DARTS-well: An open-source fully coupled well–reservoir numerical model using the Operator-based Linearization Approach

During subsurface energy-transition operations, such as carbon dioxide sequestration, underground hydrogen storage, and geothermal energy extraction, fluid flow into the well and reservoir can induce strong hydraulic and thermal interactions between the well and the near-wellbore region. These interactions are particularly critical during high-pressure CO 2 injection into depleted reservoirs, where significant temperature reductions may occur along and around the well, leading to complex phase behavior and potentially reducing injectivity. Conventional standalone well or reservoir models, however, fail to resolve these well–reservoir interactions. A fully coupled well–reservoir model is therefore essential to ensure accurate, safe, and efficient subsurface energy-transition operations. To address this need, we develop an open-source, computationally efficient numerical model for transient, thermal, multiphase flow in a fully coupled well–reservoir system. A multiphase, transient wellbore model is developed using the Drift-Flux Model and fully coupled with the high-performance, open-source Delft Advanced Research Terra Simulator (open-DARTS). Open-DARTS uses Operator-Based Linearization (OBL), where state-dependent properties are grouped as operators and interpolated from multidimensional tables generated on the fly, reducing the cost of evaluating residuals and derivatives in Newton–Raphson iterations. The wellbore model is verified against an analytical solution and benchmarked against industry-standard well simulators for single- and two-phase, isothermal and thermal flow in vertical and inclined wells, and the coupling approach is also benchmarked against a commercial coupled simulator. We then simulate liquid CO 2 injection into a North Sea–representative depleted gas reservoir using grid and OBL resolutions selected by convergence analyses.