Occupation and Release Behavior of Guest Molecules in CH4, CO2, N2, and Acetone Mixture Hydrates: An In Situ Study by Raman Spectroscopy

Gas hydrates are a well-known problem in the oil and gas industry that cost millions of dollars in production and transmission pipelines. Of the thermodynamic models in the literature, few can predict the hydrate formation temperature or pressure for complex systems which include inhibitors. Two new correlations can calculate the hydrate formation pressure or temperature for single components or gas mixtures, with or without inhibitors. These correlations are applicable to temperatures up to 90 °F and pressures up to 12,000 psi. The results show an average absolute percentage deviation of 15.93 in pressure and an average absolute temperature difference of 2.97 °F. Introduction Gas hydrates are ice-like crystalline structures with gas components such as methane and carbon dioxide as guest molecules entrapped into cavities formed by water molecules. Whenever a system of natural gas and water exists at specific conditions, especially at high pressure and low temperature, we expect the formation of hydrates. In the oil and gas industry, gas hydrates are a serious problem in production and gas transmission pipelines because they plug pipelines and process equipment. By applying heat, insulating the pipelines and using chemical additives as inhibitors, we can keep the operating conditions out of the hydrate formation region. To remediate problems caused by hydrates, it is important to calculate the gas hydrate formation temperature and pressure accurately. This is more complex when the system includes alcohols and/or electrolytes. Hammerschmidt(1) first found that the formation of clathrate hydrates could block natural gas transport pipelines. Since then, the oil and gas industry has been more willing to investigate the problem. This paper presents two new correlations that can predict the gas hydrate formation pressure at a given temperature (p-correlation) or the gas hydrate formation temperature when pressure is available (T-correlation) for a single component or a mixture of gas with or without inhibitors. The work focuses on gas hydrate formation at the three-phase equilibrium (liquid water, hydrocarbon gas and solid hydrate). The developed correlations are applicable to a range of temperatures up to 90 °F and pressures up to 12,000 psi. The capability of these correlations has been tested for aqueous solutions containing electrolytes such as sodium, potassium and calcium chlorides (NaCl, KCl and CaCl2) lower than 20 wt% and inhibitors such as methanol lower than 20 wt% and ethylene glycol (EG), triethylene glycol (TEG) and glycerol (GL) lower than 40 wt%, since the use of higher amounts of these inhibitors is neither practical nor economic. In addition, these correlations may not be appropriate in some cases with high concentrations of inhibitors. The results show an average absolute percentage deviation of 15.93 in pressure and an average absolute temperature difference of 2.97 °F. To make the correlations easy to use, we programmed them with Visual Basic (program is available upon request). From gas compositions, the inhibitor concentrations and either temperature or pressure of the system, a user can calculate the hydrate formation pressure or temperature as quickly as clicking a key.

Experimental study of the formation and decomposition of mixed gas hydrates in water-hydrocarbon system

Formation of gas hydrates have been known to cause severe problems of blockages in natural gas pipelines, wellbores and natural gas processing units. Methanol and glycols are commonly used as hydrate inhibitors to control or prevent formation of gas hydrates, due to their ability to lower hydrate formation (or dissociation) temperatures considerably. For the same reason, they are very effective hydrate dissociation stimulants for enhancing gas production from hydrate reservoirs as demonstrated by a field study in the Messoyakha gas hydrate reservoir of the Soviet Union (Makogon, 1981). Effect of these hydrate inhibitors on the thermodynamic phase behavior of gas hydrates has been well established, however, no experimental data exist on effect of inhibitors on the rate of hydrate formation or dissociation. Hence, the current methods of prevention of hydrate formation by inhibitors have relied upon the thermodynamic data rather than kinetic data. In this study, the characteristics of hydrate dissociation process during methanol and ethylene glycol injection were investigated. After formation of methane hydrates in the synthetic cores, hydrates were dissociated by injection of inhibitor solution of known initial concentration, at a constant rate. The pressure was held constant during dissociation and gas production, inhibitor solution temperature and dissociating hydrate front position were monitored continuously. Experimental results show that the instantaneous rate of hydrate dissociation is function of inhibitor concentration, inhibitor injection rate, pressure, temperature of inhibitor solution and hydrate-inhibitor interfacial (contact) area. Based upon the unsteady state dissociation data, empirical correlations for the rate of hydrate dissociation in presence of inhibitors are developed. The correlations can be used to compute the degree of enhancement in the rate of hydrate dissociation by inhibitors in applications such as hydrate prevention or production of natural gas from hydrates.

The purpose of this paper is to study the thermodynamic modeling of the conditions for methane and ethane gas hydrate formation and their mixtures in a porous and non-porous environment. In this paper, the Van der Waals- Platteeuw thermodynamic model was used for prediction of gas hydrate formation conditions. Also, the SRK and PTV equations of state were used for calculations of driving force. In this research, the results of thermodynamic modeling in a porous were compared with the non-porous environment and laboratory data in the literature. Studies have shown that the results of the modeling are in good agreement with the laboratory data and the percentage of errors is low. The results also showed that with increasing pressure of porous and non-porous media, the equilibrium temperature increases. In addition, the effect of the pore diameter of porous media on the results of modeling was investigated for methane, ethane and their mixtures during gas hydrates formation. The results showed that by increasing the pressure for any size of the pore diameter of the porous medium, hydrate formation temperature increases. In addition, by increasing the pore diameter of the porous medium, hydrate formation temperature methane, ethane and their mixture increase at a constant pressure. The results also showed that the equilibrium temperature of the non-porous medium is higher than the equilibrium temperature of the non-porous medium. This shows that the hydrate formation in the porous medium has a deterrent effect and leads to lower temperatures and higher temperatures conditions for gas hydrate formation. The results showed that by increasing the percentage of methane in a porous or non-porous medium, the temperature of hydrate formation of the binary gas mixture of methane and ethane decreases.

Laboratory and field results have confirmed the advantages of kinetic gas hydrate inhibitors compared to classical thermodynamic inhibition methods. Kinetic inhibitors being used currently are high molecular weight water soluble polymers and they require a polar carrier solvent. In situations where the solvent is allowed to evaporate, i.e. hot spots in a gas pipeline, the solid polymer will plate out of the solution and may cause problems. This paper describes new low molecular weight kinetic gas hydrate inhibitors. These newly developed products, being liquid, will not plate out of solutions, can be used either in a polar solvent or a hydrocarbon carrier. Simulated hydrates laboratory testing has been done on a tetrahydrofuran/salt water system in a simple and unique testing loop. Comparable laboratory results were obtained for experimental and established kinetics gas hydrate inhibitors. Positive laboratory results were followed with successful field testing. Laboratory and field results indicate the experimental inhibitors have potential commercial applications in gas hydrate prevention technology. Introduction Gas hydrates form when water molecules crystallize around guest molecules. The water/guest cocrystallization process has been recognized for several years, is well characterized and occurs with sufficient combinations of temperature and pressure. Significant amounts of natural gas are deposited in the form of hydrates and are considered as future energy sources. Light hydrocarbons, methane-to-heptanes, nitrogen, carbon dioxide and hydrogen sulfide are the guest molecules of interest to the natural gas industry. Depending on the pressure and gas composition, gas hydrates may build up at any place where water coexists with natural gas at temperatures as high as 80 F. Long gas transmission lines are particularly vulnerable to being blocked with hydrates during extended cold weather conditions. Underwater pipelines and equipment may be exposed to hydrate forming conditions all the time. Extensive studies on gas hydrates were conducted by B. Dendy Sloan Jr. and coworkers at the Colorado School of Mines (CSM). These studies sponsored by several oil and gas companies included both the theoretical and practical aspects of hydrates and methods of hydrates inhibition. Gas hydrates are being studied worldwide at several research laboratories. There are few methods of preventing gas hydrates formation. The obvious ones like removing the water component, heating the system above the temperature of hydrate formation and lowering the pressure, are often not practical. Another method, the addition of large amounts of ethylene glycol or methanol, decreases the hydrate stability and effectively lowers the temperature of hydrate formation. The above mentioned methods are called thermodynamic inhibition because they destabilize water/gas clathrates by changing the composition or conditions. Thermodynamic inhibition with chemicals requires relatively large amounts of methanol and/or glycol; hence it is quite expensive. Kinetic methods of gas hydrates prevention have been developed by CSM and other laboratories. Kinetic inhibitors prevent a growth of hydrate nuclei to form larger crystals. Kinetic inhibitors are usually water-soluble copolymers. They are effective at concentrations typically ten to one hundred times less than the effective concentrations of ethylene glycol or methanol. With thermodynamic inhibitors increasing in price, the kinetic ones are becoming economically favorable. Known kinetic inhibitors like poly(N-vinylpyrrolidone (PVP), poly(N-vinylpyrrolidone/N-vinylcaprolactam/N, N-dimethylaminoethylmethacrylate (VC-713), poly(N-vinylcaprolactam) (PVCap) or poly(N-vinylpyrrolidone/Nvinylcaprolactam) (VP/VC) inhibit gas hydrates formation by coating and commingling with hydrate crystals nuclei, thereby interfering with agglomeration of small particles into large ones which would result in plugging the gas pipeline and equipment.

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 90422, "Enhanced Hydrate Inhibition in Alberta Gas Field," by Dana Budd, EnCana Corp., and Danica Hurd, SPE, Marek Pakulski, SPE, and Thane D. Schaffer, SPE, BJ Chemical Services, prepared for the 2004 SPE Annual Technical Conference and Exhibition, Houston, 26-29 September. Operating gas wells in southern Alberta poses challenges. High bottomhole pressure in new wells, low bottomhole temperatures, and the Joule-Thompson expansion-cooling effect (which often lowers the gas-stream temperature below the brine freezing point) create conditions favorable for the formation of gas hydrates in wells and transportation pipelines. The problems are aggravated during cold winter months, when wells and pipelines have strong tendencies to plug with hydrates and ice. Systematic laboratory work was undertaken to explore synergistic effects between methanol and low-dosage hydrate inhibitors (LDHIs). A strong effect was discovered at a certain ratio of methanol and low-molecular-weight oligomer-type hydrate inhibitor. Introduction Gas hydrates form when water molecules crystallize around “guest” molecules. The water/guest crystallization process occurs at many combinations of temperature and pressure. Light hydrocarbons (methane to heptanes), nitrogen, carbon dioxide, and hydrogen sulfide are the guest molecules of interest to the natural-gas industry. Depending on pressure and gas composition, gas hydrates may build up at any place where water coexists with natural gas at temperatures as high as 30°C. Gas-transmission lines and new gas wells are particularly vulnerable to being blocked with hydrates. Formation of gas hydrates can be eliminated or slowed by several methods. Thermodynamic prevention methods control or eliminate elements necessary for hydrate formation: the presence of hydrate-forming guest molecules, the presence of water, high pressure, or low temperature. Eliminating any one of these four factors from a system precludes the formation of hydrates. Unfortunately, elimination of these hydrate elements is often impractical or even impossible. For long subsea transmission lines, heating and insulating is a common mechanical solution to hydrate problems. Hydrates will not form if the gas/water system is kept at a temperature greater than the hydrate-formation temperature. Gas dehydration is another method of removing a hydrate component. However, in a practical field operation, water can be economically removed only to a certain vapor pressure, and residual water vapor always exists in a dry gas. Hydrate plugs in “dry” gas lines have been reported. Adding chemicals to the gas/water streams is the most common method of preventing hydrate formation. Large amounts of alcohols, glycols, or salts are used. These additives thermodynamically destabilize hydrates and effectively lower the hydrate-formation temperature. They function by bonding to water molecules through hydrogen bonds or solvation.

Natural Gas Hydrates

Seismic constraints on the effects of gas hydrate on sediment physical properties and fluid flow: a review

The effect of biodegradable polysaccharides – sodium (NaCMC) and ethanolammonium salts of carboxymethylcellulose, dextran and arabinogalactan on the process of gas hydrate formation was studied in order to search for new "green" inhibitors of low-concentration gas hydrate formation. The ability of polysaccharides to inhibit gas hydrate formation was studied in a quasi-equilibrium thermodynamic experiment. A mixture of hydrocarbon gases with a composition typical of the composition of petroleum gas and containing 78% methane was used as a gas-hydrate-forming model medium. It was found that in concentrations of 0.005, 0.0065 and 0.008%, dextran, NaCMC and arabinogalactan as thermodynamic inhibitors exceed methanol by 170-270 times in inhibitory properties. Dextran is superior to NaCMC and arabinogalactan in terms of inhibition efficiency, reduction of gas hydrate formation rate and induction time. Since with an increase in the concentration of polysaccharides, the pressure drop of gas hydrate formation increases and the rate of formation of gas hydrates decreases according to the mechanism of action, the studied polysaccharides can be attributed to both thermodynamic and kinetic inhibitors. It is established that the molecular weight of water-soluble polysaccharides has a significant effect on their inhibitory properties. A polysaccharide with a molecular weight of 250,000 demonstrated the highest inhibitory activity among the studied samples of NaCMC, which is 400 times more effective than methanol. NaCMC with a mass of 700 thousand did not have any effect on the formation of hydrates. Among the ethanolammonium salts, the monoethanolammonium salt CMC showed the greatest effectiveness in inhibiting the formation of tetrahydrofuran hydrates. An increase in its concentration from 0.02 to 0.1% leads to an increase in the induction time required for the nucleation and subsequent growth of crystals by 10 times. When switching from mono - to di - and triethanolammonium salts of carboxymethylcellulose, the inhibition efficiency decreases. It is shown that sodium and ethanolammonium salts of carboxymethylcellulose, arabinogalactan and dextran are promising for creating new "green" highly effective inhibitors of gas hydrate formation on their basis. The results of laboratory and field tests of the preparative form of the "green" gas hydrate formation inhibitor at the fields of Western Siberia are presented. It was found that at dosages of 500 g/m3 or less, there is no formation of hydrate plugs in the annulus of wells.

Gas hydrates are the object of continuous research in the oil and gas industry. Gas hydrates can form in gas production systems: in the bottomhole zone, in the wellbores, in plumes and infield reservoirs, in gas treatment systems, as well as in main gas transport systems, causing serious problems associated with the disruption of these processes. However, recently, the gas industry has found new uses for hydrates (for example, energy recovery, separation, storage, and transportation of gas) prompting scientists to conduct new and more detailed studies of the hydrate formation process.
 Understanding the conditions and mechanisms of hydrate formation makes an engineer able to control this process. Therefore, in practice, simplified models are often required to predict hydrate formation. This paper presents various correlations that are widely used in the oil and gas industry to determine the temperature of hydrate formation. The accuracy of the presented models is compared. The influence of the features of the gas composition on the conditions for the hydrates formation is studied.
 It is shown that for the considered gases, the most accurate results can be obtained using the G.V. Ponomarev correlation. When the relative density of gases is greater than 0.6, it is possible to use the Towler and Bahadori correlations. Correlations proposed by Hammerschmidt, Mottie and Berge show the worst results.
 Since the temperature of hydrate formation in the considered correlations does not depend on the gas composition, but only on the relative density, the influence of the content of non-hydrocarbon components on the temperature of hydrate formation was studied in this work. It was found that the content of hydrogen sulfide has the greatest influence. Moreover, at high contents of hydrogen sulfide in the gas composition, the temperature of hydrate formation shifts towards higher values. The content of nitrogen and carbon dioxide to a lesser extent affect the value of the temperature of hydrate formation.

The next generation of hydrate prediction IV: A comparison of available hydrate prediction programs

Application of statistical learning theory for thermodynamic modeling of natural gas hydrates

Gas hydrate is an ice-like solid and a kind of inclusion compounds of which the cage-like structure formed by hydrogen-bonded water molecules can include various kinds of guest gas molecules. In general, gas hydrates are formed with “host” water and “guest” gas molecules under lower temperature and higher pressure conditions, but sometimes large differences in the hydrate formation conditions are observed among guest gases. In such cases, if gas hydrate is formed with such a gaseous mixture, it can be anticipated that the component of which the hydrate formation condition is milder (that is, higher temperature and lower pressure conditions relatively) could be enriched in the hydrate phase. Effective gas separation, or higher selectivity, can be achieved for gas mixtures with larger differences in the hydrate formation conditions. On the other hand, multi-component gas hydrates are formed under higher pressure and lower temperature conditions in which any component of gaseous mixture can change to hydrate. Several applications have been proposed in environmental and energy fields by using the inclusion abilities in the framework of gas hydrates; natural gas transport (Gudmundsson & Borrehaug, 1996), gas storage (Lee et al., 2005), and gas separation (Kang & Lee, 2000) and so on, and thus many investigations for gas hydrate formation, especially thermodynamics and gas hydrate formation kinetics, have been carried out in batch systems. The solid hydrate can be dissociated to recover a product gas. The selectivity and production rate are key factors in determining the performance of hydrate-based applications. Although the selectivity is limited by the thermodynamic equilibrium of the hydrate phase and the feed vapour phase (Nagata et al., 2009), the production rate is dependant on the hydrate formation rate and the system design. Gas hydrate-based applications would require an efficient formation or production process of gas hydrates, and the elucidation of the formation mechanism of gas hydrates. Gas hydrate formation is similar to crystallization from liquid mixture, and gas-liquid system changes to liquid-solid or gas-solid systems. In general, it is known gas hydrate forms on gas-liquid interface, and thus the gas-liquid interfacial area, the driving force, and kinetic constant can affect hydrate formation. Therefore, an efficient way to increase these factors is necessary for continuously forming gas hydrate solid in gas-liquid system. For example, several efficient processes to increase the interfacial area for gas hydrate formation have been demonstrated, including a spray (Fukumoto et al., 2001) or jet reactor (Szymcek et al., 2008; Warzinski et al., 2008), and a bubble column (Luo et al, 2007; Hashemi et al., 2009)

Natural gas hydrates have garnered significant attention as a potential new source of alternative energy, and understanding their formation mechanism is of paramount importance for efficient utilization and pipeline transportation. However, there is no consensus among academics on the formation mechanism of natural gas hydrates. In this paper, we propose a method for promoting the rapid formation of natural gas hydrates based on the rupture of methane nanobubbles, which creates local high temperature and pressure to facilitate the mixing of methane and water. The rapid decrease in system temperature and pressure during the process further enhances the formation of gas hydrates. Using molecular dynamics simulations, we theoretically verify the formation of natural gas hydrates. Our results indicate that the instantaneous rupture of methane nanobubbles induced by shock waves leads to a dramatic increase in the local molecular motion velocity around the bubbles. This results in extreme local high temperature and high pressure, leading to complete mixing of methane and water and rapid formation of gas hydrates during the cooling and pressure drop of the mixture. We confirm our findings by analyzing F3-order parameters, F4-order parameters, and water cage statistics.

Prevention and disposal technologies of gas hydrates in high-sulfur gas reservoirs containing CO2