Abstract: The assessment and exploitation of natural gas hydrate resources depend on the accurate identification of the distribution and thickness of the hydrate stability zone. This study develops a calculation method for determining the hydrate reservoir stability zone, grounded in the generalized phase equilibrium theory. Furthermore, the analysis software has been developed, taking into account the effects of seawater salinity, pore capillarity, and temperature offset on the hydrate stability zone. The experimental results from reservoir samples in the Qiongdongnan sea area reveal that the phase equilibrium characteristics of hydrates can be consistently characterized by a three-dimensional generalized phase equilibrium surface, encompassing temperature, pressure, and unhydrated water content. By comparing with mature software, the reliability of the proposed stability zone calculation method and software is confirmed. Stability zone calculation and analysis are subsequently performed based on the measured sample phase equilibrium curve and field conditions. The results show that, in addition to water depth, seabed temperature, geothermal gradient, and seawater salinity, the effective pore radius of sediments and temperature offset exert a significant influence on the lower boundary of the stability zone. Failure to account for the effects of pore effective radius and temperature offset results in an overestimation of the lower boundary of the stability zone by 77.4 m within this gas hydrate target area, significantly affecting the accurate assessment of resource quantity. These findings can provide theoretical support and a data reference for predicting the stability zone and accurately assessing the resource quantity of marine natural gas hydrates.

Key words: natural gas hydrate, phase equilibrium, stable zone, effective pore radius, resource quantity