Kinetics Of Hydrate Dissociation Research Articles

Experimental studies and pore-scale modeling of hydrate dissociation kinetics at different depressurization rates in a cubic hydrate simulator

Natural gas hydrate (NGH) is a promising energy resource that has triggered a race for scientific innovation in energy and environmental research. However, there still faces the challenges of low-efficiency hydrate dissociation and unstable gas production during the hydrate production. Clarifying the kinetic mechanism of hydrate dissociation is the key to improving the efficiency of “phase-transition to gas-supply” in hydrate-bearing sediment. This study successfully conducts hydrate dissociation experiments with different depressurization rates in the Cubic Hydrate Simulator (CHS). A novel dissociation kinetic model integrating the hydrate pore-scale morphology and its evolution is established, and the sensitivities of the kinetic and thermal parameters are discussed in detail. The mathematical relationship between the hydrate saturation and reaction area is quantitatively analyzed for the hydrate formation and dissociation kinetics. The history matching simulation with the newly developed model achieves an excellent agreement with the experimental results. The full-lifecycle kinetic behaviors of the hydrate dissociation in the depressurization experiments are finely characterized. It is found that the evolutions of the hydrate saturation (SH) and temperature (T) spatial distributions exhibit strong heterogeneous characteristics in the vessel due to the phase transition and heat transfer effects. In addition, this reveals that the relatively high depressurization rate can lead to a more significant dissociation rate in the depressurization stage. This provides a new reliable and adaptable method for describing the hydrate dissociation kinetics in the porous media.

Amino acid-assisted effect on hydrate-based CO2 storage in porous media with brine.

CO2 storage as hydrates in porous media is a promising method for storing carbon dioxide (CO2). However, the sluggish formation kinetics of hydrates urge the need to focus on the use of additives (promoters) to accelerate hydrate kinetics. This study investigates the effect of amino acid solutions in brine on CO2 hydrate formation and dissociation kinetics in quartz sand particles QS-2 (0.6-0.8 mm) with 38% porosity. The amino acids l-methionine (l-meth), l-isoleucine (l-iso), and l-threonine (l-threo) were studied at 0.2 wt% using an autoclave hydrate reactor at 4 MPa and 274.15 K in the presence and absence of salt (3.3 wt% NaCl) in 100% water saturation. The hydrate dissociation kinetics was studied at a temperature of 277.15 K. These conditions represent the normal seabed temperature range in Malaysia and hence were used for testing CO2 hydrate formation and dissociation kinetics in quartz sand in this study. Further, CO2 hydrate formation and dissociation experiments were conducted with sodium dodecyl sulphate (SDS) and brine systems as standards for comparison. The findings reveal the best kinetics for l-meth exhibiting the highest CO2 hydrate storage capacity. l-meth recorded a gas-to-hydrate conversion ratio of about 93% at 0.2 wt% in quartz sand with brine. Moreover, l-meth exhibited the lowest hydrate dissociation rate compared to l-iso and l-threo systems, thereby enhancing CO2 hydrate stability in quartz sand. Comparatively, l-meth enhanced the storage capacity by 36% and reduced the induction time by more than 50% compared to conventional promoter SDS in quartz sand with brine, suggesting it to be favorable for CO2 storage applications. CO2 hydrate nucleation time was predicted in quartz sand with and without the best-studied amino acid l-meth system with high prediction accuracy and an absolute average deviation of 2.4 hours. The findings of this study substantiate the influence of amino acids in promoting the storage capacity of CO2 in sediments as hydrates.

Investigation of Hydrate Formation and Stability of Mixed Methane-THF Hydrates: Effects of Tetrahydrofuran Concentration.

Effects of tetrahydrofuran (THF) concentration on the mixed methane hydrate formation, dissociation, and stability were investigated. The experiment was conducted at 286.2 K and 6 MPa in a quiescent reactor. The presence of THF below 2.5 mol% did not show the evidence of hydrate formation. However, the concentration above 2.5 mol% enhanced the methane formation rate and the methane consumption. Increasing the THF concentration decreased the induction time as the result of the decrease in the surface tension. Moreover, the methane uptake and formation rate increased with the THF concentration due to the higher degree of hydrate nucleation. The methane recovery after the dissociation experiment showed up to 96%. Furthermore, the hydrate stability increased, and the hydrate dissociation kinetics decreased with the increase in the THF concentration. The optimum THF concentration to enhance and improve the hydrate formation kinetics and stability is its stoichiometric concentration.

Comparison of fluid production between excess-gas and excess-water hydrate-bearing sediments under depressurization and its implication on energy recovery

Methane hydrates are considered as the future energy due to its vast resource volume and high energy density. The fluid production and thermal response of two types hydrate-bearing sediments (i.e., excess-gas and excess-water) under controlled depressurization are still unclear and warrant investigation. In this study, we devised two different hydrate-bearing sediments (HBS) synthesis methods and synthesized excess-water (SA = ∼27.2%) and excess-gas (SG = ∼26.5%) HBS with SH of 72.0%. The hydrate dissociation kinetics and fluid production behavior were examined under three bottom-hole pressures, i.e., 3.0, 5.0, and 7.0 MPa. Gas production from the excess-gas HBS follows two-stage profile, while continuous gas production was observed after SG reaches 6.0% in the excess-water HBS. Water production from excess-gas HBS was significantly delayed compared with excess-water HBS and only started when SA reached above 22.5%. A logarithmic water production profile was observed in all cases. Rapid temperature drop due to hydrate dissociation is significantly delayed in excess-gas cases. Heat transfer from surroundings is relatively slow due to its low composite thermal conductivity. The findings on the contrast fluid production behavior between excess-gas and excess-water cases shed light on optimizing production strategies for future field production trials from these two different types reservoirs.

CO2-TBAB semi-clathrate hydrate dissociation behaviour in individual capsules: A new two-stage numerical model and parametric study

Gas hydrates have a wide implementation in the capture, storage, transport and utilisation of a range of gases, in which the dissociation kinetics is of great importance to gas transport and recovery efficiency. Encapsulation has been proven an effective technique to enhance gas–liquid mass transfer and thus improve gas hydrate formation kinetics. However, the dissociation behaviour of gas hydrate in individual capsules remains unknown. In this work, the dissociation kinetics of encapsulated CO2-TBAB semi-clathrate hydrates in different shapes are experimentally investigated under various operating conditions, and, for the first time, a two-stage numerical model is developed which integrates mass transfer, heat transfer and the intrinsic gas hydrate reaction kinetics to simulate the gas hydrate dissociation process. The effects of temperature, pressure, capsule volume and capsule geometry on gas hydrate dissociation kinetics are investigated. The results reveal that the proposed model is capable of capturing two distinct experimentally-observed dissociation stages, a rapid and subsequently slow stage according to the measured dissociation rate. Surface-to-volume ratio of the capsule and the dissociation driving force are the two main factors influencing the dissociation kinetics. The ring-shaped capsule exhibits the most efficient dissociation process due to its high surface-to-volume ratio and long transition time to the slow dissociation stage. This work enhances the understanding of gas hydrate dissociation behaviour in individual capsules and guides the manipulation of dissociation for efficient hydrate-based gas transport and recovery.

Roles of montmorillonite clay on the kinetics and morphology of CO2 hydrate in hydrate-based CO2 sequestration

Hydrate-based CO2 sequestration in marine sediments has emerged as a novel sustainable technology for long-term and stable CO2 sequestration. However, the role of clay minerals in CO2 hydrate formation and dissociation in clay-rich sediments, which are widely distributed in the South China Sea, remains controversial due to a lack of experimental evidence. This study investigates the effect of a representative clay mineral, sodium montmorillonite (Na-MMT), on the nucleation and growth kinetics of CO2 hydrate in suspensions with a msass fraction below 20.0 wt%. The kinetic experiments reveal that Na-MMT significantly reduces the induction time due to the additional nucleation sites provided by the delamination of clay particles and the induced surface electric field. While the average growth rate of CO2 hydrate is reduced by ∼72 % for Na-MMT mass fraction above 5.0 wt%. Morphologically, hydrate-clay stratification and gas-tunneling behaviors were observed, providing explanations for the retarded kinetics. The mass transfer of CO2 from the gas phase to the liquid phase is impeded by the high viscosity of the suspension and the clay-induced strongly-polarized water layer, which retards the overall kinetics of CO2 hydrate formation. Upon thermal stimulation, partial CO2 hydrate dissociation was observed within the pure CO2 hydrate stability region for 10.0 wt% Na-MMT suspension, indicating a possible impact of Na-MMT on CO2 hydrate thermodynamics. Moreover, the addition of Na-MMT clay promotes CO2 hydrate dissociation kinetics significantly. This study provides fundamental insights into the interaction between Na-MMT clay and CO2 hydrate, offering new perspectives for designing effective strategies for CO2 sequestration in abundant clay-rich marine sediments.

Effects of nanobubbles on methane hydrate dissociation: A molecular simulation study

Hydrate dissociation is often accompanied by the formation of nanobubbles. Knowledge of the effects of nanobubbles on hydrate dissociation is essential for understanding the dynamic behavior of the hydrate phase change and improving the gas production efficiency. Here, molecular dynamics simulations were performed to study the methane hydrate dissociation kinetics with and without a pre-existing methane nanobubble. The results show that the hydrate cluster in the liquid phase dissociates layer-by-layer. This process is shown to be independent of the temperature and nanobubble presence at the simulation conditions. Hydrate dissociation does not always lead to nanobubble formation because the supersaturated methane solution can be stable for a long time. A steep methane concentration gradient was observed between the hydrate cluster surface and the methane nanobubble, which can enhance the directional migration of methane and effectively minimize the methane concentration in the liquid phase, thereby increasing the driving force for the hydrate dissociation. Our findings indicate that the presence of a nanobubble near the hydrate surface does not decrease the activation energy of hydrate dissociation, but it can increase the intrinsic decomposition rate. The average hydrate dissociation rate is linearly correlated with the mass flow rate towards the nanobubble. The mass flow rate is determined by the nanobubble size and hydrate-nanobubble distance. Our findings contribute to the fundamental understanding of the dissociation mechanism of gas hydrates in the liquid phase, which is crucial for the design and optimization of efficient gas hydrate production techniques.

Facilitating gas hydrate dissociation kinetics and gas migration in clay interlayer by surface cations shielding effects

Knowledge of the kinetics of the gas hydrate dissociation in microporous sediments is a critical and fundamental issue for improving the gas recovery from fine-grained natural gas hydrate (NGH) reservoirs. Herein, molecular dynamics (MD) simulations were performed to investigate methane hydrate dissociation in clay sediments. Two initial configurations which comprised liquid bulk and clay mineral phases were adopted, and the later phase comprised two illite sheets and occupied by a hydrate phase with clay cations in and out of the interlayer. The results show that methane nanobubbles form at liquid water/hydrate interface for the cations in system, not like forming spherical cap-shaped nanobubbles on the mineral surface for the cations out system. This phenomenon can be attributed to the cations stably adsorbing on the clay surface, the cation film and ion hydration effect shielding the dissociated methane close to the solid surface. An increasing amount of released methane (approximately three times) diffuses from the pore phase to the bulk phase as driven by the Brownian motion in the cations in the system, as compared with the dissociation in cations out system and sandy pores; however, the diffusion of the released water is the opposite. Additionally, the injection speeds of heat flow against porosity were estimated to maintain a constant dissociation rate in the illite mineral hydrate reservoir. This study provides guidance for the scientific optimization of hydrate reservoirs, and for subsequent gas production from sediments with low permeabilities.

Kinetic insight on CO2 hydrate formation and dissociation in quartz sand in presence of brine

CO2 storage as hydrates in porous media is a promising method to sequester carbon dioxide (CO2) that need to be comprehended to guide its potential site selection prospects. In this study, CO2 hydrate formation and dissociation were studied in three quartz sand particles with varying petrophysical properties such as surface area, pore size, and porosities. The studied quartz sand particle sizes are QS-1 (<0.6 mm), QS-2 (0.6–0.8 mm), and QS-3 (0.8–2.0 mm). The hydrate formation experiments were conducted in a high-pressure hydrate reactor with and without brine (3.3 wt.% NaCl) at 4 MPa and temperatures of 274.15 K and 277.15 K in 100% sample saturation. The hydrate dissociation kinetics were studied at two different temperatures of 277.15 K and 279.15 K. The results show that hydrates form faster in deionized water (DI) than brine solution. The presence of brine reduced the CO2 storage capacity by 30% compared with deionized water. In addition, the presence of brine reduces the stability of CO2 hydrates. CO2 hydrate formation and dissociation behavior in porous media is highly controlled by the quartz sand porosity, pore size, and surface area. The quartz sand particles with low porosity were found to efficiently store more CO2 with a fast onset hydrate formation time of 6 hr and high gas to hydrate conversion of 55%. However, the CO2 hydrates in the sand particles with low porosity were less stable and averagely prolonged the time take to release 90% of CO2 of the systems with high porosity by 39%.

Experimental and Modeling Study of Kinetics for Hydrate Decomposition Induced by Depressurization in a Porous Medium

The hydrate decomposition kinetics is a key factor for the gas production from hydrate-saturated porous media. Meanwhile, it is also related to other factors. Among them, the permeability and hydrate dissociation surface area on hydrate dissociation kinetics have been studied experimentally and numerically in this work. First, the permeability to water was experimentally determined at different hydrate saturations (0%, 10%, 17%, 21%, 34%, 40.5%, and 48.75%) in hydrate-bearing porous media. By the comparison of permeability results from the experimental measurements and theoretical calculations with the empirical permeability models, it was found that, for the lower hydrate saturations (less than 40%), the experimental results of water permeability are closer to the predicted values of the grain-coating permeability model, whereas, for the hydrate saturation above 40%, the tendencies of hydrate accumulation in porous media are quite consistent with the pore-filling hydrate habits. A developed two-dimensional core-scale numerical code, which incorporates the models for permeability and hydrate dissociation surface area along with the hydrate accumulation habits in porous media, was used to investigate the kinetics of hydrate dissociation by depressurization, and a “shrinking-core” hydrate dissociation driven by the radial heat transfer was found in the numerical simulations of hydrate dissociation induced by depressurization in core-scale porous media. The numerical results indicate that the gas production from hydrates in porous media has a strong dependence on the permeability and hydrate dissociation surface area. Meanwhile, the simulation shows that the controlling factor for the dissociation kinetics of hydrate switches from permeability to hydrate dissociation surface area depending on the hydrate saturation and hydrate accumulation habits in porous media.

Modeling of hydrate dissociation surface area in porous media considering arrangements of sand grains and morphologies of hydrates

The volumetric surface area of hydrates (Ah) in porous media is a key factor involved in hydrate kinetics as it determines the dissociation rate and gas production behavior. Oversimplification of the pore structure and morphologies of hydrates in porous media can cause large errors when modeling Ah. In this study, considering the multiple packing arrangements of sand grains and the structural morphologies of hydrates in porous media, a unified estimation model was developed to describe the evolution of Ah with hydrate dynamic dissociation. An average error of 4.03% was observed between the model and experimental results quoted in literature. Two evolution trends of Ah with hydrate saturation were found because of the change in the structural morphologies of the hydrates. Under the premise of the same porosity, compared with the cubic arrangement of sand grains, the Ah of the orthorhombic, tetragonal-sphenoidal, and rhombohedral arrangements were reduced by approximately 5.61%, 10.37%, and 12.15%, respectively. An increase in porosity due to the change in the packing arrangement of sand grains, and the distance between the adjacent sand grains led to a reduction in Ah by approximately 7.71% and 2.79%, respectively. Ah decreased exponentially with the size of the sand grains, which seemed to be the most critical factor in Ah estimation, and a fourfold increase in sand grain size (from 100 μm to 400 μm) reduced Ah by approximately 74.45%. These findings could help to reveal the evolution mechanism of Ah with hydrate dynamic dissociation and provide a more accurate estimation of the hydrate dissociation kinetics.

Determining the controlling mechanisms of hydrate dissociation front using optimized characteristic time

The advance of the hydrate dissociation front is crucial to both the energy recovery process and the geological hazards prevention. In this work, the role of the scale of gas hydrate reservoirs in the controlling mechanisms is investigated numerically with the lab-scale and field-scale models. It is suggested that the scale of hydrate reservoir is not reliable in discriminating the three controlling mechanisms, i.e., fluid-flow (FF), heat-transfer (HT), and hydrate-dissociation-kinetics (HDK). For instance, the controlling mechanism of both the lab- and the field-scale hydrate reservoirs both might be FF as revealed in this work. Therefore, an approach of characteristic time is proposed to discriminate controlling mechanisms by employing dimensionless analyses. Also, mathematical models of the velocity of hydrate dissociation front conditioned to the single controlling mechanisms have been established, which are used to optimize the characteristic times. According to the optimized results, single characteristic times are sufficient for the HT and HDK controlling mechanisms. For instance, the time consumed to transfer the heat in the hydrate reservoirs by heat conduction through the gas phase may serve as the characteristic time of the HT controlling mechanism. As for the FF controlling mechanism, two different characteristic times should be used, i.e., a gas-phase based characteristic time for the initial stage and a water-phase based one for the later stage. The optimized characteristic times are capable of capturing the variation of hydrate reservoir controlling mechanism with geological and production parameters.

Investigation of the methane hydrate surface area during depressurization-induced dissociation in hydrate-bearing porous media

The surface area of hydrate dissociation in porous media has a dependence on the morphology of hydrate (blue) in sediments (brown): (a) grain-coating type, (b) pore-filling type and (c) cementing or bridging type. The surface area of hydrate during dissociation in porous media is essentially important for the kinetics of hydrate dissociation. In this study, the methane hydrate surface area was investigated by the comparison results of experiments and numerical simulations during hydrate decomposition in porous media. The experiments of methane hydrate depressurization-induced dissociation were performed in a 1D high pressure cell filled with glass beads, an improved and valid 1D core-scale numerical model was developed to simulate gas production. Two conceptual models for hydrate dissociation surface area were proposed based on the morphology of hydrate in porous media, which formed the functional form of the hydrate dissociation surface area with porosity, hydrate saturation and the average radius of sand sediment particles. With the establishment of numerical model for depressurization-induced hydrate dissociation in porous media, the cumulative gas productions were modeling and compared with the experimental data at the different hydrate saturations. The results indicated that the proposed prediction equations are valid for the hydrate dissociation surface area, and the grain-coating surface area model performs well at lower hydrate saturation for hydrate dissociation simulation, whereas at higher hydrate saturation, the hydrate dissociation simulation from the pore-filling surface area model is more reasonable. Finally, the sensitivity analysis showed that the hydrate dissociation surface area has a significant impact on the cumulative gas production.

Molecular dynamics simulation of the effects of different thermodynamic parameters on methane hydrate dissociation: An analysis of temperature, pressure and gas concentrations

Molecular dynamics (MD) simulation plays an important role in the microscopic analysis of the gas hydrate phase transition. This technology enables the feasible analysis of the effects of thermodynamic parameters (such as temperature, pressure and chemical potential) on the hydrate dissociation kinetics. Molecular dynamics simulations were performed in this study to analyze the effects of multiple factors on methane hydrate (MH) dissociation in an aqueous environment using NPT ensembles and to further understand the mechanism of hydrate dissociation at the microscopic level. Three thermodynamic factors, temperature (285–300 K), pressure (5–40 MPa) and the initial concentration of CH4 in the liquid phase (0–0.03125), were analyzed. Moreover, the potential energy, dissociation rates, and statistical analyses of concentrations, hydrogen bonds and molecule numbers in each phase were determined. The activation energy of hydrate was calculated to determine the effect of temperature on the dissociation of hydrate, and the activation energy of CH4 hydrate at 285–300 K was approximately 83.9 kJ/mol. Pressure variations exerted an obvious effect on hydrate dissociation when the pressure was less than 20 MPa. In contrast, little effect was observed when the pressure was greater than 20 MPa. The potential explanation is that an increase in the pressure will increase the solubility of CH4 and decrease the size of the gas phase, thus limiting the effect of pressure on the simulated system. Using simulations with different initial CH4 concentrations, an increase in the number of methane molecules in the surrounding aqueous phase decreased the dissociation rate when the initial methane concentration was less than 0.02 (molar fraction). However, it exerted the opposite effect when the initial concentration was greater than 0.02, due to the earlier appearance of gas bubbles.

3D measurements of hydrate surface area during hydrate dissociation in porous media using dynamic 3D imaging

A better understanding of the kinetics of hydrate dissociation is essential to reliably predict gas production potential from natural hydrate reservoirs. Most hydrate dissociation models assume hydrates to be a constant number of equal-sized spheres dissociating at a constant rate. This paper uses dynamic 3D synchrotron micro-computed tomography (SMT) imaging to study hydrate surface area evolution during Xenon hydrate dissociation. Hydrates are formed inside a high-pressure low-temperature cell filled with partially saturated ASTM 20-30 Ottawa sand. Hydrate dissociation is initiated through depressurization in the first experiment and through thermal stimulation in the second experiment. During dissociation, continuous full 3D SMT images were acquired where each scan took 45 s to complete. A combination of cementing, pore-filling, and surface coating pore habits were observed for the depressurization experiment and pore-filling for the thermal stimulation experiment. Surface coating hydrates dissociate faster than hydrates with pore-filling pore habit due to the higher specific area which allows for more surface for hydrates to dissociate it. Direct measurements of hydrate volume and hydrate surface area suggest that even with a combination of hydrate pore habits formed within the 3D porous media, estimation of hydrate surface area as a linear relation with (hydrate volume)2/3 is best for hydrate saturation less than a threshold value depending on the dissociation method and driving force. (hydrate volume)2 and (hydrate volume)3 were found to better estimate hydrate interfacial area in comparison to (hydrate volume)2/3 for the depressurization experiment and thermal stimulation experiment, respectively.

Dynamic 3D imaging of gas hydrate kinetics using synchrotron computed tomography

The availability of natural gas hydrates and the continuing increase in energy demand, motivated researchers to consider gas hydrates as a future source of energy. Fundamental understanding of hydrate dissociation kinetics is essential to improve techniques of gas production from natural hydrates reservoirs. During hydrate dissociation, bonds between water (host molecules) and gas (guest molecules) break and free gas is released. This paper investigates the evolution of hydrate surface area, pore habit, and tortuosity using in-situ imaging of Xenon (Xe) hydrate formation and dissociation in porous media with dynamic three-dimensional synchrotron microcomputed tomography (SMT). Xe hydrate was formed inside a high- pressure, low-temperature cell and then dissociated by thermal stimulation. During formation and dissociation, full 3D SMT scans were acquired continuously and reconstructed into 3D volume images. Each scan took only 45 seconds to complete, and a total of 60 scans were acquired. Hydrate volume and surface area evolution were directly measured from the SMT scans. At low hydrate saturation, the predominant pore habit was surface coating, while the predominant pore habit at high hydrate saturation was pore filling. A second-degree polynomial can be used to predict variation of tortuosity with hydrate saturation with an R2 value of 0.997.

Hydrate dissociation conditioned to depressurization below the quadruple point and salinity addition

Techniques have been developed to evaluate the hydrate dissociation behaviour in a brine system by reducing pressure below the quadruple point. Experimentally, hydrate with low initial pressure and temperature is dissociated by a deep depressurization method (DDM) in a one-dimensional (1D) model to evaluate the gas and water production. Theoretically, simulation techniques have been developed to determine the two-phase relative permeability, permeability variation, intrinsic hydrate reaction constant by piecewisely fitting the experimental measurements with consideration of the temperature depression induced by NaCl. Subsequently, the existing semi-empirical permeability reduction models are compared with the simulated average permeabilites to identify the occurrence of hydrate in pores. It is shown from experiments that salinity enhances the potential of gas recovery by enhancing the dissociation driving force (i.e., increasing equilibrium pressure and decreasing equilibrium temperature). Temperature decreases almost in a constant rate at sensible heat dominant stages, while, after reaching the freezing point, it is decreased lowly and even recovered slightly due to heat conduction from boundary and ice fusion. By approximating the energy contributions, it is found that sensible heat is supplied for hydrate dissociation at the initial stage and that heat conduction from boundary and ice fusion accelerates hydrate dissociation at the final stage. It is concluded that the salinity lowers the freezing point from −1.7 to −1.3 °C in the experiment. For numerical simulation, the newly developed dissociation model is found to be able to not only identify fluid flow mechanisms and kinetic hydrate dissociation, but also reproduce ice fusion. With hydrate dissociation and water production, salinity continuously decreases and finally becomes less than its initial value. The Kozeny grain center-filling model is found to be able to capture the permeability variation, implying that hydrate tends to be pore filling dominant in the experiments. The fluid flow and hydrate dissociation kinetics are found to play a dominant role above the freezing point, while, below the freezing point, the significance of heat conduction from boundary and ice fusion is enhanced during the experiments.

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.

Simulating the chemical kinetics of CO2-methane exchange in hydrate

Carbon dioxide exchange with methane in the clathrate structure has been shown beneficial in laboratory experiments and has been suggested as a field-scale technique for production of natural gas from gas-hydrate bearing sediments. Furthermore, the method is environmentally attractive due to the formation of CO2-hydrate in the sediments, leading to the geosequestration of carbon dioxide. However, the knowledge is still limited on the impact of small-scale heterogeneities on hydrate dissociation kinetics. In the present study, we developed a model for simulating laboratory experiments of carbon dioxide injection into a pressure vessel containing a mixture of gas hydrate and quartz sand. Four experiments at different temperature and pressure conditions were modeled. The model assumes that the contents are ideally mixed and aims to estimate the effective dissociation rate of gas hydrate by matching the model results with the experimental observations. Simulation results indicate that with a marginal offset the model was able to simulate different hydrate dissociation experiments, in particular, those that are performed at high pressures and low temperatures. At low pressures and high temperatures large discrepancies were noticed between the model results and the experimental observations. The mismatches were attributed to the development of extremely heterogeneous flow patterns at pore-scale, where field-scale models usually assume the characteristics to be uniform. Through this modeling study we estimated the irreversible dissociation rate of methane- and CO2-hydrate as 0.02 and 0.03 mol m−3s−1, respectively.

Surface area controls gas hydrate dissociation kinetics in porous media

This study integrates X-ray radiography and 3D computed tomography (CT) to measure xenon hydrate dissociation kinetics in a porous medium (sand) through depressurization at the quasi-isothermal condition. Before dissociation, we use X-ray CT to measure hydrate volume and surface area in the sand and find [hydrate volume]2/3 is a linear approximation of hydrate surface area regardless of differences in hydrate specific area. During dissociation, we use X-ray radiography to quantify the evolutions of hydrate volume and dissociation rate. We find [hydrate volume]2/3 (i.e., hydrate surface area) is a linear metric of hydrate dissociation rate. Additionally we observe two circumstances when dissociate rate is not linear with [hydrate volume]2/3. First, above a critical hydrate saturation of 35–45%, hydrate dissociation rate initially stays constant and then decreases with [volume]2/3. Second, the dissociation rates of patchy hydrates in large pores initially stay constant and are significantly slower than hydrates in small pores with the same sizes. Since methane hydrates in nature are patchy and heterogeneously distributed, the new knowledge will facilitate modeling of hydrates production with a simple metric of dissociation rate (i.e., [volume]2/3) and constraints of hydrate saturation and pore size.