Numerical Modelling of the Response of Marine Gas Hydrate Bearing Sediment to Dissociation after Depressurisation: Case Studies for Tectonic Uplift, Sea Level Lowering, and Gas Hydrate Production

Abstract

This thesis is a conceptual study of the thermodynamic controls on gas hydrate dissociation and formation from natural geological and human-driven causes at different time scales. I focused on understanding and shedding light on the effects of gas hydrate dissociation as it affects climate, seafloor stability, biodiversity at the seafloor, and gas production from gas hydrate geological systems. This research is premised into three subheadings: (1) Investigating the response of hydrate-bearing sediment lithologies (sand, silt, and clay) from the base of the gas hydrate stability zone, Nankai Trough offshore Japan to tectonic uplift. (2) Investigating the behaviour of sub-surface fluid flow in hydrate reservoirs with varying overburden permeabilities to depressurisation, located within the Hikurangi Margin offshore of New Zealand. (3) Investigating the possible role of gas hydrate dissociation during Pleistocene sea-level and bottom-water-temperature changes in the formation of pockmarks on the Chatham Rise, New Zealand. These studies were investigated using thermohydraulic modelling of gas hydrate dissociation and formation using the academic code, TOUGH+HYDRATE v1.5. Geological processes such as tectonic uplift have been used to explain hydrate accumulation caused by hydrate recycling and the occurrence of double bottom-simulating reflectors in tectonically active areas, e.g. the Nankai Trough Japan, Hikurangi Margin New Zealand etc. with little is known about the underlying mechanism behind its formation. In this study, I investigated the response of hydrates to tectonic uplift over the last 8 kyrs (8,000 years). I assumed that the effect of a pressure decreases from uplift started when the pressure increases from rapid post-glacial sea-level tapered off _8,000 years ago. My results show that during uplift, hydrate dissociates and fluids (water and free gas) released migrates towards the seafloor forming a shallow gas hydrate layer. Gas hydrate, water and free gas co-exist over a thick layer in which excursions in salinity, pore water temperature, and pore pressure keep pressure-temperature conditions at the phase boundary. For most scenarios, hydrate was still detected at the end of the model runs after _8000 years. In the last decade, several studies have been focusing on production of methane gas from methane hydrates bearing sediments and geo-mechanical related issues such as compaction and subsidence of the reservoir during fluid (gas and water) withdrawal. The role of fluid flow at the seafloor during natural gas production from hydrates has not been studied. Little importance has been put on effect of depressurisation on overburden permeabilities. In this study, I investigated what happens to fluid flow at the seafloor. The motivation was to study whether there might be an effect on chemosynthetic communities living on the seafloor. In the natural gas production from gas hydrate simulation, I varied the permeability of the overburden between silts and clays. I observed that increasing overburden permeability to that of silts leads to sea water intrusion into the reservoir. This scenario can have significant implications for seafloor biologic communities and could potentially constitute a major challenge for possible gas hydrate production. Seafloor instability and depressions such as pockmarks in the Chatham Rise, New Zealand, have been linked to hydrate dissociation in shallow waters during the glacial-interglacial periods. In this study, I investigated the response of hydrates bearing system in the Chatham Rise, to changes in Plio-Pleistocene sea-level and ocean bottom temperature to determine whether gas from hydrate dissociation could cause pockmarks in shallow waters and increase of gas-release during the last glacial low stands. The Chatham Rise study stretched over 120,000 years. My main finding show that, gas is released at shallower water depth at increase bottom water temperatures, which relates to the position of the large pockmarks in the Chatham Rise. However, the results from paleoceanographic studies indicate that bottom temperatures may have varied locally. These temperature changes may have a more significant effect on the shallow gas hydrate system in the study area than the relatively gradual decrease of pressure associated with sea-level lowering. In summary, my thesis show that depressurisation of seafloor depth for permeable (sand and silt) shallow hydrate-bearing sediments from tectonic uplift and sealevel lowering before the last glacial maximum, causes hydrate recycling to occur. This occurrence causes a second hydrate layer above the pre-existing hydrate layer, for the tectonic uplift scenario. For the impermeable (clays) hydrate-bearing sediments, pore pressure increased due to hydrate dissociation and by advective fluid flow, resulting in keeping hydrate in place for thousands of years during tectonic uplift. For the sealevel change scenario, increase in pore pressure could lead to naturally hydrofrack hydrate bearing sediment and could support pockmark formation due to temperature changes. For the natural gas production case, depressurisation could cause an influx of seawater into an unsealed (permeable overburden) hydrate reservoir. This would have implications to the chemosynthesis processes and to the survival of other biodiversity at the seafloor.