Dissociation Zone Research Articles

An ultrafast H* migration channel and oxidation activity driven by multifunctional Co atoms on twin Co0.01Mn0.29Cd0.7S homojunction surface for photocatalytic overall water splitting

Low kinetics of water dissociation, unfavorable hydrogen adsorption, and high water-oxidation barriers restrict the photocatalytic overall water splitting (OWS) efficiency over sulfides. Herein, multifunctional Co atoms are doped on the twin Mn0.3Cd0.7S homojunction surface for efficient photocatalytic hydrogen (H2) and oxygen (O2) evolution. An ultrafast hydrogen atom (H*) migration channel bridging the water dissociation/H2 evolution step is established between cubic-S and hexagonal-S sites by introducing Co atoms, facilitating the HER kinetics. Specifically, Co atoms regulate the electron properties of neighboring S atoms and weaken their charge accumulation, which promotes cubic-S sites to capture sufficient H* from the water dissociation zone and accelerates hexagonal-S sites to release H2 through the H* migration channel. Meanwhile, unsaturated Co atoms on the cubic phase as oxygen evolution reaction (OER) sites promote water deprotonation and optimize the kinetics of the rate-determining step to form OOH* intermediate, enhancing water oxidation activity. Co atoms strengthen the internal electric field of the homojunction and local polarized electric field by increasing the work function difference between the two phases and introducing the high spin polarization, inhibiting the bulk and surface recombination of photogenerated carriers. The Co0.01Mn0.29Cd0.7S exhibits visible-light-driven H2 and O2 evolution rates of 443.12 and 219.67 μmol g−1 h−1, about 50 times that of the Mn0.3Cd0.7S. This work provides some guidance for the design of homojunction photocatalysts with efficient OWS performance.

Radially Symmetrical Heat Hydrate Dissociation Model with a Density Difference

The hydrate dissociation is viewed as a phase change process in which hydrates transform from a solid phase into gas and liquid phase at a moving dissociation boundary. The boundary separates the dissociation zone containing gas and water from the undissociated zone containing the hydrates, leading to a density difference. Based on the assumption of a density difference between the dissociation zone and the hydrate zone, the authors propose a mathematical model to study hydrate dissociation under thermal stimulation in an infinite radially symmetrical reservoir. Analytical solutions to the temperature distribution are derived by using the self-similarity transformation. Considering the effect factors of the initial heated-water temperature and hydrate density, the authors conducted a thorough investigation of the temperature distribution and the location of the dissociation front for a sample hydrate reservoir. The results from our model show that the heated-water temperature and hydrate density exert significant influence on the hydrate dissociation. With the injection time unchanged, the dissociation distance tends to be increased as the heated-water temperature is increased, leading to a larger dissociation zone. Additionally, a smaller hydrate density can result in a larger dissociation distance. For hydrate thermal stimulation, a higher heated-water temperature and a lower hydrate density can lead to a larger dissociation distance with the injection time unchanged. As the hydrate dissociation proceeds, the dissociation rate is decreased.

Frozen n-Tetradecane Investigated by Cryo-Atom Probe Tomography

Abstract Atom probe tomography (APT) has been established in the microscopic chemical and spatial analysis of metallic or semiconductors nanostructures. In recent years, and especially with the development of a transfer shuttle system and adapted preparation protocols, the field of frozen liquids has been opened up. Still, very limited knowledge is available about the evaporation and fragmentation behavior of frozen liquids in APT. In this work, efforts were made to extend the method toward organic and biological soft matter, which are mostly built from hydrocarbon chains, the evaporation and fragmentation behavior of simple alkane chains (n-tetradecanes). Tetradecane shows a very complex evaporation behavior whereby peaks of C1–C15 can be observed. Based on multihit events and the representation of these in correlation plots, more detailed information about the evaporation behavior and the decay of molecules into smaller fragments in the region near the tip can be studied. A variety of different dissociation tracks of larger molecules in their excited state and their subsequent decay in low-field regions, on the way to the detector, could be observed and the dissociation zone in the low-field region was calculated.

Stability analysis of seabed strata and casing structure during the natural gas hydrates exploitation by depressurization in horizontal wells in South China Sea

Natural gas hydrates (NGHs) are a new type of clean energy with great development potential. However, it is urgent to achieve safe and economical NGHs development and utilization. This study established a physical model of the study area using the FLAC^3D software based on the key parameters of the NGHs production test area in the South China Sea, including the depressurization method, and mechanical parameters of strata, NGHs occurrence characteristics, and the technological characteristics of horizontal wells. Moreover, this study explored the law of influences of the NGHs dissociation range on the stability of the overburden strata and the casing structure of a horizontal well. The results are as follows. With the dissociation of NGHs, the overburden strata of the NGHs dissociation zone subsided and formed funnel-shaped zones and then gradually stabilized. However, the upper interface of the NGHs dissociation zone showed significant redistribution and discontinuity of stress. Specifically, distinct stress concentration and corresponding large deformation occurred in the build-up section of the horizontal well, which was thus prone to suffering shear failure. Moreover, apparent end effects occurred at the end of the horizontal well section and might cause the deformation and failure of the casing structure. Therefore, it is necessary to take measures in the build-up section and at the end of the horizontal section of the horizontal well to prevent damage and ensure the wellbore safety in the long-term NGHs exploitation.

Effect of lithological rhythm of a confined hydrate reservoir on gas production performance using water flooding in five-spot vertical well system

The lithological features of sediments control the heterogeneity in permeability. This study numerically investigates the effect of lithological rhythm of a confined hydrate reservoir on gas production for warm water injection scenarios. The investigated lithological rhythm structures range from simple rhythms to complex composite rhythms. The depressurization in multiple wells causes significant gas production. However, the decrease in pressure gradient constrained the production of gas and water. The warm water injection enhances the production of gas and water with various lag times, though it results in a low energy efficiency. The positive and reverse rhythms have similar energy efficiencies. The gas and water production rate slightly decrease with the increase in the permeability contrast. The maximum gas production rate increases with the increase in number of composite rhythmic layers. However, the water production changes only slightly. The rhythm structures significantly affect the gas and water production rates. A high permeability region in the bottom part of the reservoir is conducive to warm water breakthrough. However, it is unfavorable for the dissociation of remnant hydrate in low permeability regions. Moreover, the results show that the rhythm structures also affect the gas productivity of each rhythmic layer. The results presented in this study help us understand the effect of complex and real rhythm structures on the gas production performance and the dynamic evolution of hydrate dissociation zone.

Research into Dissociation Zones of Gas Hydrate Deposits with a Heterogeneous Structure in the Black Sea

Ukraine is an energy-dependent country, with less that 50% of its energy consumption fulfilled by its own resources. Natural gas is of paramount importance, especially for industry and society. Therefore, there is an urgent need to search for alternative and potential energy sources, such as gas hydrate deposits in the Black Sea, which can reduce the consumption of imported gas. It is necessary to refine the process parameters of the dissociation of gas hydrate deposits with a heterogeneous structure. The analyzed known geological–geophysical data devoted to the study of the offshore area and the seabed give grounds to assert the existence of a significant amount of hydrate deposits in the Black Sea. An integrated methodological approach is applied, which consists of the development of algorithms for analytical and laboratory studies of gas volumes obtained during the dissociation of deposits with a heterogeneous structure. These data are used for the computer modelling of the dissociation zone in the Surfer-8.0 software package based on the data interpolation method, which uses three methods for calculating the volumes of modelling bodies. A 3D grid-visualization of the studied part of the gas hydrate deposit has been developed. The dissociation zone parameters of gas hydrate deposits with different shares of rock intercalation, that is, the minimum and maximum diameters, have been determined, and the potentially recoverable gas volumes have been assessed. The effective time of the process of gas hydrate deposit dissociation has been substantiated. The obtained research results of the dissociation process of gas hydrate deposits can be used in the development of new technological schemes for gas recovery from the deep-water Black Sea area.

Explosive processes in the permafrost zone as a new type of geocryological hazard

The natural and human-induced explosive processes in the permafrost area were studied. A new approach is proposed to the theory of formation of gas emission funnels. It is based on the mechanism of migration of gas fluids from the gas hydrates dissociation zone to the overlying permafrost. The dissociation zone is formed in the areas of local heating due to the heat input from above. The preparation stages of the explosive process that formed the Yamal crater are shown. The preparation phases of natural explosive processes have been identified and characterized. Different scenarios are considered for the preparation of natural explosive processes in the cryolithozone. The main conclusions of the conducted research are the following: the explosions of hydrolaccoliths and the formation of gas emission funnels belong to one group of processes, i.e., physical explosions of natural origin; preparation of explosive processes in the permafrost goes through several stages; the migration of gas fluids plays an important role in the preparation of natural explosions in the permafrost. With the ongoing economic development of the Arctic, thermal effects on permafrost increase, and so does the hazard of explosive processes for engineering structures. However, this group of processes is not taken into account when choosing design solutions and predicting the interaction of an engineering structure with permafrost soils; moreover, these processes are even not included in the group of hazardous geological processes.

Comparison of Simplistic and Geologically Descriptive Production Modeling for Natural-Gas Hydrate by Depressurization

Summary Marine-gas-hydrate-drilling exploration at the Eastern Nankai Trough of Japan revealed the variable distribution of hydrate accumulations, which are composed of alternating beds of sand, silt, and clay in sediments, with vertically varying porosity, permeability, and hydrate saturation. The main purposes of this work are to evaluate gas productivity and identify the multiphase-flow behavior from the sedimentary-complex hydrate reservoir by depressurization through a conventional vertical well. We first established a history-matching model by incorporating the available geological data at the offshore-production test site in the Eastern Nankai Trough. The reservoir model was validated by matching the fluid-flow rates at a production well and temperature changes at a monitoring well during a field test. The modeling results indicate that the hydrate-dissociation zone is strongly affected by the reservoir heterogeneity and shows a unique dissociation front. The gas-production rate is expected to increase with time and reach the considerable value of 3.6 × 104 std m3/d as a result of the significant expansion of the dissociation zone. The numerical model, using a simplified description of porosity, permeability, and hydrate saturation, leads to significant underestimation of gas productivity from the sedimentary-complex hydrate reservoir. The results also suggest that the interbedded-hydrate-occurrence systems might be a better candidate for methane (CH4) gas extraction than the massive hydrate reservoirs.

Wellbore stability analysis during drilling through marine gas hydrate-bearing sediments in Shenhu area: A case study

Because of the metastable character of natural gas hydrate-bearing sediments (GHBS), they can be easily affected by drilling operations, thereby inducing the wellbore instability risk. This issue is studied in this paper by means of numerical simulations based on the first exploration well performed in the Shenhu area of the South China Sea in 2007. The corresponding simulated results were compared against the field caliper logging curve. Besides this analysis, the effects of both drilling mud properties (i.e., temperature, density, and salinity) and initial reservoir conditions (i.e., permeability, and saturation) on the wellbore stability were further investigated under the condition of drilling mud invasion into the GHBS. It is anticipated that the coupled multiphysics phenomena triggered under operation conditions can lead to wellbore instability and may also trigger the formation of fractures. The simulation results show that the predicted yield region around the borehole of well SH2 conforms to the borehole enlargement observed in the field. The sensitivity analyses indicate that a local increase in the pore pressure is likely to be generated in the wellbore vicinity, triggered by the large amount of free gas to be released during hydrate dissociation, which is in turn induced by heating (i.e., coming from the drilling-friction effects) and the high salinity of the drilling muds typically used. This study shows that by engineering the salinity of the drilling mud, the amount of free gas to be produced from marine GHBS can be controlled, preventing in this way further wellbore instability issues. This research also shows that an increase in the drilling-mud density not only impact on the development of pore pressures near the borehole, but it also affects the extent of drilling-mud invasion. This effect will extend the hydrate dissociation to a wider area and will also reduce the resistance of the formation, leading to further deformation and potential failure. The analyses also show that the increase in the initial hydrate saturation is associated with a stronger sediment and a more stable wellbore. The model also predicts that GHBS with higher absolute permeability will dissipate faster the excess of pore pressure in the surrounding sediment, contributing to wellbore stability; however, the extent of the hydrate dissociation zone will expand and thereby will increase the yield region.

Key Findings of the World’s First Offshore Methane Hydrate Production Test off the Coast of Japan: Toward Future Commercial Production

Marine methane hydrate in sands has huge potential as an unconventional gas resource; however, no field test of their production potential had been conducted. Here, we report the world’s first offshore methane hydrate production test conducted at the eastern Nankai Trough and show key findings toward future commercial production. Geological analysis indicates that hydrate saturation reaches 80% and permeability in the presence of hydrate ranges from 0.01 to 10 mdarcies. Permeable (1–10 mdarcies) highly hydrate-saturated layers enable depressurization-induced gas production of approximately 20,000 Sm3/D with water of 200 m3/D. Numerical analysis reveals that the dissociation zone expands laterally 25 m at the front after 6 days. Gas rate is expected to increase with time, owing to the expansion of the dissociation zone. It is found that permeable highly hydrate-saturated layers increase the gas–water ratio of the production fluid. The identification of such layers is critically important to increase the en...

Numerical simulation on the marine landslide due to gas hydrate dissociation

Marine landslide due to gas hydrate dissociation is a kind of potential heavy hazards. Numerical simulation was processed to investigate the deformation, landslide and effects of main factors on the critical scale of dissociation zone to induce marine landslides during/after dissociation of gas hydrate. A simple method for analyzing the critical scale was presented based on the limit equilibrium method. It is shown that the maximum settlement is located near the upper side of the dissociation zone. The soil near the lower side of the dissociation zone uplifts. There is a critical scale of dissociation zone over which landslide will occur under given conditions. Expansion modes of the dissociation zone have obvious effects on the critical scale.

Multiphase Flow Behavior of Layered Methane Hydrate Reservoir Induced by Gas Production

Gas hydrates are expected to be a potential energy resource with extensive distribution in the permafrost and in deep ocean sediments. The marine gas hydrate drilling explorations at the Eastern Nankai Trough of Japan revealed the variable distribution of hydrate deposits. Gas hydrate reservoirs are composed of alternating beds of sand and clay, with various conditions of permeability, porosity, and hydrate saturation. This study looks into the multiphase flow behaviors of layered methane hydrate reservoirs induced by gas production. Firstly, a history matching model by incorporating the available geological data at the test site of the Eastern Nankai Trough, which considers the layered heterogeneous structure of hydrate saturation, permeability, and porosity simultaneously, was constructed to investigate the production characteristics from layered hydrate reservoirs. Based on the validated model, the effects of the placement of production interval on production performance were investigated. The modeling results indicate that the dissociation zone is strongly affected by the vertical reservoir’s heterogeneous structure and shows a unique dissociation front. The beneficial production interval scheme should consider the reservoir conditions with high permeability and high hydrate saturation. Consequently, the identification of the favorable hydrate deposits is significantly important to realize commercial production in the future.

Numerical study on the stratum's responses due to natural gas hydrate dissociation

ABSTRACTNatural gas hydrate (NGH) dissociation can cause many kinds of potential hazards. Dimensional analysis is used first to obtain the controlling factors for NGH dissociation-induced stratum's responses. And then the effects of the controlling factors on the deformation and stress distribution have then been numerically investigated. A simplified critical condition for judging the instability of gentle inclined stratum after NGH dissociation was proposed. It is shown that the maximum settlement is located near the upper part of the slide body while the soil uplifts near the lower part. The fast change of the stress and deformation is located at the interface between hydrate-bearing stratum (HBS) and over layer. The slope angle has the largest effects on the responses of the stratum. The effects of modulus and water depth on the stratum's instability are small. There is a critical extension length of dissociation zone over which landslide will occur which is determined by the strength parameters, slope angle, friction coefficient and height of the over layer.

Numerical study on the deformation of soil stratum and vertical wells with gas hydrate dissociation

Gas hydrate (GH) dissociates owing to thermal injection or pressure reduction from the well in gas/oil or GH exploitation. GH dissociation leads to, for example, decreases in soil strength, engineering failures such as wellbore instabilities, and marine landslides. The FLAC3D software was used to analyze the deformation of the soil stratum and vertical wells with GH dissociation. The effects of Young’s modulus, internal friction angle, cohesion of the GH layer after dissociation, and the thickness of the GH layer on the deformation of soils were studied. It is shown that the maximum displacement in the whole soil stratum occurs at the interface between the GH layer and the overlayer. The deformation of the soil stratum and wells increases with decreases in the modulus, internal friction angle, and cohesion after GH dissociation. The increase in thickness of the GH layer enlarges the deformation of the soil stratum and wells with GH dissociation. The hydrostatic pressure increases the settlement of the soil stratum, while constraining horizontal displacement. The interaction between two wells becomes significant when the affected zone around each well exceeds half the length of the GH dissociation zone.

Dynamic behavior of hydrate dissociation for gas production via depressurization and its influencing factors

Natural gas hydrate can be a potential energy resource to be developed in the near future. Gas production is feasible via depressurization, which has been applied for gas extraction from gas hydrate deposits in Messoyakha. Understanding the dynamic behavior of hydrate dissociation, i.e. the variations of pressure, temperature and gas production rate during the depressurization process, is important for process optimization and predications of geological hazards caused by hydrate dissociation. In this study, methane hydrate formation and dissociation experiments were conducted using a self-designed apparatus under different conditions in simulated porous media. Hydrate formation in sand packs with different hydrate saturations were simulated, and the dynamic behavior of hydrate dissociation process and its influencing factors were analyzed, illustrated by gas production rate, accumulated gas production, and the associated temperature and pressure changes in the gas production process. The experimental results show that hydrate dissociation can be inhibited in the early stage if the production pressure is low, which can cause a decrease of the local temperature in the dissociation zone and restabilize the hydrate, and the efficiency of gas production can be improved by reducing the production pressure gradually. High permeability of the sand packs is conductive to increase gas production rate, but it can also induce a fast decrease on temperature and pressure. The time for hydrate dissociation becomes longer in the case of a higher hydrate saturation, and depressurization along with other stimulation methods, such as thermal stimulation, may be required for efficient gas production.

Mathematical models of the heat-water dissociation of natural gas hydrates considering a moving Stefan boundary

This paper presents mathematical models for radial, quasi-steady state heat transfer in a semi-infinite hydrate reservoir with a moving boundary that is related to the dissociation of natural gas hydrates. The exact solutions of the temperature in the dissociation zone and hydrate zone, using the Paterson exponential integral function, are obtained, and the dissociation frontal brim location of the hydrates is determined by combining the Deaton method with the Clausius–Claperyron equation. A sample calculation shows that the reservoir temperature falls sharply to the dissociation temperature and then drops gradually with increasing distance to the reservoir temperature. With respect to time, the temperature increases slowly to the dissociation temperature, after which, the dissociation temperature falls sharply to the temperature close to that of the injected hot-water. By increasing the temperature of injected hot-water, more hydrates participate in dissociation; with an increase in time, the radius quickly increases, but the radius of hydrate dissociation increases slowly.

Centrifuge experimental study on instability of seabed stratum caused by gas hydrate dissociation

Gas hydrate (GH) is a kind of solid energy resource with huge reserve. Stratum instability (such as marine landslide) may be caused by hydrate dissociation due to the softening of stratum and the build-up of excess pore pressure. In this paper, a centrifuge experiment was conducted to study the evolution of stratum instability during hydrate dissociation. The hydrate dissociation zone expands from the heating source in hydrate bearing sediment (HBS). The pore fluid pressure increases significantly accompanying the slow seepage and soil layer׳s softening and deformation. Large horizontal displacements and vertical displacements can be observed and increase with the expansion of hydrate dissociation zone. The results can be used for the verification of numerical simulations and a reference for engineering design.

Gas outburst with sediments because of tetrahydrofuran hydrate dissociation

SummaryMethane hydrate (MH) is a new energy resource in the 21st century. But the dissociation of MH from sediments during the MH exploration or oil/gas exploration under a hydrate layer accompanied by the softening of soils and formation of excess pore gas pressure may lead to ground failures and environmental disasters. In this study, experiments on modeling the weakening and failure of the sediment by heat‐induced dissociation of tetrahydrofuran (THF) hydrate were presented. The failure mode of gas outburst was observed. Gas outbursts is a process where gas and soils in hydrate‐dissociation zone burst out after the continuous skeleton of over‐layer is fractured during the expansion of the dissociation zone and the formation of gas zone and excess pore gas pressure. An analytical method is presented by decoupling heat transfer and soil deformation. The geometrical and mechanical similarities for gas outburst are obtained. An empirical criterion for the occurrence of outburst is proposed using the theory of thermal conduction, rigid plastic mechanics, and the experimental data. Copyright © 2015 John Wiley & Sons, Ltd.

The impact of faulting on the stability conditions of gas hydrates in Lake Baikal sediments

The phase transition problem of methane hydrate in porous sediments is solved. Based on the obtained solution, the impact of faulting on the stability conditions of gas hydrates is investigated by the numerical modeling of the filtration and thermal regimes in the sedimentary cover of the Central Basin of Lake Baikal within the segment of the anomalous behavior of the bottom simulating reflector (BSR). It is assumed that such behavior is caused by the tectonic action. The calculations testify to the plausibility of the proposed model of formation of the anomalous area with total decomposition of the contained hydrates. It is shown that dissociation of gas hydrates in sediments due to faulting and the subsequent uplift of the products of these transformations along the incipient channel toward the bottom of the lake can result in the extensive accumulation of gas hydrates on this surface. It is also shown that if the total amount of the free gas, which left the hydrate dissociation zone, reached the level of the lake surface at normal pressure and temperature, its volume could be equivalent to the resources of a medium-size gas field. The results of numerical modeling the violation of the gas-hydrate stability conditions in Lake Baikal sediments can also be valid for the other regions with hydrate-bearing sediments if the case specific conditions and regional tectonic activity are taken into account.

窒素圧入によるメタンハイドレート貯留層からの新規ガス回収法に関するシミュレーション研究

Methane hydrate (MH) is one of the potential resources of natural gas in the near future, because it exists in marine sediments or in permafrost regions worldwide. Some extraction methods of natural gas from the MH reservoir have been proposed, such as depressurization, thermal stimulation and inhibitor injection. These are all based on the in-situ dissociation process of MH that is transformed into methane gas and water. However, there are some technical and economical problems for operation of these methods. Therefore, we have proposed a new enhanced gas recovery method by nitrogen injection. Nitrogen has the effect as an inhibitor as well as methanol and salts to shift an equilibrium condition of hydrate to high temperature and low pressure.In this study, we constructed a numerical model for simulating MH dissociation process in porous media by nitrogen injection on the basis of experimental observations. The gas phase was treated as a two-component system calculation of methane and nitrogen, and equilibrium calculation of methane-nitrogen system was introduced into the numerical model. Through the history-matching of temperature change and gas production behavior in laboratory-scale experiments, we confirmed the validity of the constructed numerical model.Then, using the numerical model for nitrogen injection process, we carried out field-scale simulations. From calculation results, it was found that 1) MH dissociation zone extended depending on pressure gradient to production well, 2) At early-stages of production, water was produced depending on relative permeability characteristics, and 3) Later of the production gas front containing nitrogen and dissociated methane reached to production well and gas production was initiated. Furthermore, we discussed gas production behavior for the gas recovery method that nitrogen injection and depressurization were combined. As a result, we obtained the important knowledge that this combination method had a large advantage for dissociated gas production when well distance was longer.