A) Problems to be solved  ->

The three research tasks defined below, will address three crucial effects of characterization and quantification of NBS gas hydrate and associated seepage systems: 1) Dynamics of the system, 2) Influence on seabed stability, and 3) Resource potential

Task A (UiT/NGU): Geophysical characterisation and quantification of natural gas hydrates
Recent results from Integrated Ocean Drilling Program (IODP) operations indicate that the distribution of gas hydrates might be much more complex than previously assumed (IODP Leg 311 Preliminary Reports). The generally accepted model that highest gas hydrates concentrations are expected to occur close above the base of the hydrate-stability zone is too simple. Much higher gas hydrate concentrations may occur at much shallower positions depending on a number of key controlling factors that vary along the margin. These include local methane solubility, fluid flux rates and availability of suitable host material. These new findings fundamentally change the approach to seismic detection of gas hydrates in marine sediments. Therefore, characterisation and quantification efforts of gas hydrate systems on the NBS margins require state-of-the-art technology in both acquisition and analysis of such data. Information about the distribution of gas hydrates can be obtained using high-resolution 3D acoustic imaging techniques and multi-component seismology. The 3D seismic data will provide more detailed images of hydrate- and gas-bearing sediments, which will lead to a better understanding of hydrate distribution and hydrate system dynamics. Multi-component systems allow recording of S-waves in addition to the P-waves. This approach provides more acoustic information about the properties of rocks and fluids than is obtained by recording only one component. Consequently, this will lead to an improved identification and understanding of gas hydrates and free gas in marine sediments. Particularly multi-component seismic data will provide important seismic parameters (Vp, Vs, Qp and Qs) for the quantification and resource assessment of gas hydrates and associated free gas. Thoroughly tested rock-physics models (Chand et al., 2004, 2006; Chand and Minshull, 2004) will allow the accurate quantification of the amount of gas hydrates. These models will be further developed through the incorporation of state of the art models for hydrate nucleation and growth (Kvamme et al., 2004, Buanes and Kvamme, in press) as improved/verified through experimental data on growth and dissociation of hydrate in Bentheimer sandstone (Kvamme et al., 2004) and other types of porous media (Llamedo et al., 2004, Østergaard et al., 2002). To achieve these goals, the following sub-task are defined:
A1: Acquisition, processing and interpretation of OBS, OBC and 3D seismic imagery data from gas hydrate systems in the target areas.
A2: Development of advanced seismic inversion methods for hydrate quantification
A3: Inversion of seismic data, characterisation of gas hydrate seismic propagation parameters, and seismic attribute anaylsis.
A4: Quantification and resource assessments

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Task B (UiB/NGU/SINTEF): Geological and geochemical characterisation and quantification of natural gas hydrate
Gas migration from reservoirs towards the seabed is known to influence the local sedimentary regime, seabed morphology, seabed stability and the diversity and number of benthic organisms (e.g. Hovland and Svensen, 2006). Geological characterisation by sediment coring and high-resolution 3D seismic imagery (tomography) in areas with seafloor expression of fluid flow above know gas hydrate reservoirs, as well as in control areas with no indication of such flow, are crucial to reach further knowledge on this task. Migration of gas from reservoirs to the seabed is known to have influenced the local sedimentary regime and the diversity and number of benthic micro-organisms. Such information is important in order to determine what these seabed structures reflect in terms of migration of gas as the microfossils are expected to bear the signature of the history seeping activities in the area. Knowledge on gas migration, and its rate, is therefore crucial for evaluation of prospects and geohazards, however so far few detailed studies on this topic have been carried out for cold water systems like the NBS margin (e.g. Vogt et al. 1999, and contributions therein). Geochemical information is needed for diagnostics of seep targets, assessment of migration pathways and characterization of sources of gaseous hydrocarbons. Knowledge of the source of the gases incorporated in the hydrates is crucial for developing precise quantitative models and understanding their origin. The molecular and isotopic composition of the gases will clearly indicate whether their origin is thermogenic, biogenic or mixed, since the microbial gas will be strongly dominated by methane while natural gas seeps contain the whole sequence of hydrocarbon gases. Carbon isotopic compositions (e.g. Whiticar et al. 1986) and presence of CO2 and acetate in the porewaters are also important parameters, together with the monitoring of C15+oil hydrocarbons in the sediments. Analysis of water samples for hydrocarbon gas compositions which can be used as a proximity indicator for hydrate occurrence can be developed into a valuable diagnistic tool. Thus, the goals of this task can be summarized: (1) to study any gas migration along transects of known abundances of gas hydrates, and (2) to demonstrate the apparent relationship between subsurface gas hydrate reservoirs and seep features observed at the seabed as specified with the following sub-tasks:
B1: Characterisation of the architecture and geometry of sedimentary facies where gas hydrates and natural seeps develop
B2: Detection and description of gas hydrate and seep-related features, their origin at the seabed and estimation of flux rates
B3: Identify the origin of gaseous hydrocarbons in hydrate and non-hydrate charged sediments
B4: Improved understanding on the influence of seeps on micro-benthic communities in gas hydrate system
B5: Determination of the role of tectonic, stratigraphy and sediment type in gas hydrate and vent systems
B6: Provide geochemical/sedimentological input data for quantitative modelling and laboratory testing.

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Task C (UiB/NGI/NGU/SINTEF): Laboratory testing and modelling of hydrate reservoir properties
Hydrate stability depends on pressure, temperature and hydrate composition, which has to be in equilibrium with the surroundings. Natural hydrates are therefore rarely in a state of complete stability and are more typically in a state of very slow dynamics due to trapping mechanisms. For instance, in the Nankai Trough core samples show that the hydrate layers are divided and trapped by intermediate clay layers (Takahashi et al, 2001; Matsumoto, 2002). The local hydrate composition will therefore be different from that measured at corresponding laboratory experiments for the same conditions of temperature and pressure due to the non-equilibrium state of the hydrate. In addition to factors like the total size and connectivity of hydrate layers, the actual composition of the hydrates is the crucial factors that determine the resource potential of any hydrate reservoir. Interactions between mineral surfaces and surrounding molecules determine whether or not the hydrate will stick to the mineral surfaces, and correspondingly whether or not the hydrate then will serve as glue between solid particles. Every hydrate reservoir is therefore unique and the corresponding resource potential equally individual. Because of this, we need to understand the gas hydrate dynamics involved, and improve the geomechanical understanding of hydrate formation and dissociation effects on the surrounding sediments through experiments and development/modification of numerical modelling tools. The couplings between the hydrate kinetics and the geomechanics is mainly through local pressure changes caused by altered densities of pore filling materials. Other changes in flow situations may for instance be caused by effects of the dissolution of hydrate “glue” and corresponding changes in properties and mobility of solid material. In order to address these issues we have five sub-goals:
C1: Material models for clay behaviour during and following gas hydrate dissociation
C2: Experimental measurements of hydrate dissociation kinetics when exposed towards water and methane respectively.
C3: Guidelines for evaluation of seabed stability and foundation design in gas hydrate areas
C4: Simulation of possible effects of hydrate dissociation on seafloor installations and the development of guidelines for design of subsea foundations in regions of hydrate.
C5: Theoretical modelling of the properties of hydrate-bearing sediments based on laboratory measurements to integrate seismic field measurements and rock physics.

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B) Scientific objectives and Approach ->

i) Empirical approaches
Geophysics:
Conventional marine-towed 2D streamer systems do not provide adequate insights into marine gas hydrate systems. However, multi-component systems also allow recording of S-waves in addition to the P-waves. The concept involves placing four-component sensing systems on the seafloor to record the full vector wavefield of passing stress waves. This approach provides more acoustic information about the properties of rocks and fluids than is obtained by recording only one component. The Department of Geology at UiT has excellent experience with applying multi-component seismic technology in gas-hydrate research (Andreassen et al., 2003; Bünz and Mienert, 2004, Bünz et al., 2005). The multi-component technology allows us to distinguish a proper rock physics model for the micro-scale distribution of gas hydrates. Moreover, multi-component seismic data sets allow discrimination between fluids and lithology and will lead to a better understanding of fluid seeps that are associated with gas hydrate systems. The multi-component seismic data will be inverted using traveltime, full-waveform and tomographic inversion methods to obtain velocities and the seismic quality parameter Q of P- and S-waves of hydrate-bearing sediments. These parameters can be used to accurately estimate gas hydrate concentrations using advanced inversion schemes, e.g. differential effective medium theory. Thoroughly tested rock-physics models (Chand et al., 2004) will allow the accurate quantification of the amount of gas hydrates present. These models will be further developed through the incorporation of state of the art models for hydrate nucleation and growth (Kvamme et al., 2004, Buanes and Kvamme, in press) as improved/verified through experimental data on growth and dissociation of hydrate in Bentheimer sandstone (Kvamme et al. 2004) and other types of porous media (Llamedo et al., 2004, Østergaard et al., 2002). A new 3D high-resolution system by UiT that has been jointly developed by the UiT, the National Oceanography Centre, Fugro, and Volcanic Basin Petroleum Research (VBPR AS) will provide detailed acoustic images of the sediments containing gas hydrates and free gas. This, together with multicomponent seismic data allows determining the distribution of hydrates and gas, their accumulation and migration pathways. Moreover, the 3D seismic data will be integrated with a velocity volume obtained from tomographic inversion of OBS data, in order to provide 3D images of the amounts and distribution of hydrate- and free gas-charged sediments. The application of full-waveform inversion not only to compressional-wave data but also to converted-wave data will result in a higher-resolution characterisation of sub-surface velocities than was achievable before. Also application of improved AVO modelling using new theoretical approaches describing hydrate bearing sediment physical properties will be an added advantage in the development of theory and application of these models (NGU). A method for estimating the overpressure from OBS data will also be further developed, and applied to estimate hydraulic gradients and subsequently flow rates (UiT). As an alternate we will investigate the possibility of acquiring an OBC system (NGI/UiT). Experience with the different multi-component systems has shown a superior imaging technique of the OBC due to decreased lateral sampling intervals and the multi-channel geometry, which drastically reduces the processing efforts.

Geology/Geochemistry: As decomposition of gas hydrates, faults, and slides have been considered as major triggers for gaseous hydrocarbon release at the seafloor, the relation of these factors to potential gas anomalies in near-surface sediments will be investigated using seismic/core data and swath bathymetry/sonar in pre-selected seep target areas in cooperation with Task A. Sampling and observation will also be performed through the use of UiBs ROV Aglantha (video and sampling), ROV Bathysaurus (high-resolution multibeam, video, new micro sampling system of gass/fluids). The UiB/IMR new research vessel G.O.Sars has proved (cruises 2005) to have unique capabilities using the new generation of hull-mounted chosounder systems to locate and map shallow/deep marine vent fields due to an exceptionally low noise level (98 % below that of traditional research vessels). The large range of acoustic frequency and calibrated focus of the beam will make it possible to use the hull-mounted system to measure the size and quantify the gass bubbles released from the seabed. Additionally, the new remote Acoustic Doubler system will be lowered down to the most active vent systems for measuring the rate of flow. These acoustic facilities will combined with multibeam mapping and new ROV paltforms undoubtedly revolutionize the search and study of active vent areas. This equipment will be used at defined target/transect areas along the NBS margin. Sedimentological characteristics will be retrieved by physical properties, mineralogical (XRD, grain-size) and geochemical (stable isotopes, XRF core scanner) data. Benthonic foraminiferal assemblages will be analysed in samples from selected cores to study the imprint of gaseous leakage on stable isotope chemistry. On active seep target the molecular and isotopic signatures of low-molecular-weight hydrocarbons in near-surface sediment pore space (free gas) and the sediment matrix (adsorbed gas) across the seep targets is to be analysed. We will perform the geochemical/biological prospecting for sources of methane in subsurface sediments that provides a detailed view on (a) the diagnostics of their origins, and (b) the spatial distribution of gas-charged sediments. Sediment coring equipment will be used to obtain sediment samples for gaseous hydrocarbon analyses. The residual gas (C1-C5 headspace, pore space, sediment matrix) will be investigated by means of gas chromatography (GC) and gas chromatography/isotope ratio mass spectrometry (GC/IRMS). Occurrence of C15+ saturated hydrocarbons in near surface sediments provides insights into the impregnation of mature organic matter on the surface sediments. In addition to isotopic and compositional studies on the organic and gas fractions, clay and total mineralogical studies will be used to investigate the molecular attraction of adsorbed methane on variable particle surfaces and decipher fluctuations in methane adsorption with respect to changes in the organic and/or clay mineral source regions. RV G.O. SARS has recently proved to be uniquely able to locate deep submarine hydrothermal vent fields (Reed, 2006). Moreover, 18 kHz fish echo sounders clearly image gas seepage from the Håkon Mosby Mud Volcano at the Norwegian continental margin (Mienert et al., 2005). Calibrated detection of the acoustic signal makes it realistic to calculate flux rates of seeping hydrocarbons from at least areas with a well defined vent systems. This capability is likely to revolutionize exploration for seeps and quantification of gas seepage. The SEABED III industry consortium will give access to the semi-regional industry 3D seismic dataset with the associated regional 2D seismic lines available in the Nyegga-South Vøring target area. The Norwegian Petroleum Directorate (NPD) will assist with seismic and shallow boring data from the Barent Sea margin to meet the demand in Tasks A and B.

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ii) Modelling approaches

The theoretical concept
Magnetic resonance imaging (MRI) has proven to be a powerful tool for 3D visualisation of hydrate formation and hydrate dissociation kinetics (Kvamme et al., 2004) in real porous media as well as in more controlled surroundings. It is probably the best available fundamental experimental approach for measuring phase transition kinetics in this type of system. Relative to a fundamental understanding of the detailed mechanisms involved in the phase transition kinetics it is limited by the micrometer scale resolution and thus needs to be supplemented with a theoretical tool. The experiments provide detailed information on the progress of the phase transition front. In combination with measurements of the mass exchange it also provides a detailed kinetic profile that can be used as verification of a theoretical model. Phase field theory (PFT) (Kvamme et al., 2004) has demonstrated unique capabilities of predicting phase transition kinetics for a number of different systems with no adjustable parameters. The theoretical concept is also able to predict crystal morphologies and corresponding effects on the kinetics of phase transitions. The theory has also been applied to hydrate systems but the lack of available experimental results limits the possibility for extensive comparison. Comparisons with the few available results for CO2 hydrate are very promising. The advantage of the approach is that it does not need any predefined hypothesis for mechanisms since it applies the first and second laws of thermodynamics to trace the dynamics of the system towards stability and as such samples the natural mechanisms through this pathway.

Practical experiments
The combination of empirical data (tasks A and B) and theory (task C) is intended to create the necessary confidence in the theoretical approach so that it can also be linked to studies of real reservoir conditions. For some selected target areas (e.g. Nyegga, Svalbard), real core samples can add information about composition of the hydrate as well as contacting clay or minerals. In contrast to clay minerals, which may actually stick to hydrate (Odriozola et al., 2004, Titiloye et al., 2005), typical sandstone minerals (Techmer et al, 2005) may have surface characteristics that favour a liquid film between the mineral surface and hydrate. These liquid films, as well as gas pockets inside hydrate structure may provide channels for gas transport through the reservoir. Detailed analyses of core samples relative to corresponding theoretical analyses of the fluid/hydrate/mineral interaction can thus provide useful insight into the history of development of the hydrocarbon migrations and thus gas hydrate development.

Moreover, geomechanical experiments with sediments containing hydrates and gas have been conducted at the NGI since 2000. The experiments provide data for development of material models to be implemented in numerical analysis tools. Finite element and finite difference methods (commercial as well as in-house developed tools) will be applied and modified to be able to describe the experimental results. This will be followed by laboratory scale model test, for calibration and verification of the material models and numerical analysis tools. An initial series of laboratory test for visualisation and illustration of hydrate dissociation effects has been carried out within the Euromargins-SPACOMA project.

Finally a series of simulations of the behaviour of typical subsea foundation and wells will be modelled to improve understanding of the effects gas hydrate dissociation on deformations and foundation capacity.

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