The Influence of Geochemistry of Gas Hydrates on the Shear Strength and Stability of Marine Sediment Movement

Abstract

Methane obtained from hydrate will become a significant energy resource in the near future. It will become a critical sustainable source of energy possibly by 2025. In this inter-disciplinary research paper the influence of Environmental Geochemistry of gas hydrates on the shear strength and stability of marine sediment movement is discussed. Methane, CH4, the most common form of natural gas associated with petroleum reservoirs, is in gaseous state at standard temperature and pressure conditions. But under high pressure and low temperature, it combines with water molecules to form an icy – white compound called methane hydrate. Taking a continental margin (continental shelf, slope and raise) and abyssal depth the geo environment is represented as four zones namely 1. Continental shelf has high temperature and low pressure 2. Continental slope has low temperature low pressure 3. Continental rise with low temperature high pressure 4. Abyssal depth with high temperature and high pressure (it does not exist in nature in the case of gas hydrates). The same zones can be classified geo-technically for sediments as 1. Continental shelf with high angle of internal friction (φ) and low cohesion (c) 2. Continental slope with low angle of internal friction (φ) and low cohesion (c) 3. Continental rise with low angle of internal friction (φ) and high cohesion (c) 4. Abyssal depth with high angle of internal friction (φ) and high cohesion (c) (which does not exist in nature). These four zones are analyzed geotechnically for stability of sediments in the sea floor environment. The inter-disciplinary equation of shear strength and gas hydrate environment is τ = c + σ tan φ where τ = shear strength of the sediment, c and φ are geotechnical parameters and σ = to vertical stress of sea water depth and also sediment thickness above the point of consideration with geo thermal gradient. This paper also deals with the decomposition of gas hydrates which decreases the shear strength, making the sediment move towards failure. Basically soil is a three phase system consisting of solids of the soil, water and air. Gas hydrates will behave as solid or gas depending upon the environmental conditions. Gas hydrates will (1) change pore pressure, (2) cause dilation (3) weaken sediments and (4) break down initiates major slumps in continental slopes. The application of Skempton’s pore pressure parameters A and B are verified through environmental considerations. The sediment movement from coastal areas to abyssal plain should be as natural as possible. The conclusion is that, accelerated sediment movement is dangerous to marine environment.