NUMERICAL MODELLING FOR CARBON ACCOUNTING FROM GEOTHERMAL POWER PLANTS

Abstract

Geothermal fields naturally emit greenhouse gasses through surface expression. When a geothermal power plant is installed on a geothermal field, the fluid extracted contains these gases, but generally, the reinjected fluid does not. For this reason, power companies producing from geothermal fields are considered an ‘emitter’ within the Emissions Trading Scheme (ETS) in New Zealand. They are hence liable to purchase carbon credits to offset emissions. At present, emissions are naively calculated from the emissions from production but do not consider the system as a whole.
When fluid is taken from a geothermal system, the pressure in the system is reduced, and some of the natural surface manifestations such as hot pools, fumaroles and steaming ground may decrease. This process lowers the natural emissions from the system, but this change is neglected in current emission calculations in the ETS. Numerical modelling of geothermal reservoirs provides a mechanism for more accurately accounting for total emissions provided the model has been well-calibrated to the emissions from the field. Numerical models solve conservation equations across the entire domain. Therefore, they can quantify emissions from geothermal wells and surface features to better explain total emissions from geothermal fields.