The interdependence of lithologic heterogeneity and methane migration on gas hydrate formation in marine sediments

• Main Authors: Nole, Michael Anthony, 0000-0003-4696-5791
• Format: Article in Journal/Newspaper
• Language: English
• Published: The University of Texas at Austin 2021
• Subjects: Methane hydrate; Marine geoscience; Numerical simulation; Reservoir simulation
• Online Access: https://dx.doi.org/10.26153/tsw/11654; https://repositories.lib.utexas.edu/handle/2152/84682

Despite the ubiquity of methane hydrate in the pore space of shallow marine sediments worldwide, the processes governing the transport of methane from source to reservoir are still poorly understood. Methane migration mechanisms constitute important links in the evolution of a natural gas hydrate system because they control how gas hydrate distributes in sediment pore space as well as the quantities in which gas hydrate forms. Without a thorough understanding of methane migration, it is impossible to accurately predict how a methane source interacts with a reservoir, which makes it very difficult to reliably predict where hydrate will form in a given environment. When trying to understand a gas hydrate system as a potential natural gas prospect, as a geohazard, or as an agent of global climate change, it is essential to accurately characterize the distribution and volume of hydrate present. Thus, methane migration mechanisms must be properly understood if a hydrate system is to be evaluated for any of these purposes. The work presented here develops 3D, multiphase, multicomponent fluid transport simulation software to investigate the impact of three different methane migration mechanisms on the transport dynamics and distribution of gas hydrate in marine geosystems: diffusion, short-range advection, and methane recycling. I find that the expressions of gas hydrate systems in nature are sensitive to small-scale heterogeneities in sediment lithology and capillary effects. Properties of a hydrate-bearing unit including pore size distribution, unit thickness, dip, and proximity to other layers in multilayered systems all contribute to preferential flux of methane toward and within certain hydrate-bearing sediment strata, which impact the distribution of hydrate throughout these units. When sediments are overpressured, permeability contrasts can focus methane-charged fluids along high permeability pathways and precipitate hydrate through short-range advection. Capillary phenomena can produce a region near the base of the hydrate stability zone where hydrate, water, and free gas coexist over a range of pressures and temperatures, which can drive recycling of free gas derived from dissociated hydrate back into the hydrate stability zone