Wettability Quantification in Organic-Rich Mudrocks by Water Adsorption Isotherms
Authors: Araujo I. S. , Heidari Z. 
Published: September 2024

A precise wettability assessment of organic-rich mudrocks (ORM) is still an ongoing challenge. The intricate complex composition of these rocks is one of the aspects that complicates quantification of wettability of ORM. In this paper, we introduce a new method to experimentally estimate wettability of complex reservoir rocks by utilizing water adsorption isotherm measurements. We also verify the reliability of this method by comparing it against conventional wettability assessment methods. Moreover, we assess the influence of mineral composition and Total Organic Content (TOC) on the measured water adsorption isotherms. Powdered rock samples collected from two ORM formations, A and B, and a carbonate formation were used for a set of experimental measurements. X-ray diffraction (XRD) was performed to obtain weight concentrations of mineral components. TOC was quantified using pyrolysis measurements and EDS measurements were carried out to analyze the spatial distribution of rock components. Then, we performed N2 adsorption- desorption measurements to obtain the surface area of the solid components in the samples. Water adsorption isotherms were computed using a Dynamic Vapor Sorption Analyzer and water contact angle was measured using a goniometer. Two carbonate samples were added to the set of rocks analyzed for the purpose of comparison and verification of the results. One of the carbonate samples was kept on its original water-wet state and the other sample was chemically modified to render an oil-wet sample.

The water adsorption isotherms performed on the ORM samples were compared against water contact angle measurements. Results demonstrated that the three samples from formation A that displayed overlapping water adsorption isotherms also present very similar contact angle, indicating that they have similar affinity to water. The sample from formation A that showed the lowest water adsorption was found to have the highest contact angle among the samples from formation A, underscoring its low affinity to water. Contact angle measurements obtained for the water-wet and the chemically-altered oil-wet carbonate samples were 0° and 120°, respectively. The Amott index of the unmodified carbonate sample was 0.5. After wettability alteration, it decreased to 0, indicating a mixed-wet behavior. However, water adsorption isotherms show only 25% decrease at 70% relative humidity, suggesting a moderate change in the wettability towards the oil-wet state. Both isotherms and Amott index agrees that the sample did not reach a completely oil-wet behavior, while contact angle measurements suggest a significantly oil-wet state. We demonstrated that water adsorption isotherms effectively capture changes in wettability resulting from alterations in the composition of the rock. Our results also revealed that the introduced method effectively identifies the different wetting characteristics of reservoir rocks. By quantifying water adsorption isotherms, we can overcome challenges such as surface roughness/contamination that often lead to uncertainties in contact angle measurements. It also eliminates the limitations of imbibition-based method, which makes them unreliable in tight rocks. The technique introduced here can be used as a new method of wettability characterization that honors the complex geochemistry of ORM.