Pressure Evolution Characteristics of Gas Invasion Shut-in in Deepwater Ultra-deep Horizontal Wells

Deepwater gas invasion is highly sudden, concealed, and characterized by complex pressure–flow coupling, and failure to accurately capture its pressure evolution can rapidly lead to uncontrolled blowout events. To enhance well-control reliability, this study develops a transient gas–liquid two-phase flow model for deepwater ultra-deep horizontal wells based on mass and momentum conservation, and employs numerical simulation to analyze the temporal and spatial evolution of bottomhole pressure, annular pressure, and wellhead casing pressure during gas invasion and shut-in. Results show that bottomhole pressure continuously decreases after gas invasion, with a faster decline at higher circulation rates, while longer horizontal sections generate higher baseline bottomhole pressure due to increased friction. As gas invasion progresses, the annular pressure profile shifts downward and the invaded gas zone expands to nearly three times its initial extent within 45 min; lifting the drill bit off bottom significantly increases gas influx and mud-pit gain and accelerates bottomhole pressure decline. Horizontal wells exhibit higher post-invasion bottomhole pressure and a distinct inflection point compared with vertical wells, enabling determination of the gas invasion location through the proposed multiphase-flow-based method. The shut-in process comprises overflow, pressure-stabilization, and rapid pressure-rise stages, with the stabilization stage representing the critical “golden window” for well killing. Delaying shut-in from 15 to 35 min increases wellhead casing pressure by 81%, amplifies bottomhole pressure rise by 200%, and advances rapid pressure escalation by 60%. Additionally, the allowable mud-pit gain of a 10,000 m well is about 78% lower than that of a 4,000 m well, underscoring the need for high-sensitivity pit-volume monitoring and stricter alarm thresholds. This work clarifies the shut-in pressure evolution mechanisms of deepwater gas invasion and provides essential insights for gas-influx diagnosis, shut-in optimization, and well-control safety design in deep and ultra-deep drilling operations.