Post Melting Behavior of Gas Hydrates in Soft Ocean Sediments
Authors: Chaouch A. 
Published: October 2008

The release of gas in soft ocean sediments associated with the dissociation of gas hydrates was modeled in a geotechnical centrifuge at Texas A&M University (TAW). The use of the centrifuge allowed the simulation of varying sediment and water depth combinations that would have been cost prohibitive in full scale field testing. It also lead to the experimental identification of failure patterns in soft sediments subsequent to hydrate melting.

The TAW centrifuge was instrumented, calibrated and modified to accommodate the experiments needs. Features such as pressure sources, miniature pore pressure transducer sand video imaging were mounted on the centrifuge mobile arms. Gas cavities were induced at the bottom of cylindrical soil samples subjected to gravitational fields reaching 100 g-levels. Results showed two predominant types of failure patterns and their dependence on shear strength properties. Experimental findings were confirmed by theoretical formulations.

Introduction

Gas hydrates are composed of gas molecules trapped in a water lattice. This ice-like crystalline material is also identified as "clathrates" in reference to the Latin word Clathratus that means encaged (Sloan1) further explains that the host structure or hydrogen bonded water molecules surrounds the gas molecule or guest molecule. Gas molecules stabilize the cavities they occupy under a limited range of pressures and temperatures (Makogon2).

The occurrence of gas hydrates has been established in several locations around the world through indirect evidence such as geological and geophysical records or through irrefutable evidence such as recovered samples (Kvenvolden et al.3) Several models for gas hydrates dissociation and formation were developed by studying artificially created hydrates under controlled laboratory conditions. These experiments were conducted to evaluate the potential industrial production of gas from hydrates in the solid state and the plugging of high pressure natural gas pipelines. Studied properties included thermal properties such as thermal conductivity and thermal diffusivity, electric properties (Tzirita4), seismic wave properties for bottom ocean detection. The recent trend in offshore production towards deep water exploration renewed the interest in gas hydrates research. Indeed, for water depths in excess of 500 m, hydrates can form at seafloors showing 2 to 5°C in temperature. However, the geothermal gradients limit the existence of gas hydrates to a depth where it becomes too hot for them to be stable.

In offshore environments, porous or fissured sediments allow the seepage of gases such as methane, propane, etc. from oil reservoirs to shallow sediments depths where temperature and pressure conditions are adequate for the engagement of gas molecules in a water lattice. A subsequent thermodynamic process slowly creates the gas hydrates and can bring disseminated small nodules into layers. These layers can mature to several meters in thickness (Kvenvolden and McDonald5).

Installation of offshore production units on such formations allows a constant flow of hot oil to the platform.