Wettability Index Assessment Based on Water Absorption Isotherm Measurements in Rocks with Complex Compostition

Wettability is an essential petrophysical property that controls the relative mobility of water and hydrocarbons in oil and gas-bearing formations. However, applying conventional wettability assessment techniques to tight rocks with complex mineralogy is challenging and often yields inconsistent results. Water adsorption isotherms have shown superior sensitivity to water affinity compared to the commonly used contact angle method, making them a strong candidate for wettability quantification in such formations. The objectives of this paper are to (i) test our recently developed adsorption-based forward model and inversion algorithm method for evaluation of wettability using synthetic mineral mixtures covering a wide range of wettability and (ii) introduce and validate a new wettability index which can be calculated using water adsorption isotherms for rock components. We perform water adsorption isotherm measurements on 15 mixtures with varying concentrations of pure quartz, calcite, illite, and kaolinite minerals to represent water-wet conditions. To investigate oil-wet behavior, organo-silane and naphthenic acid treatments are performed to modify the wettability of quartz and calcite powders, respectively. We then repeated the isotherm measurements on 7 additional mixtures prepared by blending the treated quartz and calcite with the pure clay minerals. The experimental data are used to evaluate a new effective medium wettability model and inversion algorithm that reconstruct the adsorption isotherm of a given sample from the mineral composition. Next, we compute cumulative adsorption for each sample and rescale it to a dimensionless wettability index ranging between 0, which indicates oil-wetness, and 1, indicating high water-wet conditions. This workflow enables depth-by-depth wettability estimation using well logs integrated with a few lab measurements of adsorption isotherms. The water adsorption isotherm measurements revealed significant differences in water affinity among the four end-member minerals, ranging between 0.19% and 6.12%. Inversion results confirmed that the effective-medium forward model reproduces mixture adsorption isotherms using the Leave-One-Out Cross-Validation (LOOCV) with strong accuracy for the 15 water-wet mixtures. The inversion-predicted cumulative adsorption at 70% RH agrees closely with experimental values, typically within 0.5-4% relative differences for 80% of the mixtures; NRMSE = 4.82%, R2 = 0.96. Furthermore, an effective medium-based forward model was tested to reproduce the adsorption isotherms of wettability-altered mixtures, which found closely tracking the measured curves across 0-70% RH and capturing the systematic reduction in the adsorption relative to the unmodified counterparts. The cumulative adsorption of the model-estimated adsorption closely matched the experimental values for treated mixtures, typically within approximately 10% relative error. Finally, the adsorption-based wettability index (WIads) is developed to convert cumulative water adsorption into a normalized measure of wettability, ranging between 0 and 1. The findings show that water adsorption isotherms provide a more reliable wettability assessment than conventional contact-angle methods, especially for compositionally complex rocks. These results support the potential for depth-by-depth wettability quantification using well logs, enabling improved hydrocarbon-water production evaluation while honoring fluid mobility and wettability. In addition, the workflow helps quantify the water sensitivity of clay-rich rocks, supporting better decisions on water-based drilling and completion fluids tailored to formation mineralogy and wettability.