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Gas hydrates have been considered promising in gas storage, gas separation, and water desalination. The hydrates' technical application is affected by the long kinetics time. Of interest is gas hydrates show strong facilitation in dissociated water containing a high concentration of micro-nanobubbles. Here, it was discovered that the nano-particles present in the dissociated water were identified as nano-bubbles, characterized by nanoparticle track analyzer and infrared spectrum. The presence of bubbles increased the nucleation probability, likely attributed to the provision of nucleation sites. Results indicated that the ability of facilitation was related to the nanobubble types: the bubble solution with the same guest molecules exhibited similar facilitation to dissociated water; specifically, the average induction time was shortened by 27.48 %, the nucleation probability was increased by 50 % compared to deionized water. Moreover, the time hydrate filled the field of view with high bubble concentration was reduced by 60 % compared with low bubble concentration. The facilitation effect mechanism could be related to the presence of gas-dense regions inside the bubbles, based on the Raman results. This finding may offer valuable insights for the application of hydrate-based energy storage technology and shed light on the potential role of bubbles in causing memory effects. • Nano-bubbles are generated as a result of gas hydrate dissociation. • These bubbles have the potential to facilitate hydrate nucleation, with the specific bubble type influencing the process. • The bubbles play a role in promoting hydrate growth, reducing the growth time by 60 %. • The existence of high-pressure zones within the bubbles may be confirmed. [ABSTRACT FROM AUTHOR]

Several trial productions have been conducted on marine natural gas hydrate reservoirs, demonstrating its enormous potential as an alternative energy source. Nonetheless, sustained and efficient production still remains challenging, with crucial issues on low gas production efficiency and blockages. The large amount of hydrate decomposition and resulting heat absorption can lead to prolonged subzero conditions close to the well; this can readily induce ice formation and blockage. This study aims to investigate the issue of ice blockages caused by insufficient energy supply during the recovery process. A systematic analysis of the energy consumption and ice blockage quantification at each stage of the traditional multi-stage direct depressurization and constant speed depressurization methods is conducted. A hybrid optimized scheme is proposed, which enhances energy utilization efficiency by appropriately utilizing reservoir sensible heat during the initial stage of production. Additionally, a multi-stage constant speed depressurization method is introduced in the later stages to mitigate production fluctuations and reduce blockage duration. Results showed a 69.5 % increase in energy efficiency and an average 67.1 % decline in gas blockage duration with comparable gas productivity. The proposed method could be a viable solution to mitigate challenges of icing blockages and fluctuating production during trial tests of marine hydrate reservoirs, achieving improved energy efficiency and sustainable production. • Ice blockage in hydrate reservoirs reduced production efficiency during direct and constant-speed depressurization methods. • Proposed a hybrid method to address gas production fluctuations caused by ice blockage through energy consumption analysis. • Multi-stage analysis revealed 70% energy efficiency improvement and 67% reduced ice blockage duration with the hybrid method. [ABSTRACT FROM AUTHOR]

Increasing the efficiency of gas production from marine gas hydrate still faces significant problems. Based on the geological survey of marine reservoirs, gas hydrates are generally associated with underlying free gas layers with high temperature and water saturation. Here the effect of this gas layer and its characteristics on the gas and water production from the hydrate layer are studied. It was found that a large temperature difference existed between the two layers during gas production due to the different heat demand and consumption in each layer. Slower gas production rate was observed in case of dense distribution of hydrates in the hydrate-bearing layer and the resulting sluggish gas permeation and enormous heat supply. Notably, this was not present when the free gas layer was more water-saturated. The resulting maximum gas production rate could be effectively enhanced by 41 % at most as well. This was attributed to the increased sensible heat available from the pore water and the interbedded heat exchange. It was thus indicated that a hydrate reservoir with an underlying water-rich layer could be more favorable for gas production in terms of heat supply. • Joint exploitation through pre-embedding compartment device and vertical well. • A significant temperature difference between the two layers during exploitation. • Highly water-saturated underlying shallow gas layer enhance hydrate decomposition. [ABSTRACT FROM AUTHOR]

An accurate description of the reservoir characteristics is essential to the precise prediction of gas production from nature gas hydrate (NGH) deposits. In most numerical studies related to large-scale onshore or offshore NGH production cases, the reservoir permeability was usually assumed to be homogeneous. In this work, a multi-layered heterogeneous-permeability reservoir model (including a hydrate-bearing layer, a three-phase layer, and a free gas layer) was established, and the gas production behavior was analyzed by using the TOUGH + HYDRATE simulator. It is found that hydrate and free gas tend to be scatteredly distributed in this reservoir model. Reservoirs with heterogeneous permeability have better gas production performance than homogeneous ones. The 10-year cumulative gas production from the heterogeneous reservoir with the heterogeneity degree of 0.4μ (622 × 104 m3) is 15.19% higher than that from the homogeneous reservoir (540 × 104 m3). Moreover, the layered production results show that the free gas layer contributes the most to gas production among the three layers. The effect of heterogeneity on gas production in high-permeability reservoirs is more obvious than that in low-permeability reservoirs. Therefore, the permeability heterogeneity of the reservoir, especially the permeability heterogeneity of the free gas layer, should be considered in the long-term prediction of NGH production. • A multi-layered permeability heterogeneity hydrate reservoir model was built. • Gas production behaviors and evolution of reservoir characteristics were analyzed. • The sensitivity of permeability heterogeneity was discussed. • Gas production in heterogeneous reservoirs was better than in homogeneous ones. • Heterogeneity should be especially concerned in reservoirs with free gas. [ABSTRACT FROM AUTHOR]

Among deep-sea oil and gas transportation, kinetic inhibitors of gas hydrates are widely added in transportation pipelines to prevent gas hydrates from clogging pipelines. Unfortunately, the micro-and nano-sized clay particles will enter the wellbore to adsorb the practical components of the inhibitors, thereby weakening the inhibitory effect of the inhibitors on hydrates. This work shows that 1:1-type layered silicates can only adsorb inhibitors through the surface, while 2:1-type layered silicates can adsorb inhibitors through both surface and intercalation. The clay particles with adsorbed inhibitors tend to be aggregated and settled because the surface charge density and the potential energy between the particles are reduced. Meanwhile, the inhibitors adsorbed on the surface of the clay lose their inhibitory effect on gas hydrates. Therefore, the suppression effect of layered silicate minerals on inhibitors should be considered in clay-rich oil and gas transportation pipelines. • Inhibitory effect of different clays on polyvinyl caprolactam hydrate was studied. • The inhibitory effects were weakened in the presence of clas due to adsorption. • The inhibitor adsorption ability of 2:1 clay is stronger than that of 1:1 clay. [ABSTRACT FROM AUTHOR]

There have been several field tests worldwide to recover natural gas from marine gas hydrate reservoirs. The resulting intrinsic characteristics of the sediments are recognized as the major challenge hindering the large-scale gas production; of particular interest is the heterogeneous spatial distribution of natural gas hydrates in the sediments. Open issues still remain on mimicking the locally varying reservoir and revealing its role in the production process. Here in this work, reservoirs with a heterogeneous distribution of gas hydrates were prepared by controlling the uneven distribution of moisture in the fine natural sediments drilled from the South China Sea; special interest was put in the effects of the well location and depressurization schemes on the gas production performance. The results showed that the production efficiency in the heterogeneous reservoirs was significantly weakened compared with the homogeneous case; this was ascribed to the ununiform distribution of methane hydrates and pore fluids and a resulting locally different energy demand. Consequently, the production behavior showed a strong dependence on the depressurization location. A wellbore arranged in the layer with a high permeability can help improve the gas production efficiency by up to 38.25%. This yet resulted in an uneven temperature distribution; the rapid gas production and methane hydrate dissociation decreased the reservoir temperature to −3.33 °C. Further applying a cycling and step-wise depressurization could raise the minimum reservoir temperature up to 1.12 °C. Whereas, relieving temperature decline only through consuming the sensible heat in the reservoir and the surroundings was found to seriously weaken the gas production rate; an external heat supply was thus suggested in the hydrate-abundant regions for efficient gas production. Our findings expand the understanding of the spatially varying gas production behavior from the vertically heterogeneous hydrate-bearing fine marine sediments and may guide the operation of well location and depressurization schemes for field production in similar reservoirs. [Display omitted] • Heterogeneous hydrate samples were prepared by unevenly distributing the moisture. • Production behavior from the vertically heterogeneous hydrate reservoir was studied. • The optimized wellbore location would increase the production efficiency by 38.25%. • Heat supply was suggested in the hydrate-rich area to compensate the sensible heat. [ABSTRACT FROM AUTHOR]