Hydrate Decomposition Research Articles

Study on the Evolution Law of Wellbore Stability Interface during Drilling of Offshore Gas Hydrate Reservoirs

The study of wellbore stability in offshore gas hydrate reservoirs is an important basis for the large-scale exploitation of natural gas hydrate resources. The wellbore stability analysis model in this study considers the evolution of the reservoir mechanical strength, wellbore temperature, and pressure parameters along the depth and uses plastic strain as a new criterion for wellbore instability. The wellbore stability model couples the hydrate phase transition near the wellbore area under the effect of the wellbore temperature and pressure field and the ‘heat–fluid–solid’ multifield evolution characteristics, and then simulates the stability evolution law of the wellbore area during the drilling process in the shallow seabed. The research results show that, owing to the low temperature of the seawater section and shallow formation, the temperature of the drilling fluid in the shallow layer of the wellbore can be maintained below the formation temperature, which effectively inhibits the decomposition of hydrates in the wellbore area. When the wellbore temperature increases or pressure decreases, the hydrate decomposition rate near the wellbore accelerates, and the unstable area of the wellbore will further expand. The research results can provide a reference for the design of drilling parameters for hydrate reservoirs.

Rapid decomposition of methane hydrates induced by terahertz bidirectional pulse electric fields

Electric field improving hydrates decomposition has broad applications in methane extraction and transportation, which benefits from polarization of water molecular and subsequent low energy cost. However, it is still challenging whether there are approaches to further accelerate decomposition of methane hydrates. Herein, molecular dynamics simulations are performed to investigate the effects of five electric fields on methane hydrates, including terahertz bidirectional pulsed electric field (BPEF), terahertz unidirectional pulsed electric field (PEF0.5), terahertz sinusoidal electric field (SEF), terahertz sinusoidal electric field with duty cycle of 50 % (SEF0.5) and static electric field (DC). We propose that BPEF, at amplitude of 0.06 V/Å and frequency of 18.5 THz, achieve highest efficiency in hydrate decomposition. The underlying mechanism is revealed to involve BPEF-induced structural phase transition of hydrate from crystalline to liquid due to resonance of electric field with vibration modes of water. Furthermore, Amplitudes of BPEF show positive correlation in promoting methane hydrate decomposition, while frequencies close to 1.8, 7.0 and 18.5 THz exhibit superior decomposition efficiency. Increasing amplitude from 0.12 to 0.16 at 1.8 THz accelerates the hydrate decomposition by 29-fold. These findings are expected to improve hydrate extraction and transportation and promote the application of terahertz electric field to hydrate domains.

Study on microwave heating energy supplement technology for gas hydrate reservoir

This study proposes a microwave heating energy supplement technology for gas hydrate reservoir. The microwave radiation simulation model of the leaky coaxial antenna is established by HFSS. Based on the simulation results of microwave antenna structure parameters on the radiation performance, the optimized shape, angle, length, width and fillet of the slot are rectangle, 80°, 38 mm, 12 mm and 2 mm, respectively. To compare the heating performance of microwave antenna before and after optimization, a microwave heating simulation model for hydrate reservoir is developed, which is validated by experimental results. The comparison results illustrate that, after microwave heating 10 h with the optimized antenna structure, the average temperature within a 1-m radius of natural gas hydrate reservoir increases to 10.613 °C, which is about 5.5 °C higher than that before the optimization. The aforementioned results suggest that the optimized microwave antenna structure significantly increases the temperature of the hydrate reservoir, providing the necessary energy to drive hydrate decomposition. The proposed microwave energy supplementation technique holds promise for advancing the efficient development of natural gas hydrates, the further investigation of effect of which on gas production within hydrate reservoirs is needed for future application.

Gas hydrates in shallow sediments as capacitors for cold seep ecosystems: Insights from in-situ experiments

Cold seep ecosystems are characterized by a dense accumulation of chemosynthetic communities that utilize the chemical energy contained in fluids. Due to various technical challenges, the direct monitoring of these communities and their activity shifts during the venting of cold seeps has not been achieved. In this study, an integrated in-situ long-term observation platform was used to monitor seep venting activity, associated gas hydrates, and chemosynthetic communities inhabiting the Formosa Ridge in the South China Sea. In-situ Raman spectral data obtained over 14 days revealed two periods during which cold seep venting formed gas hydrates, interspersed with periods of hydrate decomposition during non-active intervals. The methane concentration in the open seawater column near the cold seep vent fluctuated, with an average of 23.07 μM (variance 28.71 μM). Furthermore, the average coverage ratio of the dominant cold seep macrofauna Shinkaia crosnieri was 22.94 % (variance 0.11 %). We hypothesize that the methane concentrations and biological cover in chemosynthetic communities exhibit stability. This phenomenon may be related to the role of natural gas hydrate deposits as methane capacitors, as proposed by earth scientists.

Numerical study of the gas production process from a gas hydrate deposit in the presence of thermal and depression effects

Today the issue of gas production technology from existing gas hydrate deposits discovered on the shelf of the World Ocean and in permafrost areas is still very significant since the methane reserves in the free state are significantly inferior to its reserves in the form of its gas hydrates. One of the tasks for possible gas production from a hydrate-containing porous medium is to study the process of gas hydrate decomposition under thermal and depression effects since they are most commonly used ones. It is necessary to conduct a theoretical study including the development of a mathematical mode and its algorithmization, the creation of a computational program and the conduct of numerical experiments. The paper presents one-dimensional axisymmetric problem of heating and/or pressure reduction at the bottom of a well passing through the entire thickness of a porous formation when its pores are initially filled with methane and its hydrate. The utilized mathematical model includes the continuity equations for methane, its hydrate and water; the equation of the gas phase motion in a porous medium as the Darcy filtration law; the state equation of methane and water, the energy conservation equation considering the Joule–Thomson effects and adiabatic cooling for gas, the latent heat of the “gas hydrate  methane + water” phase transition. A numerical implementation of the proposed mathematical model and a numerical study of the thermal and/or depression impact on the studied hydrate-bearing deposit are carried out. The results of calculations show that the size of a zone containing only the gas hydrate decomposition products (gas and water) slightly increases with a smaller length of a porous layer. They also show that the thermal effect (increasing the temperature at the bottomhole of production well) on the hydrate-saturated reservoir simultaneously with the depression effect is not efficient enough due to the intensive flow of cold gas (with a temperature equal to the initial temperature of the reservoir) from the hydrate-containing deposit to the well.

Coupling Submarine Slope Stability and Wellbore Stability Analysis with Natural Gas Hydrate Drilling and Production in Submarine Slope Strata in the South China Sea

This research explores the geomechanical challenges associated with gas hydrate extraction in submarine slope zones, a setting posing a high risk of significant geological calamities. We investigate slope and wellbore deformations driven by hydrate decomposition within a subsea environment. Utilizing Abaqus, a fluid-solid-thermal multi-field coupling model for gas hydrate reservoirs was created. Hydrate decomposition during drilling is minimal, resulting in minor formation deformation near the wellbore. However, a year of hydrate production caused a maximum displacement of up to 7 m in the wellbore and formation, highlighting the risk of submarine landslides. This indicates the need for meticulous surveillance of formation subsidence and wellhead equipment displacement. In the aftermath of a hydrate-induced submarine landslide, both the hydrate layer and the overlying strata descend together, inflicting considerable damage on the formation and wellbore. Our study presents a holistic examination of the interplay between environmental geomechanics risks and engineering structure risks for submarine slope instability and wellbore stability during hydrate development, providing crucial insights for enhancing safety measures in hydrate drilling and production, and ensuring wellbore stability.

Kinetic mechanisms of methane hydrate replacement and carbon dioxide hydrate reorganization

The carbon dioxide replacement is an ideal way to mine natural gas hydrates. However, current challenges of low replacement efficiency and immature technology compromise its application spectrum. This study aims to simulate the displacement of methane hydrate by carbon dioxide in a porous media system and verify the effect of water saturation on methane hydrate through experimental tests. By constructing a molecular dynamics model to investigate the replacement process of methane hydrate with carbon dioxide, the role of free water in this process was examined. Additionally, the study elucidated the factors contributing to suboptimal methane storage rate and low carbon dioxide recovery rate from a heat exchange perspective. The results show that methane replacement from methane hydrate can be divided into four stages. When the water saturation approach to hydrate saturation, and the temperature and pressure are between the phase curves of methane hydrate and carbon dioxide hydrate, the carbon dioxide storage rate and methane recovery rate can be significantly improved. The participation of free water can make the replacement more complete. The primary factor determining the replacement effect is the area of contact between carbon dioxide and methane hydrate, which is followed by the newly created carbon dioxide hydrate, which will impact the subsequent mass transfer. Although the heat released by the formation of carbon dioxide hydrate favor the replacement reaction proceed, most of the heat may get lost with the volatilization of carbon dioxide, resulting in the inability or incomplete decomposition of methane hydrate far away from free water. This research will surely have a profound impact on how hydrate replacement mining is monitored and managed as well as the ecosystem around it.

Phase equilibrium conditions in carbon dioxide + cyclopentane double clathrate hydrate forming system coexisting with sodium chloride aqueous solution

The four-phase equilibrium conditions were measured in carbon dioxide + cyclopentane double clathrate hydrate forming systems coexisting with either sodium chloride aqueous solution or pure water as a technological basis for developing seawater desalination technology utilizing clathrate hydrate formation and decomposition. The equilibrated phases included sodium chloride aqueous solution or pure water, carbon dioxide + cyclopentane double clathrate hydrate, liquid cyclopentane, and carbon dioxide gas. The experiments were performed using the batch isochoric procedure. Three different concentrations of sodium chloride in the aqueous solution were tested: 0.035, 0.070, or 0.105 in mass fraction. Pressures ranged from 0.385 MPa to 2.488 MPa. The results were compared with the equilibrium conditions for carbon dioxide and cyclopentane simple clathrate hydrate forming systems. The comparison revealed that the double clathrate hydrate is more thermodynamically stable than the simple clathrate hydrates, under a given sodium chloride concentration in the aqueous solution.

Experimental study of the formation and decomposition of mixed gas hydrates in water-hydrocarbon system

The phase transition of gas hydrate with two or more guests is associated with gas recovery, storage and separation. In this work, the formation and decomposition of binary and ternary gas hydrates containing CH4, C2H6 and C3H8 were investigated experimentally in high-pressure reactors equipped with visual windows. The gas compositions during mixed hydrates formation and decomposition were analyzed to track the occupation of guests in hydrate structures and the release of guests from hydrate cages. It was found that of CH4 was adsorbed in the hydrate cages constructed by C2H6 and C3H8 in the initial stage. In the depressurized decomposition of CH4-C3H8 hydrates and CH4-C2H6-C3H8 hydrates slurry, CH4 and C2H6 molecules were preferentially released from hydrate structures while more C3H8 molecules were able to stay in hydrate phase, which was in accordance with the order of the diameter ratio of guest molecule to hydrate cage. The preservation of CH4-C3H8 hydrates and CH4-C2H6-C3H8 hydrates above 0 ℃ was observed under a low pressure. It was proposed that the reconstitution or reformation of hydrate layer rich of C3H8 may encrust the remaining hydrate particles, preventing hydrate from further quick decomposition. In-situ Raman spectra confirmed the enrichment of C3H8 molecules at the surface of CH4-C3H8 hydrates, which revealed that the metastable hydrate layer rich of C3H8 molecules may act as mass transfer resistance of gas release from hydrate decomposition. Differently, the decompositions of CH4-C2H6 hydrates and C2H6-C3H8 hydrates were uniform as different guests were produced concurrently without preservation. The new insights into mixed hydrates decomposition in this work may provide a new basis for mixed gas production from gas hydrate, and gas storage and separation with hydrate.

Macroscale insights into heterogeneous hydrate formation and decomposition behaviors in porous media

Hydrocarbon fuels are distributed unevenly in the earth, known as heterogeneity. How to harness the nature of heterogeneity for sustainable development of energy sources is the key issue. Herein, the controllable process of heterogeneous hydrate phase transition is firstly realized at macroscale, and three typical hydrate distribution patterns (dispersed, massive, and layered) with the same hydrate saturation are successfully achieved. Macroscale observations revealed the kinetic of heterogeneous hydrate phase transition governed by water and gas transfer in pores. Specifically, a new sequence of “nucleation-growth-migration” driven by water migration for heterogeneous hydrate formation in porous media is proposed. In contrast, gas transportation capacity and reformation probability determine the heterogeneous hydrate decomposition efficiency. Significantly, the phase transition rates between heterogeneous and homogeneous hydrate differs by 2 to 3 times due to mass migration. These results suggest that the porous sediments with facilitated water migration under strong capillary effect are preferable for heterogeneous distribution of hydrocarbon fuels, providing explanations for the enrichment of methane hydrate in marine coarse-grained sandy sediment. Nevertheless, the distribution heterogeneity could be challenging to efficient and economic gas production, raising the demand for targeted approaches to develop natural gas hydrate with different distribution patterns. More significantly, the enhanced hydrate formation rate in porous media by controllable heterogeneous hydrate formation method offered by this work also indicates the feasibility in hydrocarbon capture, storage and transport via hydrate-based technologies.

Characterization of the quasi-liquid layer on gas hydrates with molecular dynamics simulations

Understanding the properties of the surficial quasi-liquid layer (QLL) is a prerequisite in proper handling of gas hydrates, a potential alternative future energy resource. Despite of its critical importance, the characterization of QLL has long been known to be highly challenging. In this work, the evolution of QLL during hydrate decomposition was systematically investigated by Molecular Dynamics simulations. Using the instant displacement of water molecules as a measure of the fluidity of the molecular layer adjacent to hydrate surfaces, QLL thickness was accurately measured using the dynamic properties of water molecules for the first time. The QLL thickness obtained by this new molecular dynamics-based approach takes amorphous water molecules close to hydrate surfaces into account, which was often ignored in the previous results based on the static structural order parameters. The variation of QLL thickness throughout the hydrate decomposition process at different temperatures and pressures was then evaluated. The thickness of QLL was found to increase with elevated temperature and pressure, owing to the varied changes of the amorphous and hydrate-like molecular content in this important layer. The results regarding the dynamics of the QLL with molecular resolution improved the current understanding on the stability of gas hydrate and shed new light on the physical fundamentals relevant to future hydrate exploration as well as anti-hydrate materials design.

Research on the Influence of Depressurization on the Decomposition of Nondiagenetic Hydrate in the Process of Solid Fluidized Mining

Research on the Influence of Depressurization on the Decomposition of Nondiagenetic Hydrate in the Process of Solid Fluidized Mining

INJECTION OF SUPERHEATED WATER VAPOR INTO A POROUS RESERVOIR SATURATED IN THE INITIAL STATE WITH METHANE AND ITS HYDRATE

The work presents a mathematical model of the process of pumping superheated water vapor into a semi-infinite natural porous reservoir, which in its initial state is saturated with gas (methane) and its gas hydrate, in a flat-one-dimensional approximation. The most general case is considered, when four zones of different composition of the saturating phases and three moving boundary surfaces separating these zones appear in a natural reservoir: between the first and second zones, on which condensation of superheated water vapor occurs, between the second and third zones, where displacement takes place condensed methane water, as well as between the third and fourth zones, where the dissociation of the gas hydrate occurs. In the considered formulation of the problem, the first zone of the porous reservoir is saturated with superheated water vapor, the second zone is saturated with condensed water, the third zone is saturated with methane and still water (released during the dissociation of gas hydrate), and the fourth zone of the reservoir is saturated with methane and its gas hydrate. On the basis of a numerical solution, the hydrodynamic and temperature fields that arise in a porous reservoir are studied. It is shown that solutions with the indicated areas and boundaries exist only at relatively low values of injection pressure and reservoir permeability. It has been established that an increase in both the injection pressure and the reservoir permeability leads to a noticeable increase in only the coordinates of the boundary separating the second and third regions; in this case, the coordinate of the methane hydrate decomposition front is practically independent of the indicated parameters. An increase in the values of these parameters leads to the confluence of the boundaries of methane displacement and gas hydrate decomposition. The dependence of the limiting value of the injection pressure on the permeability at which these boundaries merge occurs is obtained.

Study of CO2 injection to enhance gas hydrate production in multilateral wells

Multilateral wells offer promising opportunities for the commercial development of natural gas hydrates. The reservoir strength and stiffness weaken as hydrates decompose during the depressurization production process, potentially leading to formation subsidence, seabed inclination, and other associated geomechanical risks. Thus, understanding the geomechanical issues surrounding the wellbore is crucial for attaining safe and efficient hydrate development. This paper develops a three-dimensional numerical model to simulate the production capacity of hydrate reservoirs through CO2 injection in multilateral wells. The study compares the pressure and temperature response of the reservoir, the characteristics of gas and water production, the variation in hydrate and methane saturation, the productivity, and the geological subsidence patterns during the extraction process. Results indicate that hydrates generated through CO2 injection inhibit methane hydrate decomposition, with delayed injections resulting in higher cumulative yields. CO2 injection can restore reservoir pressure and mitigate formation subsidence. The combined depressurization method, which involves CO2 injection in multilateral wells, can increase hydrate production while preserving formation stability, offering a potential approach for commercial hydrate exploitation. This approach is anticipated to advance the industrialization of gas hydrate extraction and propose a new possibility for CO2 utilization to curb climate change.

Experimental study on the intrinsic dissociation rate of methane hydrate

This work developed a new method that can efficiently separate gas-liquid-hydrate mixture and obtain solid gas hydrate, making it possible to measure the intrinsic dissociation rate of gas hydrate directly. The results demonstrate that the classical Kim’s model can reflect the relationship between driving force and hydrate dissociation rate when at same temperature, but can’t accurately reveal the effect of temperature on hydrate decomposition rate when under the same driving force. In Kim’s model, the driving force of hydrate dissociation was expressed as the difference between equilibrium fugacity (feq) and the fugacity of guest molecules in gas phase (fg). A new hydrate dissociation model was established in this work, and the driving force of hydrate dissociation was modified into (feq-fg)/feq, which has a higher universality than feq-fg. The intrinsic rate constant and activation energy of methane hydrate decomposition were determined to be 18905.35 mol/(m2‧s) and 22.149 kJ/mol, respectively.

The direct observation and interpretation of gas hydrate decomposition with ocean depth

The direct observation and interpretation of gas hydrate decomposition with ocean depth

Experimental Study of Sand Migration under Distinct Sand Control Methods during Gas Hydrate Decomposition by Depressurization

Experimental Study of Sand Migration under Distinct Sand Control Methods during Gas Hydrate Decomposition by Depressurization

Undrained Triaxial Shear Tests on Hydrate-Bearing Fine-Grained Sediments from the Shenhu Area of South China Sea

Changes in undrained shear strength are important to the stability analysis of hydrate reservoirs during natural gas hydrate production. This study proposes a prediction model of undrained shear strength of hydrate-bearing fine-grained sediments based on the critical state theory. Several consolidated undrained triaxial shear tests are conducted on hydrate-bearing fine-grained samples from the Shenhu area of the South China Sea. The effects of effective consolidation stresses and hydrate saturations on the undrained shear strength are investigated. The results show that the undrained shear strength increases linearly with increasing effective consolidation stress. When the hydrate saturation is greater than the effective hydrate saturation, the undrained shear strength significantly increases with increasing hydrate saturation. The undrained shear strength of hydrate-bearing fine-grained sediments is a two-parameter function of effective hydrate saturation and a void ratio. The instability risk of the hydrate reservoir under undrained conditions is greater than that of under-drained or partially drained conditions. Furthermore, low-porosity reservoirs face more shear strength loss from hydrate decomposition yet lower risk than high-porosity ones. These results can improve the understanding of mechanical properties of hydrate-bearing fine-grained sediments under undrained conditions. This study also has implications for the design of marine structures in areas with hydrate-bearing sediment.

Gas permeability of alumina–spinel refractory castables bonded with hydratable magnesium carboxylate

AbstractThe high level of gas permeability can effectively reduce the explosive spalling risk of refractory castables. The hydratable magnesium carboxylate (HMC) is expected to improve the permeability of castables owing to the thermal decomposition of the HMC hydrates. This study compared the gas permeability and explosive spalling resistance of HMC bonded refractory castables (HMCC) with calcium aluminate cement bonded refractory castables (CACC). Thermal decomposition of (Mg3(C6H5O7)2∙11H2O) (hydrates of HMC), drying behavior, and the pores size distribution of castables were investigated. The level of gas permeability of HMCC is higher than that of CACC, which was confirmed by the higher values of Darcian k1 and non‐Darcian k2. The degas temperatures of HMC hydrates (156°C) and HMCC (432°C) are lower than those of CAC hydrates (289°C) and CACC (536°C) at a heating rate of 20°C/min, respectively. The large‐size and more permeable pores in HMCC were obtained according to the mercury intrusion porosimeter (MIP) results, which formed the connected paths for gases (H2O, CO2, C2H4, CO, CH4) released from the castables.

Impact Assessment of Hydrate Cuttings Migration and Decomposition on Annular Temperature and Pressure in Deep Water Gas Hydrate Formation Riserless Drilling

The accurate prediction of wellbore temperature and pressure is important for safe drilling. However, annulus temperature and pressure changes are more complicated due to phase transition. To study this problem, a prediction model of temperature and pressure in deep water riserless drilling is established by considering hydrate cuttings decomposition, interphase mass transfer, and phase transition heat. Based on this model, the effects of hydrate cuttings decomposition on the temperature and pressure of drilling in a hydrate reservoir are explored. The results show that the influence of hydrate cuttings decomposition increases significantly with an increase in the inlet temperature. The influence of hydrate cuttings decomposition on temperature and pressure decreases with an increase in displacement. A small range in the variation of density and penetration rates has little impact on the annulus pressure but mainly affects the temperature. The influence of hydrate cuttings decomposition increases with an increase in the penetration rate. In normal drilling conditions, hydrate cuttings decomposition has little impact on annulus temperature and pressure, but under the conditions of a high inlet temperature, high hydrate saturation, low displacement, and high penetration rate, it is necessary to consider the impact of hydrate cuttings decomposition. This study can provide reference for the prediction of temperature and pressure in deep water hydrate reservoir riserless drilling.