Carbfix 2: A transport model of long-term CO2 and H2S injection into basaltic rocks at Hellisheidi, SW-Iceland

Abstract

The Carbfix 2 project was developed to demonstrate the cost effectiveness and technical maturity of CO2 and H2S direct capture from emission sources and permanent geological storage by in-situ mineralization in basalts. CO2 and H2S emissions of the Hellisheiði Geothermal Power Plant are separated from the other non condensable gases and captured by water dissolution. It is then transported to an injection well where it is co-injected with geothermal brine from the power station. The injection takes place in the Húsmúli re-injection area also known as the Carbfix demonstration site and has been an integral part of the operations of the power station since 2014. Field scale three-dimensional transport models of the injection at Húsmúli are developed to characterize the storage reservoir and constrain the flow paths between the Carbfix re-injection site and the nearby Skarðsmýrarfjall production zone. The model shows a good match to the tracer recovery and enthalpy from the monitoring wells and suggests a strong structural control at Húsmúli associated with faults acting as preferential pathways for the fluid. Implementation of boron, calcium, CO2, and H2S as solutes in the transport model and comparison between the modelled and the measured concentrations in the samples from the monitoring wells allows to assess the conservative and reactive behavior of each solute. By comparing expected (modelled) and measured CO2 concentrations we can estimate the fraction of dissolved gas mineralization between the injection well and monitoring wells. The model confirms values recorded in the literature of a mineralization degree in excess of 70% for the well closest to the injection and 95% for the furthest well. This suggests that non-reactive simulation schemes can be used to effectively monitor in-situ mineralization, CO2 containment and long term storage security of the gelogical reservoir. Modelling the long-term injection of fluids at Húsmúli suggests a drop in electricity generation from the closest well to the injection zone by 2040 and provides a theoretical maximum estimate of the storage capacity of the reservoir of 300 megatons of CO2 by 2050 if 10% of the space available is filled.