The dual action of N2 on morphology regulation and mass-transfer acceleration of CO2 hydrate film

Abstract

The morphology characteristics of CH4, CO2, and CO2+N2 hydrate film forming on the suspending gas bubbles are studied using microscopic visual method at supercooling conditions from 1.0 to 3.0 K. The hydrate film vertical growth rate and thickness along the planar gas-water interface are measured to study the hydrate formation kinetics and mass transfer process. Adding N2 in the gas mixture plays the same role as lowering the supercooling conditions, both retarding the crystal nucleation and growth rates, which results in larger single crystal size and rough hydrate morphology. N2 in the gas mixture helps to delay the secondary nucleation on the hydrate film, which is beneficial to maintain the pore-throat structure and enhance the mass transfer. The vertical growth rate of hydrate film mainly depends on the supercooling conditions and gas compositions but has weak dependence on the experimental temperature and pressure. Under the same gas composition condition, the final film thickness shows a linear relationship with the supercooling conditions. The mass transfer coefficient of CH4 molecules in hydrates ranges from 4.54 × 10^-8 to 7.54 × 10^-8 mol·cm^-2·s^-1·MPa^-1. The maximum mass transfer coefficient for CO2 + N2 hydrate occurs at the composition of 60% CO2 + 40% N2, which is 3.98 × 10^-8 mol·cm^-2·s^-1·MPa^-1.

1. Introduction

Gas hydrate is a non-stoichiometric crystalline compound consisting of polygonal cage structures, where the guest molecules such as small molecule hydrocarbons trapped in Ref. [1]. Natural gas hydrate is considered as an important global backup energy resource due to its vast reserves, which occurs in a wide variety of geologic settings including continental slopes and permafrost regions [2]. Conservative estimates suggest that the amount of CH4 in natural gas hydrate is ~200000 trillion cubic feet (TCF) [3], which is at least twice as much as the energy contained in the known fossil fuel reserves [4]. The proposed exploitation methods include thermal stimulation [5,6], thermodynamic inhibitor injection [7], depressurization [8], and CO2-CH4 replacement [9,10]. The CO2-CH4 replacement method is injecting CO2 into natural gas hydrate reservoir, since CO2 hydrate is thermodynamically more stable than CH4 hydrate at relevant temperature-pressure condition, it can achieve the dual goals of carbon sequestration and methane hydrate exploitation [11]. From an environmental perspective, the gas replacement is one of the most promising hydrate recovery methods due to its advantages of low energy consumption and non-destructive characteristic. Despite these advantages, its further application is still hampered by the low replacement rate [12]. Several microscopic studies using GC [13], Raman [14,15], and NMR [16,17] have been carried out to investigate the CO2-CH4 replacement process. Although these studies vary in terms of experimental procedures and replacement conditions, they reach consensus that the replacement process can be divided into two stages [18-20]. In the first stage, CH4 hydrate on the surface can be easily replaced by CO2 with relatively high rate and form a dense mixed hydrate shell. Heat-transfer limited surface reaction rate is the controlling factor in this stage. In the latter stage, the mass transfer rate is the controlling factor, which is hindered by the mixed hydrate shell, resulting in low replacement rate. Therefore, overcoming mass transfer limitation is the key to enhance the gas replacement rate [21].

In this work, we mainly focus on the effect of small molecular gas N2 on reforming the CO2 hydrate film, which is seen as the mass transfer obstruction in natural gas hydrate exploitation. Adding small molecule gas N2 into CO2 to improve CH4 recovery rate was initiated by Park et al. [14], the recovery rate showed a great improvement from 64% to 85%. Then the mixture of CO2-N2/H2 replacing CH4 hydrate has been studied at both the laboratory-scale [22-24] and the pilot-scale in the field [25,26]. A series of studies have been conducted to understand the role of N2 in the replacement process, but the underlying mechanism remains unclear. This study aims to elucidate the dual action of N2 on morphology regulation and mass-transfer acceleration of CO2 hydrate film, providing insights for optimizing gas replacement processes.