Sediment pore-structure and permeability variation induced by hydrate formation: Evidence from low field nuclear magnetic resonance observation

The macro-scale physical properties of hydrate-bearing sediments is in fact controlled by their micro-scale pore-structures. Understanding the changes in pore-structure characteristics of the sediments during the process of hydrate formation is essential to the analyzing and predicting of the sediment properties. In this paper, the formation processes of Xenon hydrate in different sandy samples are measured with the low-field nuclear magnetic resonance (LFNMR) method. The obtained transverse relaxation time (T₂) spectra are interpreted for study of the changes in pore-structure and physical properties of the sediments during the hydrate formation. The results show that Xenon hydrates preferentially form in larger pores and only little amount of hydrates formed in smaller pores; the forming rate of hydrate is higher at the early stage of formation but decrease slowly at the later stage; the hydrate formation process also leads to the changes in pore size and pore-size distribution patterns, for examples, the maximum radius and mean radius of the water-phase pores decrease with increasing hydrate saturation, while the fractal dimension of the effective water-phase pores increases with the increasing hydrate saturation; the water-phase permeability decreases rapidly in the early stage of hydrate formation, but slowly decrease since then; the changes of water-phase permeability during hydrate formation are affected by the pore-structures of the sediment; compared to the SDR model and the Kozeny-Carman model, the fractal model of permeability performs better in showing the influences of pore-structure characteristics on the changes of water-phase permeability during the hydrate formation.