CO2 sequestration into hydrothermal system at Ogachi HDR site

Abstract

Field experiments of CO2 sequestration into the Ogachi hot dry rock (HDR; the temperatureis 200 °C) site were performed to investigate mineralization of a part of CO2 as carbonatesthrough interaction with rocks (Georeactor; Ca extraction from rocks and carbonate fixation).In 2007, carbonated water (1 wt% CO2; river water with dry ice) with several tracers was directly injected into well OGC-2, Test #1 (from September 2nd to 9th) and Test #2 (from September 11th to 16th). During the Test #2 experiment, additional river water was injected into well OGC-1, 100 m apart from well OGC-2, at 2 days after the injection of CO2 water into well OGC-2 so as to push back carbonated water around this well. Water samples were collected at the depth of ~850 m by a sampler (500 ml in volume) and monitored for their chemical and isotopic compositions. The CO2 concentrations in fluids collected decreased with duration time and were almost 2/3 of the expected concentration from tracer concentrations. This means that CO2 injected into well OGC-2 can be removed from fluid by carbonate flxation. During the field experiments, dissolution or precipitation rates of calcite were determined by using a technique of "in situ observation of crystallization". Calcite crystals covered with Au film were hold in a crystal cell and set in a crystal sonde. The crystal sonde was filled with He gas under a certain pressure and then put into well OGC-2. The fluid at depth of -850 m is introduced into the sonde and calcite can start to react with them. After 1 hour, the sonde was recovered to the surface, where the fluid in the sonde is pushed to outside by He gas. The recovered calcite crystal is analyzed for the dissolution or precipitation rates by a newly developed phase shift interferometer. The "in situ analyses" show that calcite precipitation was observed within 2 days after the injection. This probably indicates that the most of CO2 injected has been fixed as carbonate, supporting our idea that HDR system can be useful for CO2 sequestration from inland emission sources.