The Delft Geothermal Project is a consortium of governmental and industrial partners that aims to develop an innovative geothermal system at the campus of Delft University of Technology (DUT). Annually, DUT consumes 11 million m3 of gas, thus producing approximately 22000 tons of CO2 of which 15000 tons are attributed to the generation of electricity. The planned geothermal system is designed to contribute up to 5 MW, which results in a CO¬2 emission reduction of approximately 10000 tons.
To reduce CO2 emission further, a feasibility study is ongoing to also capture the CO2 and coinject it with the cooled-down-return water of the geothermal system. In this way, synergy is established between geothermal energy production and subsurface CO2 storage. To estimate the storage potential of the aquifer under Delft, and to assess the influence of coinjection of CO2 on the performance of the geothermal system, we simulate mixed CO2-water injection into the aquifer for various injected CO2 concentrations, taking into account the spatial distribution of the fluvial sandstone bodies, their connectivity, and their internal permeability heterogeneity. For this purpose, we have applied a new and effective solution approach to deal with phase disappearance and appearance, called the “negative saturation” (NegSat) solution approach. In the NegSat solution approach, a single-phase multi-component fluid is replaced by an equivalent fictitious two-phase fluid with specific properties. The properties of the fictitious two-phase fluid are such that in the single-phase aqueous region, the extended saturation of the fictitious phase is negative. Our results show that as long as the injected CO2 remains completely dissolved in the aqueous phase for the entire process, as the overall injected-CO2 mole fraction increases, the useful energy extraction decreases slightly, but the maximum stored CO2 increases accordingly. However, formation of a CO2-rich phase changes the character of the solution in terms of useful energy production/CO2 storage.

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