Molecular Characterization of Dissolved Organic Matter in Produced Water from Heavy Oil Recovery under Varied Oil Recovery Displacement Strategies

Abstract

This study employed gas chromatography–mass spectrometry and high-resolution Orbitrap mass spectrometry to characterize the molecular composition of dissolved organic matter in produced waters from both heavy and conventional crude oil reservoirs subjected to various displacement strategies, including water flooding, carbon dioxide flooding, steam stimulation, steam-assisted gravity drainage (SAGD), in situ combustion, and polymer flooding. Heavy-oil produced waters had higher dissolved organic carbon concentrations and greater polarity than conventional produced waters. For nonpolar to weakly polar fraction, phenolic and nitrogen-containing heterocyclic compounds predominated in steam stimulation and SAGD produced waters, whereas alkanes were the dominant compounds in produced waters from other displacement methods, particularly with a higher proportion of high-carbon-number alkanes in heavy oil samples. Analysis of polar components revealed that acidic oxygen-containing compounds in heavy oil produced water contained a higher degree of oxidation, with thermal processes generating highly condensed species. Basic nitrogen-containing compounds displayed higher aromaticity and condensation, and their distribution was distinctly governed by process conditions. Furthermore, polypropylene glycol ether compounds, originating from exogenous chemical additives, were detected in all produced water samples except those from in situ combustion. This finding provides fundamental molecular data to support the development and optimization of treatment processes for oilfield produced water.