Investigation of Wettability of Organic-Rich Mudrocks via Fourier-Transform Infrared Spectroscopy

The complex composition of organic-rich mudrocks (ORM) presents a significant challenge in hydrocarbon exploration and production, leading to uncertainties in wettability assessment. Investigating the interactions between key rock components, such as kerogen, and water could significantly improve our understanding about the wettability of organic-rich mudrocks. While traditional wettability assessment methods such as contact-angle measurements have been widely used for wettability quantification, they do not provide information about the underlying physicochemical mechanisms that govern rock-fluid interactions. In contrast, elemental analysis provides a more detailed perspective on these interactions. In this paper, Fourier- transform infrared (FTIR) spectroscopy and gravimetric water adsorption isotherms were used to investigate the physicochemical mechanisms that control the wettability of ORM. By experimentally evaluating water adsorption behavior, we quantify the impacts of ORM composition, ORM thermal maturity, and kerogen wettability on the overall wettability of ORM. ORM from different formations were analyzed in this work. To evaluate the impact of thermal maturity on wettability, one of the rock samples was synthetically matured through a heat treatment process. The kerogen samples evaluated were chemically isolated from ORMs. The adsorption behavior of water on ORM and kerogen was investigated using both FTIR and gravimetric techniques. First, water vapor was continuously flowed over the samples for a specified duration and the infrared spectrum was recorded every minute. Then, the height of the infrared absorbance peak of the OH stretching mode of liquid water was plotted against time to observe the dynamics of water adsorption. Additionally, water adsorption isotherms were quantified using a gravimetric method - Dynamic Vapor Sorption Analyzer. The results from both techniques were then used to evaluate the adsorption behavior of water on ORM and kerogen. The results show that the sample with the highest clay content and total organic carbon (TOC) exhibited the greatest water adsorption. The cumulative water adsorption on the sample with higher clay content and TOC was approximately six times greater than that of the sample with lower clay content and TOC. After heat- treating one of the ORM samples to achieve a high level of thermal maturity, we observed a decrease in water adsorption. This suggests that increasing thermal maturity reduces the rock’s affinity for water. To further investigate the impact of thermal maturity on wettability, we analyzed the water adsorption behavior on kerogen samples with naturally distinct thermal maturity levels. Our findings confirm that water adsorption is higher in immature kerogen compared to mature kerogen. Continuous monitoring of the kerogen infrared spectrum during water adsorption and desorption revealed peak shifts in the region between 900 and 1300 cm-1. We show that these shifts are attributed to the disruption and formation of hydrogen bonds in kerogen upon interaction with water. These findings demonstrate the impacts of ORM composition and thermal maturity on wettability. Additionally, we elucidate the physicochemical mechanisms governing the interaction between ORM and water and highlight the role of kerogen in the wettability of organic-rich mudrocks. We also demonstrate that FTIR and gravimetric methods are capable of capturing the interfacial interactions between rock and water and can be used for assessment of wettability. The outcomes of this work provide valuable insights into the key factors influencing the wettability of ORM, which are critical to consider in both formation evaluation and the production methods used for organic- rich mudrocks.