Effects of wettability of rock-CO₂-water systems on the migration and distribution of geologically sequestered supercritical CO ₂

CO₂ sequestration (capture, injection, and long-term storage) in various geologic media including depleted oil reservoirs, saline aquifers, and oceanic sediments is being considered as a possible solution to reduce greenhouse gas emissions. Residual or capillary trapping is one of the most important mechanisms to impede the migration of supercritical (SC) CO ₂ and reduce leakage risks. Capillary pressure and relative permeability are key factors in sequestration, and they are strongly affected by the interfacial properties between SC CO₂, saline fluids and formation rocks. Interfacial properties are usually described by interfacial tension and contact angle. Preliminary analyses regarding the effects of wettability and entry pressure on the capacity and sealing efficiency have been presented in the literature. A few experimental observations illustrate that interfacial properties are functions of pressure, temperature, salinity and pH of formation fluids, as well as the formation rock mineralogy. The injection of SC CO₂ can alter the in-situ conditions of P, T, pH, etc. in target formations, and consequentially change the interfacial tension and contact angle. In this study, numerical simulations on idealized formations are conducted to investigate the effects of SC CO₂ injection induced interfacial property alteration on the capillary pressure and relative permeability, and consequently on the extent of SC CO₂ migration and storage capacity. Simulations and analyses in this study are mainly concentrated on three effects of SC CO₂ migration: 1) reservoir rock formation wettability; 2) capillary pressure variation; and 3) variations in relative permeability. To date, experimental measurements in the literature have been focused mostly on the variation of interfacial tension and contact angle. These effects are investigated in the research presented here by scaling the capillary pressure, calculated from the van Genuchten model, with parameters estimated from in-situ conditions (P, T, pH and salinity) using correlations determined from previous studies. By contrast, experimental investigations of the dependence of relative permeability on wettability are relatively sparse in the literature. In order to facilitate our investigation, the modified Burdine model is adapted, in which the effects of contact angle are explicitly described. It is found that the SC phase irreducible saturation increases as the SC CO ₂ wettability of the reservoir formation rock is elevated, thus enhancing storage efficiency.