Effect of Pressure on the Effectiveness of the Kinetic Inhibition of Hydrate Formation by Polymer Reagents
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In this paper, we suggest the advanced method of methane hydrate formation by cyclodextrin solutions. The structures of the methane hydrate were experimentally investigated by Raman spectroscopy. The induction time of the methane hydrate formation becomes by shorter 10–30 times and formation rate become by faster 2–4 times originated in the increased methane concentration of hydrate formation water by adding cyclodextrins. The results by the Raman spectroscopy indicate that the structure I methane hydrate is produced and methane molecules exist in both Large and Small cages.
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Methane hydrate is the ice-stated solid crystals composed by water molecules and molecules of methane gases. Water molecules make a cage where space holds inside, molecules of methane gases are encircled in this space. Commonly, a chemical compound that gas molecules are taken into a cage made by water molecules is called as gas hydrate, gas molecules belong to methane are called as methane hydrate. In this paper, structure and chemical properties of methane hydrate, the synthesized experiments and a stable region of methane hydrate were mentioned. Methane hydrate in seabed deposit has still been attracted an attention as a recent methane resource, a marine earthquake-detecting method was introduced by using the submarine pseudo-reflection as a surveying method of methane hydrate in seabed. 10 refs., 7 figs., 1 tab.
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Density functional theory calculation has been carried out to elucidate the thermodynamic properties of the methane clathrate hydrate(51262 and 512).At the B3LYP/6-31G**level,methane clathrate hydrate is fully optimized and the potential energy curves are scanned.The theoretical calculations showed that one methane molecule encaged in the 51262 cavity is most stable.The preserved condition for safely storing methane hydrate is 190 K,200 MPa.The calculated values of symmetric C—H stretching frequencies of the methane molecule in the two cluster cavities are agreed with the experimental data.The narrow potential wall in the 512 cavity leads to restricting the motion of the methane molecule in the small cavity.
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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.
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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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In this study, methane-hydrate is produced by the mixed gases of methane and xenon, and the productive conditions of methane-hydrate is clarified. Methane-hydrate is noticed as new energy in recent years. The molecule of methane enter the crystal of the molecule of water then methane-hydrate is formed. Methane-hydrate is crystal of solid like ice and stable at low temperature and high pressure. In normal temperature and normal pressure, methane-hydrate decompose and become gas and water. As purpose of use of methane-hydrate, a cold heat source, storage of a natural gas, etc. are considered.
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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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Methane hydrate is successfully prepared through the reaction of Al4C3 and H2O using a laboratory designed high pressure hydrate apparatus.Experimental results indicate that this method is an effective solution to introduce CH4 in the synthetic experiments of methane hydrates,and the pressure-temperature formation conditions of the methane hydrate approach the natural situation.In addition,the products of the reaction of Al4C3 and H2O can simulate the real formation process of methane hydrate in the ocean environment and offer an effective route to natural gas hydrate research.With this method,a preliminary study is presented concerning the formation,dissociation and dynamic processes of methane hydrate under different temperature and pressure conditions.
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The exchange of methane between clathrate phase and gas phase on artificial methane hydrate was studied. Artificial methane hydrate samples were formed in a pressure cell from methane gas with stable carbon isotope composition δ13C of －42.3‰（ methane A）. Methane A remaining in the gas phase was removed from the cell at 77 K and methane gas with stable carbon isotope composition δ13C of －66.9‰（ methane B） was introduced into the cell. The carbon isotope compositions of both clathrate phase and gas phase methane gases were measured after the samples had been maintained at 274.7 K for 7-50 days. The δ13C of the clathrate phase methane was smaller than the initial value of －42.3‰ and that of the gas phase methane was larger than its initial value of －66.9‰. These results suggest that the exchange between the clathrate phase and gas phase methane proceeded in the period of the scale for several weeks.
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In situ Raman spectroscopy is employed to study the phase behavior of methane hydrate at high pressure. The structure 1 of methane hydrate can be maintained up to 950 MPa at 299 K. The transformation of structure I<-->structure H+water+CH4 occurs at 880 MPa and 323 K. The structure H of methane hydrate, however, decomposes to methane and water at 960 MPa and 348 K. The initiation mechanism of methane hydrate sI is also discussed.
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