Natural gas hydrate systems as pressure valves: a multi-scale study

GSTDTAP

项目编号	NE/R016615/1
	Natural gas hydrate systems as pressure valves: a multi-scale study
	Joe Cartwright
主持机构	University of Oxford
项目开始年	2018
	2018-01-08
项目结束日期	2018-07-07
资助机构	UK-NERC
项目类别	Research Grant
项目经费	50147(GBP)
国家	英国
语种	英语
英文摘要	In methane-rich geological environments, methane and water can combine at high pressures and low temperatures to generate natural gas hydrates, a solid compound that usually fills the primary and secondary pore space of the sediment. Beneath areas of gas hydrate occurrence, a zone characterised by free gas-bearing sediments is usually observed in seismic reflection data. The presence of gas hydrates is thought to profoundly influence the fluid dynamics, due to the reduction of the pore space available for fluid flow. Therefore, free gas can be mechanically trapped at the base of the gas hydrate stability zone (BGSHZ) and result in overpressure generation. Therefore, evaluating the distribution of free gas and gas hydrates at the BGHSZ is crucial to identify the existence of a pressure valve capable of contributing to the movement of submarine landslides and induce an episodic release of free gas towards the seafloor. These events constitute major geohazard issues, as well as critical processes in the carbon cycle dynamics at the interface between the seabed and the underlying sedimentary column. The integration of three-dimensional seismic, petrophysical well-log and core data is the best approach to assess such scenario. However, to date, such integrated studies are relatively rare, and there are only a few boreholes which successfully penetrate both a submarine landslide and the BGHSZ. The Tuaheni landslide complex (TLC) is a submarine landslide exhibiting characteristics attributable to creeping movements identified in the northern portion of the Hikurangi Margin, offshore New Zealand. Compelling seismic evidence indicates that part of the TLC is located just above the BGHSZ. The IODP Expedition 372 will evaluate the role of gas hydrates and free gas in the genesis and movement of the TLC by acquiring well-log and core data at three sites, located in the extensional and in in the compressional domain of the slide, as well as in a 'background' site, outside the TLC. Well-log and core data will be further jointly acquired by the Expeditions 372 and 375 from a gas hydrate stability zone located in a completely different setting, at the border of the forearc basin of the Hikurangi Margin. Seismic evidence suggests that free gas, and therefore potential overpressured zones, occur both below the TLC and at the border of the forearc basin. The available three-dimensional seismic data will be used to map the possible location of overpressured zones, generated by the fluid flow of methane-rich fluids and the accumulation of free gas. The well-log data acquired onboard will instead provide essential information on the gas hydrate and free gas distribution, as well as on the variation of pore fluid pressure with depth. Finally, the retrieved cores will provide clues on the porosity, permeability and compositional structure (i.e. the reservoir quality) of the gas hydrate and free gas-bearing sediments. For example, the observation of sedimentary layers with high hydrate saturation near the BGHSZ could indicate the existence of a barrier to fluid flow. Conversely, the presence of a fracture network and methanogenic minerals across the drilled intervals will suggest that overpressure has been released in the past and methane has migrated towards shallower depth. Considering such amount of data, the principal aim of this proposed research is to assess the role of hydrates and free gas in accumulating and releasing overpressures at present-day and in the past, with a specific focus at the BGHSZ. These overpressures could trigger or influence the movement of submarine landslides but, more generally, can profoundly impact fluid-flow in shallow sediments.
来源学科分类	Natural Environment Research
文献类型	项目
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/87009
专题	环境与发展全球科技态势
推荐引用方式 GB/T 7714	Joe Cartwright.Natural gas hydrate systems as pressure valves: a multi-scale study.2018.