Characterization of Light Hydrocarbons in Brazilian Pre-Salt Crude Oils: Applications in Reservoir Geochemistry

Abstract

The analysis of crude oil, especially light hydrocarbons (LHs) fractions or ranges, remains a challenging task in reservoir geochemistry. The large number of isomers hinders chromatographic separation, highlighting the necessity and urgency of developing comprehensive and rapid assessment methods. Advanced analytical techniques are vital in providing practical answers for monitoring oil production from enhanced oil recovery. The present study employs a new approach for the detailed characterization of LHs (C₅–C₁₅) in crude oils using comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC×GC-TOFMS) with a 30 m column length in the first dimension (¹D) and multiple modulation periods in a single run with simple sample preparation. The work aimed to obtain a rapid analysis with adequate resolution and selectivity to apply to LHs monitoring and to understand the behavior of the oil under the influence of injection of water alternating gas (WAG) into a reservoir over time (time-lapse geochemistry study). Adjusting the temperature program associated with multiple modulation period parameters provided a considerably shorter overall runtime analysis. In a repeatability study, the ratios between saturated and aromatic hydrocarbons (LHs) showed 15% RSD, being important data for inferring the hydraulic communication process in wells. Seven presalt crude oils from the Brazilian Santos Basin were collected from the same well during a seven-year production period, and the WAG recovery process showed compositional variations in LHs per carbon atom distributions, mainly for aromatic, naphthenic, and iso-alkane classes. The individual saturated and aromatic hydrocarbon ratios revealed differences between the crude oils from the same well. The developed method achieved rapid LH assessment and has promise in enhancing oil field management.