Compression-induced dynamic change in effective permeability of hydrate-bearing sediments during hydrate dissociation by depressurization

Abstract

Methane hydrate, a form of clean energy also called flammable ice, has drawn global interest as an alternative energy resource of traditional fossil energy. The effective permeability of formation is a key factor to determine the gas production rate, which is controlled by not only hydrate saturation but also the porosity changes of the host sediment. A decrease in pore pressure leads to an increase in the effective stress and the collapse of the bonded structure made by the hydrate resulting in the volume contraction and permeability reduction. On the other hand, the pore pressure drop induces hydrate dissociation, which increases the porosity and permeability. In this study, we conducted a series of experiments to measure the effective permeability of hydrate-bearing sediments with different hydrate saturation. The experiment results show that the effective permeability of specimens is 30 Darcy without hydrate under 1 MPa effective stress. It decreases with the increasing of hydrate saturation from 0 to 0.396 or effective stress from 1 to 9 MPa. The relationship between void ratio, hydrate saturation and effective permeability is derived. Combining the confined compaction analysis, we proposed a simple formula to estimate the change in effective permeability of hydrate-bearing sediments during hydrate dissociation by depressurization. The formula is embedded the thermal-hydraulic model to predict the gas production under the effective stress. The amount of gas production reaches the maximum value of 1.45 × 10−3 m3 at 50 min and 4.82 × 10−3 m3 at 95 min with soil compaction from experiment, the fitting degree of numerical simulation are 0.997 and 0.998 when hydrate saturation equals 10% and 30%, respectively. This study could evaluate the compression-induced dynamic change in effective permeability and predict the gas/water production under the effect of effective stress during hydrate dissociation by depressurization.