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•A fully coupled wellbore-reservoir system for mixed CO2and water was simulated.•A sensitivity analysis of parameters affecting reservoir pressure management was carried out.•CO2injection temperature has a large effect on the thermosiphon characteristics.

The most accepted conceptual model to explain surface degassing of cold magmatic CO2in volcanic-geothermal systems involves the presence of a gas reservoir. In this study, numerical simulations using the TOUGH2-ECO2N V2.0 package are performed to get quantitative insights into how cold CO2soil flux measurements are related to reservoir and fluid properties. Although the modeling is based on flux data measured at a specific geothermal site, the Acoculco caldera (Mexico), some general insights have been gained. Both the CO2fluxes at the surface and the depth at which CO2exsolves are highly sensitive to the dissolved CO2content of the deep fluid. If CO2mainly exsolves above the reservoir within a fracture zone, the surface CO2fluxes are not sensitive to the reservoir size but depend on the CO2dissolved content and the rock permeability. For gas exsolution below the top of the reservoir, surface CO2fluxes also depend on the gas saturation of the deep fluid as well as the reservoir size. The absence of thermal anomalies at the surface is mainly a consequence of the low enthalpy of CO2. The heat carried by CO2is efficiently cooled down by heat conduction and to a certain extent by isoenthalpic volume expansion depending on the temperature gradient. Thermal anomalies occur at higher CO2fluxes (>37,000?g?m-2?d-1) when the heat flux of the rising CO2is not balanced anymore. Finally, specific results are obtained for the Acoculco area (reservoir depth, CO2dissolved content, and gas saturation state). CO2fluxes at the surface are highly sensitive to the dissolved CO2content of the deep fluidThe sensitivity of CO2fluxes on reservoir geometry depends on the depth of gas exsolutionHeat conduction and isenthalpic volume expansion balance the heat carried by CO2

Previous work has shown that a transformation of the pressure head greatly improved the efficiency of the algorithms for simulating one‐dimensional infiltration into very dry heterogenous soils. We expanded the approach to solve two‐dimensional problems and introduced an adaptively optimum relaxation procedure to improve the efficiency and the robustness of the numerical solutions. A theoretical framework was developed for analysis of convergence problems in solving Richards' equation. Based on this framework, several simple calculation methods are proposed to estimate the relative convergence of different transformation methods. They can also be used to select “good” transformation parameters and a “good” time steps. Two transformation‐based methods, Kirkland's ϕ‐based method and Ross' p‐based method, as well as Celia's h‐based method are used as comparisons. Test cases are infiltrations into very dry heterogenous porous media. The results show that the Pt‐based method is the most effective of the four methods and the simplest of the three transformation methods. It is several orders of magnitude faster than the h‐based method and also reduces the truncation error using the same grid. The proposed adaptively optimum relaxation procedure enhances the efficiency and robustness of all four methods to different degrees. Combination of the Pt‐based method with the adaptively optimum relaxation procedure results in a very efficient and robust algorithm. Using a coarser grid in the horizontal direction can further improve the efficiency in both computer time and memory requirements of the two‐dimensional model without loss of accuracy.