Wettability Quantification in Rock Components via Water Adsorption Isotherms

The inconsistency in wettability assessment using conventional methods, such as contact angle measurement and imbibition tests on core samples, challenges the wettability quantification in mixed-wet rocks. New methods for wettability assessment have yet to be developed for reliable wettability quantification in both homogeneous and heterogeneous rocks and in core samples with spatial variation in wettability. The concept of adsorption is fundamental to understanding fine-scale interactions between solids and fluids and potentially can be used for wettability assessment. In this paper, we experimentally compute water adsorption isotherms and perform sensitivity analysis on the impacts of (i) wettability levels, (ii) types of rock components, and (iii) concentrations of mineral constituents on water adsorption. We synthetically change the wettability of pure quartz powder to obtain samples with variable wettability levels. Subsequently, we used part of the chemically treated powders to create pellets and measure the contact angle. On the remaining portion of the treated powders, adsorption isotherms are measured using a dynamic vapor sorption analyzer. We also obtain water adsorption isotherms on several types of pure minerals and organic components, including diverse clay types, quartz, calcite, and kerogen. Finally, different minerals are mixed at distinct concentrations to evaluate the effect of composition on adsorption capacity. The results of water adsorption isotherms on quartz samples with unaltered and altered wettability revealed a decrease of approximately 72% in water adsorption at 98% relative humidity for the chemically altered quartz sample. Both isotherms and contact angle measurements confirmed that the sample’s wettability changed after the chemical treatment. Contact angle and water adsorption isotherms were also quantified on minerals typically found in reservoir rocks. Both methods indicated that these minerals are water-wet. However, the water adsorption isotherms were particularly effective in capturing varying levels of water-wetness among these minerals. Cumulative water adsorption for Na-montmorillonite and kerogen was found to be extremely higher compared to that of quartz. These findings indicate that rock components such as clay minerals and kerogen play crucial roles in controlling the wetting behavior of organic-rich mudrocks. Water adsorption isotherms estimated on mixtures of minerals also demonstrated high sensitivity to both the concentration and wettability of the minerals in the mixture. The outcomes of this work demonstrate that water adsorption isotherms provide valuable insights into the interfacial interactions between a solid surface and water, offering information about the water-wetness of both pure minerals and their mixtures. This technique shows significant potential for developing new methods to assess the wettability of mixed-wet rocks and rocks with complex mineral compositions.