Exploration and evaluation of the eastern mediterranean sea gas hydrates and the associated deep biosphere

A simulator was developed, which was originally based on a numerical model for the description of the precipitation of minerals in porous media. The simulation work was mainly oriented on the conditions prevailing in the Anaximander area. To identify guidelines for future simulation work, directed on the description of the generation or decomposition of gas hydrate deposits in past times, sensitivity studies were performed. The 3D-model includes the following topics: - Methane fugacity at gas hydrate ls-equilibrium; - Growth and decomposition of gas hydrates; - Changes of transmissibilities due to gas hydrate content; - Transport by convection and diffusion; - Heat transfer including enthalpies of formation; - Aerobic and anaerobic methane consumption. The Kihara-parameter used for the calculation of ls-equilibria was adjusted to include structural changes at low ethane concentrations. To fit the parameter data sets of ethane/methane- and propane/methane-mixtures including the pure components were applied from literature. For the growth of gas hydrates in pore space of consolidated sediments, with permanent pore geometry, different modules were developed and used to describe the changes of transmissibilities due to gas hydrate content. This is especially essential for the simulation of the growth behaviour in millimetre length scale. The stochastic generation of gas hydrate nuclei is incorporated into the model, to apply a more realistic scenario for the initial condition of the growth of gas hydrates in sediments. A reaction kinetic module was implemented, to study the effect of the microbial activity on the release of methane from the sediment into the overlying seawater column. The following conclusion can be drawn from the simulation work and are to be taken in account for future simulations: - The slow growth processes are governed by the slow mass transport. The habitus of the hydrates is interconnected with the coupling of convection and diffusion at a millimetre scale. - For acceptable predictions a large set of data is needed. The sensitivity studies point out, that it is paramount to describe the vertical flux properly. With large distances in the range of 100 m the mass transport is governed by convection at low velocities in the range of 1 x 10-7-1 x 10-9 m x s-1, valid for mud volcanoes without heavy eruptions. Therefore detailed information about the permeability variation in the sediment is needed. In case of free mobile gas the relative permeabilities must be accurately evaluated. - The distribution of gas hydrates in deposits is strongly dependent on the nucleation probability. The knowledge about the nucleation probability, which is responsible for the distribution of hydrates in the sediment, is poor. Especially for porous media, the dependencies on concentration and pore size distribution are unknown. - The microscopic growth behaviour derived for porous media of permanent geometry must be taken into account, to evaluate the time dependent transport properties of larger blocks with deep lying sediments. - The description of the quasi-steady state of gas hydrate deposits are related to states close to equilibrium and slow mass transfer. Therefore reliable ls-equilibria calculations are essential including structural changes. - The modelling of field data underline, that the reduction of the methane release from the sediment due to microbiological activity becomes only relevant at rather low convection velocities less than 1 x 10-8 m x s-1.

MAC-A (Multi-Autoclave-Corer) and APC-A(Autoclave-Piston-Corer) ANAXIMANDER are designed to be winch-operated on offshore platforms to obtain, gravity driven, seabed samples conserving the in situ conditions. The MAC is able to secure up to four pristine cores of 80mm in diameter and max.1m in length. Its universal joint and attenuated motion promises undisturbed sampling in particular in uneven environment. The aluminium sample vessels are, certified to sustain operational pressures up to 250 bars and facilitate non-destructive screening (e.g. CT) while light enough to be handled easily. To investigate physical parameters of the samples and to take in situ sub samples (e.g. water or sediment) it can be coupled to the ARS (Autoclave Rod Sub-sampler. The APCA is certified for 200bar of operational pressure and capable to achieve samples of up to 2.5m length by falling free into the seafloor. Both gadgets can be interfaced with degassing installations.

An online conductivity measuring system was developed, to follow up quantitatively the hydrate growth in porous media by determining the changes of the salt concentration caused by the gas hydrate formation, and to study the accessibility of gas hydrates investigating the transport properties. A novel high-pressure flow through conductivity cell based on a six lead circuitry was implemented for continuous conductivity measurements with a resolution < 0.01%. The pressure bearing part laid out for 70MPa is fabricated from corundum. To operate at two phase flow conditions a low dead volume high-pressure gas/liquid-separator was developed and fabricated. A high-pressure degasser operates in series with a high-pressure gas/liquid separator. Regulating the flow rate of the liquid the pressure difference across a porous ceramic tube is controlled, so that the capillary threshold pressure is not exceeded. The response of the tracer detection equipment including the peak broadening by the gas/water separation exhibits a half width of only 0.4 cm3 under two phase flow conditions. This device is universally usable for studies on processes in porous media under high pressure, which are accompanied with conductivity variations. (Technical drawings are available.) The following conclusions can be drawn from the tracer experiments with porous media: - The growth- and decomposition processes in the pore space are governed by transport phenomena. - The symmetry of tracer elution profiles at the initial growth stage proves, that accessible liquids are involved during the growth of gas hydrates under single-phase flow. Stagnant liquid phases are not generated before the permeability is considerably reduced by gas hydrate formation. - Under dual phase flow condition initial rapid growth of gas hydrates in the pore space decreases at an early stage due to the formation of transport barriers, gas hydrate films. - Decomposition experiments prove, that stagnant liquids enriched with NaCl exist under dual phase flow.

On basis of an existing high-pressure cell a novel optical cell was developed and fabricated to determine the growth rate of gas hydrates in defined spaces referring to the condition in sediments. The cell is universally usable for studies on growth processes of gas hydrates and minerals under high pressure. The narrow glass capillary with an inner diameter of 1.7mm is installed in a slotted brass container generating a defined constant temperature gradient along the capillary axis by heating at the top and cooling at the bottom. The capillary can be rotated on its vertical axis accurately by means of a play free gear at the top of the housing. Similarly, the microscope equipped with a photo camera is positioned vertically by means of a thread. A double layered window is moved mutually with this microscope. The growth rate in the optical cell is controlled by diffusion along a temperature gradient to generate precisely adjusted steady state conditions suitable for measurements at slow linear rates in the range of 5 x 10-5 ¿ 5 x 10-4 m x s-1. Temperatures are measured by thermocouples at the circumference of capillary at three locations. The thermocouples are led through the acryl glass shield, which suppresses detrimental air convection at the glass capillary. The temperature deviation from linearity amounts to < 0.1K. The cell is installed in an insulated air bath, which atmosphere is kept dry by an influx of nitrogen. (Technical drawings are available.) In order to calculate the true distances from the microscopic photographs, the determined lengths are corrected numerically for distortion by the cylindrical optics of the glass capillary. A gas water interface can be placed at a predetermined level by withdrawing liquid from the inner of the capillary using an installed needle. Fast formation of gas hydrate can be initiated at the gas liquid interface or at an additional vl- interface by placing a methane bubble e.g. at the bottom of the glass capillary. The nucleation probability under ls-condition is low in comparison with the probability on the gas liquid interface. After long periods (> one month), gas hydrate crystals can be formed, adhering at the glass wall, without any contact to a gas liquid interface. The kinetic studies reveal, that structural changes from SII to SI were observed applying a gas with a composition of almost 0.5 % ethane and 99.5 % methane. At low temperature <280K gas hydrates of the structuire SII are formed. The thermodynamically stable 100-crystal faces were rather exceptional, and predominantly 111-faces were found. Only after long residence time (>600h) hexaeder were observed, that means, 100-faces became evident at vanishing overall growth rate. In a special case SII gas hydrate was transferred from a cold zone (276 K) to a hot zone (284 K) by buoyancy to a place 4.5mm below the hydrate free gas water interface. After this transfer a nearby competing SI crystal grew, whereas the SII crystal decomposed slowly. The horizontal growth rate determined under ls-condition at 9.49MPa and 283.0K, amounted to (1.01 +/-0.03) x 10-10 m x s-1, the vertical rate to (1.96 +/-0.08) x 10-10 m x s-1.

In this result, we will address the investigated aspects to determine the geological hazards associated to mud volcanoes containing gas and gas hydrate. That is to say, find out what hazardous events may occur. The aspects are related to the geological framework and they are following: Related to sediment - Morphology - Stratigraphy - Instability features - Geotechnical properties - Structural features Related to fluids - Free gas - Gas hydrate. Although this result focuses on mud volcanoes, it is not meant that other problems associated to the regional geological framework can be unimportant. This result, may provide useful information to other environments on the continental margins and abyssal plains in particular, in order to define concepts/parameters to determine geological hazards.

A novel, universally useable high pressure CT-cell was developed and adapted to the X-ray CT-tool manufactured by Bio Imaging Research. Using this high pressure CT-cell equipped with a temperature and a pressure control, the growth behaviour of gas hydrates in porous media and the displacement of sediment constituents by the formation of gas hydrate crystals were accurately investigated. Additionally a volumetric measurement in the annular space is applied to follow up possibly occurring expansion of sediment models. Related to the time dependent displacement of sediment constituents, the detection limits of this equipment amount to X-ray: £Gl/l = 2*10-3, volumetric measurement: 1/V dV/dt = 1*10-10 s-1 This device is universally usable for studies on processes in porous media, especially precipitation of gas hydrates and minerals.The novel high-pressure ceramic cell for high resolution CT-studies is fabricated from an Al2O3- tube (Alsint). The x-ray beam is not affected by any strongly absorbing metallic component assuring a high contrast level for quantitative measurements. The sediment model, sand packs or consolidated sandstone cores are inserted into shrinking tubes made from FEP. A floating end piece serves for compensation of axial expansion. For studies on gas hydrate behaviour accurate thermostating is essential. To enable long-term measurements (>2000 h) at constant temperature below ambient and constant pressure in the annular space of the cell, an air bath was installed and implemented into the CT-device. The thermostating / cooling system including fans and radiator grille was installed in an insulating polystyrene housing below the free rotating core holder. A rather universal set up was chosen as peripheral device to respond adequately on changing experimental necessities. The total peripheral device is mounted on a cart close to the CT-apparatus. (Technical drawings are available.) The device allows flooding of the core with gas, water, or water containing an adjustable amount of dissolved methane. For the studies proppants, England sand 20/40 mesh, were used to construct sandpacks with a diameter of 20 mm and a length of 350 mm. To achieve settled packing, the installed sediment models were compacted by an overburden pressure of 4 MPa. A method was developed for generating specific large cavities in sandpacks, in which large bubbles of free gas can be entrapped. The method is based on the fact, that large voids can be stable in sandpacks due to the large friction at the contact areas of grains, proven by the stability of proppant faces in fractured reservoirs. By inserting single NaCl-crystals into the sandpack the large voids were preformed. These crystals were dissolved by water after the compaction of the model. The following conclusions can be drawn from the CT-studies with sandpacks using this equipment: - The growth of gas hydrates is preferentially initiated at the inner surface of the voids and the first adjacent grain layer. After a short time (less than 200 h) the large pores are surrounded by grain layers cemented by gas hydrates. - The gas hydrate layers between grains form effective transport barriers. - Due to the increasing pressure differences between the periphery and the inner space of the void, a compaction of the pore system is resulting. - In collapsed pores further displacement of grains occur. - The compaction causes a further reinforcement of the transport barriers. - Gas included in the large pores with unchanged geometry is not accessible (time scale 2000 h).

To study the formation and decomposition of gas hydrates in porous media a novel, universally useable NMR cell was developed, and adapted to the dimensions of the magnetic field of a low frequency NMR-imaging tool manufactured by Magnetic Resonance Instruments. Using this high pressure NMR-cell, equipped with a precise temperature and pressure control including temperature gradient regulation, the growth behaviour of gas hydrates near to equilibrium condition was accurately investigated. A rather universal set up was chosen as peripheral device to be able for fast adaptation on new experimental necessities. This equipment is universally usable for studies on processes under high pressure. Because non-conductive pressure bearing material is a prerequisite to perform NMR-measurements, a ceramic high-pressure cell for NMR-studies on the distribution of gas hydrate was developed and fabricated. The novel core holder cell is fabricated from a silicon nitride tube. Tests with this type of tube showed, that the pressure bearing part can withstand 1000 bar. The sediment model, sand packs or consolidated sand stone cores are inserted into shrinking tubes made from FEP. For application of overburden pressure the liquid D2O is used in the annular space, to avoid any additional proton signal during NMR-measurements. For studies on gas hydrate behaviour accurate thermostating is essential. Since the maximum diameter of the core holder is limited to 40mm, the core holder has got only narrow outer annular spaces. For this reason a thermal insulation by evacuation of the outer annular sleeve is applied. D2O was chosen as thermostating fluid. To avoid condensation of H2O into the coolant a closed circuit is installed. A gear pump with a maximum flow rate of 4 L∙min-1 is used at the inlet of the heat exchanger coil to apply a high velocity in the narrow annular space of the NMR-cell. To induce a temperature gradient the top of the cell can be heated additionally. Because a quantitative measurement of axial distributions is restricted to 40 mm, due to the magnetic field characteristic, the vertical position of the cell can be changed accurately during the high pressure experiments simply by using a lifting truck bearing the mounting of the cell. By this procedure profiles with a total length of 160 mm can be achieved. The peripheral equipment allows flooding of the core from the top or bottom with gas, water, or water containing a predetermined amount of dissolved methane. The total peripheral device is mounted on a cart near by to the NMR-apparatus. Experiments can be carried out at low formation / decomposition rates. The changes in the vertical distribution of gas hydrates can be measured during these experiments. The following conclusions can be drawn from the NMR-studies using this equipment: - The growth- and decomposition processes in the pore space are governed by transport phenomena, - At high initial gas saturation growth processes expand rapidly over large areas after nucleation. Largely extended pore arrays are commonly involved in unsteady growth behaviour. Under these condition channels for the transportation of free gas remain open. - At low initial gas saturation the start of the gas hydrate formation is a local phenomenon. Convective transport of the gas component is not initiated; transport channels for gas are not formed. - After nucleation at the gas/liquid interface solid gas hydrate are formed hindering or even blocking the transport process for some time. Gas bubbles enclosed by such films collapse only slowly accompanied by a slow formation of gas hydrates accordingly. - The gas hydrate layers between grains form effective transport barriers. - NMR-studies with single-phase methane transport show, that the liquid H2O-phase in the zone containing gas hydrates can be almost completely displaced by D2O applying 1.2 PV.

In this result, we will address the aspects of the use of the sedimentary structures and stratigraphy for detection of sediments containing gas hydrate: - Detection of mud breccia layers and their vertical distribution. - Detection of physical boundaries and types (transitional or sharp). - Detection of vertical grain size trending. - Detection of clasts of the mud breccia, their vertical distribution, and their quantification. - Detection of biogenic structures - SEM analysis. The innovative aspects of this result is mainly related to definition of those useful qualitative parameters related to lithostratigraphy and elemental analysis of sedimentary structures in sediments recovered from mud volcanoes. This result can be used to determine the presence of individual mud flow events, frequency of mud flow events, and the type of activity (dormant, presently active, exhumated) of the mud volcano. The knowledge of sediments containing gas hydrate from mud volcanoes has improved after the set of measured parameters. In particular it may provide useful information for layers containing gas hydrate, and make important observations regarding grain fabric and texture. This result has been has been compared with those stratigraphic results obtained from those cores recovered in the Gulf of Cadiz, validating its potential use for other similar areas.

An experimental setup was developed to study gas hydrate equilibria of multi-component gas at under-saturated conditions, i.e. where no free gas phase is present. It consists of a thermally controlled autoclave vessel, which is magnetically stirred and equipped with suitable sampling ports for sampling water and gas. Two piston vessels provide the necessary amounts of multi-component gas and water, to maintain the pressure at the set value. A syringe pump controls the pressure of the piston vessels and transmits the pressure, volume of displacement and flow data to a computer. A novel procedure for the formation of gas hydrates from a multi-component gas was designed and tested, in order to produce homogeneous hydrate phases. The effect of each one of the following parameters on thermodynamic stability was assessed: duration of formation, stirring rate, transition to another set of conditions. The results reveal that a homogeneous hydrate phase can be produced in the laboratory at about 3 days.

Four aspects of the use of sedimentological experiments for the study of sediments containing marine gas hydrate can be mentioned. The same aspects have been used for sediments non containing gas hydrate in order to establish and assess differences and the relative importance of quantifiable parameters from marine sediments. The four aspects are the following: - Determination of textural parameters that favour gas hydrate presence, from grain size (gravel, sand, silt and clay) to statistical parameter aspects (mean, sorting, skewenes and kurtosis). - Determination of three physical parameters of those sediments containing gas hydrates using a continuous, non-destructive high-resolution Multi-Sensor Core Logger (MSCL). The measured parameters include wet-bulk density (by Gamma Ray Attenuation, GRAPE), magnetic susceptibility (MS) and P-wave velocity - Discrete measurements of geotechnical properties of sediments containing gas hydrate. The parameters comprise the following index properties: water content, grain density, porosity and undrained shear strength. - Determination of sand fraction composition and calcium carbonate content. Composition of sand fraction was determined by visual identification and by counting 350 grains per sample with the aims of a binocular. The following grains were considered to identify in sediments containing gas hydrate: quartz, light minerals, pyrite, glauconite, rock fragments, and forams (well and bad/broken preserved). The innovative aspects of this result are three: - Definition of those useful quantificable parameters to be measured in sediments containing gas hydrate; - Comparison of sediments containing and non-containing gas hydrate; and - Relationship between physical/geotechnical properties and the sedimentary processes and Texture of marine sediments. The linkage between these properties is not necessarily straightforward and their correlation must be considered in the context of the type of measurement and its scale, and sedimentary environment. Sedimentological methods are a widely used of approach for direct detection of gas hydrate in marine sediments. The fact that natural gas hydrate is affected by changes in pressure and temperature makes its observation very difficult under laboratory conditions. This fact increases the value of the results obtained from the natural conditions, because it provides ground truth measurements for constraining geological framework. Although this result focuses on sediments from mud volcanoes, many of the concepts/parameters will be able also to apply to marine sediments from other environments on the continental margins and abyssal plains. The knowledge of sediments containing gas hydrate from mud vocalnoes has improve significantly after the set of measured parameters. In particular it may provide useful information for: - Identification of hydrate and hydrate-berating sediment in other sedimentary environments and their distribution with depth - Source area of the gas - Conduits that supply fluid gas and gas into the hydrate stability zone - Estimation of porosity and methane saturation.

The ARS (Autoclave Rod Sub-sampler) facilitates investigations of samples taken by the MAC-A (Multi Autoclave Corer ANAXIMANDER) without demolishing the in situ conditions (temperature, pressure). Its maximal operational pressure is 250 bar. It is coupled to the ball-valve equipped MAC-A pressure vessel and induces an investigation rod which can be furnished with different examination tips. Depending on the tip either physical parameters (e.g. shear resistance, sample length&) or sub samples (e.g. water, sediment&) can be taken. The main sample inside the pressure vessel persists almost undisturbed after the intervention.

In order to transfer barophilic micro-organisms from field work, a special inexpensive vessel was designed. The vessel has a volume capacity of 200 cc, it is lightweight and equipped with a manometer and a valve. The main advantages of this equipment are: - It can be operated by a single person. - It is suitable for work on any ship deck, where several scientific processes are running in parallel. - It is safe, as it is pressurized to the set value of 20 MPa by pure water using a very compact HPLC pump. - It maintains the pressure with a small gas cap. - It can be quickly and easily depressurized and accessed, to minimize the time that samples remain at atmospheric pressure. Six vessels of this type were manufactured and used during Anaximander project. The pressurized sample 100cm length and 8cm diameter -, which was retrieved during the 2nd cruise of the project with the MAC piston corer (designed and manufactured by TUB-MAT) was subjected to a CT scan in Athens. In order to maintain and transport the pressurized (13 MPa) core sampler at constant temperature, a refrigerator was modified and adjusted in a small truck easy to handle by two persons and suitable for urban transportation. Subsequently, the refrigerator was used for storing the pressurized sample under constant temperature at the IGME premises. In case the pressurized sample contained in situ formed gas hydrates, a procedure was determined for the transportation of the pressurized sampler to the TUC premises.

An experimental setup was constructed for the formation and dissociation of gas hydrates confined in natural and artificial sediments at 20 MPa. It consists of a 15-inch core holder and two piston vessels. Natural sediment is confined in the core holder at controlled compaction and pore pressures. The temperature is controlled by a cooling jacket connected with a temperature controlling bath. Preliminary testing was conducted in collaboration with Heriot Watt University of Edinburgh in order to assess the mechanical behaviour of the natural sediment at several pressures and its relation with the hydrate formation within the sediment pores. The results reveal a significant shift of pressure, at which the sediment remains in the elastic region, when gas hydrates form in the sediment pore space. By conducting these tests, the necessary experience and competence were gained for testing and studying multi-component gas hydrates, contained in natural and artificial porous media.

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