Combining Optical Microscopy and X-ray Computed Tomography Reveals Novel Morphologies and Growth Processes of Methane Hydrate in Sand Pores

Thi Xiu Le,Michel Bornert,Andrew King,Daniel Broseta,Anh Minh Tang,Ross Brown,Baptiste Chabot,Patrick Aimedieu

doi:10.3390/en14185672

Abstract

Understanding the mechanisms involved in the formation and growth of methane hydrate in marine sandy sediments is crucial for investigating the thermo-hydro-mechanical behavior of gas hydrate marine sediments. In this study, high-resolution optical microscopy and synchrotron X-ray computed tomography were used together to observe methane hydrate growing under excess gas conditions in a coarse sandy sediment. The high spatial and complementary temporal resolutions of these techniques allow growth processes and accompanying redistribution of water or brine to be observed over spatial scales down to the micrometre—i.e., well below pore size—and temporal scales below 1 s. Gas hydrate morphological and growth features that cannot be identified by X-ray computed tomography alone, such as hollow filaments, were revealed. These filaments sprouted from hydrate crusts at water–gas interfaces as water was being transported from their interior to their tips in the gas (methane), which extend in the µm/s range. Haines jumps are visualized when the growing hydrate crust hits a water pool, such as capillary bridges between grains or liquid droplets sitting on the substrate—a capillary-driven mechanism that has some analogy with cryogenic suction in water-bearing freezing soils. These features cannot be accounted for by the hydrate pore habit models proposed about two decades ago, which, in the absence of any observation at pore scale, were indeed useful for constructing mechanical and petrophysical models of gas hydrate-bearing sediments.

Highlights

Marine sediments bearing methane hydrate (MH), a clathrate of methane gas in an ice-like aqueous matrix, have long been considered as an alternative source of energy as well as a source of potential geo-hazards [1]
The majority of marine methane hydrate is in dispersed form in fine sediments at low overall occupation, but for practical reasons the potential target of gas exploration is hydrate at high occupation in coarse sandy sediments [1,5]
Unless potassium iodide is present, the aqueous phase is hardly distinguishable from methane hydrate by grey levels alone in synchrotron X-ray computed tomography (XRCT) cross-sections

Summary

Introduction

Marine sediments bearing methane hydrate (MH), a clathrate of methane gas in an ice-like aqueous matrix, have long been considered as an alternative source of energy as well as a source of potential geo-hazards [1]. The majority of marine methane hydrate is in dispersed form in fine sediments at low overall occupation, but for practical reasons the potential target of gas exploration is hydrate at high occupation in coarse sandy sediments [1,5]. Physical and mechanical properties of such sediments are often accounted for by assuming some model distributions of the hydrate within the pore space, referred as the hydrate pore habit, which is important for interpreting geophysical data and reservoir-scale simulations of methane production [6,7,8,9,10,11,12,13,14,15]. Even though it is helpful to build geophysical and reservoir models, this categorization of hydrate distribution within the pore space lacks direct experimental confirmation. The very scarce high-resolution imaging studies by means of synchrotron X-ray computed tomography (XRCT) point to more complex features of gas hydrate formation and growth in sediment pores [16,17,18,19,20,21]

Methods

Results

Discussion

Conclusion