ABSTRACT

Natural gas hydrates (NGH) are crystalline compounds of water and natural gas, mainly methane, formed under low temperature and high pressure in marine and permafrost environments. Methane extraction from NGH holds great energy potential. Methods include depressurization, thermal stimulation, CO2 injection, and chemical inhibitor injection, with CO2 injection combined with depressurization being particularly promising. Injecting CO2 into NGH sediments releases CH4 and forms CO2 hydrates, increasing methane extraction and CO2 capture. This benefit boosts energy security and climate change mitigation. This double advantage emphasizes the need to improve methane recovery from NGH and promote methane as a cleaner energy source. This potential for methane to serve as an alternative to traditional hydrocarbons has attracted significant attention from researchers, spurring efforts to develop better techniques for extraction and utilization. In this paper, we investigate the methane recovery by employing depressurization following CO2 injection, both with and without surfactants, into the gas hydrate. We conduct the experiment under high-pressure and low-temperature conditions. CO2 foam helps to improve the sweep efficiency of CO2 through NGH reservoirs, which stops methane hydrate zones from being quickly bypassed. This controlled mobility ensures a more efficient displacement of methane from the hydrate structure. Our experiments show that surfactants enhance methane recovery and CO2 sequestration. Surfactants significantly increased methane yield, demonstrating their efficacy in improving methane recovery from NGH.

INTRODUCTION

Energy is essential for both the survival of humans and the progress of society. The extensive dependence on fossil fuels has resulted in increased levels of carbon dioxide (CO2) emissions in the atmosphere. In the latter half of the 21st century, we face two significant obstacles: a worldwide shortage of energy resources and increasing environmental worries. Conventional energy sources are being used up rapidly, indicating a forthcoming and serious energy shortage (M. Zhang et al. 2023; X. Zhang et al. 2024). Exploring alternative energy sources has become a highly effective approach to addressing the current challenges in energy and the environment. Natural Gas Hydrates (NGHs) have recently become widely recognized and valued worldwide because of their plentiful reserves, eco-friendliness, and promising future (D. Yang et al. 2023; X. Zhang et al. 2024; Zhong et al. 2022). Gas hydrates form under conditions of high pressure and low temperature, resembling ice-like structures. These structures consist of a cage formed by water molecules, within which molecules of gas like methane, ethane, and propane are trapped. The water molecules in this lattice structure are interconnected by strong hydrogen bonds, which are stable under low temperatures and high pressures. Additionally, the bonding between the gas and water molecules is strengthened by van der Waals forces (Goode 2007; S. Merey and Sinayuc 2016). In the majority of cases (99%), gas hydrate reservoirs are made up entirely of methane, although natural gas can also contain impurities such as ethane and propane (Collett et al. 2015; Demirbas 2010; Kvenvolden 1988; Ş. Merey 2016). Methane gas hydrates are commonly present in marine environments and locations with permafrost. Approximately 99% of methane (CH4) hydrates are predominantly found in marine environments, according to estimates (Ş. Merey 2016; Ruppel 2015). Kvenvolden (Kvenvolden 1988) estimates that the global hydrate-bound methane is approximately 2.0 × 1016 m3, which is twice as much carbon found globally in proven fossil fuels (coal, oil, and natural gas) (Englezos 1993; Li et al. 2019). The safe and efficient use of NGHs can effectively address a number of global challenges, including energy crises and climate change mitigation (X. Zhang et al. 2024). The arrangement of cavities in hydrates result in three distinct structures: I, II, and H. Structure I hydrates are the most common in nature due to their simplicity and stability, with the majority containing smaller gas molecules such as methane (Gajanayake et al. 2022; Swaranjit Singh 2015). Three main types of NGH deposits have been identified: pore filling, fractured, and massive/nodule types. Their presence has been confirmed in various drilling samples from around the world. Most field tests have been carried out in reservoirs with pore filling type deposits, which are the most common (L. Yang et al. 2019). Scientists first found gas hydrates in 1810 while doing experiments in the lab. But they didn’t get much attention until the 1930s, when petroleum engineers noticed that they could block natural gas pipelines (Fan and Wang 2006; Song et al. 2014). Over the past four decades, as significant deposits of natural gas hydrate sediments have been discovered and researchers have gained a better understanding of their properties, there has been a growing interest in studying gas hydrates. This increased focus has led to notable scientific advancements and the publication of numerous high-quality papers. Concurrently, new technologies based on hydrate research have been developed, including gas storage and transportation, gas separation, and CO2 sequestration (Song et al. 2014). Currently, commonly used production methods include the depressurization technique, thermal stimulation technique, chemical inhibitor injection technique, CO2–CH4 exchange technique, and their combinations (L. Yang et al. 2019). The main drawbacks of the initial three methods mainly revolve around the insufficient efficiency of heat transfer within the reservoir, which restricts the rate at which NGH hydrate decomposition occurs. The formation's low thermal conductivity leads to quick temperature decreases in the decomposition zone, where substantial heat absorption takes place, making it challenging to effectively compensate. This results in decreased productivity. Furthermore, these techniques depend on the principles of hydrate decomposition, which can diminish the strength of NGH reservoirs and potentially lead to environmental problems such as slope instability and disturbance of the seabed. The CO2 replacement method utilizes the combined effect of heat absorption during the decomposition of CH4 hydrate and heat release during the formation of CO2 hydrate. The formation of CO2 hydrate is a result of the heat compensation that occurs following the degradation of NGH hydrate. This subsequent formation of hydrate plays a role in preserving the stability of the reservoir. In addition, CO2 replacement enables the storage of greenhouse gases (GHGs) as hydrates underground (Khlebnikov et al. 2016). Injecting CO2 into NGH sediments induces the release of CH4 and the formation of CO2 hydrates. This process entails the partial or complete separation of CH4 hydrates and the subsequent creation of CO2 hydrates in liquid water. Under sustained pressure, there is a certain range where gaseous CH4 is emitted while CO2 continues to form hydrates. The CH4-CO2 replacement process presents a distinct opportunity to extract methane (CH4), an energy resource, while also capturing carbon dioxide (CO2), a greenhouse gas. Hydrate reservoirs can therefore function as both sources of methane and storage sites for CO2, thereby advancing the concept of a carbon-neutral fuel source (Castellani et al. 2023). As society grows quickly, it releases huge amounts of greenhouse gases. This is a big problem for the environment that people have to deal with. Storing CO2 is a good way to cut down on CO2 emissions and the greenhouse effect. The CO2 replacement method is considered a promising technique for extracting NGHs and has garnered significant interest from researchers worldwide. However, its commercial application is limited by the slow rate of replacement and its low efficiency (Zhao et al. 2012).

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