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GAS HYDRATE ANOMALIES IN SEISMIC VELOCITIES, AMPLITUDES AND ATTENUATION: WHAT DO THEY IMPLY?Chand, Shyam 07 1900 (has links)
Gas hydrates are found worldwide and many studies have been carried out to develop an efficient method to identify and quantify them using various geophysical as well as other anomalies. In this study, various seismic anomalies related to gas hydrates and the underlying gas are analysed, and correlated them to rock physics properties. Observations of velocities in sediments containing gas hydrates show that the rigidity, and hence the velocity of sediments increases with increase of hydrate saturation. The increase of velocity due to the presence of gas hydrate can be explained in terms of gradual cementation of the sediment matrix. In the case of seismic attenuation, gas hydrate bearing sediments are quite different from common sedimentary rock behaviour of low seismic attenuation with high rigidity. In contrary gas hydrate bearing sediments is observed to have increased seismic attenuation of higher frequencies with increase of hydrate saturation. This strange phenomenon can be explained in terms of differential fluid flow within sediment and hydrate matrix. Also it is observed that the presence of large amount of gas hydrate can result in an increase of seismic amplitudes, a signature similar to the presence of small amount of gas. Hence misinterpretation of these enhanced amplitudes could result in the under estimation of gas present not only as shallow drilling hazard but also on the resource potential of the region. The increase of seismic reflection amplitude results from the formation of gas hydrates in selective intervals causing strong positive and negative impedance contrasts across the formations with and without gas hydrates.
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HYDROGEN ABSORPTION BEHAVIOR OF ORGANIC-COMPOUND CLATHRATE HYDRATESKawamura, Taro, Ohtake, Michika, Yamamoto, Yoshitaka, Higuchi, Satoru 07 1900 (has links)
The hydrogen absorption behavior of organic-compound clathrate hydrates was investigated using five kinds of organic compounds as well as tetrahydrofuran (THF). These hydrates were pressurized by hydrogen, and Raman analysis, the determination of the amount of hydrogen and calorimetric measurement were carried out. The Raman results show that the samples investigated in this work formed binary clathrate hydrate of hydrogen and each organic compound. The organic-compound clathrate hydrates presented similar performances to that of THF clathrate hydrate regarding hydrogen absorption and heat of dissociation. These results suggested that the organic compounds investigated in this work may become alternatives to THF.
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EXPERIMENTAL STUDY OF ENHANCED GAS RECOVERY FROM GAS HYDRATE BEARING SEDIMENTS BY INHIBITOR AND STEAM INJECTION METHODSKawamura, Taro, Ohtake, Michika, Sakamoto, Yasuhide, Yamamota, Yoshitaka, Haneda, Hironori, Komai, Takeshi, Higuchi, Satoru 07 1900 (has links)
The inhibitor and steam injection methods have been examined using a laboratory-prepared methane hydrate bearing sediment. New experimental apparatuses have been designed and constructed. In the case of inhibitor injection, the measurement of gas production vs. time suggested that the inhibitor increased dissociation rate. Core temperature decreased upon the inhibitor injection, in contrast to that in the case of pure water injection. The observed pressure differentials between the inlet and outlet of the core sample suggest that the inhibitor effectively prevented the hydrate reformation within the dissociating core sample. In the case of steam injection coupled with depressurization, it can be seen that the effect of steam (or hot water) injection was clear in the later stage of dissociation, compared with that in the case of depressurization alone. The inner (core) temperature change indicates that the coupling of depressurization and steam injection induces MH dissociation from upstream and downstream to the center of the sample. However, it starts from an upstream region and continues downstream steadily in the case of steam (hot water) injection alone.
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FORMATION OF HYDRATE PLUG WITHIN RECTANGULAR NATURAL GAS PASSAGESeong, Kwanjae, Song, Myung Ho, Ahn, Jung Hyuk, Yoo, Kwang Sung 07 1900 (has links)
In order to obtain a better understanding of hydrate plug formation mechanism in natural gas pipelines, formation and growth of hydrate layer within a rectangular channel formed by brass bottom and top surfaces and an insulated inner and an outer surface of transparent polycarbonate tube was studied experimentally. A gas mixture of 90 % methane balanced with propane was supplied at specified flow rates while the humidity and temperature of the supply gas was controlled at desired values using bubble type saturators and heat exchangers placed in series. Hydrate formation occurred along the top and bottom brass surfaces maintained at temperatures below equilibrium hydrate formation temperature, while the transparent tube served as window for visual observation. A series of carefully controlled laboratory experiments were performed to reveal the shape of porous hydrate layer under different combinations of under-cooling and moisture concentrations. The observed transient characteristics of hydrate layer profiles will provide important data that can be used for validation of numerical models to predict hydrate plugging of natural gas pipelines.
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HYDRATE PLUGGING POTENTIAL IN UNDERINHIBITED SYSTEMSHemmingsen, Pål V., Li, Xiaoyun, Kinnari, Keijo 07 1900 (has links)
An underinhibited system is defined as a system where an insufficient amount of thermodynamic inhibitor
is present to prevent hydrate formation. Underinhibition might occur due to malfunctioning of equipment,
temporary limitations in the inhibitor supplies or operational limitations or errors. Understanding the
plugging risk of such systems is important in order to take the correct precautions to avoid blocked
flowlines. In this paper we summarize the experimental efforts for the last decade within StatoilHydro on
the hydrate plugging risk in underinhibited systems. The flow simulator has been used as the main
experimental equipment. The overall results for systems underinhibited with ethylene glycol or methanol
show that the plugging potential increases up to a maximum at concentrations around 10-15 wt%. At higher
concentrations the plugging potential reduces compared to the uninhibited system. The results can be
explained as follows: As water is converted to hydrates in a system containing a thermodynamic inhibitor,
the inhibitor concentration will increase until the remaining aqueous phase is inhibited. This self-inhibited
aqueous phase will wet the hydrate particles, giving raise to the characteristic term of “sticky” hydrate
particles. The aqueous layer surrounding the hydrate particles will form liquid bridges, by capillary
attractive forces, upon contact with other hydrate particles or the pipe wall. During the hydrate formation
period, there is also a possibility that some of the liquid bridges are converted to solid ones, strengthening
the agglomerates. Depending on the oil-water interfacial tension, the phase ratio between the aqueous phase
and the solid hydrates and the conversion of liquid bridges to solid ones, this leads to increased plugging
risk at lower concentrations of inhibitor (< 20 wt%) and reduced risk at higher concentrations as compared
to the uninhibited system.
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PHYSICAL PROPERTIES OF REPRESSURIZED SAMPLES RECOVERED DURING THE 2006 NATIONAL GAS HYDRATE PROGRAM EXPEDITION OFFSHORE INDIAWinters, W.J., Waite, W.F., Mason, D.H., Kumar, P. 07 1900 (has links)
As part of an international cooperative research program, the U.S. Geological Survey (USGS) and
researchers from the National Gas Hydrate Program (NGHP) of India are studying the physical
properties of sediment recovered during the NGHP-01 cruise conducted offshore India during
2006. Here we report on index property, acoustic velocity, and triaxial shear test results for
samples recovered from the Krishna-Godavari Basin. In addition, we discuss the effects of sample
storage temperature, handling, and change in structure of fine-grained sediment.
Although complex, sub-vertical planar gas-hydrate structures were observed in the silty clay to
clayey silt samples prior to entering the Gas Hydrate And Sediment Test Laboratory Instrument
(GHASTLI), the samples yielded little gas post test. This suggests most, if not all, gas hydrate
dissociated during sample transfer. Mechanical properties of hydrate-bearing marine sediment are
best measured by avoiding sample depressurization. By contrast, mechanical properties of
hydrate-free sediments, that are shipped and stored at atmospheric pressure can be approximated
by consolidating core material to the original in situ effective stress.
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THE MOHICAN CHANNEL GAS HYDRATE ZONE, SCOTIAN SLOPE: GEOPHYSICAL STRUCTURECullen, Janette, Mosher, David C., Louden, Keith 07 1900 (has links)
The Scotian margin of the east coast of Canada has a large theoretical gas hydrate stability zone
(GHSZ) yet review of extensive industry seismic data reveals a prominent BSR at only one location. 3D
seismic reflection and long offset (9 km) pre-stack 2D multichannel seismic data were used to study the
velocity structure and geophysical characteristics of the hydrate zone and surrounding regions. The
Mohican Channel study area shows a unique double BSR at 300 to 450 m below the seafloor in the western
section of the study area immediately adjacent to the Mohican Channel in a water depth range of 1500-
1930m. The topmost BSR (BSR 1) is the more extensive of the two covering an area of 150 km2 in the 3D
volume and a calculated area of 280 km2 using 2D industry and single-channel seismic profiles outside of
the study area. BSR 2 covers an area of ~50 km2 and occurs approximately 80m below BSR 1. A system of
polygonal faults is prominent in the area and some faults appear as conduits for gas leakage into the GHSZ.
Fluid escape features are common on the surface of BSR 1 but rare on the seafloor suggesting that fluid
flux is at lower levels than in the past.
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STRESS AND GAS HYDRATE-FILLED FRACTURE DISTRIBUTION, KRISHNA-GODAVARI BASIN, INDIACook, Ann, Goldberg, David 07 1900 (has links)
In this research, we study high resistivity fractures found in unconsolidated clay sediments on
logging-while-drilling borehole resistivity images from Indian continental margin collected
during the National Gas Hydrate Program Expedition 01. These fractures, found at Sites 5, 6, 7,
and 10 are likely filled with natural gas hydrate. Gas hydrate is identified on borehole logs and
images as high resistivity responses without associated density increases or indications of free
gas. The local state of stress at the time of fracturing can be determined by fracture orientations.
In Holes 5A, 5B, 6A an 7A the gas hydrate-filled fractures have an aligned, preferred orientation
likely associated with a local stress regime. At Site 10, where 130 m of gas hydrate-filled
fractures were observed, fracturing is chaotic, likely due to high gas flux.
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REGIONAL VERSUS DETAILED VELOCITY ANALYSIS TO QUANTIFY HYDRATE AND FREE GAS IN MARINE SEDIMENTS: THE SOUTH SHETLAND MARGIN CASE STUDYTinivella, Umberta, Loreto, Maria F., Accaino, Flavio 07 1900 (has links)
The presence of gas hydrate and free gas within marine sediments, deposited along the South Shetland margin, offshore the Antarctic Peninsula, was confirmed by low and high resolution geophysical data, acquired during three research cruises. Seismic data analysis has revealed the presence of a bottom simulating reflector that is very strong and continuous in the eastern part of the margin. This area can be considered as a useful site to study the seismic characteristics of sediments containing gas hydrate, with a particular focus on the estimation of gas hydrate and free gas amounts in the pore space. Pre-stack depth migration and tomographic inversion were performed to produce a regional velocity field of gas-phase bearing sediments and to obtain information about the average thickness of gas hydrate and free gas layers. Using these data and theoretical models, the gas hydrate and free gas concentrations can be estimated. Moreover, the common image gather semblance analysis revealed the presence of detailed features, such as layers with small thickness characterised by low velocity alternating with high velocity layers, below and above the bottom simulating reflector. These layers are associated with free gas trapped within the hydrate stability zone and deeper sediments. Thus, the use of the detailed and the regional velocity field analysis is important to give a more reliable estimate of gas content in the marine sediments.
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EFFECT OF GRAIN CHARACTERISTICS ON THE BEHAVIOUR OF DISSEMINATED METHANE HYDRATE BEARING SEDIMENTSKingston, Emily, Clayton, Chris R.I., Priest, Jeffery, Best, Angus I. 07 1900 (has links)
Results of seismic surveys are routinely used to assess the presence of methane hydrate in deep
ocean sediments. Accurate estimates of hydrate distribution and volume within the sediment are
required to assess the potential of gas hydrate as an energy resource, driver for climate change or
as a geotechnical hazard. However, seismic velocity may be affected not only by the quantity and
morphology of the hydrate, but also by the properties of the host sediment, for example its
particle size distribution and grain shape. This paper reports the results of experiments conducted
to determine dynamic geophysical properties such as compressional wave velocity (Vp), shear
wave velocity (Vs) and their respective attenuation measurements (Qp
-1 and Qs
-1) of specimens
with varying amounts of disseminated methane hydrate within materials with different particle
shapes and sizes. The results show that the impact of disseminated hydrate is affected both by
mean particle size and by particle sphericity, with the surface area of the sediment grains
influencing the spread of hydrate throughout a material and therefore it’s bonding capabilities.
The sediments with 10% hydrate content show the highest surface areas correspond to the least
increase in seismic velocity while sediments with low surface areas gives the most.
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