Kinetics Of Methane Hydrate Dissociation Research Articles

Dissociation Kinetics of Methane Hydrate in Frozen Rocks under Decreasing External Pressure: Mathematical and Experimental Modeling

Dissociation Kinetics of Methane Hydrate in Frozen Rocks under Decreasing External Pressure: Mathematical and Experimental Modeling

Experimental investigation and modeling on the dissociation kinetics of methane hydrate in clayey silt cores in depressurization

Natural gas hydrate (NGH) in clayey silts might be the majority of its global reserve. A thorough understanding on the dissociation kinetics of NGH in clayey silts is crucial for its safe and efficient exploitation. In this study, the dissociation characteristics of methane hydrate in quartz silts, montmorillonites and South China Sea (SCS) sediments made cores in multi-step depressurization was experimentally studied and modeled. In quartz silt core, methane hydrate thoroughly dissociated once the pressured is lowered to the equilibrium pressure in bulk. In montmorillonite and SCS sediment cores, methane hydrate dissociation initiated at higher pressure than quartz silt. Capillary and clay induced water activity depressions are suggested to be the major causes. Stepwise methane hydrate dissociation was discovered in the montmorillonite and SCS sediment cores in multi-stage depressurization. In one step depressurization, hydrate in the fine grain cores with larger surface area dissociated faster. In montmorillonite and SCS sediment cores, lower water content leads to faster dissociation at the same pressure, which may be attributed to the higher driven force for dissociation (fugacity difference fe-f). A kinetic model for NGH dissociation in clayey silts was established by introducing fitted equivalent diameter and surface area correction factor of hydrate particles to the Kim-Bishnoi model. In the model, the hydrate equilibrium conditions in clayey silts were predicted by the water activity measurement (WAM) technique. With a maximum deviation of 9.81% in gas production, the model prediction and experimental results are in good agreement. The experimental findings and model reported in this work may be helpful to understand the NGH phase transition mechanism in clayey silt sediments.

Impact of lighter alkanes on the formation and dissociation kinetics of methane hydrate in oil-in-water dispersions relevant for flow assurance

Various crude oil fractions (saturates, aromatics, resins, and asphaltene) affect the hydrate formation cumulatively during the multiphase phase flow in offshore flowlines. This study investigates the impact of chain length of saturated hydrocarbons (nC5, nC6, nC7, and nC10) on methane hydrate formation and dissociation kinetics in oil-in-water dispersions at 275.15 K and 8 MPa. Sodium dodecyl sulfate (SDS) has been added (at 0, 500, 1000, and 1500 ppm concentration) to mimic the natural surfactants in the crude oil. Methane hydrate formation kinetics without surfactant does not correlate well with methane solubility trend in liquid alkanes (this study) due to narrow methane solubility difference along with poorly dispersed oil droplets without surfactant and stochastic hydrate induction behaviour. Increased SDS concentration (up to ∼1000 ppm) enhances the hydrate formation during initial stages due to enhanced oil–water interface following a decrease for higher concentration (∼1500 ppm) due to possible disruptions in the structure of local water by the hydrophilic surfactant part. At fixed surfactant concentration (∼1000 ppm), the hydrate formation is enhanced with increasing carbon number (nC6 < nC7 < nC10) during initial stages (∼10 h) due to lower methane solubility (increased methane availability in water) along with synergistic mechanism of alkane and SDS surfactant, before achieving similar formation kinetics during later stages. During dissociation, the gas release has been observably delayed at higher concentrations (∼1500 ppm) compared to lower (∼1000 ppm and below), possibly due to hydrate crystals trapping by surfactant molecules and methane adsorption on the hydrophobic surfactant part. A faster gas release has also been observed for systems with higher alkane at fixed surfactant concentration, possibly aided by lower methane solubility upon gaining enough thermal energy.

Influence of pipeline steel surface on the thermal stability of methane hydrate

The thermal stability and surface adhesion of natural gas hydrate are critical for the safety of oil and gas pipelines. The roughness and hydrophobicity of the pipe surface often vary during long-distance transportation, but it remains unclear about how these variances influence the hydrate stability. In this study, twelve molecular models of solid steel pipeline surfaces with random morphology were evaluated and molecular dynamics simulations were performed to gain insights into the kinetics of methane hydrate dissociation, the nucleation and growth of gas bubbles during hydrate decomposition, and the free energy of hydrate adhesion to the solid steel surface. Results demonstrated that the stability of methane hydrate could be decreased by up to 85% by increasing the hydrophobicity of the pipe surface by 52%. The bubble nucleation site of the gas released from hydrate decomposition shifted from bulk water to the solid surface by increasing the surface hydrophobicity (εsw: 3.73–5.74 kJ mol−1), but a highly hydrophobic surface (εsw: 2.73 kJ mol−1) made it hard to form gas bubble on either smooth or rough surface. Moreover, the free energy of hydrate adhesion also depended on the roughness and hydrophobicity of the solid surface, while the largest energy barrier for the adhesion of methane hydrate was found on the hydrophobic surface with high roughness. The findings from this study provided theoretical support for better understanding the methane hydrate evolution principles when the surface properties of the pipe wall changed from naturally occurred events (e.g., metal corrosion) or artificial treatment (e.g. chemical coating).

Dissociation kinetics of methane hydrate and CO2 hydrate for different granular composition

The dissociation of gas hydrate particles (methane hydrate and CO2 hydrate) in a layer with no coalescence of particles (with “true size” of particles), as well as that of pressed pellets have been studied experimentally. The activation energy of gas hydrate at negative temperatures was determined using the statistical method. Values of the kinetic constant and activation energy Ea = 78.3–81 kJ/mol, obtained in the works of Kim, Bishnoi, Clarke et al. at methane hydrate dissociation, are valid only at positive temperatures (temperatures above the melting point of ice). At negative temperatures, only the hydrate cell dissociation and the formation of ice without its melting are realized. In this case, the activation energy is reduced several times Ea = 33.5 kJ/mol. A sharp decrease in Ea at intense heat exchange will lead to faster kinetics of gas hydrate dissociation. The pre-exponential factor for the pressed layer (or pellets) can vary by an order of magnitude and reflects the filtration rate. If the dissociation rate (for a thick layer of coalesced particles or for pressed large pellets) is known from the experiment, then at given Ea = 33.5 kJ/mol, the “true diameter” of the particles can be calculated. This technique is important for the development of modern technologies of storage and transportation of natural gas hydrates, as well as for the study of natural gas hydrates in the permafrost zone.

Methane recovery from marine gas hydrates: A bench scale study in presence of low dosage benign additives

De-pressurization is one approach which has been found to be economically feasible for methane recovery from marine hydrates. Hydrate dissociation being an endothermic process suggests that de-pressurization alone would not be sufficient and some additional stimulation would be required for sustained production from one such reservoir. Thermal stimulation may overcome the challenge posed by the endothermic dissociation process; however, economically it may not be ideal. A possible way out is to use thermal stimulation, but at relatively low temperatures as compared to conventional practice. This would be economical and can be accomplished in the presence of small doses of additives mixed in with the water stream used for thermal stimulation. In the present study, a number of benign additives were identified which when used in low concentrations enhance the kinetics of methane hydrate dissociation compared to pure water. Additives were first shortlisted from a wide potential pool using quantum mechanical calculations. These additives were later tested for their efficacy in stirred tank reactor to quickly identify the best additives for the job and few selected additives were then studied in a larger bench scale setup (fixed bed configuration) where they were injected in the form of an additive-water stream to dissociate already formed hydrates. Factors such as toxicity of the additive, fluidity of additive-water stream, foam formation on mixing of additive with water, etc. were also taken into account. An energy and efficiency analysis revealed that reported additives enhance the energy ratio and thermal efficiency of the process as compared to pure water stimulation.

Formation and Dissociation Kinetics of Methane Hydrate in Aqueous Oilfield Polymer Solutions (Polyacrylamide, Xanthan Gum, and Guar Gum) and Their Performance Evaluation as Low-Dosage Kinetic Hydrate Inhibitors (LDHI)

Hydrate formation and deposition during drilling offshore oil and gas wells has been one of the major challenges faced by the oil and gas industry. The use of low-dosage hydrate inhibitor (LDHI), s...

Methane clathrate hydrate dissociation analyzed with Raman spectroscopy and a thermodynamic mass transfer model considering cage occupancy

The objective of this work was to investigate diffusion phenomena in methane clathrate hydrates during depressurization that lead to their dissociation. A thermodynamic mass transfer model is proposed that considers cage occupancy in structure I clathrate hydrates and methane diffusion from both S-cages (dodecahedron: 12 pentagons) and M-cages (tetradecahedron: 12 pentagons, 2 hexagons). Methane occupancies during dissociation were measured in situ with Raman spectroscopy with a newly-constructed optical system and data were estimated with Langmuir constants calculated by assuming a spherically symmetric cell potential. Model results were in agreement with experimentally-measured changes in average occupancy with time, so that dissociation kinetics at the interface were concluded to be strongly related to both methane diffusion and cage occupancy. Dissociation kinetics of methane hydrate can be reliably estimated with the proposed model. The proposed model is applicable to the study of methane hydrate dissociation-limited mechanisms or to larger-scale geological systems being used to estimate gas production rates.

Methane hydrate dissociation in the presence of novel benign additives

The apparent drawbacks of the classical approaches towards dissociation of natural gas hydrates have resulted in a paradigm shift into the development of new hybrid hydrate dissociation practices combining the various basic hydrate dissociation techniques. Another approach that can be followed to maximize the efficiency of gas production from natural gas hydrate reserves is the identification of benign additives which when used even in sparingly small concentrations may enhance the kinetics of hydrate dissociation. In the present work, a class of such additives, never reported before, have been unveiled and christened as Low Dosage Hydrate Dissociation Promoters (LHDPs). The additives were first short listed from a wide potential pool using a lab scale (~250 ml) stirred tank reactor setup and then further studied using a bench scale (~2.35 l) reactor setup where they were injected in the form of a water-additive stream to dissociate hydrates. The dissociation approach followed in the case of the bench scale reactor experiments was a combination of the thermal stimulation and depressurization processes along with the element of injection of additives. For both sets of experiments (lab and bench scale), the newly identified LHDPs were found to enhance the kinetics of methane hydrate dissociation as compared to pure water. It was observed that concentration of additive and its flow rate also affect the kinetics of methane hydrate dissociation. An energy and efficiency analysis for the hydrate dissociation method in the case of bench scale rector revealed that additive presence enhanced the energy ratio and thermal efficiency four fold as compared to pure water.

Diffusion model of gas hydrate dissociation into ice and gas: Simulation of the self-preservation effect

The diffusion model of gas hydrate dissociation into ice and gas is presented. This model takes into account the possible porous structure of the formed layer of ice as well as the intrinsic kinetics of the process of gas hydrate dissociation into ice and gas. Specifically, the problem of the dissociation of a spherical gas hydrate particle into ice and gas was considered. In the framework of a quasi-stationary approximation, a simplified solution to this problem was obtained. From the comparison between the calculated data obtained in the framework of a quasi-stationary approximation and the available experimental data, the parameters of the developed model responsible for the kinetics of methane hydrate dissociation into ice and gas were estimated. In the framework of the developed diffusion model, an explanation for the anomalous preservation thermal regime of gas hydrates is given. From this explanation it follows that for some hydrate-forming gases such a regime may not occur.

Effect of molecular weight of polyethylene glycol (PEG), a hydrate inhibitive water-based drilling fluid additive, on the formation and dissociation kinetics of methane hydrate

Polyethylene glycol (PEG), a water-soluble polymer, has been widely used as the most promising polymer for designing hydrate inhibitive water-based drilling fluids. Studies involving the kinetics of methane hydrate formation and dissociation in PEG aqueous solutions may help to develop effective hydrate inhibitive drilling fluids for efficient drilling operation in deep-offshore and hydrate bearing formation. In this work, formation and dissociation kinetics of methane hydrate system have been investigated in an aqueous solution of PEG (with number average molecular weight of 200 and 600 kg/kmol; referred to as PEG-200 and PEG-600, respectively). Different concentrations of PEG, such as 0.2 and 0.4 mass fractions, have been considered for the investigations. Various experiments have been carried out at an initial formation pressure of 7.5 MPa and 5.5 MPa and 263.15 K temperature. The information on the number of moles of methane consumed during hydrate formation, rate of hydrate formation, water-to-hydrate conversion, gas-to-hydrate conversion, number of moles of gas recovered during hydrate dissociation, and rate of hydrate dissociation are being provided in this study. This study provides useful information to use effective combinations of lower and higher molecular weight polymeric systems to develop effective hydrate inhibitive water-based drilling fluid systems suitable for offshore drilling and for hydrate bearing formations.

Kinetics of methane hydrate dissociation

The methane hydrate dissociation at an external pressure of 1 bar has been experimentally studied. The methane dissociation rate depends on the heat transfer intensity in the course of dissociation. Quasi-isothermal and nonisothermal conditions of clathrate dissociation have been compared. Under nonisothermal conditions, the dissociation rate depends on the heat flux density. The lowest heat flux was achieved in a quasi-stationary mode, and the highest heat flux was achieved through combustion of methane released from the clathrate powder. The 54-fold increase in heat flux leads to the ninefold increase in the methane dissociation rate. Depending on the heat flux density, the following variants of clathrate dissociation were observed: without self-preservation, partial self-preservation with one dissociation rate minimum, and partial self-preservation with two minima.

Effect of Bubble Formation on the Dissociation of Methane Hydrate in Water: A Molecular Dynamics Study

We investigate the dissociation of methane hydrate in liquid water using molecular dynamics simulations. As dissociation of the hydrate proceeds, methane molecules are released into the aqueous phase and eventually they form bubbles. It is shown that this bubble formation, which causes change in the methane concentration in the aqueous phase, significantly affects the dissociation kinetics of methane hydrate. A large system size employed in this study makes it possible to analyze the effects of the change in the methane concentration and the formation of bubbles on the dissociation kinetics in detail. It is found that the dissociation rate decreases with time until the bubble formation and then it turns to increase. It is also demonstrated that methane hydrate can exist as a metastable superheated solid if there exists no bubble.

Nonstationary Combustion of Methane with Gas Hydrate Dissociation

Kinetics of methane hydrate dissociation at combustion in the air atmosphere at the outer pressure of 1 bar was studied experimentally. Dissociation is divided into several characteristic time stages with different rates of gas hydrate decomposition. There is a very high temperature gradient along the granule radius. At combustion the movement velocities of dissociation front and pores are high. Thermal imaging of the powder surface demonstrates significant temperature maldistribution. Spatially nonuniform escape of methane from a sample leads to local temperature and concentration maldistributions in the flame, which cause thermal instability of the flame front and the loss of hydrodynamic stability. Instability of combustion generates two equivalent mechanisms of convective transfer: the shear laminar flow and large-scale vortices, which intensify fuel and oxidizer mixing. Experimental data have revealed not only the local spatial-time nonuniformities but also significantly nonstationary character of the averaged flow. The measured instantaneous and average velocities of methane on the powder surface allowed an estimate of the stoichiometric ratio. The maximal velocity of methane propagation corresponded to 1 mm/s in experiments without combustion and to 15 mm/s with combustion.

Effect of heat transfer on the kinetics of methane hydrate dissociation

The dissociation of methane hydrate under external pressure of 1bar is studied experimentally. Non-isothermal dissociation is fundamentally different from the quasi-isothermal case. The increase in the density of heat flux from 255 to 13700W m−2 results in 9-fold increase in the dissociation rate of methane hydrate. Different variations of clathrates dissociation may be observed depending on the heat flux magnitude: (1) without self-preservation (high heat fluxes), (2) a partial self-preservation with one minimum of dissociation rate, and (3) a partial self-preservation with two minimums (low heat fluxes). When describing dissociation kinetics of the spherical granules, it is important to know the time dependence of the ice layer thickness growth. It is shown that not the curvature, but the heat flux value regulates the dissociation rate and the change in diffusion. A drastic change in dissociation rate is caused by a pressure decrease in pores.

Combustion of methane hydrates

Gas hydrates of methane were obtained under the laboratory conditions, and their structural characteristics were determined there. Kinetics of methane hydrate dissociation in the air atmosphere was studied experimentally. The combined effect of high temperature and pressure gradients was studied and it was shown that in practice it is necessary to take into account the nonstationary heat and mass transfer processes together. The direct measurements of clathrate mass and thermal imaging allowed distinguishing for several typical stages of dissociation with significantly different dissociation rates. The weakly expressed mechanism of self-preservation (the maintenance of residual state of gas in the crystalline lattice at the temperature, which exceeds significantly the equilibrium temperature) was observed in experiments.

Study on Dissociation Behaviors of Methane Hydrate in Porous Media Based on Experiments and Fractional Dimension Shrinking-Core Model

The dissociation kinetics of methane hydrate in the silica gels as the porous media are studied when the temperatures are above the quadruple-phase (hydrate(H)–water(LW)–ice(I)–vapor(V)) point temperatures. The dissociation experiments were carried out by depressurization in the temperature range of 269.15–278.15 K and the initial formation pressure range of 4.1–11.0 MPa. Four silica gels, with pore sizes of 9.03, 12.95, 17.96, and 33.2 nm (the particle size distribution is 0.105–0.15 mm) were used. The experimental results shows that the rate of methane released from the hydrate dissociation increases with the increase of the initial formation pressure, the decrease of the environmental temperature, and the increase of the pore size. The temperature in the hydrate crystallizer first immediately has an obvious drop in the process of the hydrate dissociation and then rises gradually to the environmental temperature after it reaches the lowest temperature point. Based on the fractal theory and the shrinking-core model, a fractional dimension shrinking-core model is established for the correlation and prediction of the dissociation kinetic behaviors of methane hydrate in the porous media above the corresponding quadruple-phase point temperatures. The results are in good agreement with the experimental data.