Influence of high-pressure conditions on the occurrence characteristics of shale adsorbed water: A case study of a shale reservoir in the Jiaoshiba area, Fuling, Chongqing

Objective Shale generally contains water, and it is highly important to clarify the occurrence characteristics of adsorbed water in shale to improve the drainage effect of shale gas. Methods This study analysed shale cores from the JY11-4 and JY41-5 wells in the Jiaoshiba area of Fuling. The influence of high-pressure conditions on the occurrence characteristics of adsorbed water in shale was investigated using custom-designed frozen nitrogen adsorption experiments and nuclear magnetic resonance experiments. Results and Conclusion The results show that (1) under atmospheric pressure, the average volume of adsorbed water per unit mass calculated by the "weighing method" is 0.017 3 mL/g. The volume proportion of water in the micropores and mesopores (average of 90.94%) significantly exceeds that in the macropores (average of 9.06%). This may be because water molecules cannot occupy the adsorption sites in all the shale pores when the relative pressure is relatively low. Most water molecules condense in micropores and mesoporous pores, and only a few water molecules enter the macropores. In addition, the small and medium pores in clay-rich shale become "filled and blocked" by water molecules in the process of water adsorption. (2) Under a saturated water pressure of 30 MPa, the average adsorbed water volume per unit mass of the sample calculated by the "weighing method" is 0.021 6 mL/g. The volume proportions of water in the micropores and mesopores (average of 40.26%) is lower than that in macropores (average of 59.74%), which may be related to the fact that water molecules can occupy more adsorption sites on the inner surface of micropores and mesoporous pores under the action of capillary forces when the relative pressure is significantly increased. (3) Compared with normal pressure, high-pressure conditions increase the volume of water adsorbed per unit mass of shale (approximately 25% increase in the experiment), and the proportion of water volume in large pores is greater than that in micropores and mesoporous pores. (4) During hydraulic fracturing, the relative pressure of the reservoir increases significantly. Under the action of capillary forces, fracturing fluid may enter large pores that cannot enter under the pressure of the original reservoir to "relieve" the "unsaturated state" of the original shale reservoir. After fracturing is completed, the pressure around the reservoir is gradually released, and the fracturing fluid that originally enters the adsorption pores of the shale may have difficulty overcoming the capillary resistance at the pore, making it difficult to return.