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Gas hydrate plugs in petroleum fluid pipelines are a major flow assurance problem and thus, it is important for industry to have reliable mathematical models for estimating the time required to dissociate a hydrate pipeline plug. The existing mathematical models for modelling hydrate plug dissociation treat the problem as a pure heat transfer problem. However, an early study by Jamaluddin et al. speculated that the kinetics of gas hydrate dissociation could become the rate-limiting factor under certain operating conditions. In this short communication, a rigorous 2D model couples the equations of heat transfer and fluid flow with Clarke and Bishnoi's model for the kinetics of hydrate dissociation. A distinguishing feature of the current work is the ability to predict the shape of the dissociating hydrate–gas interface. The model is used to correlate experimental data for both sI and sII hydrate plug dissociation, via single-sided depressurization and double-sided depressurization. As a preliminary examination on the necessity of including dissociation kinetics, this work is limited to conditions for which hydrate dissociation rate constants are available; kinetic rate constants for hydrate dissociation are available at temperatures above 273.15 K. Over the range of conditions that were investigated, it was found that including the intrinsic kinetics of hydrate dissociation led to only a very small improvement in the accuracy of the predictions of the cumulative gas volumes collected during dissociation. By contrast, a sensitivity study showed that the predictions of hydrate plug dissociation are very sensitive to the value of the porosity. Thus, it is concluded that unless values of the thermophysical properties of a hydrate plug are known, accounting for the dissociation kinetics need not be a priority. [ABSTRACT FROM AUTHOR]

This work examines the use of the electrolyte Trebble–Bishnoi equation of state (eTBEOS) in predicting high‐pressure phase equilibria in the presence of aqueous mixed‐salt solutions. The eTBEOS combines the Trebble–Bishnoi equation of state with a Born energy term, a mean spherical approximation term, and a cation solvation term. Shortcomings in the originally regressed set of eTBEOS parameters are identified and discussed, and a new set of equation of state parameters is subsequently regressed for 58 salts. In order to extend the eTBEOS to mixed‐salt systems, two approaches for computing the EOS parameters of the ionic species are investigated. The first approach uses the originally regressed parameter set plus parameter mixing rules, where appropriate, whereas the second approach uses the newly regressed parameters and no mixing rules. In the prediction of osmotic coefficients in mixed‐salt solutions, the maximum relative difference between the experimental and computed values was 5.70%. For the gas solubility predictions, data were available for the solubility of CO2, CH4, and N2 in a small number of mixed electrolyte solutions; the maximum relative difference was 5.79%. Finally, CH4, C2H6, C3H8, and CO2 gas hydrate formation conditions were predicted in a number of mixed‐salt solutions, with a maximum relative difference of 10.76%. Overall, it was seen that both approaches allowed for comparable accuracy in phase equilibrium calculations. However, in the case of solutions made from salt mixtures with common cations, the second approach consistently resulted in improved accuracy. [ABSTRACT FROM AUTHOR]

The existence of solid hydrate in porous media always greatly reduces the relative permeability, which limits the development of hydrate reservoirs. Therefore, to recover the gas, it is necessary to carry out reservoir reconstruction measures that induce fractures around the production wells to increase the gas production of hydrate reservoirs. At present, there is limited quantitative research on the promotion of gas production by induced fractures in hydrate reservoirs. In this paper, numerical models of Class Ⅲ hydrate reservoirs with induced fractures are established. By comparing the gas production with and without induced fractures, the effectiveness of induced fractures in promoting the depressurization production of natural gas hydrates is verified. An index of improved efficiency is established and calculated to measure the promotion effect of induced fractures during the fracture promotion stage. The multiple regression calculation formulas of improved efficiency with reservoir geological parameters are obtained by integrating nonlinear regression and linear regression. The calculation model is helpful to quickly and directly measure the rationality and economy of the induced fracture depressurization method to promote hydrate development. • Numerical models of Class Ⅲ hydrate reservoirs with induced fractures are established. • An index of Improved Efficiency is established to measure the promotion effect of induced fractures. • The improved efficiency of induced fractures under different geological parameters is calculated and compared. • The multiple calculation formulas of improved efficiency with geological parameters are obtained by stepwise regression. [ABSTRACT FROM AUTHOR]

Naturally occurring gas hydrates are regarded as an important future source of energy and considerable efforts are currently being invested to develop methods for an economically viable recovery of this resource. The recovery of natural gas from gas hydrate deposits has been studied by a number of researchers. Depressurization of the reservoir is seen as a favorable method because of its relatively low energy requirements. While lowering the pressure in the production well seems to be a straight forward approach to destabilize methane hydrates, the intrinsic kinetics of CH4-hydrate decomposition and fluid flow lead to complex processes of mass and heat transfer within the deposit. In order to develop a better understanding of the processes and conditions governing the production of methane from methane hydrates it is necessary to study the sensitivity of gas production to the effects of factors such as pressure, temperature, thermal conductivity, permeability, porosity on methane recovery from naturally occurring gas hydrates. A simplified model is the base for an ensemble of reservoir simulations to study which parameters govern productivity and how these factors might interact. [ABSTRACT FROM AUTHOR]

Abstract: The kinetics of structure II gas hydrates, formed from pure propane and a mixture of propane and ethane, were investigated and intrinsic rate constants were regressed from the experimental data. The experiments were conducted in a semi-batch stirred tank reactor equipped with an in-situ particle size analyzer and connected to an external Raman spectrometer. Experiments were conducted with pure propane at temperatures ranging from 274 to 276K and pressures ranging from 0.39 to 0.43MPa. The intrinsic rate constant for ethane in structure II was subsequently regressed from experimental data on the formation of hydrates formed from an equimolar mixture of propane and ethane at 274K and 0.35MPa. Raman spectroscopy was used to verify that ethane was present only in the large sII cavity. [Copyright &y& Elsevier]

A reversed-micellar solution is defined as a system of water, oil and an amphilphile (surfactant). It has been shown that gas hydrate formation in reverse-micelle systems provides a means for in-situ control of the droplet size, which in turn can be used to manipulate the properties of any material that has been formed in the water droplet. In this study, the incipient conditions for ethane and carbon dioxide gas hydrate formation in the presence of AOT-water-isooctane were carried out. Subsequently, the results were modelled using the model of van der Waals and Platteeuw in conjunction with a model for the activity of water in reverse micelles and the Peng Robinson equation of state. The predictions were seen to fit the experimental data well. [ABSTRACT FROM AUTHOR]

Abstract: The intrinsic kinetics of CO hydrate formation were investigated using a semi-batch stirred tank reactor equipped with a focused beam reflectance method (FBRM) probe for in situ particle size analysis. Experiments were carried out at temperatures ranging from 274 to 279K and pressures ranging from 1.6 to 3.0MPa. The experiments were designed to obtain the intrinsic rate constant using a modified version of the model of Englezos et al. (Chem. Eng. Sci. 42(1987a)2647) that incorporates the measured particle size data. It was found that the intrinsic rate constant for CO hydrate formation varied from to mol/mPas, with the minimum value occurring at roughly 277K. [Copyright &y& Elsevier]

Experimental data on the rate of decomposition of CO2 gas hydrates has been obtained using a semi-batch stirred tank reactor, with an in-situ particle size analyser, at temperatures ranging from 274 to 281 K and pressures ranging from 1.4 to 3.3 MPa. A method for calculating the moments of the particle size distribution has been presented. The experimental data was analysed using the kinetic model of Clarke and Bishnoi (Determination of the intrinsic kinetics of gas hydrate decomposition kinetics using particle size analysis, Presented at the Third International Conference on Gas Hydrates, Salt Lake City, Utah, July 18–22, 1999; Chem. Eng. Sci. 55 (2000) 4869) in its differential form in order to account for the slight change in temperature during the decomposition of CO2 hydrates. The applicability of the new instrument for measuring gas hydrate decomposition kinetics was examined by conducting experiments with ethane at conditions similar to those encountered by Clarke and Bishnoi (2000). It was seen that the previously obtained rate constants for ethane hydrate decomposition were able to predict the new obtained data. A new procedure for regressing the intrinsic rate constant and activation energy has also been described and it is seen that the activation energy is 102.88 kJ/mol and the intrinsic rate constant is 1.83×108 mol/(m2 Pa s) for CO2 gas hydrate decomposition. [Copyright &y& Elsevier]

An equation of state that can be used for phase equilibrium and other thermodynamic property calculations at high pressures is developed for systems that contain aqueous solutions of strong electrolytes and molecular species. The proposed equation of state is based upon contributions to the Helmholtz free energy from a non-electrolyte term and three electrolyte terms. The non-electrolyte term comes from the Trebble–Bishnoi equation of state and the electrolyte terms consist of a Born energy term, a mean spherical approximation term and a newly developed hydration term. The application of the proposed equation of state to aqueous systems containing mixed salts and mixed solvents is illustrated by calculating the vapour–liquid equilibrium (VLE) and solid (Clathrate hydrate)–vapour–liquid equilibrium (SVLE) conditions for several systems. The solubility of CO2 in salt water systems is examined at elevated pressures. As well, the new equation of state is used in conjunction with the model of van der Waals and Platteeuw to predict the SVLE conditions for gas hydrate forming systems in the presence of single salts, mixed salts and a mixture of aqueous salts and methanol. It is found that the new equation of state is able to accurately represent the experimental data over a wide range of pressure, temperature and salt concentration. [Copyright &y& Elsevier]

An implicit optimisation scheme is formulated to find the Kihara potential parameters for methane and ethane, using experimental hydrate formation data from both pure methane and ethane hydrates and hydrates formed from mixtures of methane and ethane. In this method, all three Kihara potential parameters, for both methane and ethane, are allowed to be adjustable parameters. Phase equilibria calculations, using the new Kihara potential parameters, exhibit a transition from structure I to structure II in hydrates formed from methane/ethane mixtures. This is consistent with experimentally observed behaviour. The use of Microsoft® Excel, an easily available tool, is illustrated for the determination of the Kihara potential parameters by implementing the formulated implicit optimisation scheme using gas hydrate equilibrium data. [Copyright &y& Elsevier]

Hydrate problems in industry have historically motivated modeling of hydrates and hydrate phase transition dynamics, and much knowledge has been gained during the last fifty years of research. The interest in natural gas hydrate as energy source is increasing rapidly. Parallel to this, there is also a high focus on fluxes of methane from the oceans. A limited portion of the fluxes of methane comes directly from natural gas hydrates but a much larger portion of the fluxes involves hydrate mounds as a dynamic seal that slows down leakage fluxes. In this work we review some of the historical trends in kinetic modeling of hydrate formation and discussion. We also discuss a possible future development over to classical thermodynamics and residual thermodynamics as a platform for all phases, including water phases. This opens up for consistent thermodynamics in which Gibbs free energy for all phases are comparable in terms of stability, and also consistent calculation of enthalpies and entropies. Examples are used to demonstrate various stability limits and how various routes to hydrate formation lead to different hydrates. A reworked Classical Nucleation Theory (CNT) is utilized to illustrate that nucleation of hydrate is, as expected from physics, a nano-scale process in time and space. Induction times, or time for onset of massive growth, on the other hand, are frequently delayed by hydrate film transport barriers that slow down contact between gas and liquid water. It is actually demonstrated that the reworked CNT model is able to predict experimental induction times. [ABSTRACT FROM AUTHOR]

Natural gas hydrates (primarily methane hydrates) are considered to be an important and promising unconventional source of hydrocarbons. Most natural gas hydrate accumulations exist in pore space and are associated with reservoir rocks. Therefore, gas hydrate studies in porous media are of particular interest, as well as, the phase equilibria of pore hydrates, including the determination of equilibrium pore water content (nonclathrated water). Nonclathrated water is analogous to unfrozen water in permafrost soils and has a significant effect on the properties of hydrate-bearing reservoirs. Nonclathrated water content in hydrate-saturated porous media will depend on many factors: pressure, temperature, gas composition, the mineralization of pore water, etc. In this paper, the study is mostly focused on the effect of hydrate-forming gas pressure on nonclathrated water content in hydrate-bearing soils. To solve this problem, simple thermodynamic equations were proposed which require data on pore water activity (or unfrozen water content). Additionally, it is possible to recalculate the nonclathrated water content data from one hydrate-forming gas to another using the proposed thermodynamic equations. The comparison showed a sufficiently good agreement between the calculated nonclathrated water content and its direct measurements for investigated soils. The discrepancy was ~0.15 wt% and was comparable to the accuracy of direct measurements. It was established that the effect of gas pressure on nonclathrated water content is highly nonlinear. For example, the most pronounced effect of gas pressure on nonclathrated water content is observed in the range from equilibrium pressure to 6.0 MPa. The developed thermodynamic technique can be used for different hydrate-forming gases such as methane, ethane, propane, nitrogen, carbon dioxide, various gas mixtures, and natural gases. [ABSTRACT FROM AUTHOR]

The correlating capabilities of four machine learning methods, the ordinary kriging method, an adaptive neuro-fuzzy interference system (ANFIS), a multi-layer artificial neural network (ANN) and a Hybrid of Fuzzy logic and Genetic Algorithm (HFGA), as well as the thermodynamics-based approach of van der Waals-Platteeuw (vdWP) are compared for CO 2 gas hydrates formed in the presence of thermodynamic promoters as well as for semi-clathrates formed from CO 2. These systems were chosen for testing the three methods due to their potential relevance in CO 2 capture and due to the expectation of them being computationally challenging. This is the first time that kriging has been tested for correlating gas hydrate equilibrium conditions. Different statistical indices, including the mean square error (MSE), an average absolute relative deviation (AARD), a correlation coefficient, and minimum and maximum errors, are employed to evaluate the performance of these methods. According to these performance indices, the ANFIS method performed the best among these methods; it predicted the equilibrium pressure with the highest accuracy. Finally, an outlier diagnosis is applied to the generated results to specify the reliability and uncertainty of the machine learning-based models. The simple-to-use machine learning tools are shown to be acceptable alternative to the vdWP methods and can be easily coupled with commercial simulation software to reduce calculation times while maintaining the accuracy. • An ANFIS, ANN, Ordinary Kriging, and a HFGA are employed for correlating the clathrate hydrate formation pressure of CO 2 in the presence of thermodynamic promoters. • The classic thermodynamic model based on the theory of van der Waals–Platteeuw is also used to correlate the same set of experimental data. • The model results are subsequently compared with the results obtained via the classical thermodynamic approach. • To the best of the authors' knowledge, no one has attempted to use these machine learning techniques and kriging method for modeling these systems. [ABSTRACT FROM AUTHOR]

Abstract: Semi-clathrates were formed in an isochoric equilibrium-cell, from gaseous mixtures of (CO2 +H2 +CO), in aqueous solutions of tetra-butyl ammonium bromide (TBAB). The experimental temperatures ranged between (282 and 291)K, experimental pressures ranged between (2.6 and 6.6) MPa and the experiments were conducted in aqueous TBAB solutions of w TBAB =(0.05, 0.10, and 0.20). The gas phase composition was analysed, using gas chromatography, and it was observed that the amount of CO2 in the gas phase was reduced. Additionally, the experimental results were compared to the formation conditions required for gas hydrates, that are formed from the same gas mixtures, and it was observed that the pressure required for semi-clathrate formation was significantly less than that required for gas hydrate formation. [Copyright &y& Elsevier]

Abstract: The three-phase (vapour+liquid+solid) equilibrium conditions for semi-clathrates formed from three mixtures of (CO2 +N2), in aqueous solutions of tetra-butyl ammonium bromide (TBAB), were measured in an isochoric reactor. The experiments were conducted at temperatures between (281 and 290)K, at pressures between (1.9 and 5.9)MPa and in aqueous TBAB solutions of wTBAB =(0.05, 0.10, and 0.20). The experimental results obtained in this study were compared with previously obtained results for gas hydrates, formed from the same three mixtures of (CO2 +N2) and it was observed that semi-clathrates formed at a substantially lower pressure than did gas hydrates. [Copyright &y& Elsevier]

Abstract: Although there are many uncertainties in hydrate dissociation process in porous media, numerical simulation gives useful information in evaluating economically feasible gas recovery processes from gas hydrate reservoirs. Furthermore, there are several unknown parameters involved in the numerical model and determination of accurate values of these parameters is essential for reliable production forecasts. One of these parameters is the variation of permeability of the porous media in the presence of hydrates. In this study the permeability to gas was experimentally determined at varying hydrate saturations in a porous medium made of packed glass beads. By comparing the experimentally determined permeability with those calculated using the empirical permeability correlations it was found that for initial water saturations less than 35%, hydrate tends to form on the grain surfaces. However, for initial water saturations greater than 35%, the experimental results indicate a pore filling tendency of hydrate formation. The experimental permeability values were also correlated with the Masuda et al.''s (1997) permeability model and a value of 3.0 was obtained for the permeability reduction exponent. To evaluate the impact of permeability reduction exponent on the dissociation process, a one-dimensional numerical model was developed for dissociation of gas hydrates in porous media by depressurization. The numerical model includes the three mechanisms i.e. kinetics of hydrate decomposition, heat transfer and fluid flow; that might be associated with the dissociation of hydrates in porous medium. The effect of permeability reduction exponent on the dissociation characteristics of hydrate was analyzed using this simulator. [Copyright &y& Elsevier]