The presence of gas hydrates in permafrost regions has been confirmed by core samples recovered from the Mallik gas hydrate research wells located within Mackenzie Delta in the Northwest Territories of Canada. Strong vertical variations of compressional and shear velocities and weak surface seismic expressions of gas hydrates indicate that lithological heterogeneities control the lateral distribution of gas hydrates. Seismic scattering studies predict that typical horizontal scales and strong velocity contrasts due to gas hydrate concentration will generate strong forward scattering, leaving only weak energy to be captured by surface receivers. In order to understand the distribution of gas hydrates and the scattering effects on seismic waves, heterogeneous petrophysical reservoir models were constructed based on the P-wave and S-wave velocity logs. Random models with pre-determined heterogeneity scales can also be used to simulate permafrost interval as well as sediments without hydrates. Using the established relationship between hydrate concentration and P-wave velocity, we found that gas hydrate volume content can be determined by correlation length and Hurst number. Using the Hurst number obtained from Mallik 2L-38, and the correlation length estimated from acoustic impedance inversion, gas hydrate volume fraction in Mallik area was estimated to be 17%, approximately 7x108 m3 free gas stored in a hydrate bearing interval with 250,000 m2 lateral extension and 100 m depth. Simulations of seismic wave propagation in randomly heterogeneous models demonstrate energy loss due to scattering. With the available modeling algorithm, the impact of heterogeneity scales on seismic scattering and optimum acquisition geometries will be investigated in future studies.

Geochemical processes associated with gas hydrate formation lead to the growth of iron sulphides which have a geophysically-measurable magnetic signature. Detailed magnetic investigation, complemented by petrological observations, were undertaken on cores from a permafrost setting, the Mackenzie Delta (Canadian Northwest Territories) Mallik region, and two marine settings, IODP Expedition 311 cores from the Cascadia margin off Vancouver Island and the Indian National Gas Hydrate Program Expedition 1 from the Bengal Fan. Stratigraphic profiles of the fine scale variations in bulk magnetic measurements correspond to changes in lithology, grain size and pore fluid geochemistry which can be correlated on local to regional scales. The lowest values of magnetic susceptibility are observed where iron has been reduced to paramagnetic pyrite, formed in settings with high methane and sulphate or sulphide flux, such as at methane vents. High magnetic susceptibility values are observed in sediments which contain detrital magnetite, for example from glacial deposits, which has survived diagenesis. Other high magnetic susceptibility values are observed in sediments in which the ferrimagnetic iron-sulphide minerals greigite or smythite have been diagenetically introduced. These minerals are mostly found outside the sediments which host gas hydrate. The mineral textures and compositions indicate rapid disequilibrium crystallization. The unique physical and geochemical properties of the environments where gas hydrates form, including the availability of methane to fuel microbiological activity and the concentration of pore water solutes during gas hydrate formation, lead to iron sulphide precipitation from solute-rich brines. Magnetic surveying techniques help delineate anomalies related to gas hydrate deposits and the diagenesis of magnetic iron minerals related to their formation. Detailed core logging measurements and laboratory analyses of magnetic properties provide direct ties to original lithology, petrophysical properties and diagenesis caused by gas hydrate formation.

Seismic reflection data from a multi-channel streamer deployed offshore Korea reveal evidence of hydrateforming gases being vented into the ocean. Numerous, localised vent structures are apparent from reduced seismic reflection amplitude, high seismic velocities, and reflector pull-up. These structures penetrate upward from the base of the gas hydrate stability zone (GHSZ) and are typically several hundred metres wide, and only a few hundred metres high. Underlying zones of reduced reflection amplitude and low velocities indicate the presence of gas many kilometers below the seabed, which migrates upward through near-vertical conduits to feed the vent structures. Where the local geology and underlying plumbing indicates a high flux of gases migrating through the system, the associated vent structures show the greatest change of reflector pull-up (the greatest concentration of hydrate) to be near the seabed; where the local geology and underlying plumbing indicates a moderate flux of gases, the greatest change of reflector pullup (the greatest concentration of hydrate) is near the base of the GHSZ. The distribution of gas hydrate in the high-flux gas vent is consistent with the recent salinity-driven model developed for a rapid and continuous flow of migrating gas, while the hydrate distribution in the lower-flux vent is consistent with a liquid-dominated system. The high-flux vent shows evidence of recent activity at the seabed, and it is likely that a substantial amount of gas is passing, or has passed, through this vent structure directly into the overlying ocean.

Experiments involving the dissociation of artificial methane-hydrate-bearing sediments were performed using X-ray computed tomography (X-CT, 40 s scanning speed at 2 min intervals) to directly observe dissociation behavior in the sediments and the gas and water flows generated by dissociation. Dissociation by depressurization was performed using a backpressure regulator, and showed that the temperature reduction induced by depressurization depends on the phase equilibrium state of methane hydrate, and that preferential dissociation occurs along the periphery of the core. This behavior is caused by heat flux from the outside of the core, and this controls the dissociation rate. A heat exchanger was installed at one end of the core to simulate thermal stimulation, and propagation of a clear and unidirectional dissociation front was observed. Depending on the heating temperature, the dissociation rate was less than that observed for depressurization. Hot water was also injected at a constant rate from the bottom of the core, and CT images showed the movement of distinct accumulations of dissociated gas being pushed by the hot water. The gas production rate increased immediately after the gas accumulation reached the opposite end of the core where the gas and water flow out.

Formations containing gas hydrates are encountered both during ocean drilling for oil or gas, as well as gas hydrate exploration and exploitation. Because the formations are usually permeable porous media, inevitably there are energy and mass exchanges between the water-based drilling fluids and gas hydrates-bearing formation during drilling, which will affect the borehole’s stability and safety. The energy exchange is mainly heat transfer and gas hydrate dissociation as result of it. The gas hydrates around the borehole will be heated to decomposition when the drilling fluids’ temperature is higher than the gas hydrates-bearing formation in situ. while mass exchange is mainly displacement invasion. In conditions of close-balanced or over-balanced drilling, the interaction between drilling fluids and hydrate-bearing formation mainly embodies the invasion of drilling fluids induced by pressure difference and hydrate dissociation induced by heat conduction resulting from differential temperatures. Actually the invasion process is a coupling process of hydrate dissociation, heat conduction and fluid displacement. They interact with each other and influence the parameters of formation surrounding the borehole such as intrinsic mechanics, pore pressure, capillary pressure, water and gas saturation, wave velocity and resistivity. Therefore, the characteristics of the drilling fluids invading into the hydrate-bearing formation and its influence rule should be thoroughly understood when analyzing on wellbore stability, well logging response and formation damage evaluation of hydrate-bearing formation. It can be realized by establishing numerical model of invasion coupled with hydrate dissociation. On the assumption that hydrate is a portion of pore fluids and its dissociation is a continuous water and gas source with no uniform strength, a basic mathematical model is built and can be used to describe the dynamic process of drilling fluids invasion by coupling Kamath’s kinetic equation of heated hydrate dissociation into mass conservation equations.

Discovery of natural methane hydrate in deepwater sediments in the east-coast of India have generated significant interest in recent times. This work puts forward a possible relationship of multi-TCF gas accumulation through destabilization of paleo-hydrate in Plio-Pleistocene deepwater channel sands of Krishna-Godavari basin, India. Analysis of gas in the study area establishes its biogenic nature, accumulation of which is difficult to explain using the elements of conventional petroleum system. Gas generated in sediments by methanogenesis is mostly lost to the environment, can however be retained as hydrate under suitable conditions. Longer the time a layer stayed within the gas hydrate stability zone (GHSZ) greater is the chance of retaining the gas which can be later released by change in P-T conditions due to sediment burial. P-T history for selected stratigraphic units from each well is extracted using 1-D burial history model and analyzed. Hydrate stability curves for individual units through time are generated and overlain in P-T space. It transpired that hydrate formation and destabilization in reservoir units of same stratigraphic level in different wells varies both in space and time. Presence of paleo hydrates is confirmed by the occurrence of authigenic carbonate cement and low-saline formation water. We demonstrate how gas released by hydrate destabilization in areas located at greater water depths migrates laterally and updip along the same stratigraphic level to be entrapped in reservoirs which is outside the GHSZ. In areas with isolated reservoirs with poor lateral connectivity, the released gas may remain trapped if impermeable shale is overlain before the destabilization of hydrate. The sequence of geological events which might have worked together to form this gas reservoir is: deposition of organic rich sediments → methanogenesis → gas hydrate formation → destabilization of hydrate and release of gas → migration and entrapment in reservoirs.

Gas hydrate formation generally involves gas dissolution, formation of nuclei and growth of new nucleus. On condition of synthesizing experiments without agitation, the formation of hydrate nuclei is comparatively difficult and needs an induction period which is considerably uncertain and random. Some additives such as surfactant sodium dodecyl sulfate (SDS) can increase the formation rate and reduce the induction time. A hydrate formation and mini drilling experimental system was used to carry on methane hydrate formation experiments with small quantity of SDS and SDS- tetrahydrofuran(THF) in deionized water. The reactor is a high pressure cell (40Mpa) made of titanium alloy with 4 transparent windows and an inner volume of about 2.8 liters. The effect of SDS and THF hydrate on the formation rate and amount of methane hydrate was studied by comparative testing and analyzing the collected data of temperature and pressure. According to the results of the tests, the formation rate of methane hydrate in the SDS-THF solution was faster than that in the SDS solution. As a water-soluble hydrate former, THF hydrate nucleation may be benefit of methane hydrate nucleation. A small amount of SDS and THF could dramatically promote the formation of methane hydrate in the pure water, and rapidly increase the amount of methane hydrate too. Therefore, a great deal of time for experiment was saved, which established a good basis for the coming mini drilling and drilling fluid experiments.

The structural and mechanical properties of s-I methane hydrate have been investigated by first principles calculations. For the first time, the fully elastic constant tensor of s-I methane hydrate is obtained entirely ab-initio. The calculated lattice parameter, bulk modulus, and elastic constants were found to be in good agreement with experimental data at ambient pressure. The Young modulus, Poisson ratio and bulk sound velocities are estimated from the calculated elastic constants and compared with wave speed measurements available.

Replacement of the traditional thermodynamic hydrate inhibitors (methanol and glycols) in multiphase applications is highly desirable for Health, Safety & Environment (HSE) considerations and for investment costs savings. Low Dose Hydrate Inhibitors (LDHI) are good candidates to achieve this objective and their interest is growing in the E&P industry. There are two types of LDHI: the Kinetic Hydrate Inhibitors (KHI) and the Anti-Agglomerants (AA) also called dispersant additives. The main challenge with LDHIs is that they require the unprocessed effluents to be produced inside the hydrate stability zone. It is then of the utmost importance to select, qualify and implement properly LDHIs, so that their field deployment is performed with success. But due to the very stochastic nature of the nucleation step, the hydrate crystallisation process leads to very large discrepancies between performances results carried out at lab or pilot scales. In order to overcome this difficulty, we have developed an in-house special protocol which is implemented prior to each qualification tests series. This in-house 15 years old protocol consists in conducting each tests series with a fluids system having previously formed hydrates in a first step but followed by a dissociation step at moderate temperature for a few hours. This paper presents results selected from several field cases studies and obtained from our 80 bara and 165 bara flow loops. They show the very good reproducibility obtained with and without LDHIs. In the case of KHI, where the stochastic nature of the nucleation step is very critical, the results show that the deviation on the “hold time” for a given subcooling is less than 15%. (Revised version of ICGH paper 5499_1)

Structure determination of powdered crystals is still not a trivial task. For gas hydrates, the difficulty lies in how to determine the rotational disorder and cage occupancies of the guest molecules without other supporting information or constraints because the complexity of the problem for the powder diffraction technique generally depends on the number of atoms to be located in the asymmetric unit. Here, the crystal structures of gas hydrates of CO2, C2H6, C3H8, and Methylcyclohexane/CH4, as determined by the direct-space and Rietveld techniques are reported. The resultant structures and cage occupancies were consistent with results found from conventional experimental methods using single crystal x-ray diffraction or solid-state 13C-NMR. It was shown that the procedures reported in this study make it possible to determine guest disorder and absolute cage occupancy of gas hydrates even from powder crystal.