To achieve the goals of Peak Carbon Dioxide Emissions and Carbon Neutrality, China's energy system will continue to accelerate the transition to a clean and low-carbon one. As the cleanest fossil fuel, natural gas is regarded as an inevitable choice for China to build a clean, safe, efficient, and low-carbon energy system and fulfill the goal of “double carbon”. However, the domestic conventional natural gas supply remains rigid while the stimulation of unconventional natural gas is still limited. If we have a firm grip on the principal line of “understanding the ocean --developing resources --ensuring security” to realize the large-scale development of 85 trillion square meters of NGH in the South China Sea, then we could not only greatly reduce China ‘s foreign dependence on natural gas, but also guarantee the safety of China ‘s natural gas multi-path supply and safeguard the sovereignty of the South China Sea. Thus, the goal of Peak Carbon Dioxide Emissions and Carbon Neutrality can be achieved in no time.

The global amount of energy contained in natural gas hydrates is huge, maybe as much as twice all known conventional fossil fuel resources. Unlike the worldwide distribution of conventional fossil fuels, hydrocarbons trapped in water as hydrate is also available in countries with limited conventional hydrocarbon resources. The development towards lower global emissions of greenhouse gases requires new strategies for the use of hydrocarbons, whether they are available as conventional resources, or in the form of natural gas hydrates. In this work we outline some possible strategies of utilizing hydrocarbon energy resources in a clean and environmentally friendly way. The use of carbon dioxide for producing hydrates is not new. Results from several experimental studies are available. In this work we shed more light on thermodynamic limitations and the need for additives in order to make the approach technically efficient. Through thermodynamic analysis we show that up to 20 mol per cent N₂ is feasible in a CO₂/N₂ injection gas based on ability to form a new hydrate with free pore water and the released enthalpy needed to dissociate in situ CH₄ hydrate. Surfactant is also needed in order to keep the injection gas front free of blocking hydrate films. Small alcohols like methanol and ethanol have surfactant properties. It is demonstrated that even 10 wt% ethanol in liquid pore water still makes it feasible to create a new hydrate from injection gas containing 20 mol per cent N₂. Consequences of other components than CH₄ in the hydrate are also discussed. And in practical cases with a well-defined source of CO₂ it is also important to investigate impact of other components like for instance H₂S on the stability of injection gas hydrate, as well as changes in enthalpy of hydrate formation. Another key element of this work is the conversion of produced hydrocarbons over to hydrogen and carbon dioxide using steam cracking. The technology for this is very old and in daily use.

Mineral surfaces adsorb water to extreme densities and corresponding low chemical potentials. This results in a dual effect in terms of hydrate. Water and slightly polar components adsorb directly on mineral surfaces and generate efficient conditions for hydrate nucleation. But due to the extremely low chemical potential of adsorbed water the hydrate nuclei formed towards mineral surfaces have to either detach from the vicinity of mineral surfaces, or be bridged by structured water in a dynamic attachment of hydrate cores some few nm outside mineral surfaces. During transport of gas (CH₄, gas mixtures, CO₂) the conventional water dew-point analysis will typically result in a substantially higher acceptable water concentration as compared to the concentration for adsorption of water from gas to rust surface. Direct formation of hydrate from water dissolved in gas is thermodynamically feasible, as discussed in open literature. In this work we demonstrate that it is also feasible in terms of mass transport. A new theory for enthalpy of hydrate dissociation has been extended to also direct hydrate formation from water dissolved in gas. The remaining question is whether direct hydrate formation from gas is also feasible in terms of transporting the hydrate formation heat away through a heat insulating medium. We propose further research strategies to enlighten this issue. Addition of glycols to critical points in processing of gas or transport is already in use by companies like for instance EQUINOR. There is, however, a need for more work on how efficient it is and if it can also be used for multiphase transport of hydrocarbons with significant water cut. Some research activities are in progress and briefly outlined here.

The large quantity of marine methane hydrates has driven substantial interest in methane-gas-fuel potential, especially with the qualified success of Shensu (2017) and Nankai-Trough (2014 & 17) production trials via depressurisation (blighted ultimately by sanding out), building on an earlier Malik-2008 trial for permafrost-bound hydrate. In particular, obviating deep-water-drilling approaches, such as the MeBO production rig (without such a drill bit), together with blowout preventers, constitutes a tantalising cost-saving measure. Tailored means of addressing sand production by customised gravel packs, wellbore screens and slotted liners with from-seafloor drilling will be expected to lead to future production-trial success. However, despite these exciting engineering advances and a few marine-mimicking laboratory studies of methane-hydrate kinetics and stabilisation from microbial perspectives, relatively little is known about the thermogenic or microbial origin of marine hydrates, nor their possible formation kinetics or potential stabilisation by microbial sources as an exponent of Gaia's hypothesis, or within the context of “Gaia's breath” as regards global methane ‘exhalations’. Here, for the first time, we elucidate the methylotrophic-microbial basis for kinetic enhancement and stabilisation of marine-hydrate formation in both deionised-and sea-water, identifying the key protein at play, which has some similarity to porins in other methylotrophic communities. In so doing, we suggest such phenomena in marine hydrates as evidence of Gaia's hypothesis.

Development of marine gas hydrate resources presents a huge challenge to the energy industry owing to the well production complications such as wellbore collapse, sand production, and low productivity. Radial lateral wells (RLW) and horizontal snake wells (HSW) have been proposed separately to mitigate these complications. We compare the productivity potentials of these two types of wells using the recently developed analytical models and field case data from a gas hydrate reservoir in the South China Sea. It is concluded that RLW will yield slightly higher gas productivity than HSW under similar conditions. Sensitivity analysis with the well models indicates that the productivity of RLW is directly proportional to the number of laterals, length of laterals, and radius of laterals, while the productivity of HSW is directly proportional to the length and radius of the horizontal wellbore. The decision of using RLW or HSW can be made based on economic analysis of well completion and production, which should be addressed in future studies.

This work focuses on the assessment of the effect of well completion types on gas productivity in subsea gas hydrate reservoirs of class 1G type where the gas hydrates have decomposed into gas and water. Three types of vertical well completions are considered: frac-packed well with vertical hydraulic fracture; frac-packed well with horizontal hydraulic fracture, and a cased-hole gravel-packed well. Sensitivity analysis was conducted with analytical well inflow models to determine factors that affect the gas well productivity. The results of the analyses indicated that proppant mass pumped during fracture treatment slightly improves well productivity for frac-packed natural gas hydrate wells. Well productivity increases nonlinearly with fracture productivity up to a threshold value of 50,000 md for frac-packed well with horizontal fracture, above which further increase in fracture conductivity would create no benefit. With a proppant mass of 50,000 Ibm and a corresponding proppant volume of 504 ft³, commercial gas production rates of 14.9 MMscf/d, 5.621 MMscf/d, and 11.35 MMscf/d are possible for frac-packed well with vertical fracture, frac-packed well with horizontal fracture, and cased-hole gravel-packed well, respectively. Because hydraulic fracture orientation depends on the in-situ formation stress, whether a well should be hydraulic-fractured or not depends on in-situ formation stress.

It is easy to change the original temperature state of marine gas hydrate reservoir by drilling, which leads to uncontrollable decomposition of gas hydrate and release of large amount of gas. The decomposition gas will further escape and expand, and the reservoir will break and collapse due to its weak cementation characteristic, which will easily lead to a series of other potential risks. Therefore, in this study, based on the drilling process of marine gas hydrate, we establish the theoretical model and numerical calculation method of wellbore temperature field, analyze the influence on wellbore temperature of drilling fluid displacement, density, viscosity and injection temperature, and seawater depth. Then the sensitivity laws of reservoir risk in marine gas hydrate drilling are obtained. The results show that with the increase of drilling fluid displacement, density, viscosity and injection temperature, the temperature in lower well section and bottom hole will increase, making the increasing amplitude of temperature in hydrate reservoir larger and the scope of influence on hydrate reservoir stability bigger. Moreover, drilling is more likely to raise the temperature of reservoirs in shallow seawater depth, posing greater risks. Thus, engineering measures to avoid risks caused by rising reservoir temperature in marine gas hydrate reservoir drilling are presented. This study is of great significance to ensure the safety of marine gas hydrate reservoir drilling.

There are many emergency risks in the process of natural gas hydrate (NGH) drilling. In order to ensure the safe and efficient exploitation of NGH, it is urgent to establish an intelligent judgment method for the risks in the process of NGH drilling. In this paper, the response relationship between monitoring parameters and risk categories of NGH while drilling is established. Based on fuzzy analytic hierarchy process (FAHP), the comprehensive weights of 10 risk monitoring parameters are obtained, including gas production, wellbore instability, hydrate ice barrier, drill string fracture, sticking, bit balling, drilling tool piercement, gas seepage, seabed subsidence and seabed landslide. Besides, the comprehensive judgment weight matrix is constructed, and the reasonable fluctuation range of monitoring parameters is formed. Thus, the intelligent judgment method of NGH drilling risk is established. The intelligent judgment and alarm of NGH drilling risks can be realized quickly and accurately by this method, namely, it can monitor the risks in the process of operation and guarantee the construction safety of NGH drilling.

The natural gas hydrate resources in the South China Sea alone are about 85 trillion cubic meters. In the drilling process of marine gas hydrate, the natural gas hydrate will decompose and produce gas, as the rising of temperature and dropping of the pressure in the annulus. This process will have a significant impact on drilling safety. Therefore, it is necessary to study the wellbore temperature distribution during the drilling of marine hydrate layer. In this paper, the wellbore temperature distribution of safe drilling in hydrated formation is taken as the research goal, and the research status of relevant domestic and international wellbore temperatures was investigated. According to the characteristics of the marine environment and reservoir-forming characteristics of hydrate reservoirs in the South China Sea, the wellbore temperature distribution model of offshore drilling wellbore under the condition of hydrate decomposition was established. The temperature distribution curve of drilling straight wellbore in hydrate layer of South China Sea was obtained. When drilling the hydrate reservoir, the distribution regularity of the wellbore temperature is similar to that of the conventional offshore drilling wellbore. However, the temperature of the wellbore annulus near the hydrate decomposition site is lower than the ambient temperature, mainly due to the hydrate decomposition endothermic. In this paper, the sensitivity analysis of several main parameters of the wellbore temperature distribution of drilling straight wellbore in hydrate layer of South China Sea was carried out. Through the conduction of experiment and numerical simulation, we have get some new findings: (1) The hydrate saturation has little effect on the wellbore temperature; (2) As the drilling fluid displacement increases, the annulus temperature of the wellbore above the mudline increases, and the temperature of the wellbore below the mudline decreases continuously; (3) As the density of the drilling fluid increases, the temperature at the wellhead decreases, and the temperature at the bottom of the well increases slightly; (4) The greater the rate of penetration of the well, the temperature at the upper part of the wellbore decreases, and the temperature at the bottom of the wellbore increases; Among them, the penetration rate has the most obvious effect on the annular temperature. The results are expected to be helpful to guide the drilling process of marine gas hydrate and offer some references.

The permeability is one of the intrinsic parameters that determines the fluid flow in the porous media. The permeability in hydrate-bearing sediments affects the gas recovery and production of hydrate reservoirs significantly. The irregular permeability characteristics are challenging for fine-grained hydrate-bearing sediments. In this study, a series of experiments was conducted using an one-dimensional pressure vessel to investigate the hydrate formation characteristics and the permeability in hydrate-bearing fine quartz sands (volume weighted mean diameter was 36.695 μm). Hydrate saturations (0-26% in volume) were controlled and calculated precisely based on the amount of injected water and gas, the system pressure and temperature. The results indicated that the hydrate nucleation induction period was completed during gas injection, and the average time of hydrate formation was within 500 min. The permeability of methane hydrate-bearing fine quartz sands was investigated by steady gas volume flow. For hydrate saturation lower than 13.94%, the hydrate mostly formed in grain-coating, the permeability reduction exponent calculated by Parallel Capillary, Kozeny Grain Coats and Simple Cubic Filling models were 2.00, 2.10 and 1.74 respectively, and Simple Cubic Filling model was in accordance with the experimental data best. However, for hydrate saturation ranged from 13.94% to 25.91%, the permeability increased due to the flocculation structure formation of fine quartz sands and hydrate, which caused the increase of effective porosity. A new relationship among hydrate saturations, effective porosity, the ratio of permeability in the presence and the absence of hydrate was developed. This study developed the mathematical models for predicting the permeability with hydrate saturation in fine quartz sands, which could be valuable for understanding the characteristics of hydrate-bearing fine-grained sediments.

Natural gas hydrate is a kind of clean energy with huge reserves, and the saturation (volume percentage of hydrate in pore space of sediments) is the key parameter for determining whether the reservoir is worthy of exploitation. In this work, rapid hydrate dissociation by the combination of heat injection and NaCl inhibitor addition was studied, and an on-site evaluation method for hydrate saturation in sediment samples was proposed by using a core sampler to transfer hydrate samples under pressure. The results showed that the average gas production rate per unit volume was increased significantly to reach 7.22 L/Lr·min-1 by the injection of NaCl aqueous solution with 50.9 °C, which was attributed to the increase of the chemical potential to further accelerate the rate of hydrate dissociation in the presence of NaCl. Furthermore, for the measurement of methane hydrate samples saturation with a volume of 673 cm³ (which contained 1.4 mol hydrates with the saturation of 58%), hydrate saturation could be accurately achieved within 30 min with a relative error lower than 11.7% This work may provide new thoughts for on-site saturation evaluation and rapid dissociation of hydrate samples during natural gas hydrate exploitation.

In this study, the Discrete Element Method (DEM) was employed to investigate numerically the effects of hydrate cementation and intermediate principal stress on the stress-dilatancy relation of grain-cementing type methane hydrate-bearing sediment (MHBS) by conducting a series of conventional and true triaxial tests. A novel 3D thermo-hydro-mechanical-chemical (THMC) contact model for MHBS was employed. The numerical results show that with increasing hydrate saturation and back pressure, or decreasing confining pressure, temperature and salinity, the stress-dilation relation of grain-cementing type MHBS evolves from dilation-dominant to bond-dominant. For the clean sand samples, the relationship between the normalized stress ratio η/M_cr and the dilatancy rate d is close under different intermediate principal stress coefficients. However, for the MHBS samples, this relationship is still affected by the intermediate principal stress coefficient b, due to the effect of hydrate cementation.

Natural gas hydrates are mostly formed in low-permeability and fractured muddy sedimentary formations. Adding suitable nanoparticles to the drilling fluid system can improve its filtrate resistance and fracture plugging, and effectively weaken the invasion of drilling fluid into the reservoir. However, it is likely that nanoparticles promote hydrate formation and accumulation in wellbores which will induce accidents. Therefore, this study investigated the effect of hydrophilic silica nanoparticles with particle sizes of 30 nm, 60 nm, and 80 nm and concentrations of 0.5-4.0 wt% on hydrate formation during upward migration of methane gas using a dynamic simulation system for hydrate formation in a wellbore. The experimental results show that under the condition of methane gas migration, hydrophilic silica nanoparticles inhibit hydrate formation. The inhibition effect increased with the growth in the particle size under a constant concentration, whereas it first increased and then decreased with increasing nanoparticle concentration under a constant particle size. The strongest inhibition effect was observed at a hydrophilic silica nanoparticle concentration of 2.0 wt%. The influence of hydrophilic silica nanoparticles on hydrate formation may be mainly determined by their hydrophilic properties, heat and mass transfer, and gas migration in the wellbore. Our research indicates that hydrophilic silica nanoparticles can be added to hydrate drilling fluid systems if their concentration can be properly controlled.

With the exploration and development of oil and gas fileds going towards into deep-water fields, high waxy reservoir has much more flow assurance issues of encourage complex solids depositions in the transportation system, especially hydrates and wax. Applying risk management such as hydrate slurry technology to control hydrate blockage, has much more economic and technical advantages, comparing to the traditional methods. It is significant to understand the viscosity of the waxy-hydrate slurry using hydrate slurry technology in high wax content reservoir. In this work, based on a simplification idea by coupling the wax content effect into the viscosity, volume and density of the water-in-waxy oil emulsion, a new viscosity model of waxy-hydrate slurry is established according to the Einstein effective medium theory, based on the experiments carried out in a high-pressure rheology system with different wax contents ranging from 0.5 wt%~2.0 wt%. The effect of the complex aggregate coupling wax-hydrate-water is considered by function the non-Newtonian coefficient by four dimensionless parameters. Well-fitting results within an improved deviation of ±15% indicate the feasibility of this method is feasibility. This work can provide a valuable reference for the application of hydrate slurry technology in deep-water fields with high wax content reservoir.

The gas hydrate formation in pipelines of industries and chemical plants can cause various operational damages and can increase economic risks. Hence, the knowledge of hydrate formation conditions has become a critical research study to overcome the problems arising from the formation of hydrates. In this study, we applied an algorithm to develop an LSSVM model to predict the formation temperature of natural gas hydrate for a comprehensive range of data points. Total 188 experimental data points were applied from the literature for the development of the LSSVM model. The input parameter was finalized based on the structure of hydrates by each gas species. The results obtained by the LSSVM model have good accuracy as compared with empirical correlations available in the literature. This model gave the squared correlation coefficient (R²), and root mean square error of 0.9901 and 0.59974, respectively. The composition of gases may affect the phase equilibrium condition of gas hydrates. The applied algorithm revealed that the developed LSSVM model could become a good alternative for calculating the formation temperature of hydrate for the range of all data sets. The results showed that the proposed LSSVM model could be applicable for the prediction of hydrate formation temperature for all data points.