EXPERIMENTAL SIMULATION OF RE-INJECTION OF NON-CONDENSABLE GASES INTO GEOTHERMAL RESERVOIRS: GREYWACKE-FLUID INTERACTION

Abstract

A laboratory experiment was conducted to simulate fluid-rock interaction between greywacke and geothermal brine containing high concentrations of CO2 and H2S at 200°C and 35 bars.
The apparatus consisted of a high T-P continuous flow through simulator containing 350 – 500 ?m greywacke fragments. The experimental fluid consisted of low pressure separator brine with a dissolved mixture of CO2/H2S/H2 (96/4/0.01%). The total experimental time was 66 days. Effluent samples contain high concentrations of CO2 and H2S averaging 4000 mg kg-1 and 70 mg kg-1, respectively. The pH values during the room T period averaged ~6.2 while after temperature increase to 200oC they decreased to approximately 4.5 – 5.0. Silica concentrations remained between 700 - 900 mg kg-1.
Upon completion of the experiment, the rock was examined for effects on its geochemistry and mineralogy by XRD and SEM-EDS analysis. Pyrite and calcite were identified as secondary run products by XRD. SEM-EDS examination showed considerable corrosion of ferromagnesian phases and pyrite precipitation from the released ferrous iron. Fluid chemistry demonstrates equilibrium with calcite and pyrite. The distribution of secondary phases suggests that reaction rates are rapid; however, aluminosilicate minerals appear not to have reached full equilibrium.