Simulating stepwise depressurization production of natural gas hydrate using a three-dimensional test system

In the three-dimensional scale, the gas hydrate reservoir can be more accurately simulated, and on this basis, the stepwise depressurization production characteristics study can provide a scientific basis for improving the efficiency and controllability of gas hydrate production. This study used a three-dimensional gas hydrate production simulation device to conduct stepwise depressurization production tests with a single horizontal well in three-dimensional scale. The results were compared with two-dimensional tests to investigate the effects of reservoir scale and stepwise depressurization on hydrate production characteristics and efficiency. The results showed that in the early stage of depressurization, free gas production dominated, and the pressure change rate was easy to control. However, in the middle and late stages, hydrate decomposition led to more complex pressure changes. Compared to the two-dimensional tests, the Joule–Thomson effect had a more significant impact on temperature changes in the three-dimensional tests, making temperature stratification more noticeable than pressure stratification. The cumulative gas production in the three-dimensional tests was only 7.2 times that of the two-dimensional tests, which may be attributed to the combined effects of reservoir heterogeneity, low heat transfer efficiency, complex gas migration pathways, and dynamic changes in pore structures, all of which reflect the inherent complexity of hydrate production in realistic geological conditions, characterized by nonlinear production behavior and strong coupling among thermal, hydraulic, mechanical, and chemical fields. The average gas production rate changed in stages over time, with higher rates during depressurization and lower rates during constant pressure. Unlike the two-dimensional tests, the gas production capacity gradually weakened in the three-dimensional tests, aligning more closely with real-world conditions. The peak gas production rate showed a sequential change trend throughout the test, reflecting the system's dynamic behavior.