Mechanisches Tiefseesedimente-, marine Rohstofflagerstätten- und/oder Unterseehang- Stabilisierungsverfahren und/oder Regulierungs-/Konditionierungsverfahren der hydraulischen Eigenschaften von Tiefseesedimenten

Die Erfindung betrifft ein mechanisches Tiefseesedimente-, marine Rohstofflagerstätten- und/oder Unterseehang- Stabilisierungsverfahren und/oder Regulierungs-/Konditionierungs-verfahren der hydraulischen Eigenschaften von Tiefseesedimenten aufweisend ein Injizieren einer gashydratbildenden Substanz in marine oder submarine Sedimente, wobei Gashydrat-Sediment-Verbünde gebildet werden

Magnetic Resonance Imaging of Gas Hydrate Formation and Conversion at Sub-Seafloor Conditions

The production of natural gas from sub-seafloor gas hydrates is one possible strategy to meet the world’s growing demand for energy. On the other hand, climate warming scenarios call for the substitution of fossil energy resources by sustainable energy concepts. Burning natural gas from gas hydrates could be emission neutral if it was combined with a safe storage of the emitted CO2. Laboratory experiments, that address corresponding strategies, need to be performed under high pressures and low temperatures to meet the thermodynamic conditions of the sub-seafloor environment. In this paper, we present a high-pressure flow-through sample cell that is suitable for nuclear magnetic resonance (NMR) experiments at realistic marine environmental conditions, i.e. pressures up to 15 MPa and temperatures from 5 to 20 °C, and we demonstrate its suitability in applied gas hydrate research

CO2 injection into submarine sediments: disturbing news for methane-rich hydrates

The production of natural gas via injection of fossil-fuel derived CO2 into submarine gas hydrate reservoirs can be an example of tapping a hydrocarbon energy source in a CO2-neutral manner. However, the industrial application of this method is technically challenging. Thus, prior to feasibility testing in the field, multi-scale laboratory experiments and adapted reaction-modeling are needed. To this end, high-pressure flow-through reactors of 15 and 2000 mL sample volume were constructed and tested. Process parameters (P, T, Q, fluid composition) are defined by a fluid supply and conditioning unit to enable simulation of natural fluid-flow scenarios for a broad range of sedimentary settings. Additional Raman- and NMR-spectroscopy aid in identifying the most efficient pathway for CH4 extraction from hydrates via CO2 injection on both microscopic and macroscopic level. In this study we present experimental set-up and design of the highpressure flow-through reactors as well as CH4 yields from H4-hydrate decomposition experiments using CO2-rich brines and pure liquefied CO2

Technical aspects of gas hydrate conversion and secondary gas hydrate formation during injection of supercritical CO2 into CH4-hydrate-bearing sediments

The injection of CO2 into CH4-hydrate-bearing sediments has the potential to drive natural gas production and simultaneously sequester CO2 by hydrate conversion. Currently, process conditions under which this goal can be achieved efficiently are largely unknown. While the recent Ignik Sikumi field test suggests that a combination of N2/CO2 injection with depressurization yields effective CH4 production, in a previous study (Deusner et al., 2012) we showed that a combination of CO2 injection and thermal stimulation eliminates mass transfer limitations observed at cold reservoir temperatures. These high-pressure flow-through studies revealed that the injection of supercritical CO2 at 95 °C triggers dissociation of CH4-hydrates and counters rapid CO2-hydrate formation in the near-injection region. We also observed a strong effect of reservoir temperature on CH4 production and CO2 retention. The efficiency and yield of CH4 production was highest at a sediment temperature of 8 °C compared to 2 °C and 10 °C. At 2 °C CO2 hydrate formation was rapid and clogged the sediment at the injection spot. Outside the CO2-hydrate stability region, at 10 °C, we observed fast CO2 breakthrough and a comparably low CH4 production. Experiments comparing discontinuous and continuous CO2 injection showed that alternating periods of equilibration and CO2 injection improved the overall CH4 production. We hypothesize that slow formation of secondary CO2-rich hydrate improves the accessibility of the CH4-hydrate distributed in the sediment by locally changing permeability and fluid flow patterns. In situ measurements showed dynamic changes of local p-/T-gradients due to gas hydrate dissociation or dissolution and secondary gas hydrate formation. In addition, continued reconfiguration of guest molecules in transiently formed mixed hydrates maintain elevated gas exchange kinetics. Online effluent fluid analysis under in-situ pressure conditions indicated that CH4 released from CH4-hydrates is largely dissolved in liquid CO2.. It is a current objective of our studies to further elucidate rheological properties and gas exchange efficiencies of CO2-CH4 mixed fluids that approach equilibrium with gas hydrates and to study the effect of in situ CH4-CO2-hydrate conversion and secondary gas hydrate formation on sediment geomechanical parameters

Methane Production from Gas Hydrate Deposits through Injection of Supercritical CO2

The recovery of natural gas from CH4-hydrate deposits in sub-marine and sub-permafrost environments through injection of CO2 is considered a suitable strategy towards emission-neutral energy production. This study shows that the injection of hot, supercritical CO2 is particularly promising. The addition of heat triggers the dissociation of CH4-hydrate while the CO2, once thermally equilibrated, reacts with the pore water and is retained in the reservoir as immobile CO2-hydrate. Furthermore, optimal reservoir conditions of pressure and temperature are constrained. Experiments were conducted in a high-pressure flow-through reactor at different sediment temperatures (2 °C, 8 °C, 10 °C) and hydrostatic pressures (8 MPa, 13 MPa). The efficiency of both, CH4 production and CO2 retention is best at 8 °C, 13 MPa. Here, both CO2- and CH4-hydrate as well as mixed hydrates can form. At 2 °C, the production process was less effective due to congestion of transport pathways through the sediment by rapidly forming CO2-hydrate. In contrast, at 10 °C CH4 production suffered from local increases in permeability and fast breakthrough of the injection fluid, thereby confining the accessibility to the CH4 pool to only the most prominent fluid channels. Mass and volume balancing of the collected gas and fluid stream identified gas mobilization as equally important process parameter in addition to the rates of methane hydrate dissociation and hydrate conversion. Thus, the combination of heat supply and CO2 injection in one supercritical phase helps to overcome the mass transfer limitations usually observed in experiments with cold liquid or gaseous CO2

Microscale Processes and Dynamics during CH4–CO2 Guest Molecule Exchange in Gas Hydrates

The exchange of CH4 by CO2 in gas hydrates is of interest for the production of natural gas from methane hydrate with net zero climate gas balance, and for managing risks that are related to sediment destabilization and mobilization after gas-hydrate dissociation. Several experimental studies on the dynamics and efficiency of the process exist, but the results seem to be partly inconsistent. We used confocal Raman spectroscopy to map an area of several tens to hundreds µm of a CH4 hydrate sample during its exposure to liquid and gaseous CO2. On this scale, we could identify and follow different processes in the sample that occur in parallel. Next to guest-molecule exchange, gas-hydrate dissociation also contributes to the release of CH4. During our examination period, about 50% of the CO2 was bound by exchange for CH4 molecules, while the other half was bound by new formation of CO2 hydrates. We evaluated single gas-hydrate grains with confirmed gas exchange and applied a diffusion equation to quantify the process. Obtained diffusion coefficients are in the range of 10−13–10−18 m2/s. We propose to use this analytical diffusion equation for a simple and robust modeling of CH4 production by guest-molecule exchange and to combine it with an additional term for gas-hydrate dissociation

Experimental investigation of water permeability in quartz sand as function of CH4-hydrate saturation

Water permeability in gas hydrate bearing sediments is a crucial parameter for the prediction of gas production scenarios. So far, the commonly used permeability models are backed by very few experimental data. Furthermore, detailed knowledge of the exact formation mechanism leads to severe uncertainties in the interpretation of the experimental data. We formed CH4 hydrates from a methane saturated water solution and used Magnetic Resonance Imaging (MRI) to measure time resolved maps of the three-dimensional gas hydrate saturation. These maps were used for 3D Finite Elements Method (FEM) simulations. The simulation results enabled us to optimize existing models for permeabilities as function of gas hydrate saturation

Strain Rate‐Dependent Hardening‐Softening Characteristics of Gas Hydrate‐Bearing Sediments

The presence of gas hydrates (GHs) increases the stiffness and strength of marine sediments. In elasto‐plastic constitutive models, it is common to consider GH saturation (Sh) as key internal variable for defining the contribution of GHs to composite soil mechanical behavior. However, the stress‐strain behavior of GH‐bearing sediments (GHBS) also depends on the microscale distribution of GH and on GH‐sediment fabrics. A thorough analysis of GHBS is difficult, because there is no unique relation between Sh and GH morphology. To improve the understanding of stress‐strain behavior of GHBS in terms of established soil models, this study summarizes results from triaxial compression tests with different Sh, pore fluids, effective confining stresses, and strain histories. Our data indicate that the mechanical behavior of GHBS strongly depends on Sh and GH morphology, and also on the strain‐induced alteration of GH‐sediment fabrics. Hardening‐softening characteristics of GHBS are strain rate‐dependent, which suggests that GH‐sediment fabrics dynamically rearrange during plastic yielding events. We hypothesize that rearrangement of GH‐sediment fabrics, through viscous deformation or transient dissociation and reformation of GHs, results in kinematic hardening, suppressed softening, and secondary strength recovery, which could potentially mitigate or counteract large‐strain failure events. For constitutive modeling approaches, we suggest that strain rate‐dependent micromechanical effects from alterations of the GH‐sediment fabrics can be lumped into a nonconstant residual friction parameter. We propose simple empirical evolution functions for the mechanical properties and calibrate the model parameters against the experimental data. Plain Language Summary Gas hydrates (GHs) are crystalline‐like solids, which are formed from natural gas molecules and water at high pressure and low temperature. GHs, and particularly methane hydrates, are naturally abundant in marine sediments. It is known that the presence of GH increases the mechanical stiffness and strength of sediments, and there is strong effort in analyzing and quantifying these effects in order to understand potential risks of sediment destabilization or slope failure. Based on our experimental results from high‐pressure geotechnical studies, we show that not only the initial amount and distribution of GH are important for the increased strength of GH‐bearing sediments but also the dynamic rearrangement of GH‐sediment fabrics during deformation characterizes the stress‐strain response and enables strength recovery after failure. We propose that different microstructural mechanisms contribute to this rearrangement and strength recovery of GH sediment. However, we consider these complicated processes in a simplified manner in an improved numerical model, which can be applied for geotechnical risk assessment on larger scales

Anaerobic methane oxidation inducing carbonate precipitation at abiogenic methane seeps in the Tuscan archipelago (Italy)

Seepage of methane (CH4) on land and in the sea may significantly affect Earth's biogeochemical cycles. However processes of CH4 generation and consumption, both abiotic and microbial, are not always clear. We provide new geochemical and isotope data to evaluate if a recently discovered CH4 seepage from the shallow seafloor close to the Island of Elba (Tuscany) and two small islands nearby are derived from abiogenic or biogenic sources and whether carbonate encrusted vents are the result of microbial or abiotic processes. Emission of gas bubbles (predominantly CH4) from unlithified sands was observed at seven spots in an area of 100 m(2) at Pomonte (Island of Elba), with a total rate of 234 ml m(-2) d(-1). The measured carbon isotope values of CH4 of around -18 parts per thousand (VPDB) in combination with the measured delta H-2 value of -120 parts per thousand (VSMOW) and the inverse correlation of delta C-13-value with carbon number of hydrocarbon gases are characteristic for sites of CH4 formation through abiogenic processes, specifically abiogenic formation of CH4 via reduction of CO2 by H-2. The H-2 for methanogenesis likely derives from ophiolitic host rock within the Ligurian accretionary prism. The lack of hydrothermal activity allows CH4 gas to become decoupled from the stagnant aqueous phase. Hence no hyperalkaline fluid is currently released at the vent sites. Within the seep area a decrease in porewater sulphate concentrations by ca. 5 mmol/l relative to seawater and a concomitant increase in sulphide and dissolved inorganic carbon (DIC) indicate substantial activity of sulphate-dependent anaerobic oxidation of methane (AOM). In absence of any other dissimilatory pathway, the delta C-13-values between -17 and -5 parts per thousand in dissolved inorganic carbon and aragonite cements suggest that the inorganic carbon is largely derived from CH4. The formation of seep carbonates is thus microbially induced via anaerobic oxidation of abiotic CH4

Alkaline mineral addition to anoxic to hypoxic Baltic Sea sediments as a potentially efficient CO2-removal technique

Recent studies have begun to explore the potential of enhanced benthic weathering (EBW) in the Baltic Sea as a measure for climate change mitigation. To augment the understanding of EBW under seasonally changing conditions, this study aims to investigate weathering processes under anoxia to hypoxia in corrosive bottom waters, which reflect late summer conditions in the Baltic Sea. Dunite and calcite were added to sediment cores retrieved from Eckernförde Bay (Western Baltic Sea) with a constant flow-through of deoxygenated, CO2-enriched Baltic Sea bottom water. The addition of both materials increased benthic alkalinity release by 2.94 μmol cm−2 d−1 (calcite) and 1.12 μmol cm−2 d−1 (dunite), compared to the unamended control experiment. These excess fluxes are significantly higher than those obtained under winter conditions. The comparison with bottom water oxygen concentrations emphasizes that highest fluxes of alkalinity were associated with anoxic phases of the experiment. An increase in Ca and Si fluxes showed that the enhanced alkalinity fluxes could be attributed to calcite and dunite weathering. First order rate constants calculated based on these data were close to rates published in previous studies conducted under different conditions. This highlights the suitability of these proxies for mineral dissolution and justifies the use of these rate constants in modeling studies investigating EBW in the Baltic Sea and areas with similar chemical conditions. Generally stable pH profiles over the course of the experiment, together with the fact that the added minerals remained on the sediment surface, suggest that corrosive bottom waters were the main driving factor for the dissolution of the added minerals. These factors have important implications for the choice of mineral and timing for EBW as a possible marine carbon dioxide removal method in seasonally hypoxic to anoxic regions of the Baltic Sea