Abstract

Experimental and field observations evidence the effects of capillarity in narrow pores on inhibiting the thermodynamic stability of gas hydrates and controlling their saturation. Thus, precise estimates of the gas hydrate global inventory require models that accurately describe gas hydrate stability in sediments. Here, an equilibrium model for hydrate formation in sediments that accounts for capillary inhibition effects is developed and validated against experimental data. Analogous to water freezing in pores, the model assumes that hydrate formation is controlled by the sediment pore size distribution and the balance of capillary forces at the hydrate–liquid interface. To build the formulation, we first derive the Clausius–Clapeyron equation for the thermodynamic equilibrium of methane and water chemical potentials. Then, this equation is combined with the van Genuchten’s capillary pressure to relate the thermodynamic properties of the system to the sediment pore size distribution and hydrate saturation. The model examines the influence of the sediment pore size distribution on hydrate saturation through the simulation of hydrate formation in sand, silt, and clays, under equilibrium conditions and without mass transfer limitations. The results show that at pressure–temperature conditions typically found in the seabed, capillary effects in very fine-grained clays can limit the maximum hydrate saturation below 20% of the host sediment porosity.

Highlights

  • Gas hydrates are naturally occurring crystalline compounds that contain low molecular weight gases in excess of saturation, within a lattice of hydrogenbonded water molecules

  • The discrepancies observed in these sites between the depth of the bottom simulating reflector (BSR), indicating the actual base of the gas hydrate stability zone (GHSZ), and its theoretically predicted depth under bulk conditions have been in part attributed to capillary effects [8,11,12]

  • A new equilibrium model for methane hydrate stability in porous media is presented here to examine the importance of the capillary pressure developed at the hydrate–liquid interface on inhibiting hydrate formation in pores and controlling its saturation

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Summary

Introduction

Gas hydrates are naturally occurring crystalline compounds that contain low molecular weight gases (commonly methane) in excess of saturation, within a lattice of hydrogenbonded water molecules. To better understand the relation between gas hydrate saturation and distribution and the physical properties of the hosting sediment, several fluid flow models have been developed over the last two decades, e.g., [22,26,27,28,29,30,31] These models account for capillary effects on the thermodynamic stability of gas hydrates through the decrease in water activity with decreasing the pore size of the sediment. The model is applied to simulate methane hydrate formation in sand, silt, and clays (with different contents of fines), under equilibrium conditions and without mass transfer limitations This is, to the best of our knowledge, the first attempt to compare quantitatively the effect of the host sediment pore size distribution on inhibiting the maximum methane hydrate saturation expected in natural sediments at a given P-T-S combination due to capillary effects. Offering the opportunity to enhance their ability to predict the maximum hydrate saturation that any lithology can have accounting for capillary effects on hydrate formation

Equilibrium Model for Gas Hydrate Formation in Pores
Phase Distribution in Pores
Methane Hydrate Thermodynamic Equilibrium
Clausius–Clapeyron Equation for the Methane-Water System
Capillary Effects and Equilibrium Pressure Relation
Capillary Effects and Methane Hydrate Saturation Relation
Results and Discussion
Hydrate Stability in Discrete Pores
Conclusions
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