Effect of the Physicochemical Structure of Mudstone on Readsorption Behavior of Water

Mudstone can store formation water when acting as a caprock, which is a part of water sources in early oil exploitation. To improve crude oil quality, it is necessary to understand readsorption behavior of water. The behavior of water is closely related to the physicochemical structure of mudstone. Interactions between water and chemical structures are hard to detect because of the overlap of O–H infrared vibration bands (IR) in H2O and that in mudstone, thus D2O was innovatively used to interact with mudstone. In this study, mineral compositions and pore structures of mudstone were identified by X-ray diffraction, X-ray fluorescence, and low-temperature N2 adsorption/desorption, respectively. Furthermore, water retention contents at different relative humidities (RH) were investigated by comparing hysteresis loop areas at low (S1) and high (S2) RH based on water adsorption/desorption isotherms. Interactions between water and mudstone were investigated by studying the IR spectra changes and release of D, HD, OD, and HOD during drying of mudstones containing adsorbed D2O using in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and thermogravimetric coupled with mass spectrometry (TG-MS), respectively. The results showed that the specific surface area (SBET) and mineral compositions controlled S1, while S2 was related to capillary pore volume. The detection of OD and HOD by in situ DRIFT at 300 °C and by TG-MS at 450 °C manifested that hydrogen bonds were formed between water and mudstone. Based on this study, we suggested that production wells should be constructed as far as possible on top of mudstone caprocks with relatively lower pore volumes and higher kaolinite contents to improve crude oil quality in early exploitation.