Elsevier

Applied Energy

Volume 326, 15 November 2022, 119971
Applied Energy

Effective CH4 production and novel CO2 storage through depressurization-assisted replacement in natural gas hydrate-bearing sediment

https://doi.org/10.1016/j.apenergy.2022.119971Get rights and content

Highlights

  • Depressurization-assisted replacement in hydrate-bearing sediment was investigated.

  • Longitudinal distributions of replacement efficiency and hydrate fractions were observed in the 1-D reactor.

  • The efficiencies of CH4 production and CO2 sequestration were enhanced by depressurization-assisted replacement.

  • Gas hydrate saturation in the sediment rapidly recovered after depressurization-assisted replacement.

Abstract

In this study, the guest exchange behaviors in the hydrate-bearing sediment of a one-dimensional reactor specially designed for depressurization-assisted replacement were experimentally investigated for CH4 production and CO2 storage. The longitudinal distributions of vapor compositions, replacement efficiency, and hydrate weight fractions, as well as the average efficiency for depressurization-assisted replacement, were examined using gas chromatography and powder X-ray diffraction (PXRD). The immediate re-formation of gas hydrates after CO2 injection implied a rapid recovery of the geo-mechanical strength of the sediment. The unique changes in the gas compositions in the pore space caused by the re-formation of gas hydrates resulted in a distinctive longitudinal distribution of the replacement efficiency in the hydrate-bearing sediment. PXRD analysis of the hydrate samples revealed that the gas hydrate saturation nearly recovered after depressurization-assisted replacement. Despite CH4 re-enclathration, the replacement efficiency was remarkably enhanced through depressurization-assisted replacement, and a larger amount of CO2 was stored in the hydrate-bearing sediment than the amount of CH4 produced. Furthermore, the production rate of CH4 through depressurization-assisted replacement was significantly higher than that through replacement only, and the guest exchange rate increased with an increase in the initial hydrate dissociation ratio. The experimental results demonstrated that depressurization-assisted replacement could solve the weakening of the geo-mechanical strength of the sediment for depressurization only and the slow production rate for replacement only; thus, it would be useful for low-carbon energy production from natural gas hydrate-bearing sediments.

Introduction

Recently, many countries have attempted to shift their major fossil fuel sources from coal and oil to natural gas and to develop carbon capture, storage, and utilization technologies to cope with global warming [1], [2], [3], [4], [5]. CH4-CO2 replacement in natural gas hydrates (NGHs), which enables natural gas production and simultaneous CO2 sequestration into subsea gas hydrate reservoirs, has attracted significant attention as a promising option for both energy production and carbon storage [6]. NGHs are naturally occurring crystalline inclusion compounds consisting of cage-like hydrogen-bonded water frameworks and captured hydrocarbon molecules [7]. Since a tremendous amount of NGHs can be found in permafrost regions and deep ocean sediments, extensive efforts have been made to extract natural gas effectively from these next-generation energy resources [8], [9], [10], [11], [12]. NGH reservoirs can also function as reliable and long-term CO2 storage sites because they have stored natural gas for millions of years [13], [14], [15]. Therefore, CH4-CO2 replacement can also solve the problem of insufficient storage space for CO2, which has been considered a major obstacle to actual CO2 sequestration.

In CH4-CO2 replacement, low-carbon energy production can be achieved through a spontaneous guest exchange between CH4 enclathrated in NGHs and CO2 produced from fossil fuel combustion [16], [17]. As replacement does not accompany the dissociation of gas hydrates in the hydrate-bearing sediment during natural gas production, it can reduce the potential environmental impact and prevent geological hazards such as seafloor subsidence [18]. Many research groups have investigated the efficiency of replacement, guest exchange behaviors, and structural transition in different hydrate structures to reveal the replacement mechanism and to optimize the replacement process [19], [20], [21]. Recently, replacement using flue gas from fossil fuel-fired power plants and fuel gas from CH4 reforming processes has also been studied to achieve more efficient low-carbon energy production [22], [23], [24], [25]. Although many previous studies on replacement have been conducted on a laboratory scale and many factors still need to be identified to scale up, the pilot-scale test production project in the Ignik Sikumi region on the north slope of Alaska successfully demonstrated the technical feasibility of CH4-CO2 replacement in an actual gas hydrate reservoir [26]. However, the slow reaction rate and low mass transfer in CH4-CO2 replacement are considered major obstacles to large-scale commercialization [27].

Depressurization is another NGH production method well known to be the most economical and effective owing to its fast production rate [28]. The technical feasibility of the depressurization method has already been demonstrated through several previous production tests conducted in Mallik (Canada), Nankai Trough (Japan), and the South China Sea (China) [29], [30], [31]. However, the weakening of the mechanical strength of the hydrate-bearing sediment during depressurization can be induced by the dissociation of gas hydrates [32]. In addition, the driving force for NGH dissociation continues to decrease as depressurization proceeds, resulting in gradually decreasing productivity because the reservoir temperature is lowered by the endothermic reaction of NGH dissociation [33].

Even though several hybrid production methods have been tested to overcome the technical limitations of conventional production methods [34], [35], [36], [37], this study was aimed at developing a depressurization-assisted replacement method as a significant breakthrough for NGH production that combines depressurization for the rapid production rate and subsequent replacement for CO2 storage and secure seafloor stability. Depressurization in the first step can contribute to increasing the production rate and creating pore space in the sediment for CO2 sequestration. Replacement in the next step can be used not only in natural gas production from residual NGHs through guest exchange but also in CO2 sequestration in a stable solid form. Furthermore, gas hydrate re-formation during replacement can aid in the recovery of the geo-mechanical strength of the sediment.

In this study, depressurization-assisted replacement was closely investigated to show its technical feasibility and to understand guest exchange for natural gas production and CO2 storage in hydrate-bearing sediments. A specially designed one-dimensional (1-D) reactor, which was firmly packed with silica beads, was used to observe the longitudinal behaviors of CH4 production and CO2 storage in the hydrate-bearing sediment. The replacement efficiency and guest exchange kinetics in depressurization-assisted replacement were examined by measuring gas compositions in real-time using gas chromatography. The longitudinal re-formation of gas hydrates during depressurization-assisted replacement was detected using temperature sensors. Hydrate saturation and hydrate weight fraction distributions were quantitatively analyzed using Rietveld refinement of the powder X-ray diffraction (PXRD) patterns for the hydrate samples.

Section snippets

Materials

CH4 gas with a purity of 99.999 % and CO2 gas with a purity of 99.99 % were supplied by Korea Noble Gas Co. (Republic of Korea) and MS Gas Co. (Republic of Korea), respectively. Silica beads with an average particle diameter of 89 μm were purchased from DAIHAN Scientific Co. (Republic of Korea). Double-distilled, deionized water was used for initial water saturation and hydrate formation in the 1-D reactor.

Apparatus

As shown in Fig. 1, a high-pressure 1-D reactor system was specially designed for the

Longitudinal guest exchange behaviors in depressurization-assisted replacement

The prompt recovery of gas hydrate saturation during NGH production is important in maintaining the geo-mechanical strength of hydrate-bearing sediment to prevent geological hazards [41]. As shown in Fig. 3(a), the gas hydrate re-formation in the 1-D reactor during replacement after depressurization was observed by tracing the temperature and pressure profiles. The gas transfer behaviors in the pore space of the hydrate-bearing sediment are represented in Fig. 3(b) as measurements of the

Conclusions

This study investigated the longitudinal behaviors of CH4 production and CO2 storage in the hydrate-bearing sediment in a specially designed 1-D reactor for depressurization-assisted replacement. For depressurization-assisted replacement with 50 % dissociation of initial gas hydrates, immediate re-formation of gas hydrates sequentially occurred from the inlet to the outlet of the 1-D reactor as injected CO2 was transferred throughout the sediment. The average efficiency of

CRediT authorship contribution statement

Wonjung Choi: Conceptualization, Writing – original draft, Investigation, Formal analysis, Validation, Visualization. Junghoon Mok: Investigation, Formal analysis, Visualization. Jonghyuk Lee: Investigation, Formal analysis, Validation. Yohan Lee: Conceptualization, Formal analysis, Validation. Jaehyoung Lee: Investigation, Formal analysis, Validation. Amadeu K. Sum: Investigation, Validation. Yongwon Seo: Supervision, Validation, Writing – review & editing, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported by the National Research Foundation of Korea (NRF) (NRF-2021R1A2C2005856) and also by the Korean Ministry of Ocean and Fisheries (KIMST Grant 20210632).

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