Dissertations / Theses on the topic 'Hydrate Saturation'

Gas hydrates are ice–like compounds found in marine sediments and permafrosts. A significant fraction of all known hydrocarbons in nature is in the form of hydrate. Gas hydrates are a potential energy resource, with possible roles in seafloor slope stability and climate change. As such, improved geophysical methods are needed to identify and quantify in situ natural hydrates to better study their potential impacts. Current estimates of the distribution and volume of gas hydrates vary widely, by orders of magnitude, largely because of uncertainties in geophysical inversion results. The presence of hydrate affects the geophysical properties of the host sediment, creating anomalies that can be detected by seismic or electrical methods measurements. However, the precise relationships between measured geophysical properties and hydrate content (and distribution) are not fully understood, leading to uncertainties in hydrate estimates. Previous studies have shown that both the hydrate saturation (content) and its distribution (morphology or habit) affect the geophysical properties of the host sediment, and separating these effects presents a challenge to geophysical data interpretation. As this knowledge is generally required to interpret field data, this thesis instead seeks to gain this understanding from controlled laboratory experimental studies. I studied laboratory hydrate formation and dissociation in Berea sandstone and Leighton Buzzard sand to understand their effect on P- and S-wave velocities and attenuations, and on electrical resistivity. I used high resolution synchrotron radiation X-ray tomography (SRXCT) to visualize the pore-scale evolution of hydrate morphology with saturation. These observations could be important for seismic data interpretation in terms of hydrate content and sediment strength, which are needed for natural resource and geohazard assessments (also for joint seismic and electromagnetic survey data interpretation). Hence, I was able to observe how hydrate distribution within the pores (morphology or habit) changes with hydrate formation and dissociation, and how these changes affect the P- and S-wave velocities and attenuations. I calculated hydrate saturation continuously from changes in pressure and temperature and independently from electrical resistivity during hydrate formation and dissociation. I applied a new rock physics model to relate P- and S-wave velocities and attenuations with changes in hydrate saturation and morphology. I found that not all the gas formed hydrate, even when the system was under hydrate stability conditions with excess water. The synchrotron CT results suggest that the dominant mechanism for co-existing gas is the formation of hydrate films on gas bubbles; these bubbles either rupture, releasing trapped gas, or remain trapped within an aggregate of hydrate grains. From a geophysical remote sensing perspective, such co-existing gas could cause errors in hydrate saturation estimates from electrical resistivity as both gas and hydrate are resistive compared to saline pore fluid. I saw that hydrate starts forming in the pore-floating morphology (where hydrate grains are surrounded by brine) and evolves into the pore-bridging morphology (where hydrate connects mineral grains). Eventually, hydrate from adjacent pores joins and forms a pore hydrate framework, interlocking with the sand grain framework and separated by thin water films. I was able to relate these changes in morphology to our elastic wave measurements using the HBES (Hydrate Bearing Effective Sediment) rock physics model. For low hydrate saturations, both P and S wave velocity follows the pore-floating model curve. As hydrate formation continues, the P-wave velocity follows the pore-bridging model curve, similar to other studies. In contrast, the S-wave velocity was lower than the pore-bridging model but higher than the pore-floating model curves. I think that the presence of water films between hydrate and the rock frame inhibited the ability of pore-bridging hydrate to increase the frame shear modulus. The higher S-wave velocity than the pore-floating model predictions is likely due to interlocking rock and pore-bridging hydrate frameworks. The magnitude of relative changes in attenuation is much higher than that of velocity due to changes in hydrate content and distribution. Elastic wave attenuation frequency spectra between 448 and 782 kHz show systematic and repeatable changes during hydrate formation and dissociation. In our experiments, the dominant mechanism of attenuation and velocity changes with an increase in hydrate saturation is (i) a decrease in methane gas bubble radius and (ii) an increase in secondary porosity with hydrate formation. The accurate measurement of both velocity and attenuation at multiple frequencies in the pulse-echo system allow us to constrain the dominant attenuation mechanisms using the HBES rock physics model. Overall, I conclude that hydrate-sediment systems are complex with interlocking solid hydrate aggregate and host grain frameworks separated by water films, with isolated pockets of gas within the hydrate. Such an interlocking pore hydrate framework and co-existing gas, if widespread in nature, should be considered in hydrate quantification from elastic wave velocities. For more reliable estimates of in situ hydrate, multiple geophysical parameter measurements are required (e.g., P and S wave velocities and attenuation, electrical resistivity, and at multiple frequencies), and hydrate estimates from seismic velocities alone could lead to significant errors at low hydrate saturations (< 40%).

L'analyse de données de sismique réflexion 3D sur la marge active de Nankai (Japon) a permisde mettre en évidence le BSR (bottom simulating reflector) et le double BSR. Le BSR est un contrastedimpédance acoustique à linterface séparant les sédiments riches en hydrates de gaz de forte vitesseau dessus et les sédiments riches en gaz libre en dessous. Le double BSR peut être considéré commeun BSR fossile ou résultant d'un mélange dans les sédiments des gaz de composition différente. LeBSR est par suite utilisé pour contraindre le régime thermique dans la boîte 3D (5km x 42. 5 km) de lamarge de Nankai. Le flux de chaleur calculé à partir des profondeurs du BSR donne des valeurscomprises entre 20-68 mW/m2. Des fortes amplitudes de BSR sont localisées dans les zones où le fluxde chaleur est relativement faible, et des faibles amplitudes du BSR sont par contre localisées dans leszones où le flux de chaleur est relativement important. La circulation des fluides chauds perturberaitl'amplitude du BSR. Par ailleurs, le BSR est absent au voisinage de la faille de Tokai dans la zone dubassin de pente, et est discontinu tantôt absent au niveau de la faille de Kodaiba dans la zone du bassindavant-arc. Dans la zone du bassin davant-arc où la distribution du BSR est plus importante, lesrésultats de linversion des formes d'onde ont permis de confirmer la présence des zones à fortevitesse (en rapport avec les hydrates de méthane) au dessus du BSR et des zones à faible vitesse (enrapport avec le gaz libre) en dessous du BSR. La présence du gaz libre sous jacent augmenteraitl'amplitude du BSR. La concentration des hydrates de méthane estimée est inférieure à 25 %. Levolume moyen des hydrates de gaz calculé est de 0. 85 km3. La concentration du gaz libre varie entre0. 7 et 8 %. Le volume moyen du gaz libre calculé est de 0. 06 km3. Au regard de la superficie de lazone étudiée, on conclut que ces concentrations/volumes sont énormes mais, ne peuvent constituer unréservoir économiquement exploitable, car les hydrates de gaz sont disséminés dans les sédiments
The analysis of 3D seismic reflection data on the Nankai (Japan) active margin showed evidenceof a BSR (bottom simulating reflector) and a double BSR. The BSR is an acoustic impedance contrastat the interface separating sediments rich in gas hydrate, having a high velocity above, and sedimentsrich in free gas, having a low velocity below. The double BSR can be considered as a fossil BSR orcan result from a mixture of gases of different compositions within the sediments. The BSR depth isused to constrain the thermal regime in the 3D box (5 km x 42. 5 km) of the Nankai margin. The heatflow calculated from BSR depths gives values between 20-68 mW/m2. Strong BSR amplitudes arelocalized in the zone where the heat flow is relatively low, and weak BSR amplitudes are localized inthe zone where the heat flow is relatively high. The circulation of warm fluids would perturb theamplitude of BSR. The BSR is absent around the Tokai fault in the slope basin zone, and issometimes discontinuous or absent around the Kodaiba fault in the forearc basin zone. In the forearcbasin where the distribution of the BSR is more important, full waveform inversion results allowed toconfirm the presence of a zone with high velocity above the BSR, which could be due to the presenceof gas hydrate in sediments. Just below the BSR, we find a low velocity zone, which could be due tothe presence of the free gas in sediments. Strong BSR amplitude could be correlated with the presenceof underlaying free gas. The estimated concentration of gas hydrate is lower than 25 %. The meanvolume of gas hydrate calculated is about 85 x 107 m3. The estimated concentration of free gas variesbetween 0. 7 and 8 %. The mean volume of free gas calculated is about 6 x 107 m3. In the study area,we conclude that these concentrations/volumes are enormous but, they cannot constitute aneconomically exploitable reservoir, because gas hydrates are disseminated in the sediments

Kyoto University (京都大学)
0048
新制・課程博士
博士(農学)
甲第19046号
農博第2124号
新制||農||1032(附属図書館)
学位論文||H27||N4928(農学部図書室)
31997
京都大学大学院農学研究科応用生命科学専攻
(主査)教授 小川 順, 教授 加納 健司, 教授 植田 充美
学位規則第4条第1項該当

Many Arctic gas hydrate reservoirs such as those of the Prudhoe Bay and Kuparuk River area on the Alaska North Slope (ANS) are believed originally to be natural gas accumulations converted to hydrate after being placed in the gas hydrate stability zone (GHSZ) in response to ancient climate cooling. A mechanistic model is proposed to predict/explain hydrate saturation distribution in “converted free gas” hydrate reservoirs in sub-permafrost formations in the Arctic. This 1-D model assumes that a gas column accumulates and subsequently is converted to hydrate. The processes considered are the volume change during hydrate formation and consequent fluid phase transport within the column, the descent of the base of gas hydrate stability zone through the column, and sedimentological variations with depth. Crucially, the latter enable disconnection of the gas column during hydrate formation, which leads to substantial variation in hydrate saturation distribution. One form of variation observed in Arctic hydrate reservoirs is that zones of very low hydrate saturations are interspersed abruptly between zones of large hydrate saturations. The model was applied on data from Mount Elbert well, a gas hydrate stratigraphic test well drilled in the Milne Point area of the ANS. The model is consistent with observations from the well log and interpretations of seismic anomalies in the area. The model also predicts that a considerable amount of fluid (of order one pore volume of gaseous and/or aqueous phases) must migrate within or into the gas column during hydrate formation. This work offers the first explanatory model of its kind that addresses "converted free gas reservoirs" from a new angle: the effect of volume change during hydrate formation combined with capillary entry pressure variation versus depth. Mechanisms by which the fluid movement, associated with the hydrate formation, could have occurred are also analyzed. As the base of the GHSZ descends through the sediment, hydrate forms within the GHSZ. The net volume reduction associated with hydrate formation creates a “sink” which drives flow of gaseous and aqueous phases to the hydrate formation zone. Flow driven by saturation gradients plays a key role in creating reservoirs of large hydrate saturations, as observed in Mount Elbert. Viscous-dominated pressure-driven flow of gaseous and aqueous phases cannot explain large hydrate saturations originated from large-saturation gas accumulations. The mode of hydrate formation for a wide range of rate of hydrate formation, rate of descent of the BGHSZ and host sediments characteristics are analyzed and characterized based on dimensionless groups. The proposed transport model is also consistent with field data from hydrate-bearing sand units in Mount Elbert well. Results show that not only the petrophysical properties of the host sediment but also the rate of hydrate formation and the rate of temperature cooling at the surface contribute greatly to the final hydrate saturation profiles.
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Gas hydrate can precipitate in pore space of marine sediment when gas concentrations exceed solubility conditions within a gas hydrate stability zone (GHSZ). Here we present analytical expressions that relate the top of the GHSZ and the amount of gas hydrate within the GHSZ to the depth of the sulfate-methane transition (SMT). The expressions are strictly valid for steady-state systems in which (1) all gas is methane, (2) all methane enters the GHSZ from the base, and (3) no methane escapes the top through seafloor venting. These constraints mean that anaerobic oxidation of methane (AOM) is the only sink of gas, allowing a direct coupling of SMT depth to net methane flux. We also show that a basic gas hydrate saturation profile can be determined from the SMT depth via analytical expressions if site-specific parameters such as sedimentation rate, methane solubility and porosity are known. We evaluate our analytical model at gas hydrate bearing sites along the Cascadia margin where methane is mostly sourced from depth. The analytical expressions provide a fast and convenient method to calculate gas hydrate saturation for a given geologic setting.

Downhole acoustic data were acquired in very low-velocity, hydrate-bearing formations at five sites drilled on the Cascadia Margin during the Integrated Ocean Drilling Program (IODP) Expedition 311. P-wave velocity in marine sediments typically increases with depth as porosity decreases because of compaction. In general, Vp increases from ~1.6 at the seafloor to ~2.0 km/s ~300 m below seafloor at these sites. Gas hydrate-bearing intervals appear as high-velocity anomalies over this trend because solid hydrates stiffen the sediment. Logging-while-drilling (LWD) sonic technology, however, is challenged to recover accurate P-wave velocity in shallow sediments where velocities are low and approach the fluid velocity. Low formation Vp make the analysis of LWD sonic data difficult because of the strong effects of leaky-P wave modes, which typically have high amplitudes and are dispersive. We examine the frequency dispersion of borehole leaky-P modes and establish a minimum depth (approx 50-100 m) below the seafloor at each site where Vp can be accurately estimated using LWD data. Below this depth, Vp estimates from LWD sonic data compare well with wireline sonic logs and VSP interval velocities in nearby holes, but differ in detail due to local heterogeneity. We derive hydrate saturation using published models and the best estimate of Vp at these sites and compare results with independent resistivity-derived saturations.

A series of studies have been carried out to elucidate the sediment effect on the saturation level of methane hydrate in sediments. The specimens tested covered most of the natural sediment types, with various combinations of particle size and mineral composition. The results obtained indicate that particle size and clay contents are the two key factors determining the saturation level of gas hydrate in sediments: the finer the particle size and/or the higher the clay content, the lower the hydrate saturation. The observed particle size effect and clay effect on hydrate saturation can be accredited to the specific surface area of a sediment.

The pore saturation of natural gas hydrate in sediments is a key parameter for estimating hydrate resources in a reservoir. For a better understanding of gas hydrate distribution, the experimental study of the pore saturation of methane hydrate in sediments from a hydrate reservoir in the Eastern Nankai Trough have been carried out. In total, eleven samples, comprising sand, silty sand, silt, and representative of the main sediment types identified in the Eastern Nankai trough, were tested. The results obtained clearly indicate a particle size and clay content dependent trend: almost 100% of pores were saturated with methane hydrate in sand when little silt and clay were present, decreasing to ~ 13% in silty sand (sand 54%, silt 41% and clay 5%), and ~ 4% in clayey silt. These results are generally consistent with NMR logging results for high-saturation samples, but somewhat different for samples with medium or low saturation levels.

碩士
國立中央大學
地球物理研究所
99
Interest in natural gas hydrates has grown in recent years with the recognition of the gas-hydrate bearing sediments offshore the southwestern Taiwan. Several oceanographic cruises were conducted in the aim of acquiring more information about the presence of gas hydrate. The collected reflection and refraction seismic data are extensively used to map and interpret the gas hydrate zones for the area of the Hengchun Ridge deformation front, accretionary wedge of the eastern subducting Maniala subduction zone, and the passive China continental Margin. Among them, a wide angle seismic experiment consisted of 24 recovered Ocean Bottom Seismometers (OBSs) were acquired along 4 in-line transects offshore southwestern Taiwan in November 2004 and several off-lines and cross-lines data were recorded simultaneously. In this study, we use PStomo_eq software to perform a 3-D acoustic inversion velocity analysis of the first arrivals records of the OBS by using all the in-lines, off-lines and cross-lines information. By analysing the inverted 3-D dimensional Vp models, we estimate the density and the thickness of the marine sediments and crust. Based on the core data, the porosity, the lithology and other physical properties were assumed and the Weighting equation is applied for the estimation of the gas hydrate concentration. In our study , the gas hydrate saturation is about 10%~30%, most of them are concentrated below the Yuan-An ridge and the anticlines.

碩士
臺灣大學
海洋研究所
98
The area offshore southwestern Taiwan is the place where active Luzon accretionary wedge province meets the passive China continental margin province. Not only marine seismic reflection data indicate that large amount of gas hydrates and free gases may exist beneath the seafloor sediments, but geochemical data also reveal high methane flux and shallow sulfide-methane interface (SMI) at many cored locations. In this study, we intend to investigate detail distribution and possible formation and migration of gas hydrates in both active and passive continental margins. We implement pre-stack depth migration technique to derive depth sections with accurate velocity information from large-offset multichannel seismic data collected during the 2009 TAIGER survey. We focus our study on the Yung-An Ridge and the Formosa Ridge areas in active and passive margin settings, respectively. Large variations in acoustic velocities are observed in both areas where BSRs are prominent. The high velocity values derived in shallow sedimentary layers above BSR range from 1750 to 2000 m/s, and this high velocity zone may indicate the existence of gas hydrates. The low velocity values ranging from 1450 to 1550 m/s below BSR are interpreted to be the free gases zone. Through rock physics modeling, we establish the relationships between velocity and gas hydrate saturation values. Estimated gas hydrate saturation values suggest that gas hydrate could occupy up to 50 % of pore space in the Yung-An Ridge area and 35 % of pore space in the Formosa Ridge area. Comparing seismic images of the constructed depth sections with corresponding velocity models, we suggest that the distribution of gas hydrates and free gases in the Yung-An Ridge area are controlled by complex fault systems and porous sand layers. Distribution of gas hydrates and free gases in the Formosa Ridge area are controlled by submarine erosion and mass movement processes and development of faults and fractures in the Formosa Ridge area.

碩士
國立臺灣海洋大學
應用地球科學研究所
101
In this study, we analyze four-component (hydrophone, vertical and two horizontal components) data recorded from 25 ocean-bottom seismometers (OBS) in 2008 for investigating converted shear waves and for estimating gas-hydrate saturation west of the Yuan-An Ridge off SW Taiwan. Firstly, initial Vp/Vs models west of the Yuan-An Ridge were constructed for P-wave velocity models based on the previous studies. Subsequently, travel time and amplitude of the P-S converted waves were synthesized based on initial Vp/Vs models and were superimposed on four-component OBS data for determining the phases of P-S converted waves. Secondly, we picked the travel times of the P-S converted waves from the four-component OBS data to invert the Vp/Vs model. Finally, we reduce the travel time errors to obtain a better Vp/Vs model. Furthermore, the gas-hydrate saturation was estimated by the four-phases four-materials (water, free gas, hydrates and rock matrix) of Wood’s equation based on the P-wave velocity (1.80-1.95 km/s) and the Vp/Vs (3.18-3.38) of gas-hydrate layer. Therefore, hydrate saturation of 5 -17% are estimated in the west sedimentary basins at the Yuan-An Ridge. Since the Vp/Vs ratio west of Yuan-An Ridge in the second sedimentary layer is higher than that in the other layers, free gas may be migrated along the thrust faults and to accumulate the hydrate below the Yuan-An Ridge.