Key Takeaways & Executive Findings
- •• Class I reservoirs exhibit two-stage gas production with 38.4%–78.3% higher cumulative production and superior gas-to-water ratios compared to Class II and III. • The free gas layer in Class I accelerates pressure and heat transfer, while Class II's water layer causes water blocking and impairs heat flow. • Class III reservoirs show rapid initial dissociation but quick decline without fluid support, highlighting the need for external energy input. • Low temperature, low hydrate saturation, and high production pressure reduce efficiency; over-depressurization risks hydrate reformation and ice blockage.
Abstract
Natural gas hydrate in Class I reservoirs holds significant commercial potential, as demonstrated by production trials in the South China Sea. However, experimental studies have focused largely on Class III systems, with Class I/II reservoirs remaining underrepresented due to the difficulties in simulating the geothermal gradient and interlayer interactions. This study investigates depressurization performance across all three classes using a novel 360° rotatable reactor with segmented temperature control, enabling precise simulation of reservoir conditions. Results reveal: (i) Class I shows two-stage gas production, with 50% from early free gas enabling rapid depressurization, followed by dissociated gas dominance. They achieve 38.4%–78.3% higher cumulative production and superior gas-to-water ratios due to efficient energy use. (ii) The free gas layer in Class I accelerates pressure and heat transfer. Class II’s water layer provides sensible heat but causes water blocking, impairing heat flow. Class III exhibits rapid initial dissociation but a quick decline without fluid support. (iii) Low temperature, low hydrate saturation, and high production pressure collectively reduce efficiency by increasing flow resistance, limiting gas supply, and reducing dissociation drive. Over-depressurization risks hydrate reformation and ice blockage. This work bridges experimental gaps for Class I/II reservoirs, offering key insights for optimizing recovery.
1. Introduction
Natural gas hydrate, a crystalline compound formed under low-temperature and high-pressure conditions, represents a vast potential energy resource, with global estimates on the order of n×1015 m3 [1–4]. Successful production trials in the South China Sea have demonstrated the technical feasibility of exploitation, highlighting its significant commercial potential, particularly in specific reservoir types [5,6]. Among various exploitation methods, including depressurization, thermal stimulation, CO2 injection, and inhibitor injection [7–9], depressurization has emerged as the most promising and widely adopted technique due to its efficiency and economic feasibility [10].
Current geological exploration reveals that natural gas hydrate reservoirs are frequently underlain by free gas layers/shallow gas accumulations [11,12]. This configuration was observed during depressurization production tests of Wells CMGS6-SH02 and SHSC-4 (Fig. 1a) in the South China Sea in 2017 and 2020 (Fig. 1b) [5,6]. Geological data confirm that the developed reservoirs comprise three distinct strata: A hydrate-bearing layer (HBL) containing gas hydrates, a three-phase coexistence zone containing gas hydrates, and an underlying free gas layer (FGL) containing only free gas and pore water. These associated gas sources (Fig. 1c) effectively mitigate the gas supply limitations inherent in singular hydrate reservoirs. Generally, reservoirs with potential for commercial exploitation are classified into three categories (Class I–III) based on sediment types, hydrate density/concentration, and formation trap structures (Fig. 1e) [13,14]. Class I reservoirs feature an upper hydrate-bearing layer (H+W) and a lower free gas-water mixed layer (W+G). It is noteworthy that the HBL in Fig. 1a often contains a three-p
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Hongyu Ye, Jie Li, Yuanxin Yao, Daoyi Chen, Jun Duan, Xuezhen Wu, Dayong Li, Mucong Zi (2025). Distinct gas production characteristics from laboratory-synthesized Class I, II, and III hydrate reservoirs: A novel thermally-segmented rotatable approach. SinoTechIntel Verified Research. https://doi.org/10.1016/j.ijmst.2025.12.016
Research & Educational Purpose Only:The translations, structured abstracts, analytical annotations, and data reports provided by SinoTechIntel are intended exclusively for academic research, internal corporate R&D, and educational benchmarking. They do not constitute formal engineering, chemical safety, legal, or professional advice.
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Frequently Asked Questions
What are the key differences in gas production characteristics among Class I, II, and III hydrate reservoirs?
Class I reservoirs show two-stage gas production with higher cumulative production and gas-to-water ratios due to the presence of a free gas layer. Class II reservoirs have a water layer that provides sensible heat but causes water blocking, impairing heat flow. Class III reservoirs exhibit rapid initial dissociation but quick decline without fluid support.
How does the novel rotatable reactor improve experimental simulation of hydrate reservoirs?
The 360° rotatable reactor with segmented temperature control allows precise simulation of reservoir conditions, including geothermal gradients and interlayer interactions, which were previously difficult to replicate in laboratory experiments.
What operational parameters affect gas production efficiency in hydrate reservoirs?
Low temperature, low hydrate saturation, and high production pressure collectively reduce efficiency by increasing flow resistance, limiting gas supply, and reducing dissociation drive. Over-depressurization can lead to hydrate reformation and ice blockage.
Why are Class I reservoirs considered commercially promising?
Class I reservoirs have a free gas layer that accelerates pressure and heat transfer, leading to higher cumulative gas production (38.4%–78.3% higher) and superior gas-to-water ratios, making them more efficient for energy recovery.
What are the implications of this study for optimizing hydrate recovery?
The study provides key insights into the distinct production behaviors of different reservoir classes, guiding the selection of appropriate production strategies and operational parameters to maximize gas recovery while minimizing risks such as water blocking and ice formation.
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