Description: The exchange of hydrate-bond CH4 with CO2 is one possible method for the production of CH4 from hydrate-bearing sediment which was investigated on different scales in the SUGAR project. Tubular polydimethylsiloxane (PDMS) membranes were utilized to monitor the spatial and temporal gas distribution in a large-scale experimental simulation on CO2–CH4 gas hydrate exchange. The suitability of PDMS membranes for the measurement of gaseous and dissolved CO2 and CH4 concentrations in pure and mixed gas systems was evaluated in lab-scale experiments. The results reveal a strong interacting mutual influence of CO2 and CH4 in CO2–CH4 mixed feed composition and in the presence of water. The competitive absorption between CO2 and H2O as well as membrane plasticization, which increases CH4 permeability and reduces CO2 permeability, makes a direct correlation of mixed systems to pure systems and a quantification of the gas concentration in the feed reservoir impossible. The successful run of five tubular PDMS membranes, employed in a large test reservoir during an experimental simulation of CO2-driven CH4 hydrate decomposition, demonstrates the high stability of the material in harsh conditions. Also, a time-resolved observation of the progressing CO2 front is possible and makes membrane incorporation a valuable addition to conventional ex situ gas measurements in reservoir tests of various dimensions. The monitoring technique can significantly contribute to a comprehensive process understanding with respect to the spatial distribution of hydrate formation, dissociation and reformation in the presence of CO2 and CH4.

Description: We developed a new thermostated ring-shear-apparatus for investigation on hydrate- or ice-bearing sediments. A fluid inlet at the bottom of the static part of the cell and a fluid outlet at the top of the rotating half-cell allow exchanging and pressurizing the pore fluid in the sample cell to a certain value below the pressure providing the normal load that is applied hydraulically via a seal disk using a syringe pump. The volume change in the sample can be derived from the volume received or injected by the pump. The system allows the use of different methods for the formation and controlled decomposition of hydrate before, after, and during a shear test. The ring-shear-cell is designed for a maximum hydraulic pressure of 30 MPa. A commercial hollow shaft servo actuator applies the torque to the rotating half of the sample cell, and a rotary encoder provides information for determining the shear displacement. Stress path investigations in shear rate controlled experiments with large strain are possible with shear rates up to 12.6 mm/s and torques up to 1440 N m. The system design allows for complex experiments studying the behavior of a shear plane in hydrate- and/or ice-bearing sediments, including the decomposition and reformation of hydrate and/or ice under varying pressure and temperature conditions. It is a useful tool to provide experimental data to support research and engineering in solving problems related to permafrost and hydrate-bearing formations. The system performance is demonstrated by examples of tests on hydrate- and ice-bearing sand samples.

Description: The “guest exchange” of methane (CH4) by carbon dioxide (CO2) in naturally occurring gas hydrates is seen as a possibility to concurrently produce CH4 and sequester CO2. Presently, process evaluation is based on CH4−CO2 exchange yields of small- or mediumscale laboratory experiments, mostly neglecting mass and heat transfer processes. This work investigates process efficiencies in two large-scale experiments (210 L sample volume) using fully water-saturated, natural reservoir conditions and a gas hydrate saturation of 50%. After injecting 50 kg of heated CO2 discontinuously (E1) and continuously (E2) and a subsequent soaking period, the reservoir was depressurized discontinuously. It was monitored using electrical resistivity, temperature and pressure sensors, and fluid flow and gas composition measurements. Phase and component inventories were analyzed based on mass and volume balances. The total CH4 production during CO2 injection was only 5% of the initial CH4 inventory. Prior to CO2 breakthrough, the produced CH4 roughly equaled dissolved CH4 in the produced pore water, which balanced the volume of the injected CO2. After CO2 breakthrough, CH4 ratios in the released CO2 quickly dropped to 2.0−0.5 vol %. The total CO2 retention was the highest just before the CO2 breakthrough and higher in E1 where discontinuous injection improved the distribution of injected CO2 and subsequent mixed hydrate formation. The processes were improved by the succession of CO2 injection by controlled degassing at stability limits below that of the pure CH4 hydrate, particularly in experiment E2. Here, a more heterogeneous distribution of liquid CO2 and larger availability of free water led to smaller initial degassing of liquid CO2. This allowed for quick re-formation of mixed gas hydrates and CH4 ratios of 50% in the produced gases. The experiments demonstrate the importance of fluid migration patterns, heat transport, sample inhomogeneity, and secondary gas hydrate formation in watersaturated sediments.

Description: In recent years gas hydrates have been in the focus not only of scientific but also of economic research as they are considered to be a potential source of energy. Production scenarios are being tested in the field as well as in laboratories. In addition to production efficiency, the possible effect of the presence of gas hydrates in sediments on slope stabilities is of major interest. The geo-mechanical parameters of hydrate-bearing sediments do not only concern gas hydrate production but also the increasing usage of the continental slope sediments in general, since gas hydrates occur in marginal sediments worldwide. Shear stress is one of the geo-mechanical parameters of interest and there are a number of publications on selected laboratory and natural samples. However, systematic investigations to determine the dependencies of shear strength and hydrate saturation and/or effective stress, all of which influence the shear stress, are rare. This is based on small numbers of natural samples and difficulties in the formation of gas-free, hydrate-bearing sediments with high saturations and pore-filling character. This is due to small numbers of natural samples and difficulties in the formation of gas-free, hydrate-bearing sediments with high saturations and pore-filling character. In the study presented here hydrate of pore-filling and load-bearing character was produced in water saturated sandy samples and examined for changes in shear stress, peak and residual shear strength using a ring shear test. The available external ring shear test rig (ESTER) was constructed and build at the GFZ to study shear stress, friction angle and cohesion in relation to hydrate saturation. The circular sample even allows for studies on the shear strength of healed hydrate-bearing sediments. A series of tests were carried out on gas-free sands with hydrate saturations of 40 – 100%, clearly showing a non-linear dependency of the maximum shear strength on the hydrate saturation with a strong increase between 70-80%. Experiments on “healed” hydrate-bearing sediments did not reach comparable maximum shear strengths but reached values slightly above the original residual strength.

Description: Gas hydrates are naturally-occurring solid compounds of gas and water within almost all sediment-rich continental margins. Due to the large amounts of methane stored in submarine gas hydrates, they might serve as future reservoirs for offshore marine gas production. Assessing the reservoir characteristics requires reliable estimates of both the gas and gas hydrate concentration, which can be best addressed using geophysical and geological investigations. Here, we demonstrate the power of joint interpretation of interdisciplinary geophysical techniques and geological laboratory experiments. Regional 2D multichannel seismic data provide the broad overview of a hydrate-bearing area. High-resolution 2D and 3D seismic reflection data provide detailed images of two working areas, the buried S1 channel-levee system at 1500 m water depth (well within the gas hydrate stability zone) and a slope failure location, located at 665 m water depth (top limit of the hydrate formation) next to the S2 channel. Detailed compressional and shear wave (Vs) velocity-depth models were derived from four component ocean-bottom seismic data, the latter from P- to S-conversion upon reflection. Due to their steep reflection angles, shear wave events result in less resolved Vs models. Nevertheless, in case of a change in elasticity of the sediment matrix due to gas hydrate cementation, shear wave events can be used as an indicator. As such, Vs can give insight into the nature of hydrate formation throughout the GHSZ. We present new developments in the application of common reflection surface, normal-incidence-point tomography and full waveform inversion techniques to enhance model resolution for the seismic data sets. 2D and 3D controlled-source electromagnetic measurements provide volume information of the resistivity-depth distribution models. Electrical resistivity of the sediment formation depends on its porosity and the resistivity of the pore fluid. Gas hydrate and free gas generally have much higher electrical resistivities than saline pore fluid, and can be assessed using empirical relationships if the porosity and pore fluid salinity are known. Calibration with logging data, laboratory experiments on hydrate- or ice-bearing sediments, and resulting velocity and resistivity values, guide the joint interpretation into more accurate saturation estimations. Beyond that, a joint inversion framework supporting forward calculation of specialized geophysical methods at distributed locations is under development. In this paper, we summarize these individual components of a multi-parameter study, and their joint application to investigate gas hydrate systems, their equilibrium conditions and preservation of bottom-simulating-reflectors. We analyze data from two working areas at different locations and depth levels along the slope of the Danube Fan, which are both characterized by multiple bottom simulating reflectors indicating the presence of gas hydrate. In the first working area we located two depth windows with indications for moderate 16%–24% gas hydrate formation, but no vertical gas migration. In the second working area we observed fluid migration pathways and active gas seepage, limiting gas hydrate formation to less than 10% at the BSR. Some discrepancies remain between seismic-based and electromagnetic-based models of gas and gas hydrate distribution and saturation estimates, indicating that further in-situ investigations are likely required to better understand the gas hydrate systems at our study areas and to calibrate the inversion processes, which will be required for a joint inversion framework as well.

Description: Natural gas hydrates occur at all active and passive continental margins and are supposed to contain tremendous amounts of methane, which makes them attractive as a potential source of energy. During the last two decades several methods have been developed and tested to produce methane from gas hydrates in lab experiments and field trials. In principle, three different approaches, namely depressurization, thermal stimulation and chemical stimulation have been tested. The injection of CO2 into hydrate-bearing sediments as a variant of a chemical stimulation appears to be particularly favorable because it combines the production of methane from the hydrate phase with the storage of CO2 as a solid clathrate. Despite the fact that numerous experimental and modelling studies have been conducted worldwide in order to understand the process on different scales and to evaluate its efficiency, some issues remain pending. We obtained laboratory data from micro-scale experiments using analytical methods such as Raman spectroscopy and X-ray diffraction providing information about the structural changes of the hydrate phase and the gas exchange processes on a molecular scale. The results indicate that the processes related to the exchange of the guest molecules are quite complex. It is generally accepted that the driving force for the release of methane from the hydrate lattice and the incorporation of CO2 into the hydrate structure is the chemical potential gradient between the hydrocarbon hydrate phase and the injected CO2 phase. Raman spectroscopic and X-ray diffraction measurements indicate that this process correlates with a (partial) decomposition or opening of the hydrate cavities. In case of mixed hydrocarbon hydrates with structure II the exchange of the guest molecules with CO2 also induces a change of the hydrate structure (sII to sI). In general, the exchange of the guest molecules results in the formation of a secondary mixed hydrate phase containing CO2 besides methane and other hydrocarbons depending on the composition of the surrounding gas phase. We also obtained laboratory data from large-scale experiments providing information about the methane recovery and fluid migration, hydrate dissociation rates and the formation of a secondary hydrate phase, heat and mass transfer. The experimental results show a wide variation indicating that the recovery rate of methane strongly depends on the experimental conditions such as the volume of the sample, hydrate saturation and thus permeability of the hydrate bearing sediment, hydrate morphology as well as pressure and temperature of the injected CO2 phase. To avoid clogging due to the formation of a CO2 hydrate in the vicinity of the injection area we used gas mixtures containing N2 besides CO2 which also has an impact on the recovery rate of methane and the potential formation of a secondary hydrate phase. In any case, the injection of CO2 results in the production of a gas mixture rather than the extraction of pure methane because the injected CO2 forms a mixed hydrate with a certain amount of methane remaining in the hydrate phase. In addition, the long-term stability of the secondary formed CO2-rich hydrate phase also depends on the potential changes of the chemical environment which may result in the release of CO2 due to a re-exchange of the hydrate bonded gas molecules or dissociation of the hydrate phase. Maybe these results make the approach of methane production from natural gas hydrate deposits via CO2 injection less attractive with regard to efficiency and safe CO2 sequestration than originally expected. This contribution will present the experimental results of our investigations on various scales using different analytical methods and the overarching interpretation and conclusions based on these data.

Description: At the GFZ German Research Centre for Geosciences we have developed a safe and efficient method which allows for the decomposition of gas hydrates by the supply of heat inside the reservoir. The heat is generated in situ by a catalytic combustion of methane in a counter-current heat-exchange reactor. The reactor that Rudy Rogers, Professor Emeritus in Chemical Engineering at Mississippi State University, referred to as the "Schicks Combustor" is placed in a borehole in such way that the hot reaction zone is situated in the area of the hydrate layer. The counter-current heat-exchange reactor developed at GFZ generates heat via a flameless catalytic oxidation of methane at a noble metal catalyst. The system is closed i.e. there is no contact of the reactants, catalyst and environment. For safety reasons, methane and air are fed separately through a tube-in-tube arrangement into the mixing chamber. Due to its cooling effect and for safety reasons air instead of pure oxygen is used. From the mixing chamber the gas mixture arrives in defined quantities on the catalyst bed, where methane and oxygen are converted into carbon dioxide and water. The hot product gases release their heat via an aluminum foam to the outer wall of the reactor and then to the environment. Simultaneously, the incoming gases are preheated. The reaction runs stable and autonomous between 673 and 823 K. The counter-current heat-exchange reactor was designed as a lab reactor and a borehole tool. The lab reactor was tested in a reservoir simulator to investigate the heat transfer into gas hydrate bearing sediments. Therefore methane hydrate was generated in the LArge Reservoir Simulator (LARS), an autoclave with a volume of 425 L. In a test with 80% hydrate saturation, the reservoir simulator warmed up within 12 hours after the ignition of the catalyst to such an extent that the temperature of the complete sample was above the dissociation temperature of the previously formed methane hydrate which dissociated completely and methane could therefore be produced. During this test, only 15% of the produced CH4 was consumed to generate the energy needed for the thermal dissociation of the hydrates. The experience with the laboratory reactor served as basis for the design of a borehole tool which is suitable for the application in natural gas hydrate reservoirs. The borehole tool has a total length of 5120 mm, an outer diameter of 90 mm and weighs ca. 100 kg. First results from field tests at the continental deep drilling site KTB in Windischeschenbach, Germany, confirm that the borehole tool reliably produces heat at depth.

Description: The guest molecule exchange of methane (CH4) by carbon dioxide (CO2) in natural gas hydrate reservoirs is considered a desirable possibility to produce CH4 and at the same time sequester CO2. So far process evaluation is commonly based on CH4-CO2 exchange yields and rates from small- or medium-scale experiments in partly water-saturated sediments, both of which does not represent natural conditions. The experiments are presented in detail in a currently submitted manuscript by Heeschen et al. (2019). The presented data originate from two large-scale experiments (210 L) investigating the efficiency of the CH4-CO2 exchange under fully water-saturated natural reservoir conditions. For details on the equipment and the methods used see: Priegnitz et al., 2013; Schicks et al., 2011; Spangenberg et al., 2014. The reservoir conditions were 13 MPa and 8 °C, and the gas hydrate saturation in the sand (Sh) was 50% of the pore space. The gas hydrate was formed from dissolved CH4 only. About 50 kg heated CO2 was injected 1) discontinuously with intermediate soaking periods (E1) and 2) continuously (E2). In both cases, the CO2 injection periods were followed by a discontinuous depressurization of the reservoir. The experiments demonstrate the importance of fluid migration patterns, heat transport, sample inhomogeneity, reaction kinetics, and secondary gas hydrate formation in water-saturated sediments. Methane production yields of 5% were small in both experiments during the injection periods, whereas controlled depressurization following the injection of CO2 into a CH4 hydrate reservoir could be a possible approach for the production of CH4 from a gas hydrate reservoir. However, the success of this method strongly depends on the distribution of CO2, and the availability and distribution of residual pore water.

Description: All datasets provided in the operational dataset (DOVE-Phase 1 Scientific Team et al., 2023b) of the ICDP project DOVE phase 1 (ICDP 5068) consist of metadata, data and/or images. Here, we summarize explanations on the tables, data and images exported from the database of the project (mDIS DOVE) as well as some basic explanations on identifiers used in ICDP, depths corrections and measurements that are integrated into the dataset.