Subsurface – Surface Optimization for CO2 Geothermal Systems
Authors: Arekhov V. , Boehmer F. , Clemens T. , Wechsung M. 
Published: June 2024

Producing energy from geothermal sources is economically challenging. Using CO2 as a working fluid has a favorable effect on pressure losses during heat extraction. CO2 fluid properties, namely lower viscosity comparing to water and large variation of density depending on temperature, allow to achieve beneficial operational conditions and improve heat and energy extraction. However, due to the complexity of the system, finding optimum operational conditions is challenging. This requires coupling of the reservoir model with the wellbore and surface model. Moreover, optimization and selection of operational conditions must be done considering reservoir uncertainties.

We have developed a workflow for optimization of operational conditions in CO2 geothermal systems. Reservoir properties were defined under uncertainty in an analytical model to allow for fast computation. To assign reservoir parameter ranges, all available data were used including seismic, well logging, well test, and analogue data. Multiple reservoir realizations were simulated to predict possible CO2 flowrates and pressure losses in the subsurface. Wellbore modelling was performed using full enthalpy calculation and calibrated with available field test data. A sensitivity study was conducted for different tubing sizes to properly select wellbore design and equipment for each subsurface realization. Surface installations were modelled with Krawal-modular software and allowed to estimate possible energy output for the simulated operational conditions.

Energy generation is proportional to the CO2 mass flowrate and difference between pressures at production and injection wellheads. For each subsurface realization, the optimum operating conditions were iteratively found by solving for the largest net power output. The approach resulted in the optimum selection of the wellbore configuration. The results indicated that for the given reservoir, with 80% probability, the selection of larger diameter tubing will be beneficial for energy generation. At the same time, there is a 3% chance that the flow cannot be initiated with larger tubing diameter for a poor reservoir quality scenario. Furthermore, this allowed to correctly size the power generation installations and select the required gas processing and cooling systems. Moreover, the results have shown that the most influential parameter for energy extraction is reservoir permeability. The higher the permeability, the lower the pressure losses in the reservoir, which directly influences net power output of the whole system. The developed workflow has shown robust predictions of operating conditions for a variety of subsurface realizations.

The paper presents a novel approach in determination of optimum operational conditions for CO2 geothermal systems considering reservoir uncertainties. The proposed workflow can be used for green field opportunity assessment and provide robust expected range of net power output of the CO2 geothermal process. The described approach helps identifying well and surface equipment design and size for the reservoir of interest.