Thermal conductivity of hydrate and effective thermal conductivity of hydrate-bearing sediment

Abstract

The research on the thermal property of the hydrate has recently made great progress, including the understanding of hydrate thermal conductivity and effective thermal conductivity (ETC) of hydrate-bearing sediment. The thermal conductivity of hydrate is of great significance for the hydrate-related field, such as the natural gas hydrate exploitation and prevention of the hydrate plugging in oil or gas pipelines. In order to obtain a comprehensive understanding of the research progress of the hydrate thermal conductivity and the ETC of hydrate-bearing sediment, the literature on the studies of the thermal conductivity of hydrate and the ETC of hydrate-bearing sediment were summarized and reviewed in this study. Firstly, experimental studies of the reported measured values and the temperature dependence of the thermal conductivity of hydrate were discussed and reviewed. Secondly, the studies of the experimental measurements of the ETC of hydrate-bearing sediment and the effects of temperature, porosity, hydrate saturation, water saturation, thermal conductivity of porous medium, phase change, and other factors on the ETC of hydrate-bearing sediment were discussed and reviewed. Thirdly, the research progress of modeling on the ETC of the hydrate-bearing sediment was reviewed. The thermal conductivity determines the heat transfer capacity of the hydrate reservoir and directly affects the hydrate exploitation efficiency. Future efforts need to be devoted to obtain experimental data of the ETC of hydrate reservoirs and establish models to accurately predict the ETC of hydrate-bearing sediment.

1. Introduction

Hydrates are nonstoichiometric crystalline solids that consist of water and guest molecules. The guest molecules that can form hydrate could be CH4, CO2, Cl2, H2, C2H6, C3H8, THF (tetrahydrofuran), TBAB (tetra butyl ammonium bromide), cyclopentane, etc. At low temperature and high pressure, water molecules are linked by hydrogen bonds to form a cage-like crystal lattice and encapsulate the guest molecules in them. The interaction between the guest molecule and the water molecule by van der Waals forces is the key for the formation and stable existence of the hydrate structure [1]. The size of the guest molecule determines the type of crystal structure of the hydrate. The hydrate structure can be classified into structure I (sI), structure II (sII), and structure H (sH) according to the size and structure of the hydrate cages lattice [2].

The studies of hydrates have been applied to many fields. Major applications of hydrates are in the area of natural gas hydrate exploitation [3e9], flow assurance [10e19], energy storage and transportation [20e25], mixed gas separation [26e31], seawater desalination [32e34] and other environmental aspects [35], such as global warming [36,37] and geo-hazards [38e41]. Thermal conductivity, as an important fundamental thermal property of hydrate, is of great practical importance for heat transfer characteristics of hydrate-bearing sediment during the natural gas hydrate exploitation process [42]. The studies of the ETC of hydrate-bearing sediment could directly affect the efficiency of heat transfer of the hydrate-bearing sediment, controlling on the hydrate exploitation efficiency [43]. The ETC of hydrate-bearing sediment is also one of the key parameters to evaluate the stability of hydrate reservoirs [44,45].