Ca and CO2 Transport and Scaling in the Hijiori HDR System, Japan

Abstract

Carbon dioxide (CO2) and calcium (Ca) transport in underground water or geothermal reservoirs is an important topic in water-rock interactions, especially for estimating effects on CO2 storage or scaling in engineering geothermal systems (EGS) or hot dry rock (HDR) systems.

In the Hijiori HDR test field, dissolved anhydrite near the injection well is the primary source of calcium that leads to calcite precipitation in pipelines. HDR-2 has a lower temperature, and about 2.7 tons of calcite precipitated in the pipeline and well during the 3 months of CO2 circulation. HDR-3 has a higher temperature and about 0.6 tons of calcite precipitated in the deep well.

The transport of CO2 in HDR system including river water and atmosphere was estimated. From river water, production well and atmosphere, about 6 tons of CO2 were injected over a 3 month period. Of the CO2 that reached HDR-2, 68% was precipitated as calcite and 3% CO2 was discharged as gas by the atmosphere. In HDR-3, 20% CO2 precipitated as calcite and 30% CO2 was discharged as gas to the atmosphere.