The schematic diagram of hydrate decomposition process and the gas production by using different methods. Display omitted
•Three gas production methods were evaluated with different hydrate ...saturations.•The roles of temperature, pressure, sensible heat and heat transfer were analyzed.•The driving force of hydrate dissociation at different stages was analyzed.•The combined method effectively improved the gas production and energy efficiency.
To investigate the gas production from methane hydrate-bearing sediments, the gas production processes from methane hydrate in porous media using depressurization, two-cycle warm-water injection and a combination of the two methods were characterized in this study. The methane hydrates were formed in porous media with various initial hydrate saturation (Shi) in a pressure vessel. The percentage of gas production, rate of gas production, and energy efficiency were obtained and compared using the three methods. The driving force of the hydrate dissociation at different stages of depressurization was analyzed and ice formation during the gas production was observed. For the two-cycle warm-water-injection method, the percentage of gas production and the energy efficiency increased with increasing of Shi. However, due to the large amount of warm water needed to heat the porous media at the dissociation site, the percentage of gas production was lower than the other two methods under the same experimental conditions. The experimental results proved that the combined method had obvious advantages for hydrate exploitation over the depressurization and warm-water-injection method in terms of the energy efficiency, percentage of gas production and average rate of gas production, and with increasing of Shi, the advantages are enhanced. For the Shi of 51.61%, the percentage of gas production reaches 74.87%, which had increments of 18.63% and 31.19% compared with the depressurization and warm-water-injection methods. The energy efficiency for the combined method were 31.47, 49.93 and 68.13 for Shi of 31.90%, 41.31% and 51.61%, respectively.
Mid-latitude Central Asia is characterized by an extreme arid landscape. Among the Central Asian deserts, the Taklimakan Desert is the largest shifting sand desert which is located in the rain shadow ...of the Tibetan Plateau and of the other central Asian high mountains. The formation of this desert is important for placing widespread aridification into a regional tectonic context at the western end of the Himalayan-Tibetan orogen. However, there still exists considerable controversy regarding the timing of desert formation, thus impeding our understanding of the climatic effects of the Tibetan uplift and regional environmental changes. Here we report new biostratigraphic age control and multiple high-resolution climatic records from the center of the Taklimakan Desert. Our results reveal dramatic environmental changes at ~5Ma, suggesting that an extremely dry climate has prevailed since the beginning of the Pliocene, which is consistent with findings from a high-resolution borehole record about 670km to the east in the same basin. The two records combined demonstrate that an extremely dry environment prevailed in the entire Tarim Basin from approximately 5Ma. This was related to the reduced transport of water vapor by westerlies in response to the retreat of the Paratethys Ocean driven by global climatic cooling, and the closed oceanic water-vapor pathway between the Pamir and the Tian Shan ranges driven by the ongoing India-Eurasia collision.
•New biostratigraphic age constrain on the Late Cenozoic strata in the Tarim Basin.•Multiple high-resolution records indicate extreme aridification at ~5Ma.•The extreme aridification is largely enhanced by regional tectonics.
Schematic diagram illustrating the process of gas production in hydrate-bearing sediment induced by depressurization. When depressurization occurs, the reservoir pressure and temperature change along ...the trajectory of A–B–C–D. Character of gas production process is outlined. Display omitted
•Hydrate dissociation behavior was analyzed in porous media by depressurization.•The gas production process can be divided into three main stages.•Methane hydrate first dissociates simultaneously throughout the hydrate zone, and then from the outside.•The sensible heat of the reservoir and ambient heat transfer play a dominant role in hydrate dissociation.
Natural gas hydrate is a vast energy resource with global distribution in permafrost regions and in the oceans; its sheer volume demands that it be evaluated as a potential energy source. Understanding the mechanisms of natural gas extraction from hydrate-bearing sediments is critical for the utilization of hydrate accumulations. In this work, methane hydrate dissociation was performed in three kinds of porous media at production pressures of 2.2MPa, 2.6MPa, and 3.0MPa. Results show that the methane gas production process can be divided into three main stages: free gas liberation, hydrate dissociation sustained by the sensible heat of the reservoir, and hydrate dissociation driven by ambient heat transfer. In the process of gas production, hydrate dissociation occurs simultaneously throughout the hydrate zone along the phase equilibrium curve, and then spreads radially from the outside as a result of ambient heat transfer. Hydrate reformation and ice generation always occur in the reservoir interior due to insufficient heat transfer. The use of porous media with increased thermal conductivity accelerates the gas production rate; however, it has little influence on the final percentage of gas production. Furthermore, the Stefan (Ste) number and dissociation rate constant were employed to evaluate the impact of the sensible heat of the reservoir and ambient heat transfer. Results indicate that the sensible heat of the reservoir and ambient heat transfer play a dominant role in hydrate dissociation, and that both are dependent on production pressures.
In this study, a microfocus X‐ray computed tomography‐based triaxial testing apparatus was used to observe and quantify the microstructure evolution of hydrate‐bearing sands during thermal ...dissociation of hydrate. Three triaxial shear tests with X‐ray computed tomography were conducted to study the influence of hydrate dissociation on the mechanical behavior of hydrate‐bearing sands. The results show that hydrate covering the sand particle surface dissociates first and then at the menisci between sand particles. The secondary hydrate formation mainly occurs on the menisci between sand particles and the surfaces of the hydrate shells where hydrates already exist. Hydrate dissociation could cause fabric changes in hydrate‐bearing sands, resulting in a more isotropic orientation distribution of sand particles. The failure behavior of the hydrate‐free sand specimen is similar to that of the specimen after hydrate dissociation, which shows an obvious drum‐shaped failure pattern with X‐shaped shear bands. However, the hydrate‐bearing sand specimen exhibits a shear band with a determined thickness and inclination angle. Hydrate dissociation could cause a significant loss of supporting cementation, resulting in a decline in the stiffness and failure strength. The failure strength of hydrate‐free sand specimen is slightly higher than that of the specimen after hydrate dissociation; this may due to that the homogeneous orientation frequency distribution can enhance the stability of the force chain between sand particles. The secondary hydrate formation causes sediment deformation resulting in a decrease in absolute permeability due to pore blockage.
Key Points
Hydrate dissociation causes deformation and fabric changes in hydrate‐bearing sands
The failure behavior of hydrate‐free sands is similar to that of hydrate‐bearing sands after dissociation
Secondary hydrate formation during dissociation would influence the permeability significantly
Although carbon and oxygen isotopic compositions of carbonates are valuable for Quaternary climate reconstruction due to their sensitivity to paleoclimate changes, a comprehensive understanding ...regarding the formation, factors influencing isotopic variations, and age uncertainties of secondary carbonates in loess deposits remains elusive. This study investigates the microstructure and stable isotopes of secondary carbonates, specifically calcified root cells (CRC) and needle-fiber calcite (NFC), found within the loess deposits of the last glacial period in the northwestern region of the Chinese Loess Plateau. The average δ13C values of soil total organic matter, CRC, NFC, and total inorganic carbonates in the YuanBao sections are −22.1‰, −14.9‰, −7.0‰, −4.6‰, respectively. Radiocarbon dating results reveal that both CRC and NFC were not of synsedimentary origin. The pronounced negativity of the carbon isotopic composition and the microstructures offer insights that CRC formation is closely tied to C3 plants. The synchronous depletion of δ13CCRC and δ18OCRC values during the transition period from the last glacial to the Holocene, underscores the sensitivity of isotopic composition of CRC to local environmental changes, making it a potential candidate for paleo-climate studies. Additionally, CRC formed during the glacial period provides a close approximation to the true sedimentation age. The radiocarbon dating of NFC highlights a significant fluctuation in carbon source translocation depth, resulting in a pronounced temporal and spatial decoupling between NFC formation and the surrounding deposits. Consequently, values of δ13CNFC and δ18ONFC seem to exhibit no significant trend across the limited depth range in this research. We propose that, when using NFC for paleoclimate reconstruction, careful consideration of variations in translocation depths of carbon sources is imperative to prevent misestimations. NFC could potentially be more fitting for broader temporal-scale paleoclimate reconstructions rather than high-resolution investigations.
•Loess carbonate NFC exhibiting substantial variations in the depth of carbon source translocation.•CRC formed during loess development could provide a close approximation to sedimentation age.•The isotopic compositions of CRC are responsive to local environmental changes.•Carbonates formed with different mechanisms show distinct isotopic compositions, reflecting various climatic information.
The macromechanical properties (strength, stiffness, stress‐strain relationship, etc) of the hydrate‐bearing sediment are often correlated with the hydrate cementation failure behavior. In this ...study, a consolidated drained triaxial shear test with X‐ray computed tomography was conducted on a hydrate‐bearing sediment with a hydrate saturation of 32.1% under 3 MPa effective confining pressure for revealing the cementation failure behavior. The hydrate occurrences were clearly identified to be cementing, grain‐coating, prepatchy cluster and patchy cluster. The cementation failure behavior (morphology), deformation evolution (quantitative statistics), and localized shear deformation at different regions of stress‐strain curve were observed and analyzed using computed tomography. In the linearity region, the hydrate‐cemented clusters moved as a whole, while small hydrate particles would aggregate to the periphery of the clusters. The noncemented sand particles move disorderly during this process. Localized deformation occurred perfectly exhibit an antisymmetric bifurcation pattern. In the plasticity region, the specimen starts to deform plastically; an internal shear band occurs in the specimen. The hydrates begin to shed from the periphery of the cemented structure, and the grinding effect of hydrates begins to occur between sand particles. However, the shedded hydrates will not enter and fill the neighboring pores but hind the movement of sand particles structurally near the original position. In the yielding region, the hydrate‐cemented cluster structure is completely damaged and crushed into small pieces; a shear band with a determined thickness and inclination angle was observed.
Key Points
The cementation failure behavior of hydrate‐bearing sediment corresponding to different regions of stress‐strain curve is observed by CT
The localized deformation development at different regions of stress‐strain curve was determined and analyzed
The quantitative statistics of hydrate cementation failure behavior was analyzed
Leaf wax δDn-alkane values have shown to differ significantly among plant life forms (e.g., among grasses, shrubs, and trees) in higher plants. However, the underlying causes for the differences in ...leaf wax δDn-alkane values among different plant life forms remain poorly understood. In this study, we observed that leaf wax δDn-alkane values between major high plant lineages (eudicots versus monocots) differed significantly under the same environmental conditions. Such a difference primarily inherited from different hydrogen biosynthetic fractionations (εwax-lw). Based upon a reanalysis of the available leaf wax δDn-alkane dataset from modern plants in the Northern Hemisphere, we discovered that the apparent hydrogen fractionation factor (εwax-p) between leaf wax δDn-alkane values of major angiosperm lineages and precipitation δD values exhibited distinguishable distribution patterns at a global scale, with an average of -140‰ for monocotyledonous species, -107‰ for dicotyledonous species. Additionally, variations of leaf wax δDn-alkane values and the εwax-p values in gymnosperms are similar to those of dicotyledonous species. Therefore, the data let us believe that biological factors inherited from plant taxonomies have a significant effect on controlling leaf wax δDn-alkane values in higher plants.
We performed a global scale analysis of available leaf wax n-alkane δD data compiled from our new results, as well as from the literature and expressed as average values of D/H ratios from three ...common lipids of n-alkanes with odd carbon numbers (n-C₂₇, n-C₂₉, and n-C₃₁) from living higher plants. Our results clearly indicate multiple controls of hydrogen isotope composition and its variability in plants leaf wax. (1) At the global scale, precipitation δD values play a dominating factor that exercises the first order of control for hydrogen isotopic compositions in plant leaf wax. The hydrogen isotopic composition of plant leaf wax tracks the decreasing trend of precipitation δD with increasing latitude. (2) Because of different water acquisition systems, plant life form influences the hydrogen isotopic composition of leaf wax n-alkanes with woody plants and grasses having different responses to the change of global precipitation δD. (3) Physiological difference, due to different photosynthesis pathways or different water usage strategies, can leave an imprint on δD patterns of plant leaf waxes, causing δD variations among plants using the same source water. While these results better explain the variability of hydrogen isotope composition in leaf wax, they also have important implications for the interpretation of n-alkane δD data from fossils and ancient sediments.
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