Methane Hydrate Formation and Dissociation in Sand Media: Effect of Water Saturation, Gas Flowrate and Particle Size

Fatima Doria Benmesbah,Pascal Clain,Olivia Fandino,Anthony Delahaye,Laurence Fournaison,Livio Ruffine,Véronique Osswald

doi:10.3390/en13195200

Abstract

Assessing the influence of key parameters governing the formation of hydrates and determining the capacity of the latter to store gaseous molecules is needed to improve our understanding of the role of natural gas hydrates in the oceanic methane cycle. Such knowledge will also support the development of new industrial processes and technologies such as those related to thermal energy storage. In this study, high-pressure laboratory methane hydrate formation and dissociation experiments were carried out in a sandy matrix at a temperature around 276.65 K. Methane was continuously injected at constant flowrate to allow hydrate formation over the course of the injection step. The influence of water saturation, methane injection flowrate and particle size on hydrate formation kinetics and methane storage capacity were investigated. Six water saturations (10.8%, 21.6%, 33%, 43.9%, 55% and 66.3%), three gas flowrates (29, 58 and 78 mLn·min−1) and three classes of particle size (80–140, 315–450 and 80–450 µm) were tested, and the resulting data were tabulated. Overall, the measured induction time obtained at 53–57% water saturation has an average value of 58 ± 14 min minutes with clear discrepancies that express the stochastic nature of hydrate nucleation, and/or results from the heterogeneity in the porosity and permeability fields of the sandy core due to heterogeneous particles. Besides, the results emphasize a clear link between the gas injection flowrate and the induction time whatever the particle size and water saturation. An increase in the gas flowrate from 29 to 78 mLn·min−1 is accompanied by a decrease in the induction time up to ~100 min (i.e., ~77% decrease). However, such clear behaviour is less conspicuous when varying either the particle size or the water saturation. Likewise, the volume of hydrate-bound methane increases with increasing water saturation. This study showed that water is not totally converted into hydrates and most of the calculated conversion ratios are around 74–84%, with the lowest value of 49.5% conversion at 54% of water saturation and the highest values of 97.8% for the lowest water saturation (10.8%). Comparison with similar experiments in the literature is also carried out herein.

Highlights

Gas hydrates are non-stoichiometric crystalline structures made of water molecules forming a cavity, usually called a cage, in which lightweight gas molecules are enclathrated
This study investigated the effects of sand particle sizes, initial water saturation and methane injection flowrate on hydrate formation kinetics and gas storage capacity
This study consists of a series of methane hydrate formation and dissociation experiments in a sandy core

Summary

Introduction

Gas hydrates are non-stoichiometric crystalline structures made of water molecules forming a cavity, usually called a cage, in which lightweight gas molecules are enclathrated. They are stable under low-temperature and high-pressure conditions [1,2,3]. Natural-gas hydrate deposits represent one of the largest methane reservoirs on earth [14,15] It has been considered a potential energy resource, and intense studies are ongoing to both assess the amount of natural gas bound in these deposits and for the development of new technologies for future exploitation [16,17,18,19,20,21,22]

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Conclusion