A Novel Reservoir-Transient Wellbore Coupling Procedure for Carbon Storage in Depleted Reservoirs

Abstract Depleted oil and gas reservoirs represent one of the most promising options for CO2 storage projects, owing to their favorable characteristics, established storage integrity, and suitable subsurface conditions. A key challenge in simulating CO2 storage in such reservoirs lies in the low-pressure and low-temperature conditions, which are often near the critical point of CO2. As CO2 injection continues the near-wellbore region tends to be dominated by pure or nearly pure CO2. To accurately model this behavior, a robust isenthalpic flash formulation was developed specifically for pure or near-pure CO2 phase. Transient modeling of reservoir–wellbore interaction is essential for understanding the effects of CO2 injection, wherein variables within the wellbore tubulars are dynamically solved by accounting for flow regime, gravity, friction, and heat exchange with the surroundings. A controller was developed to couple a transient wellbore simulator with a compositional reservoir simulator. To enable seamless integration with multizone completions—particularly in heterogeneous reservoirs—a novel layer-based inflow performance relationship was formulated and implemented. The transition between transient and pseudo–steady-state flow regimes, and vice versa, based on operating conditions, is managed through controlled time-stepping in both the wellbore and reservoir simulators. This controller has been successfully employed to couple several depleted oil and gas reservoirs with transient wellbore models during CO2 storage operations, including under challenging conditions near the critical temperature and pressure of CO2 and during subzero-temperature CO2 injection. The paper presents the algorithm underlying the coupling process, along with the modeling of condensation and vaporization phenomena near the critical point. It also examines cases that exhibit complex physical behavior within the wellbore and in the near-wellbore region. Selected examples are provided to illustrate the strengths and advantages of the coupled simulation approach in supporting the design of safe CO2 storage operations and ensuring flow assurance.