The experimental and numerical investigation of THCM process coupling is important to better understand reservoir geotechnical behavior and sub-surface processes. In particular, when THCM process coupling is dominated by focused fluid migration and localized chemical or microbiological reactions, bulk sediment and, thus, reservoir geotechnical behavior becomes poorly predictable. To improve the understanding of these complicated processes and process coupling on relevant time and spatial scales, it is necessary to combine experimental and numerical simulation approaches, and to develop complementary investigation strategies.We use different high-pressure flow-through experimental systems with triaxial testing units in combination with tomographical imaging tools (e.g. X-ray CT and ERT) to simulate and analyze relevant processes in ocean and earth systems. Our geotechnical studies are carried out at high hydrostatic pressures up to 40 MPa and temperatures between -30°C and 80°C. The experimental systems allow testing of large sample specimen (up to a diameter of 150 mm and a height of 400 mm). In particular, we investigate scenarios with heterogeneous phase distributions and dynamic flow conditions, which cannot be interpreted based on the assumption of homogeneous phase distributions in a sensible manner.Here, we focus on discussing experimental and numerical strategies and problems towards understanding geotechnical behavior of heterogeneous sediments, including issues from gas migration in fine-grained sediments (e.g. silty clays), gas hydrate formation under two-phase flow conditions, and localized failure and shear banding in cemented soils. We present results from recent studies on underground usage including gas production and injection scenarios, which are relevant for the understanding of reservoir behavior, storage scenarios and, overall, marine sediment and slope stability. One of the most important aspects is to improve current strategies for combined and complementary experimental and numerical studies, considering that the overall objective is to understand processes on a reservoir scale.

Methane hydrates naturally form on Earth and in the interiors of some icy bodies of the Universe, and are also expected to play a paramount role in future energy and environmental technologies. Here we report experimental observation of an extremely fast methane diffusion at the interface of the two most common clathrate hydrate structures, namely clathrate structures I and II. Methane translational diffusion—measured by quasielastic neutron scattering at 0.8 GPa—is faster than that expected in pure supercritical methane at comparable pressure and temperature. This phenomenon could be an effect of strong confinement or of methane aggregation in the form of micro-nanobubbles at the interface of the two structures. Our results could have implications for understanding the replacement kinetics during sI–sII conversion in gas exchange experiments and for establishing the methane mobility in methane hydrates embedded in the cryosphere of large icy bodies in the Universe.

We present an extensive study of the quantum dynamics of molecular hydrogen trapped within the nanocavities of two hydrate structures at low temperatures, namely, clathrate structure II and filled ice structure C1. By inelastic neutron scattering measurements, we investigate a (i) simple H2 hydrate in clathrate structure II at ambient and high pressure and different temperatures, (ii) binary He–H2 hydrate in clathrate structure II at high pressure and different temperatures, (iii) simple D2 hydrate in clathrate structure II at ambient pressure and different temperatures, and (iv) simple H2 hydrate in structure C1 at high pressure and different temperatures. Molecular quantum rotations and translations, as well as combinations of these are identified in the spectra for hydrogen molecules in the small and large cages of clathrate structure II and in the channels of structure C1 and their energy is provided.

We investigate the effects of high pressure on the reorientational and vibrational dynamics of methane molecules embedded in methane hydrate III—the stable form of methane for pressures above 2 GPa at room temperature—by combining high-pressure Raman spectroscopy with ab initio simulations including nuclear quantum effects. We observe a clear evolution of the system from a gas-filled ice structure, where methane molecules occupy the channels of the ice skeleton and rotate almost freely, to a CH4:D2O compound where methane rotations are hindered, and methane and water dynamics are tightly coupled. The gradual orientational ordering of the guest molecules results in a complete locking-in at approximately 20 GPa. This happens along with a progressive distortion of the guest molecules. Finally, as pressure increases beyond 20 GPa, the system enters a strong mode coupling regime where methane guests and water hosts dynamics are intimately paired.

We have formed the long-sought He-clathrate. This was achieved by refilling helium into ice XVI, opening a new synthesis route for exotic forms of clathrate hydrates. The process was followed by neutron diffraction; structures and cage fillings were established. The stabilizing attractive van der Waals interactions are enhanced by multiple cage fillings with theoretically up to four helium atoms per large cage and up to one per small cage; He-clathrate hydrates can be considered as a solid-state equivalent of the clustering of small apolar entities dissolved in the liquid state of water. Unlike most other guests, helium easily enters and leaves the water cages at temperatures well below 100 K, hampering applications as a gas storage material. Despite the weak dispersive interactions, the inclusion of helium has a very significant effect on lattice constants; this is also established for helium inclusion in ice Ih and suggests that lattice parameters are arguably the most sensitive measure to gauge dispersive water-gas interactions.

A new fast diffraction-based method for the determination of crystallite size distributions (CSDs) is presented. The method is destruction-free, applicable to in situ and ex situ studies, and allows for a determination of the crystallites' volumes in powders or polycrystalline aggregates with excellent sampling statistics. The method is applied to the formation and coarsening of gas hydrates (GH) in a sedimentary matrix; both Xe-hydrates and CH4-hydrates were investigated in a time range from 2 min to 6 weeks. The GH crystallites have a size of a few micrometers when formed, followed by a coarsening process which mainly takes place at the surface of GH aggregates. Important conclusions can be drawn from the time-dependent analysis of CSDs: (1) Coarsening by normal grain growth proceeds several orders of magnitude more slowly than in normal ice at similar temperatures; this points to very slow grain boundary migration seemingly related to the complexity of topological reconstruction of the crystalline network across a disordered grain boundary. (2) The persisting small crystallites together with their known high resistance against deformation by dislocation motion must lead to grain size sensitive creep, most likely governed by grain boundary sliding. (3) The CSDs of GHs formed in the laboratory appear to have distinctly smaller sizes compared to natural GHs. As a consequence, laboratory-based studies of GH can only be safely related to the natural situation once the mutual CSDs are characterized.

Simple gas hydrates are known to adopt seven basic structures depending on the conditions of pressure, temperature and guest species. When taken at face value this may seem to be a rich range of structures, but when compared with other types of network structure it is remarkably limited. For example, there are over two hundred known zeolite structures. In my talk I will describe recent work by the Edinburgh hydrate group which has identified three more hydrate structures. One of these is a chiral channel structure which is quite unlike any other hydrate structure, and which is not found silica or zeolite systems but is instead related to a Zintl-phase structure.

We present full in situ structural solutions of carbon dioxide hydrate-II and hydrogen hydrate C0 at elevated pressures using neutron and X-ray diffraction. We find both hydrates adopt a common water network structure. The structure exhibits several features not previously found in hydrates; most notably it is chiral and has large open spiral channels along which the guest molecules are free to move. It has a network that is unrelated to any experimentally known ice, silica, or zeolite network but is instead related to two Zintl compounds. Both hydrates are found to be stable in electronic structure calculations, with hydration ratios in very good agreement with experiment.

In situ high-pressure crystallization and diffraction techniques have been applied to obtain two very structurally distinct semi-clathrates of the tert-butylamine-water system with hydration numbers 5.65 and 5.8, respectively, thereby considerably reducing a notable hydration gap between the monohydrate and the 71/4 -hydrate that results when crystallization space is explored by temperature alone. Both structures can be considered as an intriguing solid-state example of hydrophobic hydration, in which the water network creates wide tert-butylamine-filled channels stabilized by cross-linking hydrogen bonds. The existence of interconnected channels might also add low hydration structures to a list of potential targets for hydrogen storage. A detailed analysis of the topology of host water and host-guest interactions is reported and extended to those of other hydrates of the compound. This analysis offers new insight into properties of the tert-butylamine-water system and provides some clues as to the occurrence of the sizable number of hydrates of this compound.

Guest exchange in clathrates is a complex activated phenomenon of the guest–host cage interaction on the molecular-scale level. To model this process, we develop a mathematical description for the nonequilibrium binary permeation of guest molecules during gas replacement based on the microscopic “hole-in-cage-wall” diffusive mechanism. The transport of gas molecules is envisaged as a series of jumps between occupied and empty neighboring cages without any significant lattice restructuring in the bulk. The gas exchange itself is seen as two-stage swapping initiated by almost instantaneous formation of a mixed hydrate layer on the hydrate surface followed by a much slower permeation-controlled process. The model is constrained by and validated with available time-resolved neutron diffraction data of the isostructural CH4 guest replacement by CO2 in methane hydrate, a process of possible importance for the sequestration of CO2 with concomitant recovery of CH4 in marine gas hydrates.

The lattice constants of hydrogenated and deuterated CH4-, CO2-, Xe- (clathrate structure type I) and N2-hydrates (clathrate structure type II) from 10 K up to the stability limit were established in neutron- and synchrotron diffraction experiments and were used to derive the related thermal expansivities. The following results emerge from this analysis: (1) The differences of expansivities of structure type I and II hydrates are fairly small. (2) Despite the larger guest-size of CO2 as compared to methane, CO2-hydrate has the smaller lattice constants at low temperatures, which is ascribed to the larger attractive guest-host interaction of the CO2-water system. (3) The expansivity of CO2-hydrate is larger than for CH4-hydrate which leads to larger lattice constants for the former at temperatures above ∼150 K; this is likely due to the higher motional degrees of freedom of the CO2 guest molecules. (4) The cage occupancies of Xe- and CO2-hydrates affect significantly the lattice constants. (5) Similar to ice Ih, the deuterated compounds have generally slightly larger lattice constants which can be ascribed to the somewhat weaker H-bonding. (6) Compared to ice Ih, the high temperature expansivities are about 50% larger; in contrast to ice Ih and the empty hydrate, there is no negative thermal expansion at low temperature. (7) A comparison of the experimental results with lattice dynamical work, with models based on an Einstein oscillator model, and results from inelastic neutron scattering suggest that the contribution of the guest atoms' vibrational energy to thermal expansion is important, most prominently for CO2- and Xe-hydrates.

The exchange process between CO2 and methane hydrate has been observed in numerous laboratory experiments, computer simulations, and recently confirmed in a field test. Yet, to date there is no kinetic model capable of accurately predicting the swapping process at given fluid composition and p-T conditions. Major obstacles on the way to an adequate mathematical description are caused by the insufficient characterization of experimental environments and a nearly complete lack of information on the time-resolved composition of the two-phase fluid at the gas hydrate interface. Here we show that all necessary data can be provided by a combination of cryo-SEM, Raman, and neutron diffraction measurements that deliver accurate space-averaged, time-resolved in situ data on the CH4-CO2 exchange reactions at conditions relevant to sedimentary matrixes of continental margins. Results from diffraction are cross-correlated with ex situ Raman spectroscopy to provide reliable information on the preferential sites for CO2 and CH4 in the (partially) exchanged hydrate. We also show a novel approach based on scattering of neutrons to probe the fluid composition during the in situ replacement in a time-resolved, noninvasive manner. The replacement is seen as a two-step process including (1) a fast surface reaction parallel to a fast enrichment of the surrounding fluid phase with CH4 followed by (2) a much slower permeation-limited gas swapping between the gas hydrate and mixed ambient CH4-CO2 fluid. The main part of the replacement reaction takes place in the second stage. Based on our earlier experimental studies and existing literature we work toward a quantitative gas exchange model which elaborates the hole-in-cage-wall diffusion mechanism to describe the two-component gas replacement.

To date, very little is known about the distribution of natural gas hydrates in sedimentary matrices and its influence on the seismic properties of the host rock, in particular at low hydrate concentration. Digital rock physics offers a unique approach to this issue yet requires good quality, high-resolution 3-D representations for the accurate modeling of petrophysical and transport properties. Although such models are readily available via in situ synchrotron radiation Xray tomography, the analysis of such data asks for complex workflows and high computational power to maintain valuable results. Here, we present a best-practice procedure complementing data from Chaouachi et al. (2015) with data post-processing, including image enhancement and segmentation as well as exemplary numerical simulations of an acoustic wave propagation in 3-D using the derived results. A combination of the tomography and 3-D modeling opens a path to a more reliable deduction of properties of gas hydrate-bearing sediments without a reliance on idealized and frequently imprecise models.

In-situ synchrotron X-ray computed microtomography with sub-micrometer voxel size was used to study the decomposition of gas hydrates in a sedimentary matrix. Xenon-hydrate was used instead of methane hydrate to enhance the absorption contrast. The microstructural features of the decomposition process were elucidated indicating that the decomposition starts at the hydrate-gas interface; it does not proceed at the contacts with quartz grains. Melt water accumulates at retreating hydrate surface. The decomposition is not homogeneous and the decomposition rates depend on the distance of the hydrate surface to the gas phase indicating a diffusion-limitation of the gas transport through the water phase. Gas is found to be metastably enriched in the water phase with a concentration decreasing away from the hydrate-water interface. The initial decomposition process facilitates redistribution of fluid phases in the pore space and local reformation of gas hydrates. The observations allow also rationalizing earlier conjectures from experiments with low spatial resolutions and suggest that the hydrate-sediment assemblies remain intact until the hydrate spacers between sediment grains finally collapse; possible effects on mechanical stability and permeability are discussed. The resulting time resolved characteristics of gas hydrate decomposition and the influence of melt water on the reaction rate are of importance for a suggested gas recovery from marine sediments by depressurization.

Dissociation of gas hydrates below 240 K leads to the formation of a metastable form of water ice, so called cubic ice (Ic). Through its defective nature and small particle size the surface film composed of such material is incapable of creating any significant diffusion barrier. Above 160 K, cubic ice gradually transforms to the stable hexagonal (Ih) form on laboratory time scales. The annealing, coupled with a parallel decomposition of gas hydrates, accelerates as temperature rises but already above 190 K the first process prevails, transforming cubic stacking sequences in-to ordinary Ih ice within a few minutes. Remaining stacking faults are removed through very slow isothermal annealing or after heating up above 240 K. The role of the proportion of cubic stacking on the decomposition rate is discussed. A better understanding of the dissociation kinetics at low temperatures is particularly im-portant for the critical evaluation of existing hypotheses that consider clathrates as a potential medium that actively participate in geological processes or is able to store gases (e.g. CH4, CO2 or Xe) in environments like comets, icy moons (i. e. Titan, Europa, Enceladus) or on Mars. Here, we present kinetics studies on the dissociation of CO2 clathrates at isothermal and isobaric conditions between 170 and 190K and mean Martian surface pressure. We place special attention to the formed ice and demonstrate its influence on the dissociation rates with a combination of neutron diffraction studies (performed on D20 at ILL/Grenoble) and cryo-SEM. More detailed crystallo-graphic information has been acquired via a flexible stacking-fault model capable of revealing the time evolution of the defect structure of ice Ic in terms of stacking probabilities and crystal size.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.