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One of the most demanding environmental and economic challenges of this era is that of supplying the increasing global energy demand while reducing and neutralizing the CO 2 footprint. In this regard, the provision of a secure and diversified energy supply and the sequestration of CO 2 into geological formations are currently of great significance. However, the buoyant nature of CO 2 under the temperature and pressure conditions of typical geological sites leads to the risk of CO 2 leakage, and thus necessitates long-term monitoring. Therefore, the hydrate-based sequestration of CO 2 beneath oceanic sediments has become a desirable alternative to conventional geologic sequestration in some regions, with the potential to eliminate the risks of leaking CO 2 . Over the past decades, considerable progress has been made in the study of gas hydrates, including understanding their occurrence in nature, characterizing their behavior, and assessing their energy and exploitation potentials, along with the potential development of sustainable hydrate-based chemistry applications and technologies. Due to the significance and severity of plugged oil/gas flowlines, the early research was dominated by the inhibition of gas hydrates in order to provide flow assurance in pipelines. However, the discovery of vast natural gas hydrate resources has shifted scientific attention towards the possibility of storing CO 2 as a hydrate, and developing applications such as CO 2 capture and transportation, gas separation, cold storage, natural gas solidification, and CH 4 -CO 2 hydrate replacement. Hence, the present review examines the fundamental properties of gas hydrates, including their natural occurrence and distribution in geological sediments, along with their thermodynamic and kinetic behaviors with respect to the effects of promoting and inhibiting additives. In addition, natural gas hydrate (NGH) production methods and field-scale trials are reviewed. Furthermore, various concepts and experimental investigations pertaining to hydrate-based CO 2 sequestration pathways are reviewed in the final section, including sequestration in the deep ocean, sub-permafrost regions, and marine sediments, as well as the CH 4 -CO 2 hydrate replacement technique. Therefore, CO 2 hydrate storage in suitable depleted oil and gas fields has the potential to reduce infrastructure costs, with CH 4 -CO 2 hydrate replacement potentially providing additional economic incentives. However, long-term field trials in marine sediments with different compositions are required to assess the CO 2 storage potential of hydrate reservoirs, and the effectiveness of CH 4 -CO 2 hydrate replacement technique. Highlights Gas clathrate hydrates enormous energy potential and application for gas storage are introduced. Fundamental properties, occurrence, and geological distribution of gas hydrates are reviewed. Natural gas hydrate (NGH) production methods and field-scale trials are reviewed and discussed. Hydrate-based CO 2 storage pathways and experiments are reviewed and evaluated comprehensively. Field-scale trial of CH 4 -CO 2 hydrate replacement technique is reviewed and discussed. Future research is projected to be on novel techniques that enhance CH 4 -CO 2 replacement efficiency.

A gas-lifting production method was firstly proposed to transport the methane-water mixture from natural gas hydrates deposits through marine vertical pipe in this work. Aiming at UBGH2-6 site, SH7 site and GMGS2-8 site, the gas-lifting performance of methane-water mixture in the vertical pipe was investigated by numerical calculation. The potential of Natural gas hydrates (NGH) self-eruption production induced by the gas-lifting process under ideal conditions was also studied based on the energy analysis. The calculation results indicate that the gas-lifting method has great advantage in avoiding the secondary hydrates formation in marine vertical pipe and reducing energy consumption. The gas-lifting process in the vertical pipe is testified to be spontaneous in UBGH2-6 site and SH7 site during the initial 4000 and 1000 days, respectively, which indicates the energy consumption for methane-water mixture transportation is saved. Sufficient heat supply for the hydrate dissociation is crucial for the NGH self-eruption production. Sensitivity analysis indicates that the water-gas ratio has more significant influences on gas-lifting performance in the vertical pipe compared to the flow rate. With the decrease of water-gas ratio, the bottomhole pressure decreases rapidly. Thus, the reduction of water production is effective to improve the gas-lifting performance.

Natural gas hydrates, commonly called methane (CH4 ) hydrates, are ice-like materials belonging to the family of clathrates that form at low temperature and high pressure. They can be found in permafrost and oceanic environments. The amount of natural gas trapped into worldwide hydrate deposits has been estimated at 18000 trillion m3 of methane [1] and it surpasses the world natural gas proven reserves (180 trillion m3 [2]) by two orders of magnitude. Fossil fuel based energy is still a major source of carbon dioxide (CO2) emissions. Hence, it contributes greatly to the issues of global warming and climate change. Geological sequestration of carbon dioxide appears as the safest and most stable way to reduce such emissions for it involves CO2 entrapment into hydrocarbon reservoirs and aquifers. Indeed, CO2 can also be sequestered as hydrates while assisting in the dissociation of in-situ methane hydrates. This approach could help mitigate the emissions of CO 2 in the atmosphere and improve the economics of carbon dioxide sequestration and natural gas production from hydrate deposits. The proposed research focused on investigating the feasibility of the CO2-CH4 exchange in hydrates as a gas hydrate mining method through experimental studies and numerical modeling of the exchange, as well as the application of a life-cycle assessment (LCA) approach to the evaluation of CO2 emissions resulting from the use of the replacement technique. Under the limitations of our numerical model and experiments, we were able to establish the influence of initial reservoir temperature and pressure on the performance of the exchange. We also noticed the influence of the presence of excess water during the exchange. These observations have been confirmed in the relevant literature. The application of the life cycle assessment method to the process of gas production from a hypothetical hydrate reservoir allowed us to highlight conditions where the use of the exchange technique could lead to emission-neutral gas extraction from methane hydrate deposits.

Gas hydrate is anticipated to serve as a viable substitute for traditional fossil fuels in the near future. Unfortunately, some geomechanical issues may arise during its development, threatening its efficient development and the marine ecology. It is regrettable that research in this area remains inadequate. In the present work, a coupled mathematical model was used to analyze sediment stability during the prolonged extraction of natural gas from hydrate-bearing sediments. Moreover, the applicability of this model was verified by comparison. Based on this, the factors influencing sediment stability were then explored, and the corresponding mechanisms were thoroughly discussed. The comparison results showed that the results obtained by the mathematical model used were more accurate, as it included more physical fields and factors. Therefore, it was more suitable for numerical simulation of sediment stability during the long-term development of gas hydrates. Moreover, it was demonstrated that the strength weakening caused by hydrate dissociation and the stress change due to depressurization were two main mechanisms for sediment deformation or instability. Although gas production increased with increasing depressurization amplitude, permeability and hydrate saturation, as well as shallower reservoir depth, the sediment stability deteriorated accordingly. Interestingly, both sediment stability and gas production were unaffected by the heating amplitude during the prolonged development operation. This study offers a fresh perspective on mitigating the risk of sediment instability while ensuring the efficient development of marine hydrates.

Effective production of natural gas from hydrate-bearing sediments by using various strategies (such as depressurization) is an important way to solve the current global energy crisis. Nevertheless, hydrate dissociation during gas production can weaken sediment strength, influencing reservoir stability and subsequent gas production. Previous studies focused mainly on the analysis of production behavior of natural gas from hydrates, but few on reservoir stability. In this work, evolution of gas production, reservoir characteristics and sediment deformation were analyzed thoroughly with ABAQUS platform. Investigation on gas production revealed that the average production rate was 5.57 × 10 4 m 3 /day, indicating that development strategies mentioned herein can achieve the goal of commercial development of gas hydrates. Although the changes of hydrate saturation and effective stress both affected the characteristics of hydrate reservoir throughout hydrate development operation, hydrate saturation was the main influencing factor. The contour of the distribution nephogram of reservoir characteristics basically coincided with that of the hydrate saturation distribution nephogram. Meanwhile, the yield area around wellbore appearing in the early stage of development operation corresponded to the area prone to sand production. However, the yield area near the seabed appearing in the late stage of development operation corresponded to the area prone to submarine landslide. Finally, investigation on sediment deformation indicated, except for the dissociation area, which experienced significant compaction, the sediments in other areas in the confined space experienced continuous subsidence. This study is expected to lay a theoretical foundation for proposing engineering measures to avoid uncontrollable geological disasters in the process of hydrate development.

Clay-rich sediments are common in natural gas hydrate reservoirs and pose significant technical challenges in past field production tests. Among these, Na-montmorillonite significantly affects methane hydrate (MH) formation and dissociation due to its swelling and water absorption properties. However, the role of swelling clay layers on fluid production by depressurization remains unclear, thus limiting CH4 recovery efficiency. This study aims to elucidate the role of swelling clay on fluid production from natural gas hydrate deposits under depressurization. We synthesized MH-bearing sediments in the presence of sand, clay, and clay-sand alternating layers to examine fluid production and water-gas ratios under different bottom-hole pressures of 6.0 and 10.0 MPa. Results suggest that MH formation rates and final saturation are significantly lower in clay layers (14%) compared to sand layers (29%). During depressurization, MH in clay layers decomposed earlier due to the thermodynamic inhibitory effects of Na-montmorillonite. Swelling clay reduced water recovery (4.3%) and increased gas recovery (88.4%) in clay-sand layers at 6.0 MPa. Lowering pressure enhanced CH4 recovery to 88.4% in clay, 71.8% in clay-sand, and 68.3% in sand layers. Depressurization created persistent low-temperature region in clay layers (ΔT = 2.0 °C), potentially promoting MH reformation. Heat conduction was more dominant in sand layers than clay layers, indicating imbalance interlayer heat transfer characteristics. These findings offer critical insights for optimizing depressurization strategies in clay-rich hydrate reservoirs, enabling secure and efficient energy recovery from clay-rich NGH deposits.

Natural gas hydrate (NGH) has been widely considered as an alternative to conventional oil and gas resources in the future energy resource supply since Trofimuk’s first resource assessment in 1973. At least 29 global estimates have been published from various studies so far, among which 24 estimates are greater than the total conventional gas resources. If drawn in chronological order, the 29 historical resource estimates show a clear downward trend, reflecting the changes in our perception with respect to its resource potential with increasing our knowledge on the NGH with time. A time series of the 29 estimates was used to establish a statistical model for predict the future trend. The model produces an expected resource value of 41.46 × 10 12 m 3 at the year of 2050. The statistical trend projected future gas hydrate resource is only about 10% of total natural gas resource in conventional reservoir, consistent with estimates of global technically recoverable resources (TRR) in gas hydrate from Monte Carlo technique based on volumetric and material balance approaches. Considering the technical challenges and high cost in commercial production and the lack of competitive advantages compared with rapid growing unconventional and renewable resources, only those on the very top of the gas hydrate resource pyramid will be added to future energy supply. It is unlikely that the NGH will be the major energy source in the future.

This review covers the types and applications of chemical inhibitors of gas hydrate formation in the oil and gas industry. The main directions of the development of new types of highly effective and environmentally safe “green” kinetic hydrate inhibitors (KHIs) based on biopolymers are analyzed. The structure, physicochemical properties, efficiency of gas hydrate formation inhibition, and commercial prospects of polysaccharides in preventing and controlling the formation of gas hydrates are considered. The criteria for their selection, current experimental data, and the mechanism of inhibition are presented. Recent research in the development of cost-effective, efficient, and biodegradable KHIs for industrial applications in the oil and gas industry is also presented.

The low permeability of silty hydrate reservoirs in the South China Sea is a critical issue that threatens safe, efficient, and long‐term gas production from these reservoirs. Hydraulic fracturing is a potentially promising stimulation technology for such low‐permeability reservoirs. Here, we assess the gas production potential of a depressurization horizontal well that is assisted by the hydraulic fracturing using numerical simulation according to field data at site SH2 in this area. In addition, the number of horizontal wells drilled is discussed if commercial production is to be performed at this site. The results show that the production potential can be significantly stimulated at the early production stage by adopting hydraulic fracturing in this reservoir due to a better depressurization effect. However, the increase in gas recovery gradually decreases with the continuous dissociation of gas hydrates, and the evolution trend is similar to that in a reservoir without stimulation during later periods of gas production because the dissociation front gradually moves away from the fractures. From the perspective of production potential, using a horizontal well scheme assisted by the hydraulic fracturing technology for gas recovery from a hydrate deposit can sharply reduce the number of operation wells, shorten the drilling operation time, and boost the economic efficiency. The horizontal well scheme may be an effective way to increase the gas yield if the application of quickly deployed horizontal wells and hydraulic fracturing techniques in such hydrate reservoirs greatly increases in the near future. The effect of hydraulic fracturing on gas recovery and the needed minimum number of horizontal operation wells are evaluated for gas hydrate reservoirs in Shenhu area of the South China Sea. The horizontal well scheme may be an effective way to increase the gas yield if the horizontal wells and hydraulic fracturing techniques in such hydrate reservoirs are adopted in the near future.

This paper is devoted to the history of exploration of sintezed and natural gas hydrate. Academic, engineering and energy periods of the history of gas hydrates studies are described. The most significant researches in this area are described. The main practical projects in the world for the study and production of gas hydrates are reviewed.