Evaluating rock wettability in CO2-water systems using relative permeability data: Implications for geologic CO2 sequestration in saline aquifers

Abstract

Wettability of the reservoir rock plays a crucial role in CO₂ sequestration in saline aquifers, affecting trapping mechanisms, migration dynamics, spatial distribution, and injection performance. We present two methods to determine rock wettability in CO₂-water systems by analyzing water-displacing-CO₂ relative permeability data. The first method assesses whether the water saturation at the crossover point of relative permeability curves is above or below a reference value —defined as a function of endpoint saturations— to indicate water-wet, CO₂-wet, or neutral-wet conditions. The second method compares the areas under the CO₂ and water relative permeability curves, based on the principle that the wetting fluid's area should be smaller. We developed two indices for each method, ranging from −1 (strongly CO₂-wet) to 1 (strongly water-wet). Wettability was evaluated from 33 tests across sandstones, carbonates, and shales, showing a range of CO₂-wet to water-wet states that generally align with reported contact angle measurements. A key observation is that simpler water–oil systems used as proxies may not reliably represent CO₂–brine behavior, as they often remain water-wet due to cleaning procedures and the use of fluids that do not alter wettability during the experiment, even when viscosity ratios and interfacial tensions are matched. While uncertainties remain due to sampling origin (e.g., outcrop versus subsurface) and laboratory cleaning procedures, the ability to derive wettability directly from relative permeability data offers a practical advantage, as such measurements are more accessible than reliable contact angle data. This enables more robust interpretation of multiphase flow behavior, supporting improved assessments of CO₂ storage capacity, injectivity, and long-term containment. The approach presented in this research for inferring wettability from relative permeability data is not limited to CO₂–brine systems. It can also be applied to other systems, such as water displacing hydrogen sulfide (H₂S) in acid gas storage or water displacing hydrogen in underground hydrogen storage (UHS).