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• A novel NGH exploitation method by CO 2 /H 2 semi-continuous injection-production was proposed. • CH 4 recovery and concentration were enhanced dramatically when injecting low CO 2 concentration. • Fast CH 4 hydrate decomposition stage can be retained separately in the replacement process. • A circulating injection-production strategy was beneficial to CO 2 storage with high CH 4 recovery. The CO 2 replacement technique is considered as a promising approach for both gas hydrate recovery and CO 2 sequestration. Based on this technique, the continuous CO 2 /H 2 injection-production mode had been studied in our previous work. However, there are limitations on this continuous mode, such as lower CH 4 recovery and CO 2 sequestration ratios, lower CH 4 concentration in produced gas and higher injection-production ratio caused by the fast breakthrough of injected gas. Here we proposed a so called semi-continuous injection-production mode, in which continuous injection-production process is interrupted periodically by stopping injection and production operations for letting injected gas diffuse sufficiently, delaying its breakthrough and increasing its effective sweep region. A series of experimental simulations were performed with respect to this mode in a three-dimensional simulator with a volume of 10.6 L. The results indicated that for the injected gas with low CO 2 concentration, the CH 4 recovery and concentration in the produced gas could be enhanced dramatically by the semi-continuous gas injection method. Additionally, the fast decomposition stage of CH 4 hydrate could be retained separately in the replacement process, thereby effectively improving production efficiency. However, the corresponding CH 4 concentration in the produced gas decreased and the injection production ratio increased. Notably, the process combining the CH 4 steam reforming with cyclic injection-production was first simulated. The results showed an excellent CO 2 storage capability and CH 4 hydrate recovery ratio, which was mainly controlled by the gas composition of the re-injected gas. Through systematically comparing and analyzing the CH 4 concentration, recovery ratio, production efficiency and injection-production ratio, it is demonstrated that the semi-continuous injection-production mode is superior to the continuous one. The results obtained in this work are of significance for guiding future NGHs exploitation. [ABSTRACT FROM AUTHOR]

• A highly simulated unsealed marine hydrate deposits is established. • The technology of pressure-retaining gas injection is proposed and proved. • The pre-depressurization operation can significantly enhance gas recovery. • An optimized strategy for hydrate exploitation is proposed. The exploitation of unsealed marine hydrate deposits is of greater commercial value, but also more challenging because of the risks of the invading of huge body of sea water and subsidence of the seabed. In this work, we proposed a new method to recover natural gas from the usealed hydrate deposits saturated with water by pressure-retaining gas (CO 2 + H 2) injection. A series of experimental simulations was performed to prove its feasibility through establishing a highly simulated unsealed marine hydrate-bearing sediments system. The results demonstrated that overlying water invading was almost completely inhibited for the pressure-retaining approach while unimpeded for the simple depressrization process. Further, we designed a temporary depressurization process before gas injection to establish a flow field for gas in the sediments. This combined operation effectively enhanced the migration and spread of the injected gas, eventually increasing gas recovery ratio from 25% to 60% while the produced water/gas ratio remains as low as 12.6 ST kg/m3. Additionally, it is proved that the balance between CO 2 sequestered and CH 4 recovered could be achieved, which was an excellent support for reservoir stability. The combination method realized a higher CH 4 concentration in the produced gas and a lower injection-production ratio, which greatly reduced the costs of gas injection and subsequent treatment of produced gas. For comparison, similar simulation work was also performed with respect to sealed gas-rich hydrate-bearing sediments. Overall, this temporary depressurization aided pressure-retaining gas injection approach is very promising for high efficient and safe exploitation of marine hydrtates. [ABSTRACT FROM AUTHOR]

For natural gas hydrate exploitation, revealing the interaction between flow and geomechanics is the key to accurate prediction of gas production and formation stability. In this work, based on the site GMGS3-W19 of the South China Sea, a numerical model with multilayer hydrate deposits was established to analyze the evolution of geological parameters during depressurization. The results demonstrated that the reservoir underwent sequential process of expansion and compression, which resulted in the corresponding changes of porosity and permeability. The different initial geological parameters of the three hydrate layers showed respective coupling characteristics between flow and geomechanics. Additionally, we also analyzed the geological stability of reservoir. The calculation results of horizontal and vertical effective stresses were within the elastic region away from the Mohr-Coulomb yield function until 1800 d. In contrast, the layer with high hydrate saturation was more likely to produce shear failure. Finally, the gas-water production was determined after the coupled flow and geomechanical analysis. The result showed that gas productions of the two horizontal wells were on par, while water production of upper well was >7 times that of lower well. The numerical model and analysis could provide useful insight into the marine heterogeneous hydrate exploitation. • A multi-layer heterogeneous hydrate model is established. • Coupled flow and geomechanical analysis during depressurization is conducted. • The possibility of stress failure at different production pressures is analyzed. • Water production of upper well is > 7 times higher than that of lower well. [ABSTRACT FROM AUTHOR]

Ensuring reservoir stability during natural gas hydrates production is the key to balance the geology-engineering-environment nexus. Under the overlying stress of 10 MPa, a series of experiments were conducted to investigate the variations of P-wave velocity, subsidence, and stratum stiffness during hydrate depressurization mining. The influence of hydrate saturations, gas production pressure, and sediment type were examined. The stratum stiffness of hydrate-bearing sediment gradually decreased with the decomposition of hydrate, despite the continuous compaction of overlying stress. Higher hydrate saturation led to larger subsidence and slower sedimentation rate. Lower gas production pressure resulted in greater subsidence and faster sedimentation rate. In sandy sediments, a larger particle size generated a larger sedimentation rate due to the strong heterogeneity of hydrate. However, the subsidence of silty clay is smaller with slower sedimentation rate. Hence, a novel method of intermittent CO 2 /N 2 injection below the freezing point was proposed, which kept the stratum stiffness, reduced the sedimentation rate and subsidence of the reservoir, but also realized the sequestration of CO 2 simultaneously. The method greatly enhanced the recovery ratio of methane hydrate below the freezing point. These findings provide guidance for the safe and efficient exploitation of natural gas hydrate in future. • The stratum stiffness of hydrate-bearing sediment decreases with the decomposition of hydrate. • Higher hydrate saturation leads to larger subsidence and slower sedimentation rate. • Lower gas production pressure leads to greater subsidence and faster sedimentation rate. • A larger sediment particle size leads to a larger sedimentation rate. • Intermittent CO 2 /N 2 injection below 0 °C favors the stability of hydrate-bearing sediment. [ABSTRACT FROM AUTHOR]

• Hydrate phase equilibrium conditions for hydrogen-containing gas mixtures were measured. • The feed gas are binary and ternary gas mixtures, while aqueous solutions are pure water and brine. • WS and vdW mixing rules were applied in predicting hydrate phase equilibrium conditions. • The AADP of PT-WS model is 4.61%, exhibiting a better performance for available data. Hydrate-based technology is considered as one of potential ways for hydrogen separation and storage. Additionally, it has been reported that injecting CO 2 /H 2 mixture is an effective approach to the enhancement of natural gas hydrate exploitation. Hence the determination of hydrate formation conditions of hydrogen containing gas mixtures is of fundamental significance. A series of hydrate formation conditions of hydrogen-containing gas mixtures were then measured under 273.8 to 285.5 K and 2.61 to 10.52 MPa, including 3 binary (CH 4 + H 2) gas mixtures and 5 ternary (CH 4 + CO 2 + H 2) gas mixtures in pure water, as well as 8 ternary (CH 4 + CO 2 + H 2) gas mixtures in aqueous solutions of NaCl and Na 2 SO 4. Meanwhile, a thermodynamic model for predicting hydrate formation conditions of hydrogen-containing systems were developed, in which Chen–Guo hydrate model was used to calculate the fugacities of guest species in hydrate phase and Patel–Teja equation of state was used to calculate the fugacities of guest components in gas phase and those of water in aqueous solutions. Two mixing rules, van der Waals mixing rule and Wang-Sandler mixing rule, were applied in parallel to determine the values of parameters of Patel–Teja equation of state. The overall average absolute deviation on formation pressure for 273 data points is 4.61 % when using Wang-Sandler mixing rule while it is 5.70 % when using van der Waals mixing rule. The experimental data and prediction method presented might be of significance for both exploitation of natural gas hydrate via injecting CO 2 /H 2 mixture as well as the purification of hydrogen via forming hydrate. [ABSTRACT FROM AUTHOR]

Short gas production periods and high production costs are two main challenges for the commercial exploitation of natural gas hydrates (NGHs). The NGHs deposit with enriched underlying free gas has drawn more attention, because the dual-gas co-production from hydrate zone and underlying free gas zone are expected to increase gas yields. We designed a method to prepare this kind of hydrate deposits that a hydrate layer over a free gas layer, and performed a series of dual-gas co-exploitation experiments to investigate its gas production characteristics via depressurization method. Results demonstrate that the temperature of hydrate layer and free gas layer exhibit different variation tendency during gas production process and lower production pressure attributes to higher gas production rate and gas recovery ratio. The wellbore set at underlying free gas layer is recommended because it can avoid lower initial local permeability around the well and prevent ice plug or hydrate reformation during gas production process. Meanwhile, it enhances the movement of hydrate decomposition front and further improves gas production efficiency. Compared with the NGHs reservoir with a single hydrate zone, the dual-gas co-production has higher gas production efficiency and lower temperature decrease to maintain the driving force of hydrate dissociation. The dual-gas co-production with the wellbore set at underlying free gas layer can avoid lower initial local permeability around the well, prevent ice plug or hydrate reformation during gas production process, and enhance the movement of hydrate decomposition front. [Display omitted] • Dual-gas co-production from hydrate zone and underlying free gas zone was investigated. • Temperature tendency is different for hydrate and free gas layers during production. • Lower production pressure attributes to higher gas production rate and gas recover ratio. • The wellhead at underlying free gas layer avoids sharply temperature decrease of hydrate layer. • Dual-gas co-production has higher gas production efficiency and lower temperature decrease. [ABSTRACT FROM AUTHOR]

• A method to prepare hydrate deposit with heterogeneous distribution was proposed. • The influence of water migration on P-wave velocity and amplitude was examined. • Hydrate formation process was divided into unconsolidated and consolidated stages. • P-wave velocity with homogeneous distribution is faster than heterogeneous one. • Layered distribution hydrate provided direct evidence about heterogeneous degree. The relationship between the hydrate-bearing sediments with physical properties such as wave velocity is of importance for the exploration and exploitation of natural gas hydrate. The distribution modes of naturally occurring gas hydrates in sediments are diverse and have significant influence on physical properties of rocks. However, hydrates in homogeneous distribution are in general investigated in laboratory currently. In this work, to simulate the diverse distribution modes of naturally occurring gas hydrates, a novel method to prepare the hydrate-bearing sediments with heterogeneous distribution including layered distribution was proposed by controlling the temperature, water migration, and water saturation distribution. Velocity and amplitude of P-wave are investigated to evaluate the physical properties of hydrate-bearing sediments with homogeneous or heterogeneous distribution. The results showed that the hydrate formation process apparently consists of unconsolidated and consolidated steps according to the amplitude curve. The amplitude of P-wave in the unconsolidated stage increases slowly, while it increases suddenly and rapidly in the consolidated stage. The P-wave velocity of the hydrate deposits with homogeneous distribution is faster than that with heterogeneous distribution when under a same hydrate saturation. This is also verified from the experiments performed on hydrate-bearing sediments with layered distribution, which provided direct evidence and quantification about heterogeneous degree of hydrate distribution in sediment. This difference of the P-wave velocity between heterogeneous and homogeneous hydrate-bearing sediments would provide a guidance in the future development of natural gas hydrate. [ABSTRACT FROM AUTHOR]