Gas Invasion Rule and Characterization Method Under Multi-Field Coupling in Deepwater and Deep-Formation Fractured Gas Reservoirs

Abstract Gas invasion is a critical challenge affecting the safety and efficiency of drilling in deepwater gas reservoirs, particularly in fractured gas reservoirs. The mechanism of gas invasion in fractured formations under thermo-hydro-mechanical (THM) coupling remains unclear, which significantly hinders the accurate calculation of key parameters of gas invasion. This study adopts a discrete fracture network approach to establish a gas-liquid two-phase flow model and a corresponding numerical solution method for fractured gas reservoirs, incorporating THM coupling effects in the flow process from the wellbore to the formation. The accuracy of the model was validated through laboratory experiments. Based on numerical results, the influence of matrix parameters, fracture parameters, fluid properties, and bottomhole pressure differences on gas invasion rates were revealed, and the dynamic closure patterns of fracture width during gas invasion under different bottomhole pressure were obtained. Furthermore, by modifying the production equation for conventional permeable formations, a mathematical characterization method for gas invasion rates in fractured gas reservoirs was developed, which can achieve accurate prediction of the bottom hole inflow boundary after gas invasion. The research findings can provide a theoretical foundation for predicting wellbore pressure and optimizing killing parameters after gas invasion in deepwater fractured reservoir.