Effect of dissociation on the properties of hydrate bearing sediments

Abstract

Gas hydrates are clathrate hydrates, which are solid, ice-like compounds. Gas hydrates exist where there is an ample supply of gas and water combined with high pressure and/or low temperature conditions. In nature these are found in sediments where permafrost is present, and in deep-marine sediments. The morphology of gas hydrate within a sediment has a large impact on the strength and stiffness properties of hydrate bearing sediments. Gas hydrates are metastable and they dissociate if the temperature and/or pressure conditions are sufficiently altered. The dissociation of gas hydrate and its potential as a submarine geohazard have become of increasing importance as oil and gas exploration activities extend into significant water depths on continental margins and seas where gas hydrates are known to exist. Such activities may lead to dissociation of hydrate, possibly increasing pore pressure, and altering the stiffness and strength of the sediment. Due to difficulty in performing field testing and obtaining undisturbed in-situ samples for testing, at present, hydrate dissociation in the natural environment and its effects are hypothesised on the basis of remote observations. Therefore, a series of well-controlled laboratory tests were conducted on laboratory-prepared methane hydrate bearing sand sediments.

The tests were undertaken with hydrate saturation ranging from 7% to 27% in the Gas Hydrate Resonant Column Apparatus (GHRC). Factors such as effective stress were also assessed with regard to specimen stiffness. Resonant column testing during hydrate formation and dissociation processes carried out for the first time, such that not only final change in specimen properties to be determined as a function of total hydrate saturation but also the change in specimen properties as function of the percentage of hydrate formation and dissociation. Test results showed that a rapid reduction in stiffness occurred for a minor change in hydrate saturation of sand specimens where dissociation was induced by temperature increase, but for specimens that were dissociated using the pressure reduction method a slower reduction occurred. In contrast, during hydrate formation stiffness increased more gradually. In addition, test results showed that the hydrate formation using the excess gas method led to higher increases in the shear stiffness compared to the flexural stiffness of specimens, and the linear stiffness threshold limit of hydrate bearing specimens were lower than the non-hydrate bearing sands.

In addition to laboratory tests, an analytical model was built to predict the increase in pore pressure under undrained conditions within hydrate bearing sediment during dissociation. The results obtained from the laboratory tests were used to compare the predicted results from the model. Analytical model showed that the rise in pore pressure within a sediment was dependent on a number of factors: major factors were initial pore pressure, amount of hydrate dissociation, cage occupancy of gas within hydrate, stiffness of the sediment, and degree of water saturation; Minor factors were methane gas solubility in water, and methane hydrate density.

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