The Tsinghua team has made new progress in the study of extraction characteristics of Type I reservoirs for marine natural gas hydrates

Through systematic indoor experiments and theoretical analysis, the team has achieved a series of breakthroughs. First, the team innovatively developed new experimental simulation systems and sample preparation methods, breaking through the bottlenecks in reservoir simulation technology. The research team independently designed a 360° rotatable high-pressure reactor equipped with dual independent water bath jackets, and developed a comprehensive experimental approach that includes hydrogel synthesis, rotation, and depressurized extraction. This approach overcame the industry challenge posed by traditional experimental setups, which are unable to simultaneously simulate the in-situ geothermal gradient in reservoirs as well as the interactions between hydrogel layers and underlying layers. This device enables precise control of Class I reservoirs, ensuring that the temperature and pressure conditions in the hydrate layer remain within the phase equilibrium curve, while the free gas layer is on the phase equilibrium curve, thus perfectly reproducing the characteristics of in-situ marine sedimentary reservoirs in the South China Sea. Meanwhile, the research team innovatively adopted nitrogen tracing technology to achieve real-time and accurate differentiation between free gas and gas released from hydrate decomposition, thereby determining the contribution of different gas sources to gas production. This provided crucial experimental calibration and validation data for numerical simulation models in the field of hydrate extraction. Secondly, the research system revealed the differential gas production patterns of three typical hydrate reservoir types, clarifying the development advantages of Type I reservoirs. The team’s research found that, under the condition of a constant total amount of hydrates, type I reservoirs exhibit a distinct \"two-stage gas production\" pattern: in the early stages of extraction, gas production is primarily driven by the free gas layer beneath them ； In the later stages of extraction, the gas generated by hydrate decomposition becomes dominant. The study also clarified the key differences in gas generation among the three types of reservoirs: the free gas layer in Type I reservoirs can significantly accelerate pressure transmission, thereby compensating for the endothermic energy loss associated with hydrate decomposition through intense convection, and thus ensuring continuous and stable gas generation ； Although the water layer underlying Class II reservoirs can provide sensible heat buffering to prevent excessive cooling of the hydrate layer, the water locking effect severely hinders heat transfer between the layers, resulting in the lowest gas production efficiency and the highest water production volume ； The homogeneous distribution of hydrates in Class III reservoirs results in a fast initial decomposition rate and the shortest time required to achieve 90% cumulative gas production; however, due to the lack of energy and material support from mobile fluids, their production capacity declines rapidly over time, and they lack the ability to maintain stable production.

Finally, the research team clarified the mechanism by which key parameters exert their influence, providing a fundamental experimental basis for optimizing field mining. For Class I reservoirs, the research team conducted a systematic sensitivity analysis on key parameters such as initial temperature, hydrate saturation, and wellbore production pressure, thereby clarifying the regulatory mechanisms of these parameters on production efficiency. Research shows that low temperatures in the reservoir reduce the driving force for hydrate decomposition, increase fluid flow resistance, and significantly prolong the exploitation period ； Low hydrate saturation reduces the overall gas production potential, but it can effectively increase reservoir permeability, accelerating pressure propagation and the gas production process ； High production pressure weakens the driving force for decomposition, but it can significantly reduce the total water production volume, thereby optimizing the gas-to-water ratio. Meanwhile, studies have found that excessive pressure reduction increases the risk of secondary formation of hydrates and ice plugging in the reservoir, providing important experimental references for pressure control in production wells and the design of anti-plugging processes. Regarding the future development of this field, the team has proposed key research directions for the next steps: focusing on the three-phase coexistence region at the interface between the hydrate layer and the free gas layer, and conducting detailed characterization there ； Clarify the interference mechanism of gas migration from beneath during extraction on the stability of the overlying hydrate reservoir ； By integrating advanced technologies such as distributed temperature, pressure, and acoustic sensing as well as isotope tracing, the spatial and temporal evolution patterns of reservoir properties can be accurately characterized, thereby establishing a comprehensive theoretical and technical framework for the commercial exploitation of natural gas hydrates. The research findings, titled \"Differential gas production characteristics of laboratory-synthesized I, II, and III type hydrate reservoirs: A novel thermally staged rotational method,\" were published in the International Journal of Mining Science and Technology on March 19. Zi Muchong, an associate professor at the Shenzhen International Graduate School of Tsinghua University, is the corresponding author of the paper, while Ye Hongyu, a doctoral student in the class of 2023 at the same school, is the first author. Co-authors of the paper include Chen Daoyi, a professor at the Shenzhen International Graduate School of Tsinghua University; Li Jie, a doctoral student in the class of 2025; Yao Yuanxin, a doctoral graduate from the class of 2025; Duan Jun, a doctoral student in the class of 2023; Wu Xuezhen, a researcher at Fuzhou University; and Li Dayong, a professor at China University of Petroleum (Huadong). The research was funded by the **National Natural Science Foundation, Shenzhen Science and Technology Planning Projects, and special funds for the development of Guangdong’s marine economy (the six major marine industries), among others.

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