• Energy Research

Abstract CO2-rock wettability is a key factor which determines the fluid dynamics and CO2 geo-storage capacity. However, the full understanding of real reservoir CO2-wettability is yet to be gained. We thus systematically analysed the wettability of CO2/brine/South West Hub sandstones at various pressures (0.1 MPa, 5 MPa, 10 MPa, 15 MPa, and 20 MPa) at 334 K. A new procedure based on organic carbon isotope tracking (δ13Corg) was proposed to eliminate the effect of artificial organic matter introduced by drilling mud penetration. The results indicate that the advancing (θa) and receding (θr) water contact angles for the CO2/brine/South West Hub sandstone system increase with increase in pressure (ranging from 71° to 118° and 66° to 111°). It can thus be suggested that the system is weakly water-wet to intermediate-wet. When the samples were treated with dichloromethane, a slight decline in organic content was observed leading to slight decrease in water contact angles (i.e. TOC decreased from 0.019% to 0.003% for core C, and the corresponding θa and θr decreased from 118° and 111° to 110° and 104°, respectively, at 20 MPa and 334 K). This wettability analysis demonstrates that (a) of the contact angle is very sensitive to the amount of organic matter and therefore care should be taken to remove artificial organic matter from the sample, and that (b) this condition prevails in a real proposed CO2-storage site. This analysis thus has important implications for assessing the feasibility of long-term CO2 storage and enabling large-scale industrial carbon geological storage projects.

Wettability is one of the main parameters controlling CO2 injectivity and the movement of CO2 plume during geological CO2 sequestration. Despite significant research efforts, there is still a high uncertainty associated with the wettability of CO2/brine/rock systems and how they evolve with CO2 exposure. This study, therefore, aims to measure the contact angle of sandstone samples with varying clay content before and after laboratory core flooding at different reservoir pressures, of 10 MPa and 15 MPa, and a temperature of 323 K. The samples’ microstructural changes are also assessed to investigate any potential alteration in the samples’ structure due to carbonated water exposure. The results show that the advancing and receding contact angles increased with the increasing pressure for both the Berea and Bandera Gray samples. Moreover, the results indicate that Bandera Gray sandstone has a higher contact angle. The sandstones also turn slightly more hydrophobic after core flooding, indicating that the sandstones become more CO2-wet after CO2 injection. These results suggest that CO2 flooding leads to an increase in the CO2-wettability of sandstone, and thus an increase in vertical CO2 plume migration and solubility trapping, and a reduction in the residual trapping capacity, especially when extrapolated to more prolonged field-scale injection and exposure times.