Quantifying the Impact of Organic-Rich Mudrock Composition on Wettability through Water Adsorption Isotherm Measurements

Accurately assessing the wettability of organic-rich mudrocks (ORM) remains a significant challenge. Their intricate composition is a key factor that complicates the quantification of wettability using conventional methods. In this article, we introduce water adsorption isotherms as a new approach to experimentally quantify the impacts of ORM composition on wettability. Water adsorption isotherms were initially quantified on several pure rock components, including minerals and kerogen, to understand the adsorption behavior of rock constituents typically found in the ORM. Subsequently, water adsorption isotherms were measured, and compositional analyses were conducted on ORM samples with a wide range of compositions and thermal maturities. Quantitative X-ray diffraction (QXRD) was used to determine the weight concentrations of mineral components, and pyrolysis was employed to quantify total organic carbon (TOC) and hydrogen index (HI). Water adsorption isotherms were obtained by using a dynamic vapor sorption analyzer. The quantified water adsorption isotherms on the ORM, along with detailed compositional and thermal maturity information, were used to evaluate the impact of rock composition on wettability. Additionally, wettability assessments through water adsorption isotherms were compared to contact angle measurements. Quantification of water adsorption on pure rock components revealed that clay minerals and kerogen exhibit significantly higher water adsorption compared with other minerals. The high water adsorption observed on kerogen is attributed to water sorption-induced swelling. Water adsorption isotherms measured on the ORM samples showed that rock samples from the same formation exhibit similar water adsorption. Among the ORM samples analyzed, the sample with the highest water adsorption had the highest clay concentration and TOC. Conversely, the sample with the lowest water adsorption contained negligible clay content and low TOC. The results demonstrate the measurable sensitivity of water adsorption isotherms to rock composition and the significant impacts of kerogen and clay minerals on ORM wettability, supporting our observations on the adsorption behavior of pure rock components. The comparison of water adsorption isotherms with contact angle measurements further demonstrates that water adsorption isotherms are more sensitive to rock composition compared with the contact angle measurements. The findings presented in this paper demonstrate that water adsorption isotherms represent a promising approach for assessing the impact of rock composition on the wettability of an ORM. Quantifying water adsorption isotherms helps address challenges like surface roughness and contamination, which often introduce uncertainties in contact angle measurements. Water adsorption isotherm measurements also eliminate the limitations of imbibition-based methods, which makes them unreliable in tight rocks. Therefore, the method introduced in this article can potentially be used as a new method of wettability characterization that honors the complex geochemistry of ORM.