Methane hydrates are naturally-occurring ice-like substances found in permafrost and in ocean sediments along continental shelves. These compounds are often the source of cold seeps—plumes which vent methane into aquatic environments, and may subsequently release the potent greenhouse gas into the atmosphere. Methane hydrates and methane gas seeps are of particular interest both for their potential as an energy source and for their possible contribution to climate change. In an effort to improve location of hydrates through the use of seismic surveys and echo-sounding technology, this work aims to describe the low-frequency (10 Hz to 10 kHz) acoustic behavior of methane gas bubbles and methane hydrates in water under simulated ocean-floor conditions of low temperatures and high pressures. Products of the experiments and analysis presented in this thesis include (a) passive acoustic techniques for measurement of gas flux from underwater seeps, (b) a modified form of Wood's model of low-frequency sound propagation through a bubbly liquid containing real gas, and (c) low-frequency measurements of bulk moduli and dissociation pressures of four natural samples of methane hydrates. Experimental procedures and results are presented, along with analytical and numerical models which support the findings. / text

Submitted by Eduarda Figueiredo (eduarda.ffigueiredo@ufpe.br) on 2015-03-12T15:30:43Z No. of bitstreams: 2 Leite_FS-Tese2012-DOCEAN-tt.pdf: 7355705 bytes, checksum: cdd11be57564224083c3a2b8946fb3de (MD5) license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) / Made available in DSpace on 2015-03-12T15:30:44Z (GMT). No. of bitstreams: 2 Leite_FS-Tese2012-DOCEAN-tt.pdf: 7355705 bytes, checksum: cdd11be57564224083c3a2b8946fb3de (MD5) license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Previous issue date: 2012-09-26 / FACEPE / A exploração de óleo e gás vem apresentando um rápido crescimento em regiões de baixa latitude, mesmo assim pouquíssimos experimentos e modelagens envolvendo vazamento de gás têm sido publicados pela comunidade científica. Este estudo foi desenvolvido de modo a aumentar o conhecimento a respeito do comportamento da pluma de gás durante um vazamento acidental em águas rasas. Os métodos usados e os resultados obtidos são apresentados neste estudo, assim como um modelo para simular o transporte e a dispersão de uma pluma de gás liberada em águas rasas. Primeiramente, experimentos de campo foram realizados através da simulação de um vazamento de gás natural a aproximadamente 30 m de profundidade na costa nordeste do Brasil. Quatro cenários distintos, com variadas condições de forçantes geofísicas, foram associados a diferentes fluxos de gás (de 3000 a 9000 L.h-1) e períodos sazonais (verão e inverno). Num segundo estágio, a análise de dispersão da pluma de gás foi realizada com os dados obtidos in situ. O modelo usou um volume de controle lagrangiano para discretização e simulou a evolução da pluma de gás associando a termodinâmica e o impacto desta na hidrodinâmica da pluma de gás. De acordo com os dados coletados, o transporte predominante da corrente ocorreu para sulsudoeste (nordeste) durante o verão (inverno). A diferença no diâmetro da pluma ocorreu principalmente na camada mais próxima à superfície. A pluma de gás deslocou-se para sul-sudoeste no verão e para nordestenorte durante o inverno. Os fluxos de gás liberados no assoalho oceânico pareceram não afetar a hidrodinâmica local. O movimento da pluma foi sempre influenciado pelas forçantes de maré e meteorológica, nesta ordem. Os resultados de modelagem indicaram que, à medida que a pluma sobe na coluna de água, a mesma é deslocada horizontalmente na direção da corrente predominante. A situação extrema estabeleceu um raio crítico (máximo deslocamento horizontal) da fonte de gás de 35,2 m. A comparação entre os dados medidos e os calculados mostrou que o modelo representou satisfatoriamente as principais características da liberação de gás, tais como o deslocamento, o diâmetro e o tempo de ascensão da pluma. Apesar das plumas apresentarem a largura média da mesma ordem de magnitude entre as medições e os cálculos, melhorias podem aumentar o desempenho do modelo durante o desenvolvimento inicial das plumas. Dados importantes e únicos foram coletados durante os vazamentos de gás, os quais contribuíram para a caracterização do comportamento de diferentes fluxos em diferentes períodos. Os experimentos forneceram uma base de dados para um modelo computacional que foi capaz de reproduzir o transporte e a dispersão de uma pluma de gás no ambiente marinho. O modelo foi capaz de prever o transporte e destino do gás liberado no ambiente. O mesmo pode, portanto, ser usado como uma ferramenta para planos de contingência de vazamentos acidentais de gás no oceano. / Underwater oil and gas exploration has been growing fast in low latitude regions, even though very few experimental data acquisition and modeling involving gas release in tropical and shallow waters have been published by the scientific community. This study was developed to increase the knowledge concerning the gas behavior during a subsurface blowout in shallow waters. The methods used and the results obtained from this study are presented, as well as a model to simulate the transport and dispersion of a subsurface gas plume released from shallow waters. At first, field experiments were carried out by simulating a subsurface blowout with natural gas at approximately 30 m depth in the Northeast Brazilian coast. Four distinct scenarios with varied conditions of geophysical forcing were associated with different fluxes (from 3000 to 9000 L.h-1) and seasonal periods (summer and winter). As a second stage, the analysis of the gas plume dispersion was accomplished with the data obtained from the above campaigns. The model used a Lagrangian control volume for discretization and simulated the gas plume evolution, associating thermodynamics and the impact of the thermodynamics on the hydrodynamics of the gas plume. The predominant transport occurred toward the south-southwest (northeast) during the summer (winter) period. The difference in the plume width occurred mainly in the upper surface layer. The gas plume displaced toward the south-southwest (northeast-north) during the summer (winter) period. The gas flow releases seemed not to affect the local hydrodynamics. The plume movement was always influenced by the tidal and meteorological forcings, in that order. The simulation results indicated that, as the gas plume rose in the water column, the same plume was horizontally displaced toward current predominant direction. The extreme situation provided a critical radius (maximum horizontal displacement) from the gas release source of 35.2 m. The comparison between the measured and the calculated data showed that the model satisfactorily represented the main features of the gas release, such as the displacement, diameter and ascending time of the plume. Although the mean plume widths have the same order of magnitude between the measurements and the calculations, improvements may enhance the model’s performance during the earlier plume development. Important and unique data were collected during these subsurface releases, which contributed to the characterization of the behavior of different blowouts in different seasons. The experiments provided a baseline for a computational model capable of reproducing gas plume transport and dispersion in the marine environment. The model was able to predict the gas release transport and fate in the environment. Thus it can be used as a tool for contingency planning of an accidental underwater gas release.