Investigation of Multiphase Wellbore Flow Behavior Following Gas Kicks During Drilling in Deepwater Natural Hydrogen Reservoirs
Abstract
Natural hydrogen, strategically relevant to net-zero targets, complicates post-influx wellbore multiphase flow because of its extremely low molecular weight, density, and viscosity and its tendency to partially dissolve in drilling fluids. This study presents a drift-flux wellbore multiphase framework for deepwater natural-hydrogen drilling that captures gas-influx transients and explicitly represents hydrogen dissolution and exsolution during the influx process. The model employs a pressure–temperature-coupled solver, and hydrogen properties are dynamically updated with local pressure and temperature. Sensitivity analyses are conducted to evaluate the effects of influx rate, fluid density and viscosity of the mud, and circulation rate on gas holdup, pressure at the well bottom, and gas migration, and methane is used as a reference to quantify differences in transient flow behavior and pressure response under otherwise identical conditions. Simulation results indicate that, at an influx rate of 5×104 Sm3·d-1, hydrogen dissolution significantly suppresses early-time gas velocity and gas holdup, delays surface gas breakthrough by approximately 9.1 minutes, and reduces the rate of bottomhole-pressure decline. At later times, as dissolved hydrogen gradually exsolves from the drilling fluid, gas holdup increases rapidly and bottomhole pressure drops sharply, with the time from breakthrough to peak gas holdup decreasing from approximately 3.0 to 1.5 minutes. Increasing drilling-fluid density effectively limits gas expansion, whereas higher circulation rate and higher drilling-fluid viscosity accelerate gas migration. Compared with methane, hydrogen rises faster, produces higher wellbore gas holdup, and results in an 8–12% greater mud-pit volume gain. These findings elucidate the transient flow characteristics of hydrogen influx, clarify its distinct pressure response and gas-transport behavior, and provide technical guidance for safe drilling in deepwater natural hydrogen reservoirs.