Hydrate Dissociation In Porous Media Research Articles

Multiphysical evolution and dynamic competition involved in natural gas hydrate dissociation in porous media and its implications for engineering

Hydrate (shorted for natural gas hydrate) is one of the most promising future energies, and dissociating hydrate in situ is commonly recognized as the best strategy to realize commercial exploitation. So it is significant to understand the fundamental mechanism in the process of hydrate dissociation involving complicated thermal-hydraulic-mechanical-chemical phenomena. To date, the multiphysical evolution patterns are not revealed by the overall comparison among different phenomena, and we still have little knowledge on the contributions of different influence factors to hydrate dissociation rate. More importantly, the fundamental study has a considerable gap to the real engineering. Therefore, in this paper, a fully-coupled thermal-hydraulic-mechanical-chemical model is developed to further analyze the evolution of all phenomena from both global and local perspectives and find the coupled relationships among physical/chemical phenomena from a dynamic competition perspective. Results show that the global spatial distributions of pressure, hydrate saturation and strain show one simple pattern during hydrate dissociation，while those of temperature and hydrate dissociation rate manifest four and three complex patterns respectively. The effective hydrate reaction specific area, reaction coefficient and pressure difference play different roles in the domination of the trend and value of hydrate dissociation rate. Additionally, two examples are given to demonstrate the implications of the fundamental mechanism to the engineering. A negative-power-law relation is found between the finish time of hydrate dissociation and the heat transfer coefficient of outside heat source (geothermal heat in real engineering). A good linear relation between hydrate dissociation front and pressure transfer front is found, which provides a possible easy way to predict the range of dissociated hydrate by pressure in the real engineering.

Analytical model on natural gas hydrate dissociation with different phase equilibrium curves

By introducing a moving dissociation interface, we build an analytical model of natural gas hydrate dissociation in porous media under thermal chemical solution stimulation method. Analytical solution of temperature distribution, pressure distribution are obtained by employing similarity solution. We investigate the behaviors of four thermal chemical solutions stimulation on hydrate dissociation and focus on the performance of hydrate dissociation under thermal 18%AlCl3 solution stimulation method including temperature distribution, pressure distribution, gas production volume, injected heat flux and compare to the performance under thermal pure water stimulation, showing the influence of changing phase equilibrium curve to hydrate dissociation. Parameters sensitivity analysis of hydrate dissociation is also presented in this paper. The advantages of inputting thermal chemical solution are the reduction of initial energy injection and variation of hydrate phase equilibrium curve.

Full implicit simulator of hydrate (FISH) and analysis on hydrate dissociation in porous media in the cubic hydrate simulator

Natural gas hydrate is a promising strategic energy resource in marine and permafrost sediments. To precisely characterize the coupled Thermal-Hydraulic-Chemical (THC) process in hydrate reservoirs, a bona fide mathematical model with improved key parameters is needed. We developed a Full Implicit Simulator of Hydrate (FISH), a three-phase, four-component mathematical model, to simulate gas recovery from hydrate reservoirs. The experiment of methane hydrate formation and dissociation under depressurization via a single vertical well was executed in the Cubic Hydrate Simulator (CHS). FISH was employed to reproduce the experiment of hydrate dissociation in quartz sands. This study successfully obtains unified parameters that are suitable for the entire hydrate dissociation process. Simulation results of gas and aqueous production profiles and the temperature distribution agreed well with that in the experiment. The Mean Absolute Percentage Error (MAPE) of the simulated volume of gas produced was 2.845%. The mass conservation in the pressure vessel was confirmed by both experimental data and numerical simulation results. This numerical simulator can be applied to predict the behavior of hydrate-bearing-sediments in the laboratory or to evaluate the gas production potential from the marine or permafrost hydrate reservoirs.

Comprehensive effects of heat and flow on the methane hydrate dissociation in porous media

The dissociation of methane hydrate in porous media is a heat flow coupling process. In this paper, we established a heat flow coupling algorithm for hydrate dissociation. A core-scale hydrate dissociation model was established and was verified against a laboratory dissociation experiment. The evolutions of the key characteristics, such as the temperature, the energy, and the hydrate/ice saturations, were analyzed. The effects of heat conductivity and permeability on dissociation characteristics and gas production rate were studied. It is found that there are three different modes under the coordinate system of heat conductivity and the permeability of porous media. In the flow-controlling mode, the rate of hydrate dissociation is mainly related to the flow resistance. In the heat-controlling mode, the rate of hydrate dissociation is mainly related to heat supply. In the heat flow-coupling mode, both the increase in the permeability and the heat supply have positive effects on the increase in the hydrate dissociation rate. In addition, there are different hydrate dissociation front expanding characteristics for the different modes. This analysis of the dissociation mode improves the understanding of events associated with the hydrate dissociation process.

Modeling Two-Phase Flow Caused by Hydrate Dissociation in a Reservoir on the Concept of Global Pressure

Summary The classic Darcy’s two-phase flow equation has a variety of mathematically equivalent formats, such as pressure/saturation (PS), two-phase pressure (PP), and global pressure/saturation (GP). Based on the concept of global pressure, we derive a new formulation for two-phase flow caused by hydrate dissociation in porous media, coupled with the mass and energy conservation equations, thereby offering a novel theoretical frame for gas hydrate extraction simulation. The new model provides in-depth insights into complex flow fields: The dissociation of hydrates under thermal stimulation may lead to two flow fields of gas and water with different directions in the reservoir. The simulation results are in good agreement with the recorded data set of Masuda’s and Chong’s experiments, which verifies the correctness and applicability of the new model. Based on numerical simulations of the same hydrate dissociation experiments, the efficiency of the GP method was compared with the PP method published in our previous work. The results show that the GP method has more than two times the timestep size of the PP method for unsteady flow and 1.5 times for steady flow. Furthermore, the relative residual of the GP method is even two orders of magnitude lower than that of the PP method for two-phase flows with complex variations of pressure gradient and fluid flux. Therefore, the GP method is significantly more efficient than the PP method for simulating gas hydrate development. The proposed GP method improves the computational efficiency of hydrate extraction simulation at the laboratory scale and understanding the mechanisms of two-phase flow in reservoirs caused by hydrate dissociation. It may have potential advantages for field-scale simulation of hydrate development, which requires further studies to demonstrate.

Modeling and characterizing the thermal and kinetic behavior of methane hydrate dissociation in sandy porous media

Methane hydrates (MHs) are deemed to be a potential energy source to meet the overwhelmingly growing demand for natural gas. The production of MHs involves many controlling processes such as heat transfer, fluid dynamics and hydrate reaction kinetics. Local thermal equilibrium (LTE) between solid and fluid phases was typically assumed when modeling the hydrate production, however, in many cases the local thermal non-equilibrium (LTNE) caused by the complex heat transfer processes at the pore scale cannot be neglected. In this work, a LTNE model was adopted to reflect the multiple heat transfer forms associated with hydrate dissociation and gas-liquid flow, particularly the solid-fluid interfacial convective heat transfer and the Joule-Thomson effect. The LTNE phenomenon and Joule-Thomson effect were hinted by a hydrate dissociation experiment carried out for model verification. The results show that the average estimation errors of temperature, cumulative gas and water production profiles between experiment and LTNE model were 7.70%, 6.63% and 4.02%, respectively, which are considerably better than those of the traditional LTE model (16.44%, 11.74% and 10.89%, respectively). A tenfold increase of interfacial convective heat transfer coefficient could reduce the local solid-fluid temperature difference by about 78.5%. A special focus was given to the Joule-Thomson effect which seemed to have less impact at the experimental condition in this work but become more significant when the gas saturation exceeds 40%. This work could add further insights into the thermal performance during hydrate dissociation in porous media, help to reveal the thermo-kinetic relationship at the pore scale and provide a more accurate estimation of the hydrate dissociation dynamics. It could be potentially employed to optimize the methane hydrate production strategies, minimize the risk of hydrate re-formation and maximize the overall energy recovery efficiency.

Full Implicit Simulator of Hydrate (Fish) and Analysis on Hydrate Dissociation in Porous Media

Full Implicit Simulator of Hydrate (Fish) and Analysis on Hydrate Dissociation in Porous Media

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.

Numerical investigation on abnormally elevated pressure in laboratory-scale porous media caused by depressurized hydrate dissociation

Hydrate dissociation induced by artificial or environmental factors in submarine sediments may lead to drilling risks, submarine landslides, and the collapse of offshore platforms—all of which are associated with suddenly increased pore pressure. This study establishes a coupled model for the two-phase flow of fluids in porous media in which the pore pressure of water and gas are used as dependent variables and heat transfer and hydrate dissociation equations are combined. The simulation results of the model are consistent with Masuda’s experimental dataset. A quantifiable controlling hydrate dissociation specific reaction surface area (SRSA) model and an absolute permeability model are proposed to conduct numerical simulations of laboratory-scale hydrate cores, with the aim to investigate the mechanism of hydrate dissociation-induced geological hazard initiation and a law of their evolution in sediments. The results show that i) suddenly depressurized hydrate dissociation in porous media can be divided into three phases—pressure induction, transition, and temperature-controlled—and that the first two are much shorter than the third. Moreover, the lower initial permeability results in a shorter holding period for the first stage, and the transition phase becomes indistinguishable; conversely, the larger initial permeability makes the three stages more distinguishable; ii) hardly increasing the core permeability during the intermediate stage of hydrate dissociation makes the pore pressure rebound. However, the steep increase in SRSA does not result in a significant pore gas pressure rebound; conversely, a larger SRSA does facilitate pore pressure dissipation. This work provides theoretical and laboratory-scale model analyses for the safe development of natural gas hydrates bearing in marine sediments.

An improved model for the migration of fluids caused by hydrate dissociation in porous media

Natural gas hydrates may be the world's future resources, but extracting from the reservoirs involves accurately understanding the transport mechanism of the fluids caused by hydrate dissociation. In this paper, based on Darcy's unsaturated soil mechanics theory, we establish a coupling model of two-phase flow and heat transfer caused by hydrate dissociation, which considering methane gas compressibility and capillary force naturally. The new model is validated through Masuda's test dataset, and notably, the experimental phenomenon of the small peak in the gas pressure of the far-end boundary (Run 4) is reproduced for the first time. Additionally, an implicit function of pore pressure and time in the initial stage of hydrate dissociation is deduced to interpret the in-situ pore accumulation of the gas generated in dissociation, which leads to a more comprehensive description of the migration mechanism of hydrate reservoir fluids. Furthermore, the initial hydrate saturation of the Run 4 sample in Masuda's test is proved to be closer to 0.501 instead of 0.443 that the author reported, which can help Masuda's test dataset being a benchmark for the hydrate dissociation researching in future. Thus, this work provides further insight into the migration mechanism of the gases caused by hydrate dissociation and services practical support for efficiently and safely extracting methane gas from the hydrate reservoirs.

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.

Pore‐Scale Investigation of Methane Hydrate Dissociation Using the Lattice Boltzmann Method

AbstractFundamental understanding of pore‐scale methane hydrate dissociation in porous media is important to evaluate submarine slope stability and potential utilization of methane resources. In this paper, a general pore‐scale framework based on the lattice Boltzmann (LB) method is established for reactive transport coupled with nonisothermal multiple physicochemical processes in porous media. The framework combines the gas hydrate dissociation kinetic model, the single‐phase flow LB model, the mass transport LB model, and the conjugate heat transfer LB model. The pore‐scale framework is validated by several benchmark problems and then employed to investigate the endothermic dissociation process of methane hydrate with pore‐filling and grain‐coating habits in porous media. The methane hydrate endothermic dissociation behavior coupled with nonlinear nonisothermal multiple physicochemical processes involving intrinsic dissociation dynamics, gas flow, mass transport, phase change heat transfer, and conjugate heat transfer is well captured by the framework. The phase change of methane hydrate dissociation and pore structure evolution for different pore habits of hydrate are well depicted, and some insights about the dissociation front advancement and temperature distributions are also obtained. In addition, the effects of temperature field, inlet temperature, and inlet pressure on methane hydrate dissociation are investigated. The pore‐scale methane hydrate dissociation helps to advance our understanding of permeability‐saturation variation relation for continuum models.

Experimental study of methane hydrate dissociation in porous media with different thermal conductivities

Methane hydrate dissociation is an endothermic reaction. Therefore, one of important factors for methane hydrate dissociation is the heat transfer rate. In order to study the heat transfer characteristics of porous media on methane hydrate dissociation, experiments of methane hydrate dissociation using depressurization are conducted in porous media with different thermal conductivities, including quartz sand (0.926 W/(m·K)), white corundum (28.82 W/(m·K)) and silicon carbide (41.9 W/(m·K)). Experimental results show that, during the depressurization stage (DS), the temperature difference among different positions in quartz sand is larger than that in white corundum and silicon carbide. Since the heat transfer rate of quartz sand is smallest among three kinds of sand. A low temperature zone at the center of the reactor is observed in quartz sand compared to white corundum and silicon carbide. During the constant pressure stage (CPS), as the thermal conductivity of porous media increases, the temperature rising rate increases, and the duration of the CPS decreases. The dissociation rate of methane hydrate is controlled by the heat transfer rate of sediments during the CPS. The minimum gas production rate is obtained from the experimental in quartz sand, and the maximum gas production rate is obtained from the experiment in silicon carbide. This result indicates that the dissociation rate of hydrate increased with the increase of the thermal conductivity of porous media. Meanwhile, the overall rate constants (koverall) for different runs are calculated to quantify the dissociate rate of methane hydrate during the CPS. As the thermal conductivities of porous media increase, the overall rate constant of methane hydrate dissociation increases. The results of this study are important for understanding the effects of thermal conductivity of porous media on hydrate dissociation in actual field. Furthermore, it can also be used for the validation of numerical simulation in future.

Effects of water-gas two-phase flow on methane hydrate dissociation in porous media

Natural gas hydrates (NGHs) represent a rich new natural gas resource, offering tremendous potential due to their high energy density and abundant reserves. The safe and efficient exploitation of NGHs is a worldwide focus. The water-gas migration characteristics in sediment are a critical parameter for hydrate exploitation, and the promotion effect of water phase flow on NGHs exploitation has been confirmed. However, there is little research on the influence of water-gas flow on hydrate dissociation. The effects of different water-gas (H2O-CH4) flow rate ratios on methane hydrate (MH) dissociation were visually investigated in this study. The influences of temperature and pressure changes on MH dissociation were eliminated during the water-gas flow. The results showed that the water phase flow was the dominant factor for hydrate dissociation, while the gas phase flow has a negative effect. When the water-gas flow rate ratios were 1-1 (the water and gas flow rates were both 1 ml·min−1) and 1-2 (the water and gas flow rates were 1 and 2 ml·min−1, respectively), the phenomenon of MH reformation in the hydrate reservoir was apparent. Moreover, MH will dissociate with higher water-gas flow rate ratios, and there will also be a higher average MH dissociation rate and a shorter variation time in the flow channel with the high water-gas flow rate ratios. Furthermore, a suitable water-gas flow rate ratio of 2-1 (the water and gas flow rates were 2 and 1 ml·min−1, respectively) was determined for relative permeability measurements in the slow hydrate dissociation process.

Masuda’s sandstone core hydrate dissociation experiment revisited

Numerical simulation of hydrate dissociation in porous media is important to investigate future hydrate fuel extraction strategies and/or the impacts of climate change on the long-term stability of vulnerable near-surface hydrate deposits. The core-scale hydrate dissociation experiment of Masuda et al. (1999) represents an important experimental data set that can be used for benchmarking numerical simulators for this purpose. Data collected includes gas production, water production, boundary pressure and temperature from three internal observation points. At least six modeling studies exist within the literature seeking to simulate the gas production data and the temperature data. However, the pressure data and water production data are generally overlooked. In this article we present a set of numerical simulations capable of reconciling the Masuda et al. (1999) data set in its entirety. Improvements on existing modeling studies are achieved by: (1) using improved estimates of the initial hydrate saturation; (2) obtaining relative permeability parameters, a hydrate stability depression temperature and a convective heat transfer coefficient by calibration with the observed data; and (3) applying a new critical threshold permeability model, specifically to reconcile a relatively fast gas production with a relatively slow far-field boundary pressure response. A subsidiary finding is that permeability is significantly reduced in the presence of very low hydrate saturations. But more importantly, the multi-faceted effectiveness of the data set from Masuda’s experiment is clearly demonstrated for numerical simulation benchmarking in the future.

Formation of the impermeable layer in the process of methane hydrate dissociation in porous media

The problem of decomposition of methane hydrate coexisting with water in a highpermeability reservoir is considered. The asymptotic solution is obtained for the decomposition regime in the negative temperature domain. Energy estimates presented show that an impermeable layer saturated with a hydrate-icemixture can be formed in reservoirs with initial positive temperature. The mathematical model of the process of hydrate decomposition is formulated under the assumption on the presence of such a layer in a high-permeability reservoir. In this case the problem is reduced to a purely thermal problem with two unknown moving boundaries. The water-ice phase transition takes place on the leading boundary, while hydrate dissociates at negative temperatures on the slower boundary. The conditions of existence of the layer saturated with a hydrate-ice mixture which is implemented in reservoirs with the high hydrate content are investigated.

Experimental investigations on energy recovery from water-saturated hydrate bearing sediments via depressurization approach

A huge amount of natural gas hydrates remains untapped in permafrost and continental margin. While several short term field production tests have been carried out, the underlying challenges during hydrate dissociation in porous media, such as the interdependent production behavior of gas and water, is still not well understood. In this work, we employed depressurization technique to recover natural gas from a water saturated hydrate bearing sediment (40% SH, 50% SA and 10% SG) at 281.5K surrounding temperature. During depressurization, the bottom hole pressure (BHP) was maintained at constant pressures of 5.0, 4.0, 3.0 and 2.1MPa respectively to evaluate its effect on gas and water production. As expected, a higher BHP (corresponding to a lower dissociation driving force) resulted in a slower gas and water production. At a BHP of 2.1MPa, thermal buffering was observed below ice point (272.7K), accompanied by enhanced gas production. By lowering BHP from 5.0MPa to 2.1MPa, the percentage methane produced increased from 45.5% to 83.0%; whereas the cumulative water production decreased from 217mL to 157mL. The difference in gas and water production was attributed to the preferential production of aqueous phase at higher BHPs (5.0 and 4.0MPa) nearing the end of hydrate dissociation whereby less hydrates were present.

Influence of thermal stimulation on the methane hydrate dissociation in porous media under confined reservoir

The kinetics of methane gas hydrate under confined environment in porous media have been studied for understanding the formation and dissociation behavior under this condition. We have developed a replica of natural hydrate-bearing atmosphere in a laboratory scale experimental set-up and examined the silica sand size effect on the formation and gas recovery in the presence of both pure water and seawater under confined reservoir conditions. Four sizes of silica sand particles were used in the present study (S1 (0.16mm), S2 (0.46mm), S3 (0.65mm) and S4 (0.92mm)). The formation experiments were done with 70% water saturation both for pure water and seawater. All these experiments were carried out at 277.15K and 8MPa. It is perceived that the gas consumption in the presence of smaller size sand particles is higher as compared to the larger size. The total consumption of methane gas during hydrate formation has been found to be less in the presence of seawater as compared to pure water. Subsequently, dissociation experiments have been carried out under confined reservoir conditions using thermal stimulation from 277.15K to 303.15K for 2h. Gas recovery and dissociation rates are found to be higher in the smaller size silica sand bed in pure water as compared to bigger ones and seawater. Also, the maximum rate of dissociation was occurred at the near-equilibrium condition of pure methane hydrate system. Further insights into the dissociation behavior of methane hydrate under confined reservoir conditions have also been presented.

Analytic modeling and large-scale experimental study of mass and heat transfer during hydrate dissociation in sediment with different dissociation methods

An analytic model of the mass and heat transfers during hydrate dissociation in porous media by depressurization, thermal stimulation, and depressurization in conjunction with thermal stimulation without any empirical correlation is established in this work. Meanwhile, the PHS (Pilot-Scale Hydrate Simulator), a three-dimensional 117.8 L pressure vessel, is used for the investigation into the characteristics of the heat transfers and gas production behaviors during hydrate dissociation with the above dissociation methods. The model is solved analytically, and the predicted results are in good agreement with the experimental data. The results indicate that the sensible heat in porous media is firstly consumed for the hydrate dissociation, and then the heat transferred from the boundaries is employed for dissociating the hydrate in the reservoir by depressurization. The maximum deviations of the predicted gas production and the predicted temperatures with the model are 7.4% and 0.36%, respectively. With the thermal stimulation method, the maximum deviation of the predicted moles of the dissociated hydrate is 7.6%. There are two moving boundaries of the hydrate dissociation in the hydrate reservoir by depressurization in conjunction with heat stimulation. A synergistic effect of depressurization and heat stimulation on hydrate dissociation enhances the hydrate dissociation rates.

Analysis of heat transfer effects on gas production from methane hydrate by depressurization

The dissociation of natural gas hydrates is an endothermic process. This dissociation process requires the continuous absorption of heat energy from the sediment and pore fluid. This heat transfer governs the dissociation rate and affects gas production. In this study, a two-dimensional axisymmetric simulator is developed to model the effects of heat transfer on the process of hydrate dissociation in porous media by depressurization. A series of simulations are performed to study sensible heat effects on the sediment, heat flow transfer in the cap- and base-sediment, and the effects of conductive and convective heat transfer on gas production from methane hydrate depressurization. The results show that the porous media material and the water content are two significant factors that affect the sensible heat in gas hydrate dissociation: the porous media material can increase methane hydrate dissociation, but water inhibits the dissociation process by affecting the pressure on the inner sediment. A high thermal conductivity of the sediment can initially positively affect hydrate dissociation but may later partially inhibit the process. Convective heat transfer in the gas flow increases hydrate dissociation markedly compared to that in water flow.