Small Alcohols as Hydrate Promoters

Bjørn Kvamme

doi:10.1021/acs.energyfuels.1c02902

Abstract

Many methods for the production of natural gas from hydrate reservoirs have been proposed during the latest 3 decades. Reducing pressure, thermal stimulation, or injection of thermodynamic hydrate inhibitors are three examples. A typical problem is, however, that different methods for producing hydrates are not evaluated thermodynamically prior to planning expensive experiments or even more expensive pilot tests. This can be due to the lack of a thermodynamic toolbox for the purpose. On a macroscopic level, there are two criteria that need to be met: the Gibbs free energy change has to be favorable and sufficient heat must be supplied in order to fulfill the first law of thermodynamics. Another challenge is the lack of focus on the limitations of the hydrate phase transition itself. The interface between the hydrate and liquid water is a kinetic bottleneck that requires efficient breaking of water hydrogen bonds. Reducing pressure does not address this problem. Injection of CO2, however, will lead to the formation of a new CO2 hydrate. This released heat from this hydrate formation is an efficient heat source for dissociating the in situ CH4 hydrate. Adding limited amounts of N2 increases the permeability for the injection gas. Addition of a surfactant increases the gas/water interface dynamics and promotes heterogeneous hydrate formation. In this work, we demonstrate a residual thermodynamic scheme that opens up for detailed thermodynamic analysis of different routes to hydrate formation and dissociation. It is demonstrated that addition of 20 mol % N2 to CO2 is thermodynamically feasible for generating a new hydrate in the pores. The available hydrate formation enthalpy when adding N2 is reduced as compared to pure CO2 but still considered as sufficient. Up to 3 mol % ethanol in the free pore water is also thermodynamically feasible. The addition of alcohol will not significantly disturb the ability to form a new hydrate from the injection gas. The released enthalpy from the formation of the new hydrate is also considered as sufficient compared to what is needed for dissociation of the in situ CH4 hydrate. Homogeneous hydrate formation from dissolved CH4 and/or CO2 is limited in amount and is not important. However, the hydrate stability limits related to the concentration of the hydrate former in the surrounding water are important. Mineral surfaces can act as hydrate promoters through direct adsorption, or adsorption in water, which is structured by the mineral surface charges. Examples from theoretical studies are discussed.

Highlights

Natural gas hydrates are a class of composite structures in which water organizes to create cavities which trap small nonpolar molecules such as CH4, C2H6, C3H8, and i-C4H10
We demonstrate a residual thermodynamic scheme that opens up for detailed thermodynamic analysis of different routes to hydrate formation and dissociation
The effect of ethanol on thermodynamic properties and the ability for CO2/ N2 mixtures to form a new hydrate with free pore water when injected into CH4 hydrate-filled sediments have been examined

Summary

Introduction

Natural gas hydrates are a class of composite structures in which water organizes to create cavities which trap small nonpolar molecules such as CH4, C2H6, C3H8, and i-C4H10. H2S stabilizes hydrates well due to an average positive electronic charge facing outward toward the cavity walls[1] when rotating inside a cavity. Depending on the size of guest molecules relative to the available volume inside a cavity, a variety of water arrangements can be found in nature. The smallest unit cell of structure I is a cubic box with the average lengths of the sides depending on temperature. A unit cell consists of 46 water molecules that form 2 small cavities (20 water molecules) and 6 large cavities (24 water molecules). Typical guest molecules that form structure I hydrates are CH4, C2H6, CO2, and H2S

Objectives

Methods

Findings

Discussion

Conclusion