TY - JOUR A1 - Li, Zhen A1 - Wang, Yongbo A1 - Herzschuh, Ulrike A1 - Cao, Xianyong A1 - Ni, Jian A1 - Zhao, Yan T1 - Pollen-based biome reconstruction on the Qinghai-Tibetan Plateau during the past 15,000 years JF - Palaeogeography, palaeoclimatology, palaeoecology : an international journal for the geo-sciences N2 - Reconstruction of past vegetation change is critical for better understanding the potential impact of future global change on the fragile alpine ecosystems of the Qinghai-Tibetan Plateau (QTP). In this paper, pollen assemblages comprising 58 records from the QTP, spanning the past 15 kyrs, were collected to reconstruct biome compositions using a standard approach. Six forest biomes were identified mainly on the southeastern plateau, exhibiting a pattern of gradual expansion along the eastern margin during early to mid-Holocene times. The alpine meadow biome was separately identified based on an updated scheme, and showed notable westward expansions towards lower latitudes and higher altitudes during early Holocene times. Consistent patterns of migration could also be identified for the alpine steppe biome, which moved eastward during the late Holocene after 4 ka. As the dominant biome type, temperate steppe was distributed widely over the QTP with minor migration patterns, except for a progressive expansion to lower altitudes in the late Holocene times. The desert biome was inferred mainly as covering the northwestern plateau and the Qaidam Basin, in relatively restricted areas. The spatial distribution of the reconstructed biomes represent the large-scale vegetation gradient on the QTP. Monsoonal precipitation expressed predominant controls on the development of alpine ecosystems, while the variations in desert vegetation responded to regional moisture brought by the mid-latitude Westerlies. Temperature changes played relatively minor roles in the variations of alpine vegetation, but exerted more significant impacts on the forest biomes. KW - biomization KW - pollen KW - vegetation migration KW - Qinghai-Tibetan Plateau KW - holocene Y1 - 2022 U6 - https://doi.org/10.1016/j.palaeo.2022.111190 SN - 0031-0182 SN - 1872-616X VL - 604 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Li, Zhen A1 - Spangenberg, Erik A1 - Schicks, Judith Maria A1 - Kempka, Thomas T1 - Numerical simulation of hydrate formation in the LArge-Scale Reservoir Simulator (LARS) JF - Energies : open-access journal of related scientific research, technology development and studies in policy and management N2 - The LArge-scale Reservoir Simulator (LARS) has been previously developed to study hydrate dissociation in hydrate-bearing systems under in-situ conditions. In the present study, a numerical framework of equations of state describing hydrate formation at equilibrium conditions has been elaborated and integrated with a numerical flow and transport simulator to investigate a multi-stage hydrate formation experiment undertaken in LARS. A verification of the implemented modeling framework has been carried out by benchmarking it against another established numerical code. Three-dimensional (3D) model calibration has been performed based on laboratory data available from temperature sensors, fluid sampling, and electrical resistivity tomography. The simulation results demonstrate that temperature profiles, spatial hydrate distribution, and bulk hydrate saturation are consistent with the observations. Furthermore, our numerical framework can be applied to calibrate geophysical measurements, optimize post-processing workflows for monitoring data, improve the design of hydrate formation experiments, and investigate the temporal evolution of sub-permafrost methane hydrate reservoirs. KW - methane hydrate KW - temperature sensor KW - electrical resistivity tomography KW - hydrate formation KW - numerical simulation Y1 - 2022 U6 - https://doi.org/10.3390/en15061974 SN - 1996-1073 VL - 15 IS - 6 PB - MDPI CY - Basel ER - TY - JOUR A1 - Li, Zhen A1 - Spangenberg, Erik A1 - Schicks, Judith Maria A1 - Kempka, Thomas T1 - Numerical Simulation of Coastal Sub-Permafrost Gas Hydrate Formation in the Mackenzie Delta, Canadian Arctic JF - Energies N2 - The Mackenzie Delta (MD) is a permafrost-bearing region along the coasts of the Canadian Arctic which exhibits high sub-permafrost gas hydrate (GH) reserves. The GH occurring at the Mallik site in the MD is dominated by thermogenic methane (CH4), which migrated from deep conventional hydrocarbon reservoirs, very likely through the present fault systems. Therefore, it is assumed that fluid flow transports dissolved CH4 upward and out of the deeper overpressurized reservoirs via the existing polygonal fault system and then forms the GH accumulations in the Kugmallit-Mackenzie Bay Sequences. We investigate the feasibility of this mechanism with a thermo-hydraulic-chemical numerical model, representing a cross section of the Mallik site. We present the first simulations that consider permafrost formation and thawing, as well as the formation of GH accumulations sourced from the upward migrating CH4-rich formation fluid. The simulation results show that temperature distribution, as well as the thickness and base of the ice-bearing permafrost are consistent with corresponding field observations. The primary driver for the spatial GH distribution is the permeability of the host sediments. Thus, the hypothesis on GH formation by dissolved CH4 originating from deeper geological reservoirs is successfully validated. Furthermore, our results demonstrate that the permafrost has been substantially heated to 0.8-1.3 degrees C, triggered by the global temperature increase of about 0.44 degrees C and further enhanced by the Arctic Amplification effect at the Mallik site from the early 1970s to the mid-2000s. KW - gas hydrate KW - permafrost KW - methane KW - faults KW - climate change KW - Mallik KW - numerical simulations Y1 - 2022 U6 - https://doi.org/10.3390/en15144986 SN - 1996-1073 VL - 15 IS - 14 PB - MDPI CY - Basel ER - TY - THES A1 - Li, Zhen T1 - Formation of Sub-Permafrost Methane Hydrate Reproduced by Numerical Modeling BT - Insights from LArge-scale Reservoir Simulator (LARS) to Mallik Site, Canadian Arctic N2 - Natural gas hydrates are ice-like crystalline compounds containing water cavities that trap natural gas molecules like methane (CH4), which is a potent greenhouse gas with high energy density. The Mallik site at the Mackenzie Delta in the Canadian Arctic contains a large volume of technically recoverable CH4 hydrate beneath the base of the permafrost. Understanding how the sub-permafrost hydrate is distributed can aid in searching for the ideal locations for deploying CH4 production wells to develop the hydrate as a cleaner alternative to crude oil or coal. Globally, atmospheric warming driving permafrost thaw results in sub-permafrost hydrate dissociation, releasing CH4 into the atmosphere to intensify global warming. It is therefore crucial to evaluate the potential risk of hydrate dissociation due to permafrost degradation. To quantitatively predict hydrate distribution and volume in complex sub-permafrost environments, a numerical framework was developed to simulate sub-permafrost hydrate formation by coupling the equilibrium CH4-hydrate formation approach with a fluid flow and transport simulator (TRANSPORTSE). In addition, integrating the equations of state describing ice melting and forming with TRANSPORTSE enabled this framework to simulate the permafrost evolution during the sub-permafrost hydrate formation. A modified sub-permafrost hydrate formation mechanism for the Mallik site is presented in this study. According to this mechanism, the CH4-rich fluids have been vertically transported since the Late Pleistocene from deep overpressurized zones via geologic fault networks to form the observed hydrate deposits in the Kugmallit–Mackenzie Bay Sequences. The established numerical framework was verified by a benchmark of hydrate formation via dissolved methane. Model calibration was performed based on laboratory data measured during a multi-stage hydrate formation experiment undertaken in the LArge scale Reservoir Simulator (LARS). As the temporal and spatial evolution of simulated and observed hydrate saturation matched well, the LARS model was therefore validated. This laboratory-scale model was then upscaled to a field-scale 2D model generated from a seismic transect across the Mallik site. The simulation confirmed the feasibility of the introduced sub-permafrost hydrate formation mechanism by demonstrating consistency with field observations. The 2D model was extended to the first 3D model of the Mallik site by using well-logs and seismic profiles, to investigate the geologic controls on the spatial hydrate distribution. An assessment of this simulation revealed the hydraulic contribution of each geological element, including relevant fault networks and sedimentary sequences. Based on the simulation results, the observed heterogeneous distribution of sub-permafrost hydrate resulted from the combined factors of the source-gas generation rate, subsurface temperature, and the permeability of geologic elements. Analysis of the results revealed that the Mallik permafrost was heated by 0.8–1.3 °C, induced by the global temperature increase of 0.44 °C and accelerated by Arctic amplification from the early 1970s to the mid-2000s. This study presents a numerical framework that can be applied to study the formation of the permafrost-hydrate system from laboratory to field scales, across timescales ranging from hours to millions of years. Overall, these simulations deepen the knowledge about the dominant factors controlling the spatial hydrate distribution in sub-permafrost environments with heterogeneous geologic elements. The framework can support improving the design of hydrate formation experiments and provide valuable contributions to future industrial hydrate exploration and exploitation activities. N2 - Gashydrate sind eisähnliche kristalline Verbindungen, die Moleküle wie Methan (CH4) in Hohlräumen einschließen. Die Mallik-Lagerstätte im Mackenzie-Delta in der kanadischen Arktis enthält ein großes Volumen an technisch förderbarem CH4-Hydrat unter dem Permafrostboden. Das Verständnis, wie die Hydrate verteilt sind, kann bei der Suche nach idealen Standorten für Förderbohrungen zu ihrer Erschließung als saubere Alternative zu Erdöl oder Kohle helfen. Weltweit führt die Erwärmung der Atmosphäre zum Auftauen des Permafrosts und zur Zersetzung der Hydrate, wodurch CH4 in die Atmosphäre freigesetzt und die globale Erwärmung verstärkt wird. Es ist also entscheidend, das potenzielle Risiko der Hydratauflösung aufgrund der Permafrostdegradation zu bewerten. Um die Verteilung und das Volumen von Hydraten in komplexen Sub-Permafrost Umgebungen quantitativ vorherzusagen, wurde ein numerischer Ansatz zur Simulation entwickelt. Hierzu wurde der Gleichgewichtsansatz für die CH4-Hydratbildung mit einem Strömungs- und Transportsimulator (TRANSPORTSE) kombiniert. Die zusätzliche Integrierung der Zustandsgleichungen, die das Schmelzen und die Bildung von Eis beschreiben, ermöglichte die Simulation der Permafrostentwicklung während der Hydratbildung. Für den Standort Mallik wird ein modifizierter Bildungsmechanismus in dieser Studie beschrieben. Demzufolge wurden die CH4-reichen Fluide seit dem späten Pleistozän aus tiefen Überdruckszonen vertikal über geologische Verwerfungssysteme transportiert, und haben die Hydratvorkommen gebildet. Der numerische Ansatz wurde anhand eines Benchmarks zur Hydratbildung verifiziert. Messdaten eines mehrstufigen Hydratbildungsexperiments im LArge scale Reservoir Simulator (LARS) dienten zur Kalibrierung. Basierend auf der guten Übereinstimmung zwischen der simulierten und beobachteten Hydratsättigung, wurde das LARS-Modell validiert. Im Anschluss erfolgte die Übertragung auf ein 2D-Modell im Feldmaßstab, das mithilfe einer seismischen Transekte durch den Mallik-Standort erstellt wurde. Die Übereinstimmung mit den Feldbeobachtungen bestätigte den beschriebenen Mechanismus zur Hydratbildung unterhalb des Permafrosts. Das 2D-Modell wurde basierend auf Bohrlochprotokollen und seismischen Profilen zum ersten 3D-Modell des Mallik-Standorts erweitert, um die geologischen Einflüsse auf die Hydratverteilung zu untersuchen. Die Auswertung verdeutlichte den Beitrag jedes geologischen Elements zum hydraulischen System, einschließlich relevanter Verwerfungssysteme und sedimentärer Abfolgen. Die beobachtete heterogene räumliche Verteilung der Hydrate ist auf die Gasproduktionsrate der Quelle, die Untergrundtemperatur und die Durchlässigkeit der geologischen Einheiten zurückzuführen. Die Analyse der Ergebnisse ergab, dass der Mallik-Permafrost um 0,8–1,3 °C erwärmt wurde, was durch den globalen Temperaturanstieg von 0,44 °C verursacht und durch die sogenannte polare Verstärkung seit Anfang der 1970er bis Mitte der 2000er Jahre beschleunigt wurde. Der in dieser Studie entwickelte numerische Ansatz zur Bildung von Permafrost-Hydrat-Systemen kann vom Labor- bis zum Feldmaßstab und über Zeitskalen von Stunden bis zu Millionen von Jahren angewendet werden. Mit den Simulationen konnten die dominierenden Faktoren identifiziert werden, welche die räumliche Hydratverteilung in Umgebungen mit heterogenen geologischen Strukturen steuern. Der Ansatz kann die Planung von Hydratbildungsexperimenten verbessern und einen wertvollen Beitrag für zukünftige industrielle Hydraterkundungen- und -erschließungen leisten. T2 - Bildung von Sub-Permafrost-Methanhydraten dargestellt durch numerische Modellierung: Erkenntnisse aus dem LArge-sclae Reservoir Simulator (LARS), angewandt auf den Mallik-Standort, Kanadische Arktis KW - methane hydrate KW - geologic fault KW - numerical simulation KW - hydrate formation KW - climate change KW - Mackenzie Delta KW - Mackenzie-Delta KW - Klimawandel KW - geologische Verwerfung KW - Hydratbildung KW - Methanhydrat KW - numerische Simulation Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-603302 ER -