Study on Gas Invasion Behavior in Vertical Fractures in Deepwater Fracture-Cavity Formations

Deepwater drilling faces heightened risks of gas invasion due to complex geological conditions. To investigate well control safety issues during drilling in fracture-cavity formations, this paper conducted systematic experimental research and numerical simulations focusing on gas invasion behavior within vertical fractures. The results indicate that significant gravity displacement occurs between the drilling fluid and the invading gas within the vertical fracture channel. Analysis of the experimental and simulation results reveals that the gas invasion velocity and volume in vertical fractures are influenced by multiple factors, including fracture aperture and geometry, the pressure difference between the wellbore and the formation, and the drilling fluid density. Among these, the drilling fluid density has a significantly greater impact on gravity displacement compared to the pressure difference, pump rate, and gas flow rate. The gas invasion velocity shows a positive correlation with the fracture aperture and length, and a negative correlation with the fracture length. When the drilling fluid density increases from 1.1 g/cm3 to 1.2 g/cm3, the time for the gravity displacement front to reach the cave decreases by 33%. When the fracture aperture increases from 1 mm to 2 mm, the gas invasion velocity increases by approximately 2.5 times, and the gas invasion volume increases by 2.1 times. This study clarifies the dynamic characteristics and main controlling factors of gas invasion in vertical fractures, providing an important theoretical basis for optimizing well control techniques in fracture-cavity formations and accurately predicting gas invasion development.