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