Permeability change with respect to different hydrate saturation in clayey-silty sediments

Abstract

The successful pilot trial of methane hydrate production in the South China Sea has proven the technical feasibility of natural gas hydrates (NGHs) exploitation in clayey-silty reservoirs. As a key issue of NGHs exploitation in clayey-silty sediments, the variation of permeability with hydrate saturation (Sh) has not been thoroughly studied. In this study, a triaxial core holder was designed to simulate the synthesis and dissociation of gas hydrates in a clayey-silty core plug sample. In the process of hydrate synthesis and multi-step dissociation, the water content and distribution in different pores were quantitatively determined by a nuclear magnetic resonance (NMR) spectrometer, and the variation of water content under different Sh was successively monitored. Meanwhile, in each depressurization phase, the gas permeabilities k(Sh) with respect to different Sh were measured under constant effective stress. Results highlight that the change of k(Sh) is more prevalent in clayey-silty sediments than that in sandstones. As Sh increases from 0 to 10.43%, the permeability ratio (k(Sh) to sedimentary permeability ks) in clayey-silty sediments decreases to 0.01–0.02; whereas in sandstones, the permeability ratio falls between 0.4 and 0.8. To predicate the permeability change in clayey-silty sediments, the previous Cubic model is improved and a Power-exponential model is presented. In the updated model, the variable of power parameter n can characterize different reservoir types. Specifically, in artificial sandstones, sandpacks or sandy reservoirs, the n values range from 3.1 to 8.3, while in clay packs or clayey-silty sediments, the n values are from 19.1 to 30.7. The Power-exponential model provides reference values for the prediction of different types of gas hydrate reservoirs such as permafrost and marine sediments.