Studies on the Inversion Phenomenon of Physical Properties Observed in the Huagang Formation Reservoir in the Xihu Sag Based on the Water-Rock Reaction Experiments

In the Xihu Sag, the reservoirs of the Paleogene Huagang formation have entered the middle diagenetic stage A and the rock physical properties of the water layer are considerably more suitable for the gas migration and storage than those of the present gas layer, indicating the inversion of the physical properties. In this study, core samples were collected from the corresponding reservoir to conduct water-rock reaction experiments in acidic, alkaline, and neutral systems under the specific temperature and pressure. The reasons for the inversion of physical properties were investigated based on the experiment results in reservoir diagenetic environments. The inversion of physical properties can be attributed to the fact that the diagenetic environment around the gas-water interface controls the water-rock reaction effect. With different types of acidic substances, two different situations corresponding to inverted physical properties were analyzed along with the corresponding mechanisms. When the pore fluid is acidic, the physical properties make the overall water layer a better reservoir space than the gas layer, which can be referred to as the overall inversion of physical properties. When the fluid were generally neutral or weakly alkaline and the gas layer was rich in CO2, only the physical properties of the water layer adjacent to the gas-water interface were more favorable for the gas migration than those of the gas layer. This phenomenon can be referred to as the near-interface inversion of physical properties.