Influences of SO₂ contamination in long term supercritical CO₂ treatment on the physical and structural characteristics of the Zululand Basin caprock and reservoir core samples

Understanding the mineral and microstructure changes in sandstone samples during supercritical CO₂ gas treatment is an important aspect of geological CO₂ sequestration. To gain insight into the effects of SO₂ contamination in CO₂ in South Africa, two core samples extracted from the reservoir lateral seal (ZC) and Cenomanian sandstone aquifer (ZG) within the Zululand Basin were studied. The samples were treated with CO₂ and CO₂/SO₂ gas streams at typical reservoir temperature and pressures (10 MPa and 316 K, and 17,5 MPa and 346 K) under supercritical conditions. High pressure Parr reactors were used for storing the rock-gas-water mixtures for up to two months using a water:rock ratio of 23:1. Tests were conducted using pure CO₂ and a 99% by weight (wt.) CO₂ and 1% (wt.) SO₂ mixed gas stream. Due to the prohibitive costs associated with CO₂ purification, the knowledge of the consequences of key impurities relevant to geological sequestration is critical. Pre-and post- CO₂/CO₂–SO₂ treatment characterisation was conducted using X-ray Diffraction (XRD) Analyses, Fourier transform infrared spectroscopy (FTIR), and low-pressure gas adsorption (LPGA). Varying mineral alterations were observed in the CO₂ treated samples, mainly comprising of calcite, plagioclase and smectite dissolution and the precipitation of quartz, plagioclase, calcite and smectite. Dissolution pores and pore clogging were observed and increased microstructure heterogeneity was reported. Increases in the adsorption capacity, surface area and pore volume were observed in all samples. After treatment with CO₂–SO₂ gas mixture, increases in mineral reactivity were observed in the ZC sample along with gypsum precipitation, indicating potential improvement in the lateral seal's self-sealing capacity. The introduction of SO₂ lead to the increase in quartz and plagioclase precipitation and increased dissolution of smectite and stilbite in the ZG sample. The study presents a novel investigation of the changes expected to take place during CO₂ injection in sandstone basins.