Drilling costs account for the majority of geothermal project costs. Therefore, it makes sense to reuse abandoned gas and oil wells supported by in-situ heat flux. This study examines the extent to which abandoned hydrocarbon wells as well as depleted reservoirs can be used for geothermal heat recovery and/or power generation using different circulation fluids, the retrofit opportunities and challenges to make them suitable for geothermal energy and assess both based on preliminary economical comparison.

We numerically investigated open and closed geothermal systems using the characteristics of depleted German oil and gas reservoirs as examples. We generated the heat and material balances of a depleted gas reservoir with a 3D reservoir simulator with abandoned wells for injection and production. For selected wells, we also calculated heat transfer in a closed-loop geothermal system for different system configurations. In each case, we evaluated the timely performance of heat extraction. The calculated data were used for a preliminary economic evaluation that included the cost of retrofitting the wells and monitoring the critical process parameters.

Our calculations confirmed the advantage of using depleted gas reservoirs as open geothermal systems, provided that the thermal capacity of the reservoir is sufficient. Another obvious advantage is that injection and production wells can be retrofitted at relatively low cost; only the production wells must be isolated. Wellbore flow containing impurities from the reservoir appears to be one of the potential hurdles. In case of lower permeability petrophysics, the connection between injection and production wells should/could be made either by hydraulic fracturing or by hydro-shearing of existing fractures in the rock, which could increase the capital cost. In closed loop systems the heat transfer from the formations is limited. Even in the case of appropriate retrofit of the wells for coaxial design for using as heat exchanger, the sustainability of heat extraction is limited allowing for direct heating applications rather than economic power generation. The economic and energy analysis were performed to assess and compare the thermo-economic performances of both systems.

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