Characterization of Gas Hydrate Formation and Its Impact on Slope Stability in Offshore Environments

Natural gas hydrates (NGH) are predominantly methane hydrates formed at high pressure and low temperature conditions typically in submarine sediments and permafrost. This dissertation is focused on the submarine methane hydrates formed by upward migration of free and dissolved gas. The methane gas stored within natural hydrates has more energy potential than all the combined oil and gas resources around the world. Numerical simulation of fluid flow, heat transport, and geomechanical stresses is used to characterize and evaluate the saturation, distribution, reservoir quality, and geohazards associated with hydrate-bearing sediments. Coupled thermo-hydro-mechanical (THM) simulation of hydrate formation, dissociation, and slope stability can be used to characterize submarine hydrate reserves and predict conditions for submarine subsidence and slope failures. Three case studies on hydrate formation and dissociation are performed with field data from Southern Hydrate Ridge and Northern Cascadia Margin in the Pacific continental margin, and Green Canyon block 955 on the Gulf of Mexico. Formation of hydrates in the Southern Hydrate Ridge is simulated testing three cases of free and dissolved gas migration and a combination of faults acting as fluid migration pathways. In the Northern Cascadia Margin, coupled THM simulations assess the possibility of slope failure under climate change conditions, production of methane gas by depressurization, and dynamic loading. The simulated hydrate dissociation caused significant slope failures during hypothetical long-term production, and under dynamic loading. The Gulf of Mexico Green Canyon block 955 was characterized by THM modeling and test cases on bottom water temperature rise and production by depressurization for 180 days are modeled. The results indicate that the Green Canyon site can be a feasible production site for methane gas with low depletion and minor mechanical displacement. Hydrate resource potential can be evaluated, estimated, and characterized in detail with reservoir simulation, which can present us with information that is not available through well logging, seismic imaging, or core testing in the laboratory. It can help identify the resources that are not evident from large scale field expeditions, assess the reservoir quality, production potential and geohazards associated with submarine hydrate reserves