Natural Gas Hydrate Dissociation Research Articles

Application of time domain reflectometry to triaxial shear tests on hydrate-bearing sediments

Various triaxial shearing apparatus are used to quantify mechanical properties of hydrate-bearing sediments which are basically required to the assessment of risks posed by dissociation of natural gas hydrates. However, the key factor, hydrate saturation, has not been well measured during triaxial shearing tests. This study aims to show how a time domain reflectometry (TDR) method nondestructively measures hydrate saturation for triaxial shear tests on hydrate-bearing sands. Hydrate saturation is determined based on the difference of volumetric water contents measured by using a flexible TDR probe with two rods after calibration. The results suggest that values of hydrate saturation acquired by the TDR method are accurate, and the flexible TDR probe can monitor the kinetic process of hydrate formation and avoid any reinforcement effects on triaxial shearing properties of test specimens.

An improved analytical model of effective thermal conductivity for hydrate-bearing sediments during elastic-plastic deformation and local thermal stimulation

As one of the crucial thermal properties controlling the dissociation of natural gas hydrate (NGH), effective thermal conductivity (ETC) should be precisely determined to improve the efficiency of NGH exploitation. But so far, most existing analytical ETC models of hydrate-bearing sediments could not accurately characterize the evolution of ETC. In this paper, an improved analytical model is derived for quantitatively revealing physical relations between ETC and various influencing factors during elastic-plastic deformation and local thermal stimulation processes, including hydrate saturation change, complex pore structures evolution, and hydrate occurrence patterns transition. The effectiveness of the proposed model is validated against available experimental data and compared with other existing models, then effects of various critical parameters on ETC are investigated. Results show that the porosity of hydrate-bearing sediments is accurately predicted during both loading and unloading processes, and the calculated ETC values under various conditions (effective stress, hydrate saturation, and temperature) are satisfactory with the test data. Contributions of elastic deformation and plastic deformation to ETC increments are quantitatively obtained. It is noteworthy that this proposed model is significant to reveal the evolution mechanisms of ETC, helping to accurately predict gas production and offer better plans for efficient NGH exploitations.

Confined Compressibility of Fine-Grained Marine Sediments with Cavities after Complete Dissociation of Noduled Natural Gas Hydrates

Confined Compressibility of Fine-Grained Marine Sediments with Cavities after Complete Dissociation of Noduled Natural Gas Hydrates

Numerical simulation of the development of hydrate-bearing layers by depressurization of radial wells

A perpendicular bisector unstructured grid was used for meshing a model with radial wells, and the non-orthogonal correction of the flux calculation was implemented in the Tough+Hydrate software. A numerical simulation model was established based on the geological parameters of hydrate-bearing layers (HBLs) in the Shenhu area of the South China Sea. Gas and water production from the HBL, developed through depressurization of radial wells, was studied, and factors influencing gas production were analyzed. The findings indicate that employing radial wells in both hydrate and mixed layers significantly accelerated gas and water production in the HBL, facilitating rapid depressurization. Faster depressurization, in turn, promoted the dissociation of natural gas hydrate (NGH) and increased gas production. Cumulative gas production using radial wells in double layers increased by 110.03% compared with a horizontal well. In the later stage of depressurization development, NGHs in the mixed layer were almost entirely dissociated, whereas nearly three-quarters of NGHs in the hydrate layer remained undissociated. The combined method of depressurization and thermal stimulation is expected to further promote NGH dissociation and enhance gas production. Analysis of influencing factors revealed that higher gas production was associated with a greater number of laterals and larger lateral length, whereas the layout of the laterals had little effect on performance.

Permeability of surface clay-bearing sediments in Shenhu Area of South China Sea

The migration of fluids, such as the dissociation of natural gas hydrates, can affect the stability of the seabed and even induce submarine geological disasters and change the marine environment. The migration of fluids in sediments depends on its permeability properties. Therefore, it is crucial to investigate the sediment permeability and its influencing factors. This paper investigates the particle size, porosity, pore size distribution, and permeability of surface clay-bearing sediments in the Shenhu Area of the South China Sea by physical property tests, computed tomography (CT) scanning tests, constant rate of strain (CRS) consolidation tests, and nuclear magnetic resonance (NMR) tests. The permeability of the surface sediments in the Shenhu Area ranges from 3.32 × 10−15–6.76 × 10−14 m2. The pore size mainly ranges between 10−5 and 10−1 mm, exhibiting a bimodal distribution with peaks from 10−4 and 10−2 mm. The increase in clay content leads to enhanced permeability due to the bimodal distribution of pore sizes, which will cause the enlargement of pores. As the pore size increases, the sediment permeability also increases. Sediment permeability linearly decreases with increasing effective axial stress and deformation due to compression of the pore space. Additionally, a new model for calculating permeability with different sediment properties is established, which improves the accuracy of permeability calculations for clay-bearing sediments compared to previous research. The findings of our study can provide a reference for exploring the influence of sediment parameters on fluid migration mechanisms in clay-bearing sediments.

Pore-scale study of the dynamic evolution of multi-phase seepage parameters during hydrate dissociation in clayey silt hydrate-bearing sediments

Unlike conventional oil and gas sources, natural gas hydrate (NGH) is stored in solid form within the pores of loose submarine sediments. Production by any method poses a complex problem involving phase transition and multi-phase flow of gas and water. The evolution of hydrate dissociation from the solid phase to gas and liquid phases in a hydrate reservoir is a dynamic process incorporating pore structure and multi-phase seepage. The solid–gas-liquid phase dissociation of NGH affects reservoir seepage properties by altering the micropore structure of the hydrate-bearing sediments. This study, based on the lattice Boltzmann method of modeling fluids flow, clearly demonstrates the micropore-scale changes in pressure and heat transfer that take place during hydrate dissociation. Pore-scale heterogeneity is found to influence the complex development of hydrates in a porous clayey silt medium during dissociation. The effect of hydrate, gas and water saturation on the evolution of relative permeability is identified, and the relationship between relative permeability and hydrate saturation during hydrate dissociation is quantified. These findings provide theoretical support for the implementation of marine natural gas hydrate production testing and future commercial production in China.

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

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

О механизме разрушения ледяных пленок метастабильных газогидратов и его возможном приложении к процессу эмиссии метана в Арктике

The release of methane deposits in the Arctic, which is bound in the form of gas hydrates, can accelerate global warming. The threat of increasing methane content in the atmosphere is associated with the decomposition of gas hydrates located in the sedimentary layer in a metastable state. The authors formulate destruction mechanisms that can affect the dissociation of natural gas hydrates and assess the risks of possible gas emissions during the advancement of slow tectonic waves. They have studied the instability of moving granules of the sedimentary strata and obtained the condition for the bifurcation of the micropolar continuum, which may be associated with the emission of methane from zones of accumulation of metastable relict gas hydrates.

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.

An Epifaunal Community Succession Sequence Driven by the Biogeochemical Footprint With Different Methane Seepage Intensity of the Deep Seafloor

AbstractMethane seepage, generated from natural processes or gas hydrate dissociation, drives the development of cold seep ecosystems and leads to atmosphere methane emission, thereby influencing climate change. Uncovering the intrinsic interactions among methane seepage intensities, biogeochemical processes in the sediment, and benthic community characteristics at the seafloor is essential to clarify and predict the ultimate fate of methane leakage from the deep sea. Here, we conducted a systematic investigation of methane intensity, pore fluid migration characteristics, sediment mineral fraction features, and the evolution of biological communities in areas with different methane intensities. Furthermore, analyses of high‐resolution images, pore fluid geochemical feature, and lithologic characteristics of the sediment in the Haima cold seep were also carried out in this study. The results showed that, in areas with different methane intensities, organic matter sulfate reduction and anaerobic oxidation of methane were heterogeneous. The heterogeneity led to the methane transformation zones at different depths, which changed the mineral composition of the sediment and biological communities in the seabed. In addition, a hypothesis of successional sequence of biomes in cold seep was established. This study revealed that the methane seepage intensity was a key factor in determining the biogeochemical process in the sediment of cold seep. The coupling effects of biogeochemical processes and methane seepage intensities dominated the community succession of cold seep. These findings could provide important implications for understanding the dynamic deep marine methane cycle mechanism and the contribution of deep‐sea methane released to climate change.

Design and feasibility analysis of a new completion monitoring technical scheme for natural gas hydrate production tests

As a prerequisite and a guarantee for safe and efficient natural gas hydrates (NGHs) exploitation, it is imperative to effectively determine the mechanical properties of NGHs reservoirs and clarify the law of the change in the mechanical properties with the dissociation of NGHs during NGHs production tests by depressurization. Based on the development of Japan’s two offshore NGHs production tests in vertical wells, this study innovatively proposed a new subsea communication technology—accurate directional connection using a wet-mate connector. This helps to overcome the technical barrier to the communication between the upper and lower completion of offshore wells. Using this new communication technology, this study explored and designed a mechanical monitoring scheme for lower completion (sand screens). This scheme can be used to monitor the tensile stress and radial compressive stress of sand screens caused by NGHs reservoirs in real time, thus promoting the technical development for the rapid assessment and real-time feedback of the in-situ mechanical response of NGHs reservoirs during offshore NGHs production tests by depressurization.

NMR based experiment of fluid invasion to natural gas hydrate reservoir and hydrate dissociation inhibition mechanism

Hydrate dissociation inhibition has an important bearing on drilling safety for natural gas hydrate exploitation. A low-field NMR-based experimental method is developed for characterizing the hydrate dissociation inhibition effect during fluid invasion to hydrate-bearing sediment. The hydrate core sample is prepared by excess-gas method, and deionized water or hydrate dissociation inhibitor solution was injected at constant flow rate. In the presence of hydrate dissociation inhibitor, the invasion area is significantly reduced, and invasion fluid saturation was kept at 30 % instead of 80 % compared to water. Experimental techniques were used to reveal hydrate dissociation inhibition mechanism. Hydrate dissociation inhibitor at 1 % can reduce the thermal conductivity of aqueous solution from 0.5870 W·m−1·K−1 to 0.5710 W·m−1·K−1 at 4 °C, and increase the low-temperature apparent viscosity from 1 mPa·s to 10 mPa·s, reducing the heat and mass transfer ability of drilling fluid with hydrate reservoirs. The surface tension of water was decreased from 72.8 mN·m−1 to 59.94 mN·m−1, which is conducive to the multi-point adsorption of inhibitors at the hydrate interface. The water activity was reduced from 1.000 to 0.985, showing little effect on the equilibrium temperature of hydrate phase. This work showcases the significance of hydrate dissociation inhibition during fluid invasion to hydrate-bearing sediment.

Application of Dual Horizontal Well Systems in the Shenhu Area of the South China Sea: Analysis of Productivity Improvement

The horizontal well technology was successfully applied in the Chinese second natural gas hydrate (NGH) field test in the Shenhu area of the South China Sea in 2020. However, the results show that the threshold for commercial exploitation has not been broken, judging from daily gas production and cumulative gas production. Consequently, the paper presents the effects of dual horizontal well systems for exploitation in this area. The NGH reservoir model in the Shenhu area was established with CMG software. The influence of various layout options and various spacing of dual horizontal well systems on the production capacity was investigated. Further, we simulated the production effect of dual horizontal well systems joint auxiliary measures, such as well wall heating, heat injection, etc. The results show that the production capacity of dual horizontal well systems increased by about 1.27~2.67 times compared with that of a single horizontal well. The daily gas production will drop significantly, no matter which method was used, when exploitation lasts for about 200 d. Meanwhile, well wall heating and heat injection have limited effects on promoting production capacity. In conclusion, attention was drawn to the fact that the synergistic effect could be fully exerted to accelerate NGH dissociation when dual horizontal well systems are applied. The NGH reservoirs in the Shenhu area may be more suitable for short-term exploitation. The research results of this paper can provide a reference for the exploitation of the Shenhu area.

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.

Experimental study on solid particle migration and production behaviors during marine natural gas hydrate dissociation by depressurization

Sand production is one of the main obstacles restricting gas extraction efficiency and safety from marine natural gas hydrate (NGH) reservoirs. Particle migration within the NGH reservoir dominates sand production behaviors, while their relationships were rarely reported, severely constrains quantitative evaluation of sand production risks. This paper reports the optical observations of solid particle migration and production from micrometer to mesoscopic scales conditioned to gravel packing during depressurization-induced NGH dissociation for the first time. Theoretical evolutionary modes of sand migration are established based on experimental observations, and its implications on field NGH are comprehensively discussed. Five particle migration regimes of local borehole failure, continuous collapse, wormhole expansion, extensive slow deformation, and pore-wall fluidization are proved to occur during depressurization. The types of particle migration regimes and their transmission modes during depressurization are predominantly determined by initial hydrate saturation. In contrast, the depressurization mainly dominates the transmission rate of the particle migration regimes. Furthermore, both the cumulative mass and the medium grain size of the produced sand decrease linearly with increasing initial methane hydrate (MH) saturation. Discontinuous gas bubble emission, expansion, and explosion during MH dissociation delay sand migration into the wellbore. At the same time, continuous water flow is a requirement for sand production during hydrate dissociation by depressurization. The experiments enlighten us that a constitutive model that can illustrate visible particle migration regimes and their transmission modes is urgently needed to bridge numerical simulation and field applications. Optimizing wellbore layout positions or special reservoir treatment shall be important for mitigating sand production tendency during NGH exploitation.

Numerical Modeling on Dissociation and Transportation of Natural Gas Hydrate Considering the Effects of the Geo-Stress

A deep understanding of the dissociation and transportation mechanism of natural gas hydrate (NGH), taking into account the effects of geo-stress, contributes to optimizing the development strategy and increases the exploitation efficiency of NGH. In this paper, the mathematical model, coupled with fluid heat and mass transfer, multiphase flow mechanics, and reaction kinetics with phase change in the process of hydrate decomposition was established. An axisymmetric two-dimensional model was developed to simulate the depressurization decomposition process of natural gas hydrate in the Berea sandstones. FLUENT software was used to solve the fundamental governing equations of the multi-phase flow, and UDF programming was employed to program the hydrate decomposition model and the modified permeability model in the dissociation and transportation of NGH. The simulation results were then validated by Masuda’s experimental data. The effects of gas saturation, outlet pressure, temperature, absolute permeability and geo-stress on the decomposition of natural gas hydrate were studied. The results indicated that a higher absolute permeability, higher initial gas saturation, lower outlet pressure, and higher initial temperature advance the decomposition rate of hydrate. Thus, an optimized production plan is essential to promote the extraction efficiency of the NGH. The geo-stress causes a decrease in the porosity and permeability of the porous rock, which restricts the efficiency of the heat and mass transfer by the fluid flow, leading to a slower dissociation and transportation rate of the NGH. Thus, it is important to take geo-stress into consideration and balance the extracting efficiency and the well pressure, especially when the NGH is developed by depressurization.

Numerical Simulation of Gas Production and Reservoir Stability during CO2 Exchange in Natural Gas Hydrate Reservoir

The prediction of gas productivity and reservoir stability of natural gas hydrate (NGH) reservoirs plays a vital role in the exploitation of NGH. In this study, we developed a THMC (thermal-hydrodynamic-mechanical-chemical) numerical model for the simulation of gas production behavior and the reservoir response. The model can describe the phase change, multiphase flow in porous media, heat transfer, and deformation behavior during the exploitation of NGH reservoirs. Two different production scenarios were employed for the simulation: depressurization and depressurization coupled with CO2 exchange. The simulation results suggested that the injection of CO2 promotes the dissociation of NGH between the injection well and the production well compared with depressurization only. The cumulative production of gas and water increased by 27.88% and 2.90%, respectively, based on 2000 days of production simulation. In addition, the subsidence of the NGH reservoir was lower in the CO2 exchange case compared with the single depressurization case for the same amount of cumulative gas production. The simulation results suggested that CO2 exchange in NGH reservoirs alleviates the issue of reservoir subsidence during production and maintains good reservoir stability. The results of this study can be used to provide guidance on field production from marine NGH reservoirs.

3D DEM modeling on mechanical weakening of gas hydrate-bearing sandy sediments during hydrate dissociation

The dissociation of natural gas hydrates (NGHs) during production weakens the mechanical properties of hydrate-bearing sediments (HBSs). Understanding and predicting the mechanical behaviors of HBSs is crucial for the safe development of NGH reservoirs. This study presents a novel approach based on the discrete element method (DEM) to describe the mechanical weakening of cementing-type gas hydrate-bearing sandy sediments (GHBSSs). Hydrate dissociation is simulated by reducing the parallel-bond radius based on a specific damage evolution law. The Weibull statistical distribution is incorporated into the model to characterize the heterogeneity of hydrates in GHBSSs. Subsequently, a series of numerical triaxial compression tests are conducted to investigate the mechanical weakening behavior of cementing-type GHBSSs and the mechanical effect of the heterogeneous distribution of hydrates. The results show that the DEM model can successfully capture the mechanical weakening behaviors of cementing-type GHBSSs during hydrate dissociation. The peak strength and secant modulus increase with increasing homogeneity index, but their sensitivities decrease as the hydrate dissociation ratio increases. Finally, a fitting equation describing the relationship among the peak strength, initial hydrate saturation, and hydrate dissociation ratio was obtained. This study is valuable for evaluating the stability of GHBSSs during hydrate production.

Numerical analysis of gas recovery enhancement from natural gas hydrate reservoir by using a large-diameter casing

Multi-well patterns and complex structural wells can enlarge the contact area between the wellbore and natural gas hydrate (NGH) to improve productivity significantly, but the construction cost of drilling is still too high. The previous study has proposed a self-entry exploitation device (SEED) with an enlarged device diameter, allowing more NGHs to dissociate simultaneously. While for conventional wellbore, a large-diameter casing may be applied in NGH development to enlarge the wellbore diameter of the production section. This work continued to study the influences of enlarged device/wellbore diameter on gas production characteristics, temperature-pressure evolutions, and the dissociation front movement pattern. The sensitivity analysis was also carried out by various reservoir and depressurization parameters under different wellbore diameters. Results show that the enlarged wellbore diameter can enlarge the drainage area and increase the dissociation efficiency. The temperature and pressure around the wellbore remain relatively stable, meeting the conditions for long-term production. Also, the dissociation front moves farther away faster as the wellbore diameter enlarges, accelerating NGH dissociation at distant wells. Finally, the sensitivity analysis reveals that the enlarged wellbore diameter enhances gas production significantly in unfavorable production parameters: poor production pressure, low permeability, low NGH saturation, and narrow reservoir thickness.

Developing offshore natural gas hydrate from existing oil & gas platform based on a novel multilateral wells system: Depressurization combined with thermal flooding by utilizing geothermal heat from existing oil & gas wellbore

Offshore natural gas hydrate (NGH) is universally recognized as a new alternative energy resource. If methods can be found to commercially exploit it, it is expected to significantly impact the world's energy supply. In this work, an innovative method for developing offshore NGH from existing oil & gas platform is proposed, namely depressurization combined with thermal flooding utilizing geothermal heat from existing oil & gas wellbore based on a novel multilateral wells system. Meanwhile, a numerical simulation model is developed to investigate the natural gas recovery performance of the proposed method. The study shows that the average NGH concentration of the whole NGH bearing layer is reduced by about 25% after one year production by utilizing the innovative method, and 1.6 × 107 m3 natural gas is produced. Bottom hole pressure is the main controlling factor in the initial stage of NGH dissociation, while the formation temperature is the main controlling factor for the dissociation of NGH in the later stage of production, and increasing the fluid injection rate is an effective means to enhance the production performance. The study also shows that the production mode of continuous injection-intermittent production has the highest cumulative natural gas production, its production performance is proved to be the best.