Components of Critical Nitrogen, Sulfur, and Oxygen Polar Heteroatomic Compounds Affecting Sandstone Wettability

Summary While the role of polar nitrogen, sulfur, and oxygen (NSO) compounds in influencing sandstone wettability is conceptually established, their specific molecular identities and quantitative impact in natural reservoir systems remain poorly characterized due to a lack of direct experimental evidence. Diverging from prior modeling-based approaches in this stidy, we use a sequential extraction process coupled with contact angle measurement, gas chromatography (GC)-mass spectrometry (GC-MS), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to directly identify and quantify the key NSO compound classes that govern wettability in reservoir sandstones. Our results reveal a critical wettability transition threshold upon dichloromethane (DCM) extraction, with contact angles shifting abruptly from 92.7° to 154.1°. FT-ICR MS analysis unambiguously identifies the predominant compounds responsible for this shift as N1-class alkaline nitrogen species and O2-O4-class acidic oxygen compounds. These compounds, particularly O2 and O3 classes inferred as monocarboxylic acids and hydroxy oacids, function as primary anchor molecules, strongly adsorbing to mineral surfaces and facilitating the retention of nonpolar crude oil components. This work provides direct molecular-level evidence for the “anchor molecule” mechanism. This evidence moves beyond theoretical proposals to offer a precise basis for designing targeted enhanced oil recovery strategies.