Gas Hydrate Resources Research Articles

A comparative study of machine learning methods for gas hydrate identification

Gas hydrates are a kind of efficient and clean energy that is recognized as the ideal alternative energy for fossil fuels in the future. Accurate identification of gas hydrate reservoirs is a prerequisite for the application of gas hydrate resources. Artificial intelligence algorithms have been widely applied to solve geological research problems and have obtained good results. Therefore, we use machine learning algorithms to analyze whether sediments contain gas hydrates in the Oregon Hydrate Ridge. In this paper, several commonly used machine learning algorithms are selected, including the random forest, bagging, decision tree, AdaBoost, support vector machine (SVM), k-nearest neighbor (KNN) and gradient boosting decision tree (GBDT). The P- and S-wave velocities (Vp and Vs, respectively) that have high sensitivities to hydrate changes are analyzed, the parameters of different algorithm models are optimized through training, and the classification effects of different algorithm models are compared in detail. Finally, the results show that these algorithms can better distinguish whether there are hydrates in the sediments. Compared with other algorithms, the random forest has the highest accuracy, precision and f1 score in the results of testing hydrate identification models; the AdaBoost has the highest recall; and the KNN has the closest area under curve (AUC) value to 1. The combination of artificial intelligence and resource exploration greatly improves the efficiency and accuracy of hydrate identification, which provides strong support for the subsequent development and utilization of gas hydrate resources.

Research progress and development tendency of polymer drilling fluid technology for unconventional gas drilling

Unconventional gas includes tight sandstone gas, shale gas, coalbed methane, and natural gas hydrate. With huge reserves, unconventional gas has become the most important natural gas resource successor after the end of the “Easy Oil era.” The drilling fluid is an indispensable wellbore working fluid for unconventional gas drilling with multiple functions. The polymer drilling fluid (PDF) is the most common, longest developed, and most diverse drilling fluid type. With advantages of easily controlled rheology, convenient on-site performance maintenance, and specifically low cost and weak environment pollution, the PDF is gradually replacing the oil-based drilling fluid as the first choice for unconventional gas drilling. The invention of the non-disperse low-solid-content PDF in the 1960s shows that PDF technology has entered the stage of scientific development, and until now, its development has generally experienced five stages: beginning, developing, improving, re-developing, and re-improving. Dozens of polymer additives and PDF systems have been invented and applied, which have solved severe drilling problems, greatly improved drilling efficiency, and promoted exploration and development in difficult oil and gas resources. This paper first reviews the research progress of PDF technology according to the timeline by introducing the composition, feature, advantages, and disadvantages of some representative polymer additives and PDF systems, emphatically the function and mechanism of stabilizing wellbores, lubricating drilling tools, and protecting reservoirs of the biomimetic wellbore-strengthening PDF and amphiphobic high-efficiency PDF in unconventional gas drilling. Then, combining future global demands, especially China’s strategic needs of oil and gas exploration and development, the development tendency of PDF technology is critically illustrated by introducing several potential research directions including intelligent PDF, ecological PDF, and PDF for natural gas hydrate and deep layer gas resources.

Research on anchoring force of the deep-sea stratum drilling robot

To enhance the sampling and exploration of the gas hydrate resources and other seafloor mineral resources within the seafloor sediment stratum, a deep-sea stratum drilling robot inspired by the earthworm’s peristaltic movement was developed in this paper. The support anchor unit is a part of this new type of robot. When the robot moves forward, it must rely on the support anchor unit to provide enough supporting force to counteract the various resistance forces on the front body units. In this paper, the maximum anchoring force of the support anchor unit, also known as the pullout force, is determined by theoretical calculation. The soil failure area during the pulling-out was obtained by simulation for visualization. The maximum anchoring force of the unit in the soil of different properties with different speeds was also obtained by tests to verify the theoretical calculations. And the effect of the support plates’ unfolding action on the maximum anchoring force was further analyzed in this research. The test results provide a helpful research basis for subsequent analysis of the robot’s movement pattern in soil and the effect of movement at different speeds.

Development and Testing of a High-Resolution Three-Dimensional Seismic Detection System for Gas Hydrate

As a novel type of mineral resource, gas hydrate has received a considerable amount of attention worldwide. This seismic detection method can detect abnormal phenomena such as the BSR, blank zones, velocity anomalies and polarity inversion of gas hydrate and become an important method of gas hydrate detection. The occurrence area of gas hydrate in the South China Sea is usually buried deep beneath the seabed. The current method cannot meet the needs of the shape and structure detection of gas hydrate deposits. With the support of the National Key R&D Program of China, some key technologies have led to developmental breakthroughs, such as ultra-high-energy plasma sources, small-group-interval high-resolution seismic streamers, and distributed three-dimensional seismic acquisition. The seismic profile obtained north of the South China Sea shows that the stratum penetration depth reaches nearly 1000 m at a depth of 1500 m, and the vertical resolution is better than 1.5 m. This system can serve the needs of high-resolution exploration of gas hydrate resources.

Identifying Submarine Engineering Geologic Hazards in a Potential Gas Hydrate Target Area on the Southern Continental Margin of the South China Sea

The southern continental margin-slope area of the South China Sea is a complex passive continental margin with diverse tectonic structures and movements. This area is rich in gas hydrate resources and is also an area with a high incidence of potential geological hazards. Identifying and understanding the potential submarine geological hazards in this area is very important for disaster prevention and management during the future exploration and development of marine resources. In this paper, five types of potentially hazardous geological bodies are identified in the research area through high-precision two-dimensional seismic processing and interpretation, including submarine mounds, pockmarks, mass transport deposits, submarine collapses and faults. At the same time, the seismic reflection characteristics and the changes in its morphology and surrounding strata are described. In addition to the causes of geological hazards in this region and their influence on exploration and development, the research prospects of geological hazards in this region are also suggested. Special tectonic and sedimentary conditions, fluid activities and hydrate decomposition may be the conditions for geological hazards in this region, which pose a significant threat to the exploration and development of seabed resources and marine engineering construction in this region. Not only does our conclusion provide useful data for the development and utilization of gas hydrate, but it also presents theoretical suggestions for reducing geological hazards in the development process.

Gas Hydrate Resource Potential Of Deepwater Sabah, Malaysia: A Preliminary Assessment

Offshore NW Sabah is one of the localities identified in the United States Geological Survey (USGS) global hydrates database but not much work has been done on this potential source of energy for Malaysia and the surrounding region. The presence of gas hydrates in this area is mainly inferred from bottom-simulating reflectors (BSR) identified in seismic reflection profiles across the margin. BSRs have been mapped across almost the entire length of the deepwater fold-thrust belt in the Sabah Trough where they are commonly observed within the crests of fold-thrust anticlines. Based on an average geothermal gradient of 62.5 °C/km, the thickness of the gas hydrate stability zone is predicted to vary with water depth from zero at 640 m water depth to 300 m at 2900 m water depth. The total in-place methane resource from the Sabah gas hydrates is estimated to range from 72 to 852 trillion cu. ft. (TCF) (2.06 – 24.1 x 1012 m3) with a mean of 364 TCF (10.3 x 1012 m3 ) and a most likely (P50) value of 252 TCF (7.1 x 1012 m3). These preliminary estimates may seem large but they are comparable with those from other gas hydrate deposits in the region. More work is required to refine them in order to determine how much of the in situ volume is technically and economically recoverable.

Cross-hole electrical resistivity tomography as an aid in monitoring marine gas hydrate reservoirs for gas recovery: an experimental simulation study

SUMMARY The in situ reservoir status monitoring plays a critical role in natural gas hydrate resource production. Considering the complexity of the field environment, a simulation framework for monitoring gas hydrates with cross-hole electrical resistivity tomography (CHERT) was developed to monitor the hydrate distribution during hydrate formation and dissociation. The simulation study comprised both numerical and physical experiments. The optimal CHERT array was designed through a numerical experiment. The effect of applying CHERT was verified through a physical experiment (a high-resistivity medium and hydrate formation experiment). The results show that improper electrode layouts will lead to varying degrees of low amplitude and blur boundary. An optimal CHERT array of a 100-mm electrode rod spacing, 8-mm electrode ring spacing and 48 electrode rings was obtained. The inversion results obtained using this CHERT array scheme can easily distinguish the distribution of high-resistivity targets and yield satisfactory results in hydrate formation experiments. These findings guarantee data processing and interpretation for applying CHERT in gas hydrate experiments and fields.

Potential and Distribution of Natural Gas Hydrate Resources in the South China Sea

The amount of natural gas contained in the world’s gas hydrate accumulations is enormous, but these estimates remain highly speculative. So far, it is still challenging to locate spatial distribution of marine gas hydrate and quantitatively characterize the evaluation parameters and systematic improvement of evaluation systems. Considering the systematic review of the key accumulation factors, such as heat flow, deposition rate and total organic carbon in the typical passive continental margin, the evaluation results of global marine gas hydrate resources were analyzed based on the characteristics of gas hydrate geology, geophysics and geochemistry anomalies in the South China Sea. We analyze the problems on the evaluation of marine gas hydrate resources, probing into the geological characteristics and distribution laws of marine gas hydrate resources in the South China Sea, and estimate the parameters for in-place resource evaluation, in which the volume method based on Monte Carlo probability was used to evaluate the gas hydrate potential resources in the South China Sea. The probability distribution ranges from 37.6 billion (with 90% probability) to 117.7 billion (with 10% probability) tons of oil equivalent, with an expected value of 74.4 billion tons of oil equivalent. The study results show that the gas hydrate resource density in the South China Sea is similar to that in the typical sea areas, and the estimated global resources are basically consistent with the assessment results at this stag; this shows that the South China Sea has great potential for gas hydrate resources. The research results can provide guidance for the evaluation of global climate change and the exploration and development of hydrate resources.

Research on a hydrate saturation prediction method based on an analysis of factors that influence the prediction accuracy of sea area hydrate saturation

It has been predicted that China has about 80 billion tons of oil equivalent of natural gas hydrate resources in its sea. China has conducted at least six drilling and two trial exploitations in the South China Sea, achieving good results. However, these achievements do not necessarily mean that the research area can support commercial exploitation. Only when the natural gas hydrate resources in the research area reach a certain scale will it be valuable for commercial exploitation. A reliable quantitative prediction method is necessary to clarify the scale of gas hydrate in the research area. However, the classical Wood method’s application to the prediction of suspended hydrate saturation in the Shenhu maritime area of China results in a large prediction error; the analysis shows that an unreliable measurement of reservoir parameters is the main reason for the large prediction error. In order to clarify the influence of reservoir parameters, this paper—by analyzing the measurement sources of the parameters of reservoir parameters, firstly indicates that the inaccurate measurement of three reservoir parameters—matrix composition, porosity, and density—is the main cause of prediction error. Then, using the design of different reservoir parameter measurement schemes to conduct comparative analysis, this paper points out that unreliable porosity and density measurement may lead to large prediction errors, while unreliable measurements of matrix composition have a relatively small impact on prediction accuracy. Further analysis shows that the absolute value of the prediction error caused by uncertainty in synthetic reservoir parameter measurement is sometimes larger than the sum of the absolute prediction error caused by the single parameter measurement uncertainty. In addressing the problem of large prediction errors caused by the inaccurate measurement of reservoir parameters, this paper proposes a hydrate saturation prediction method based on non-hydrate correction—called the “Wood-Bao method”. Simulation and actual data studies show that the prediction effect of this method is superior to that of the Wood method.

Experimental Study on the Distribution Characteristics of CO2 in Methane Hydrate-Bearing Sediment during CH4/CO2 Replacement

CH4/CO2 replacement is of great significance for the exploitation of natural gas hydrate resources and CO2 storage. The feasibility of this method relies on our understanding of the CH4/CO2 replacement efficiency and mechanism. In this study, CH4/CO2 replacement experiments were carried out to study the distribution characteristics of CH4 and CO2 in hydrate-bearing sediments during and after replacement. Similar to previously reported data, our experiments also implied that the CH4/CO2 replacement process could be divided into two stages: fast reaction and slow reaction, representing CH4/CO2 replacement in the hydrate-gas interface and bidirectional CH4/CO2 diffusion caused replacement, respectively. After replacement, the CO2 content gradually decreased, and the methane content gradually increased with the increase of sediment depth. Higher replacement percentage can be achieved with higher replacement temperature and lower initial saturation of methane hydrate. Based on the calculation of CO2 consumption amounts, it was found that the replacement mainly took place in the fast reaction stage while the formation of CO2 hydrate by gaseous CO2 and water almost runs through the whole experimental process. Thus, the pore scale CH4/CO2 replacement process in sediments can be summarized in the following steps: CO2 injection, CO2 diffusing into sedimentary layer, occurrence of CH4/CO2 replacement and CO2 hydrate formation, wrapping of methane hydrate by mixed CH4-CO2 hydrate, continuous CO2 hydrate formation, and almost stagnant CH4/CO2 replacement.

Diatom influence on the production characteristics of hydrate-bearing sediments: Examples from Ulleung Basin, offshore South Korea

The Ulleung Basin Gas Hydrate field expeditions in 2007 (UBGH1) and 2010 (UBGH2) sought to assess the Basin's gas hydrate resource potential. Coring operations in both expeditions recovered evidence of gas hydrate, primarily as fracture-filling (or vein type) morphologies in mainly silt-sized, fine-grained sediment, but also as pore-occupying hydrate in the coarser-grained layers of interbedded sand and fine-grained systems. A commonality across many of these occurrences is the presence of diatoms in the fine-grained sediment. Here we tested fine-grained sediment (median grain size <12.5 μm) associated with hydrate occurrences at four UBGH2 sites (UBGH2-2-2, UBGH2-3, UBGH2-6 and UBGH2-11) to investigate potential impacts of diatoms on efforts to extract methane from hydrate, or to tap hydrocarbon reservoirs beneath hydrate-bearing sediment. Two key considerations are: the extent to which diatoms control sediment mechanical properties, and the extent to which pore-water freshening, which occurs as gas hydrate breaks down during resource extraction, alters the diatom control on sediment mechanical properties. We conducted experiments to measure sediment index properties, sedimentation behavior and compressibility to address these considerations. We relied on scanning electron microscope (SEM) imagery and X-ray powder diffraction (XRD) to characterize the sediment mineralogy. Our high-level findings are that at the ∼20–45% (by volume) diatom concentrations observed at these UBGH2 sites, sediment compressibility increases with diatom content, but diatoms only appear to increase porosity and permeability at the highest diatom concentration (∼45%). Our measurements suggest in situ compression indices of 0.35–0.55 and permeabilities on the order of 0.01milliDarcies (1 × 10−17 m2) can be anticipated at these sites. Importantly, these properties are not expected to vary significantly upon pore water freshening that accompanies gas hydrate dissociation during production.

Bubble Plume Target Detection Method of Multibeam Water Column Images Based on Bags of Visual Word Features

Bubble plumes, as main manifestations of seabed gas leakage, play an important role in the exploration of natural gas hydrate and other resources. Multibeam water column images have been widely used in detecting bubble plume targets in recent years because they can wholly record water column and seabed backscatter strengths. However, strong noises in multibeam water column images cause many issues in target detection, and traditional target detection methods are mainly used in optical images and are less efficient for noise-affected sonar images. To improve the detection accuracy of bubble plume targets in water column images, this study proposes a target detection method based on the bag of visual words (BOVW) features and support vector machine (SVM) classifier. First, the characteristics of bubble plume targets in water column images are analyzed, with the conclusion that the BOVW features can well express the gray scale, texture, and shape characteristics of bubble plumes. Second, the BOVW features are constructed following steps of point description extraction, description clustering, and feature encoding. Third, the quadratic SVM classifier is used for the recognition of target images. Finally, a procedure of bubble plume target detection in water column images is described. In the experiment using the measured data in the Strait of Georgia, the proposed method achieved 98.6% recognition accuracy of bubble plume targets in validation sets, and 91.7% correct detection rate of the targets in water column images. By comparison with other methods, the experimental results prove the validity and accuracy of the proposed method, and show potential applications of our method in the exploration and research on ocean resources.

Experimental and modeling investigations of hydrate phase equilibria in natural clayey-silty sediments

Rapid and accurate acquisition of hydrate phase equilibria in sediments is crucial for gas hydrate resource assessment and exploitation and hydrate-based technological applications. However, current models have significant limitations and deficiencies in predicting hydrate phase equilibria in sediments, especially clayey-silty sediments prevalent in nature. To this end, a unified hydrate phase equilibrium model was developed, which considers the combined inhibitions of surface adsorption, capillary effect, and salt solution. It is derived that water content and salinity can be used as direct input parameters to the model to estimate the hydrate dissociation temperature depression. Using the stepwise heating method, the hydrate dissociation conditions in various natural sediments were measured. It suggests that montmorillonite has the greatest dissociation temperature depression, followed by SA sediments and kaolin, while pore hydrates in silts exhibit almost the same thermodynamic equilibrium conditions as bulk hydrates unless the water content is below 15%. Special attention was also paid to the additional inhibition of surface adsorption on hydrate phase equilibria in sediments with salt solutions. This inhibition was found to be quite pronounced in clay-rich sediments, especially expansive clays, but essentially negligible in silty or sandy sediments. The maximum average error of 3.29% was observed between model predictions and measured data. Additionally, the amounts of clathrated water (CW), non-equilibrium unclathrated water (NEUW), and equilibrium unclathrated water (EUW) in sediments after the end of hydrate formation were determined using the model. It reveals that CW has the highest proportion in silts, reaching 68.9%, while only 0.231 in montmorillonite; in contrast, EUW is dominant in montmorillonite; NEUW seems to be independent of lithology but shows a strong dependence on the water content. It implies that silty sediments are more conductive to hydrate-based gas storage and CO2 sequestration than clayey sediments. This study provides some practical insights and implications for hydrate phase behaviors in natural sediments and can potentially be employed as a tool for phase equilibrium prediction with universality and accuracy.

Complex Coupled Effects of Seawater Ions and Clay Surfaces on CH4 Hydrate Formation in Kaolinite Janus-Nanopores and Bulk Solution

Molecular insights into the kinetic effect of seawater ions and its coupling with the clay surface effect on CH4 hydrate formation help to understand the complex formation process of natural gas hydrate resources in marine sediments. Molecular dynamics simulations are performed to explore CH4 hydrate formation from a homogeneous salty solution containing seawater ions (Na+, K+, and Ca2+, in the form of salts NaCl, KCl, and CaCl2) in the kaolinite Janus double-nanopore and outside bulk phase, by analyzing water order parameter, aqueous CH4 concentration, flows of CH4 and seawater ions, and formation of hydrate-cage clusters. The kaolinite double-nanopore consists of a hydrophilic nanopore with gibbsite surfaces and a hydrophobic nanopore with siloxane surfaces. Simulation results show that in fresh water, the gibbsite surfaces of hydrophilic nanopore do not inhibit hydrate formation, while the siloxane surfaces of hydrophobic nanopore are adverse to hydrate formation due to formation of a large surface nanobubble. By contrast, hydrate formation in saline water is affected by the complex coupled effects of seawater ions and kaolinite surfaces. In the hydrophilic nanopore, most seawater ions quickly adsorb to the gibbsite surfaces and exert very weak effect on hydrate formation, whereas in the hydrophobic nanopore, seawater ions show a certain inhibitory effect on hydrate formation, as few ions adsorb to the siloxane surfaces. Interestingly, more hydrate solids form in saline water than in fresh water, as hydrate solids in the outside bulk solution grow into the throat of the hydrophobic nanopore and decompose the surface nanobubble. Additionally, transport of CH4 molecules and seawater ions between the kaolinite double-nanopore and the outside bulk solution further affects hydrate formation in the systems.

Double bottom simulating reflectors and tentative interpretation with implications for the dynamic accumulation of gas hydrates in the northern slope of the Qiongdongnan Basin, South China Sea

The origin of double bottom simulating reflectors (BSRs) and their relationship with gas hydrate systems are of great significance to the exploration of gas hydrate resources in submarine settings. Based on seismic data, this paper presents the first identified double BSRs in the northern slope of the Qiongdongnan Basin (QDNB), northern South China Sea. The imaged double BSRs are located in submarine ridges associated with a canyon system. These two BSRs (BSR1 and BSR2) are characterized by two subparallel high-amplitude reflections that mimic the seafloor’s distribution but have the opposite polarity. Patches of enhanced reflections (ERs) with high acoustic impedance indicate the occurrence of gas hydrate above BSR1 and between BSR1 and BSR2. The ERs indicate that the free gas terminates against BSR2 and is located beneath BSR2. The velocity spectrum of the strata shows that the interval velocity above the BSR1 and between BSR1 and BSR2 reaches as high as ∼2450 m/s, suggesting the presence of gas hydrates. The numerical modelling results of the gas hydrate stability zone (GHSZ) show that the geothermal gradient of the study area is 31.3–48.9 °C/km. Three potential formation mechanisms for these double BSRs are proposed: (1) variations in the P-T conditions in the GHSZ caused by erosion and redeposition associated with the submarine canyon; (2) precipitation of gas hydrates with different structures due to a supply of both thermogenic gas and biogenic gas; and (3) a relic paleo-BSR formed by the upward migration of a geothermal fluid through possible gas chimneys resulting in gas hydrate dissociation. BSR1 is correlated with the BGHSZ of structure I (SI) gas hydrates, and BSR2 may be correlated with the BGHSZ of structure II (SII) gas hydrates. Therefore, SI and SII gas hydrates may coexist in the study area, and SII gas hydrates and free gas may be present between BSR1 and BSR2. The occurrence of double BSRs suggests the dynamic accumulation of gas hydrates and free gas due to the erosion and redeposition of a submarine canyon, which shifted the BGHSZ and resulted in the differential distribution of the gas hydrates. A model illustrating the migration and accumulation of the hydrocarbons and gas hydrates associated with the occurrence of double BSRs is proposed, providing a reference for future gas hydrate exploration in the deepwater QDNB.

Review of Past Gas Production Attempts from Subsurface Gas Hydrate Deposits and Necessity of Long-Term Production Testing

This paper summarizes the conditions, applied techniques, results, and lessons of major field gas production attempts from gas hydrates in the past and the necessity of longer term production testing with the scale of years to fulfill the gap between the currently available information and the knowledge required for commercial development. The temporal and spatial scales of field production test projects employing depressurization have expanded since 2002. The results from the projects have proved the applicability of these techniques in both onshore and offshore conditions. However, many technical and reservoir condition-related issues have emerged in gas production, and the gap between current status and industrial requirements is still large. Sand control, artificial lift, and related flow assurance issues are common technical issues that impact onshore and offshore production testing operations. Different reservoir responses were observed well by well, and discrepancy between model predictions and actual field measurements were seen, although reasonable matches were made for short-term behaviors. Those observations suggest that temporal change of the wellbore and near-wellbore conditions and reservoir heterogeneity that cannot be fully modeled have caused complex short-term responses to the depressurization operations. To ensure the long-term operational stability and reliability of the prediction technologies for production behaviors that are essential for commercialization of gas hydrate resources, gas hydrate production testing with comparable duration with commercial operations are necessary. Due to the locality of geological conditions in gas hydrate reservoirs, numerous gas production tests will be required to understand the factors controlling gas production.

Methane Hydrate Formation in the Salty Water Confined in Clay Nanopores: A Molecular Simulation Study

A sound knowledge of the effects of clay surfaces and salt ions on CH4 hydrate formation is vital to understand the formation of natural gas hydrate resources and develop hydrate-based technologies, such as seawater desalination. Herein, microsecond simulations are conducted to investigate CH4 hydrate formation from fresh water and NaCl solution in kaolinite nanopores and the outside bulk phase. Simulation results show that the gibbsite surface shows strong affinity for water and salt ions, while the siloxane surface exhibits certain affinity for CH4 and hence affects hydrate formation. With two siloxane surfaces in the nanopore, hydrate formation is severely inhibited by the significantly lowered aqueous CH4 concentration, caused by surface adsorption of CH4 to form nanobubbles; salt ions greatly enhance such an inhibitory effect by promoting surface nanobubble formation and suppressing CH4 dissolution from nanobubbles. However, with one siloxane surface and one gibbsite surface presented in the nanopore, no nanobubble forms on the siloxane surface, no matter salt ions are present or not, and the inhibitory effects caused by the siloxane surface are not observed. In contrast, gibbsite surfaces are more beneficial to hydrate formation, and salt ions have little effect as most ions adsorb to the surface. Hydrate formation outside of the nanopores is obviously inhibited by salt ions, especially at high salt concentrations. Some interesting phenomena are observed, such as stochastic formation of large sI and sII hydrate domains, which then blocks the nanopore throat and hinders the mass transfer of CH4 and water, and sustained growth of large hydrate solids outside of the nanopore causes decomposition of small hydrate solids in the nanopore. Additionally, densification of salt ions in solution and formation of ion-containing hydrate cages are systematically analyzed to figure out the surface effects of kaolinite on the desalination process. These molecular insights are of scientific and engineering significance to sustainable chemistry related to exploitation of natural gas hydrate and hydrate-based seawater desalination technology.

Mechanical properties of remolded hydrate-bearing clayey-silty sediments

The vast majority of global gas hydrate resources exist in clay and silty fine-grained reservoirs, so fully grasping the mechanical properties of fine-grained hydrate is the key to realize the commercialization of hydrate resources. To date, the research on mechanical properties of hydrates has focused on sandy sediments, while research on the fine-grained hydrate has been relatively less common. In this paper, clay collected from the South China Sea and quartz sand were used to remold hydrate-bearing clayey-silty sediments (HBCSS). A series of triaxial shear tests were then carried out to explore the effects of hydrate saturation, effective confining pressure and clay content on the mechanical properties of HBCSS. This study not only reveals the influence of different factors on the mechanical properties of HBCSS, but also explains the action mechanism of clay. The results show that a higher hydrate saturation could cause strain softening due to spalling and crushing of hydrate particles, and this strain softening effect will also be affected by confining pressure. The failure strength of HBCSS increases with the increase of hydrate saturation and effective confining pressure, but decreases with the increase of clay content. The action mechanism of clay is the effect of lubrication, which can change the particle relationship from crushing to rotating or sliding. This research could be valuable for the safe and efficient exploitation of clay-bearing fine-grained hydrate.

Study on the dissociation characteristics of methane hydrate in clayey silts

Most natural gas hydrate (NGH) resources worldwide are in clayey silt sediments. For the efficient production of NGH, a thorough understanding of the dissociation characteristics of NGH in clayey silt is essential. In this work, an experimental method for clayey silt NGH sample preparation and determination of the dissociation conditions has been established. The dissociation conditions of methane hydrate in clayey silt formed at different initial pressures and water contents have been measured, and the dissociation characteristics have been discussed. Methane hydrate is found to form in pores of different sizes in clayey silts. The filling status of the methane hydrate in the clayey silt pores might affect its dissociation characteristics. As the temperature increases, the hydrate in the smaller pores dissociates first, the hydrate in the larger pores dissociates later, and the final equilibrium conditions are consistent with the dissociation conditions of bulk hydrate. As the initial pressure increases or the water content decreases, the proportion of hydrate in the pores increases. When the clayey silt is saturated (water content 37%), hydrate rapidly forms on the surface, forming a mass transfer barrier and preventing further hydrate formation. The hydrate reforms when the hydrate in the small pores of surface sediments dissociates as the temperature increases. The methane hydrate in clayey silt exhibits multistage dissociation and complex reformation characteristics.

Reduction of global natural gas hydrate (NGH) resource estimation and implications for the NGH development in the South China Sea

There have been at least 29 groups of estimates on the global natural gas hydrate (NGH) resource since 1973, varying greatly with up to 10,000 times and showing a decreasing trend with time. For the South China Sea (SCS), 35 groups of estimations were conducted on NGH resource potential since 2000, while these estimates kept almost the same with time, varying between 60 and 90 billion tons of oil equivalent (toe). What are the key factors controlling the variation trend? What are the implications of these variations for the NGH development in the world and the SCS? By analyzing the investigation characteristics of NGH resources in the world, this study divided the evaluation process into six stages and confirmed four essential factors for controlling the variations of estimates. Results indicated that the reduction trend reflects an improved understanding of the NGH formation mechanism and advancement in the resource evaluation methods, and promoted more objective evaluation results. Furthermore, the analysis process and improved evaluation method was applied to evaluate the NGH resources in the SCS, showing the similar decreasing trend of NGH resources with time. By utilizing the decreasing trend model, the predicted recoverable resources in the world and the SCS are (205–500) × 10 12 m 3 and (0.8–6.5) × 10 12 m 3 , respectively, accounting for 20% of the total conventional oil and gas resources. Recoverable NGH resource in the SCS is only about 4%–6% of the previous estimates of 60–90 billion toe. If extracted completely, it only can support the sustainable development of China for 7 years at the current annual consumption level of oil and gas. NGH cannot be the main energy resource in future due to its low resource potential and lack of advantages in recovery.