Hydrate—A Mysterious Phase or Just Misunderstood?

Bjørn Kvamme,Jinzhou Zhao,Na Wei,Navid Saeidi

doi:10.3390/en13040880

Abstract

Hydrates that form during transport of hydrocarbons containing free water, or water dissolved in hydrocarbons, are generally not in thermodynamic equilibrium and depend on the concentration of all components in all phases. Temperature and pressure are normally the only variables used in hydrate analysis, even though hydrates will dissolve by contact with pure water and water which is under saturated with hydrate formers. Mineral surfaces (for example rust) play dual roles as hydrate inhibitors and hydrate nucleation sites. What appears to be mysterious, and often random, is actually the effects of hydrate non-equilibrium and competing hydrate formation and dissociation phase transitions. There is a need to move forward towards a more complete non-equilibrium way to approach hydrates in industrial settings. Similar challenges are related to natural gas hydrates in sediments. Hydrates dissociates worldwide due to seawater that leaks into hydrate filled sediments. Many of the global resources of methane hydrate reside in a stationary situation of hydrate dissociation from incoming water and formation of new hydrate from incoming hydrate formers from below. Understanding the dynamic situation of a real hydrate reservoir is critical for understanding the distribution characteristics of hydrates in the sediments. This knowledge is also critical for designing efficient hydrate production strategies. In order to facilitate the needed analysis we propose the use of residual thermodynamics for all phases, including all hydrate phases, so as to be able to analyze real stability limits and needed heat supply for hydrate production.

Highlights

The formation of hydrocarbon hydrates has been a problem for the oil and gas industry for many decades
99% of natural gas hydrate resources the world are from non-equilibrium biogenic degradation organic because the discussion in this paper focus veryinmuch on hydrate butofalso because material and the resulting hydrocarbons are almost pure methane
Free energy for CH4 hydrate formed from water solution at a given temperature and pressure will generally be higher than free energy of hydrate formed from CH4 gas and liquid water

Summary

Introduction

The formation of hydrocarbon hydrates has been a problem for the oil and gas industry for many decades. These hydrates looks like ice or snow. In structure I the smallest cavities consists of 20 water molecules in the cavity walls and for the large cavity there are 24 water molecules in the cavity walls (see Figure 1 for an illustration). The cavities are stabilized mainly by the volume of the molecules (repulsion) entering the cavities and weak van der Waal type attractions between the molecules in the cavity and the water molecules in the cavity walls. Samplings from molecular dynamics simulations [1] show that the result of the water molecules in the cavity walls is a net negative charged Coulombic field pointing inwards in the cavity. The extra attractive coulombic energy between water and H2 S [2], as compared to neutral molecules like for instance methane, is the reason why H2 S is an exceptionally good hydrate former

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