Buoyancy Driven Flow and Methane Hydrate Systems

Abstract

Marine gas hydrates have been studied intensely for more than five decades under the assumption that deep ocean basins lack the methane necessary to generate significant volumes of gas hydrates. Contrary to this, the deep waters of the Aleutian Basin in the Bering Sea alone are estimated to contain globally significant volumes of methane in gaseous and hydrate forms in velocity-amplitude (VAMP) structures. After a brief introduction to hydrate stability and seismic data acquisition, the Dissertation is divided into four parts:

Part 1. A unified effective medium model is developed to incorporate the endpoints of perfectly smooth and infinitely rough sphere components, and allow partitioning between rough and smooth grains. We incorporate the unified model into the framework for gas hydrates in unconsolidated sediments using both pore-fluid and rock matrix configurations for grain placement. The model resolves conflicting results of previous investigation from the 2002 Mallik gas hydrates projects.

Part 2. Conventional semblance for seismic velocity analysis does not have the resolving power of subspace methods due to the inclusion of the noise-signal space in conventional semblance. After nearly three decades, subspace techniques still receive little use in seismic applications due to high computational costs. We develop an approach for seismic velocity spectra based on computing the temporal covariance data matrix as an intermediate step to efficiently compute the Eigen vectors of the spatial covariance data matrix.

Part 3. The use of single channel far offset seismic images is investigated for what appears to be a more reliable, cost-effective indicator for the presence of bottom simulating reflectors than traditional CDP processing or AVO analysis. This non-traditional approach is taken to be more relevant to gas hydrate imaging. Results indicate BSRs are more easily identifiable from single channel far offset seismic images than from traditional CDP displays.

Part 4. The Aleutian Basin, though atypical from the traditional model of marine gas hydrates, provides a unique opportunity to investigate the role of buoyancy driven flow in deep water sediments and marine gas hydrate deposits. Evidence of large subbottom "VAMP" structures, abundance of structures, and presence of bottom-simulating reflectors, suggest cellular convection within the Aleutian Basin. We provide a basic stability analysis to calculate the Rayleigh-Darcy number for methane in a porous medium heated from below.