Physics-based Accounting for Natural and Anthropogenic CO2 Emissions from Producing Geothermal Systems

Geothermal systems are frequently used as a source of low-emissions energy. However, reservoirs with high amounts of dissolved CO2 can produce substantial power plant emissions, exceeding 100 kt/yr in some cases. Accurately accounting for the net anthropogenic emissions at geothermal systems is difficult because these systems also naturally flux CO2, sometimes at a magnitude similar to plant emissions, and this natural flux may increase or decrease due to plant operations. Furthermore, recent efforts in Iceland and New Zealand to capture and reinject geothermal emissions can further alter CO2 fluxes and raise the amount of CO2 stored in the subsurface.

In the absence of direct monitoring data, mathematical models can be used to estimate geothermal CO2 emissions. Here, I describe a lumped parameter model of CO2 flux through a liquid geothermal reservoir. The model parameterises CO2 influx due to magmatic degassing, CO₂ loss from vertical migration through caprock or lateral outflow, degassing due to pressure or boiling driven solubility changes, CO₂ extraction and reinjection through wells, and permanent storage through mineralisation reactions. Under constant mass extraction and suitable simplifying assumptions, the model can be solved exactly yielding exponential approximations of emissions rates (natural and plant), and reservoir CO2 content.

Calibration of this model to ten years of plant emissions and pressure decline data at Rotokawa and Ngā Tamariki geothermal fields (New Zealand), suggest that degassing trends are largely driven by CO2 dilution of the liquid reservoir. Furthermore, both depressurisation and dilution substantially lower the natural CO2 outflow, an effect not presently accounted for in greenhouse gas inventories. This means that measured plant emissions may exceed the true anthropogenic impact on geothermal emissions by up to a factor of three, which has substantial financial implications for geothermal plant operators.