Harcourt granite scCO₂ water interaction: a laboratory study of reactivity and modelling of hydrogeochemical processes

Authors: BADULLA LIYANAGE AVANTHI ISAKA, RANJITH PATHEGAMA GAMAGE

Abstract: Batch-type laboratory reactivity experiments and modelling of hydrogeochemical interactions of a granite-scCO₂-water system were conducted at 100 °C and 10 MPa in order to evaluate the geochemical and mineralogical responses of the granite to long-term reaction. The laboratory reactivity tests were conducted for a total duration of 70 days, and the continued hydrogeochemical interactions for up to 210 days were determined by geochemical simulations. The reacted granite powder and the residual solutions were subjected to several analytical techniques, including inductively-coupled plasma optical emission spectrometry (ICP-OES), X-ray diffraction (XRD), scanning electron microscopy (SEM) and pH measurements, in order to characterise the mineralogical interactions. The fluid chemistry of the residual solutions shows that significant dissolution occurs in Harcourt granite at the initial stages of saturation. Hence, the Na, K, Ca, Mg, Al, Fe and Si concentrations increase in the solution. The dissolution of anorthite, albite and biotite minerals continues with time, whereas the dissolution of quartz is comparatively slow in the system. However, the Ca, Mg, Al, and Si concentrations gradually decrease with the increase of reaction time due to the formation of secondary precipitants. Metastable clay minerals such as smectite, illite, and kaolinite precipitate in the saturation medium with time. The permanent deposition of clay minerals in the upper boundary of the geothermal reservoir is beneficial for the creation of a sealing zone for geothermal reservoirs. Importantly, calcite precipitation occurs with the long-term saturation of the system, and, hence, there is potential for CO₂ trapping in the outer uppermost periphery of CO₂-based geothermal reservoirs with long-term operation.

Keywords: CO₂-based geothermal reservoirs, mineral interactions, Harcourt granite, long-term operation of enhanced geothermal systems (EGSs), PHREEQC model

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