (referred to as hydrate slurry) has grater cooling capacity compared to cold water. The utilization of the hydrate slurry in air-conditioning system is expected to reduce the pumping power consumption dramatically. Densities of the aqueous solution , the clathrate hydrate and the hydrate slurry were measured. The measured density of hydrate slurry showed a good agreement with a calculated value.
Clathrate hydrate
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The recovering rate of CH4 from CH4-hydrate soaked in liquid CO2 was measured at 274–277 K and 4–5 MPa. The fugacity of CH4 in liquid CO2 was kept below its initial hydrate forming fugacity, while that of CO2 was much higher than the initial forming fugacity of CO2-hydrate. The CH4-recovering rate in liquid CO2 was much lower than that in gaseous N2 which dose not make hydrate under the conditions. The composition of the hydrate measured at the end of the experiment indicated that the formation of CO2-hydrate consumed all the H2O molecules which had been encaging the recovered CH4. The observed phenomena was described by a mathematical model based on nonequilibrium thermodynamics for use in feasibility studies.
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Abstract Offshore flowlines transporting hydrocarbons have to be operated very carefully to avoid the formation of gas hydrates as they are considered one of the largest concerns for flow assurance engineers. The oil and gas industry is generally relying on chemical injection for hydrate inhibition; however hydrate blockages can occur in many different places of offshore production system due to unexpected circumstances. Once hydrate blockage formed considerable efforts are required to dissociate the hydrate via depressurization. Because residual hydrate structures known as gas hydrate precursors will be present in the aqueous phase after dissociation, the risk of hydrate re-formation becomes extremely high. Although the KHIs are becoming popular in many fields as hydrate inhibitors are considered not effective to inhibit the hydrate formation in the presence of residual hydrate structures, so that the use of KHIs for shut-in and restart operations is not recommended. In this study, new experimental procedures composed of three stages are designed to simulate the dissociation of hydrate blockages and transportation of well fluids experiencing hydrate formation. The obtained experimental results have shown that gas hydrates are rapidly re-formed when the temperature of dissociated water falls into the hydrate formation region. With an injection of KHIs before transporting the well fluids, the subcooling increased significantly indicating the possible use of KHIs for transporting the well fluids after dissociation of hydrate blockage. Moreover, the inhibition performance of KHIs is also investigated with two different gases to study the effect of gas composition. This study is confirmed that KHIs are possible candidate to prevent the hydrate re-formation in well fluids experiencing hydrate formation if the KHI is carefully evaluated. Introduction Gas hydrates are nonstoichiometric crystalline compounds that are classified into three structural families of cubic structure I, cubic structure II, and hexagonal structure H. Offshore flowlines transporting hydrocarbons have to be operated very carefully to avoid the formation of gas hydrates as they are considered the largest concern for flow assurance engineers. For many years industrial practice to prevent hydrate-related risks is the injection of thermodynamic hydrate inhibitors (THI) at the wellhead, commonly methanol or monoethylene glycol (MEG), to shift the hydrate equilibrium curve toward higher pressure and lower temperature, so that the operating condition of offshore flowlines are outside of the hydrate formation condition. However as the search for hydrocarbon resources moves into deeper and colder waters further offshore, these conventional techniques are becoming uneconomic due to higher injection rate of inhibitors and accompanying operational issues such as logistics and storage requirement. Although the MEG injection is considered to be the standard method for the offshore gas production system, Kinetic Hydrate Inhibitor (KHI) is also becoming popular as its dosage rate is expected in the range of 0.5~3.0 wt%, which is much lower than MEG's 30~60 wt%. Kinetic hydrate inhibitors (KHIs) are water-soluble polymers and delay the formation of hydrate crystals. These include homo- and co-polymers of the N-vinyl pyrrolidone (PVP) and N-vinyl caprolactam (PVCap). The KHIs available to date are only effective in subcoolings up to 14 oC and their performance can be affected by the presence of other chemicals such as corrosion inhibitors. There have been attempts to develop a KHI evaluation method based on a hydrate precursor where the hydrate-forming gas was a binary mixture of methane and propane that forms structure II. In this work, we conduct experiments to investigate hydrate formation in the presence of hydrate precursors and the effects of KHI on the inhibition of hydrate re-formation simulating the cold start-up after remediation of hydratep plug.
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Flow Assurance
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The formation of methane−ethane (C1−C2) clathrate hydrate was studied with high-resolution, solid-state 13C NMR and density functional theory techniques. The 13C NMR experiments yield a number of significant findings: (1) the hydration number of C2(aq) is 26, (2) the initial quantity of C2−51262 sI hydrate cages outnumber C1−512 cages at 274 K, (3) C1−C2 sII hydrate forms at a C1−C2 gas phase composition where only sI hydrate is thermodynamically stable, (4) the initial composition of C1−C2 sII hydrate at 268 K contains less than the original amount of C1, (5) a quasi-liquid water layer solvating both C1 and C2 exists at 268 K, (6) any C1(qll) and C2(qll) present at 253 K is too small to be detected, (7) the initial amounts of C1−C2 sI and sII hydrates formed at 253 K are much smaller than those formed at 268 and 274 K, and (8) C1(aq), C2(aq) and C1(qll), C2(qll) facilitate the formation of C1−C2 sI and sII clathrate hydrate at 268 and 274 K, respectively. On the basis of these experimental observations, a model is developed that states that the aqueous hydration number of the most water-soluble clathrate hydrate former controls the structure of the clathrate hydrate that forms during the initial stages of the clathrate hydrate formation reaction. For methane−ethane clathrate hydrate, this means that ethane in a water liquid phase or quasi-liquid layer eliminates or adds two water molecules to its hydration shell to form the ethane-filled 51262 or 51264 cage building blocks of structure I or structure II clathrate hydrate, respectively. Density functional theory computations on methane-filled 512, 51262, and 51264 and ethane-filled 51262, 51263, and 51264 clathrate hydrate cages yield the stabilization energy of the gas-filled cages and provide theoretical evidence consistent with the experimentally based clathrate hydrate formation model. The proposed model is found to explain the results of other clathrate hydrate formation reactions.
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Clathrate hydrates of water-soluble guest molecules, such as ethylene oxide (EO) and tetrahydrofuran (THF), have been often investigated in experimental studies instead of gas hydrates because their dissociation temperatures are higher than the ice point under ambient pressure. We examine the formation mechanism of EO and THF hydrates using molecular dynamics simulations. The crystal growth rates are determined by the simulations of the hydrate/solution two-phase coexistence. It is found that the growth rate of EO hydrate is an order of magnitude higher than that of THF hydrate. The growth rates of THF hydrate largely deviate from the Wilson–Frenkel model, while the model well approximates the growth rates of EO hydrate, indicating that trapping of guest molecules on the hydrate surface, which causes the slowing of crystal growth of THF hydrate, is insignificant for EO hydrate. We also perform long-time simulations of aqueous EO and THF solutions to examine nucleation of clathrate hydrate. Spontaneous nucleation occurs only in the EO solution within the simulation time. Similar to previous studies on methane hydrate, the obtained solid structure exhibits no long-range order. It is found that the 512 hydrate cage, which is the most dominant cage type in the early stage of the nucleation of methane hydrate, is not a major cage type in the nucleation process of EO hydrate.
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Gas hydrate represents a mixture of natural gas and water molecules formed at high pressures and low temperatures near the freezing point of water. Physically, the hydrates are ice-shaped and among the water molecules, there is a cavity filled by a hydrate gas called clathrate. The hydrates can be formed because there is gas injected in water molecules at high pressure condition having temperature above the freezing point of water. Then it is exposed to a force that can dissolve gas inside the water. A lot of research has been conducted to investigate the performance of the gas hydrate itself. The performances include the rate of hydrate formation, the hydrate stability, and the hydrate storage capacity. Several studies have been studied, among others, to observe the effect of initial gas injection pressure on gas hydrate process, the effect of rotation in a vessel tank as a container for gas hydrate formation, and the hydrate formation process on stirrer tank. One of the most important things in the gas hydrate process is how the hydrate can be formed, which can be seen from the rate of hydrate formation by investigating how much the gas pressure will penetrate into water molecules. It is due the hydrate formation requires low temperature and high pressure. However, a conditioning of the gas hydrate formation at high pressure and low temperature is a matter that requires considerable energy. Therefore, it is needed a system in which the pressure of hydrate formation is not too high. One method to lower the hydrate pressure in order to the hydrate-forming pressure is not too high, CO2 will be mixed to the gas hydrate. It is because CO2 is soluble in water molecules. It make an effect that the pressure of gas formation will be lower. In the previous research, it is showed that CO2 was able to make the pressure in methane gas mixture lower. By decreasing its pressure, CO2 is expected to improve the hydrate performances. The study was conducted by varying the percentage of CO2 volume from 0% to 100%. Each percentage of CO2 will be seen as its effect on the gas hydrate performance. The gas used in this experiment were propane-butane gas mixture of 50% each. The mixture of propane-butane gas and CO2 were then fed into the water molecules. The water used was a demine water of 50 cm3. The initial pressure of the formation rate was 0.3 MPa and the temperature formation was 273 K. Meanwhile, the temperature used to stabilize the hydrate was 268 K. The vessel tank, used to process the occurrence of hydrate has the capability of high pressure has a cavity diameter material of 4 cm, height 12 cm, 0.5 cm thick and total volume of 150 cm3. The vessel tanks were inserted into the cooling bath that was set at a temperature of 273 K. The results showed that as greater the CO2 content, as smaller the initial hydrate initiation phase, However, it has an impact to decrease the hydrate stability. For hydrate storage capacity performance, pure CO2 hydrate has the highest storage capacity, while the lowest storage capacity was CO2 with gas-CO2 mixed percentage of 50%. It shows that CO2 is capable to decrease the pressure effect on 50% composition variation.
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To better understand clathrate hydrate mechanisms, nuclear magnetic resonance (NMR) and viscosity measurements were employed to investigate tetrahydrofuran (THF) hydrate formation and dissociation processes. In NMR experiments, the proton spin lattice relaxation time (T1) of THF in deuterium oxide (D2O) was measured as the sample was cooled from room temperature down to the hydrate formation region. The D2O structural change around THF during this process was examined by monitoring the rotational activation energy of THF, which can be obtained from the slope of ln(1/T1) vs 1/T. No evidence of hydrate precursor formation in the hydrate region was found. T1 measurements of THF under constant subcooling temperature indicate that THF hydration shells do not undergo much structural rearrangement during induction. The T1 of THF was also measured as the sample was warmed back to room temperature after hydrate dissociation. T1 values of THF after hydrate dissociation were consistently smaller than those before hydrate formation and never returned to original values. It was proposed that this difference in T1 after hydrate dissociation indicates that the THF−D2O solution is more microscopically homogeneous than before hydrate formation. In viscosity experiments, a Champion Technologies hydrate rocking cell (CTHRC) was used to probe the residual viscosity phenomenon after Green Canyon (GC) gas hydrate as well as THF hydrate dissociation. The residual viscosity reported in the literature was observed after GC hydrate dissociation but not after THF hydrate dissociation. Because GC hydrate behavior involves significant amounts of gas mass transfer while THF hydrate does not, one might conclude that the residual viscosity observed after GC hydrate dissociation was likely caused by the supersaturated gas concentration and its general effect on solvent viscosity, not necessarily by a clathrate water structure lingering from the solid.
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We observed the pattern formation of tetrahydrofuran (THF) clathrate hydrates in a mixture of a THF–17H2O solution and glass beads (2 and 50 μm diameter). The overall hydrate growth rate, V, was controlled using a directional growth apparatus. We are the first to successfully obtain a variety of hydrate patterns: a disseminated type at higher V values, a nodular type in the middle range of V, and massive and layer types at lower V values. The hydrate images obtained were analyzed by several methods to quantitatively characterize the patterns. Image processing retaining the relatively large hydrate regions (90% of the total hydrate area formed by segregated growth) was suitable for evaluating the number density of the hydrate grains and the average hydrate area, which dominated the total hydrate area. The segregated growth area of the hydrates was one of the most useful indices for quantitatively categorizing the hydrate types. The image analysis results showed that the segregated growth area of the hydrates and the average hydrate area increased and the number density of hydrate grains decreased with decreasing V. These results are qualitatively consistent with the theoretical frost heave model.
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