CO2-brine-caprock interaction: Reactivity experiments on mudstone caprock of South-west Hub geo-sequestration project

Abstract

This study aims at focusing on the geo-chemical reactions of caprock upon injection of supercritical CO2 (ScCO2) under deep saline aquifer's conditions. The caprock samples were obtained at a depth of 979 m in the DMP Harvey-2 well, which is recognized as mudstone in the Lesueur Formation of the South-west Hub geo-sequestration project located 120 kilometres south of Perth in Australia. Geo-chemical reactions were conducted in a reaction chamber to determine the chemical reactivity of brine-saturated caprock under actual reservoir conditions (40 °C, 10 MPa and salinity 4.5%). The reaction was conducted at 10 MPa ScCO2 pressure and allowed for 37 weeks. The reacted fluid samples were subjected to several chemical analyses, including alkalinity tests, pH measurements and inductively-coupled plasma optical emission spectrometry (ICP-OES) tests. The solid minerals were tested using X-ray Diffraction (XRD), Scanning electron microscopy (SEM) and Energy-Dispersive X-ray (EDX) to obtain a better understanding related to of their chemical characterization of the caprock. Finally, a geochemical model (PHREEQC) was developed to forecast mineral dissolution/precipitation equilibrium and redox reactions.Fluid chemistry showed that the concentration of major elements such as Ca, Mg and K increased with time due to the dissolution of minerals such as K-feldspar, anorthite, and chlorite. On the other hand, the release concentration of Si, Fe and Al ions decreased with time due to the precipitation of secondary minerals such as kaolinite, gibbsite, amorphous silica and Fe(OH)3. Accordingly, the dissolution of minerals is very significant compared to precipitation of secondary minerals in the short term which can increase the pore volume of the caprock.