Comparison of Coal-Derived and Petroleum Asphaltenesby 13 C Nuclear Magnetic Resonance, DEPT, and XRS (original) (raw)

The Molecular Composition of Asphaltenes in a High ly Compositionally Graded Column 1

Compositionally graded reservoirs are now observed routinely, and analysis of the gradients is used to assess reservoir architecture. Typically, the variation with depth of bulk properties of oil, such as GOR or asphaltene content, is measured and analyzed with an equation of state (EoS). The recently developed Flory-Huggins-Zuo (FHZ) EoS predicts asphaltene-concentration gradients in equilibrated reservoirs, and comparisons of those predictions with measured asphaltene concentrations (typically provided by downhole-fl uid-analysis logging tools) have been used repeatedly to assess reservoir connectivity. Crude oil is a highly chemically complex system, containing millions of unique chemical compounds, which cannot be fully described by bulk properties alone. Interpretation of bulk-property gradients therefore requires simplifying assumptions regarding petroleum chemistry. For example, the FHZ EoS relies on the assumption that the chemical composition of asphaltenes is constant throughout the reservoir, even though the concentration of asphaltenes varies throughout the reservoir. In this paper we test that assumption by examining the chemical composition of asphaltenes isolated from different locations in a reservoir with a large gradient in asphaltene concentration (15- fold variation in asphaltene content over a 300 ft depth range). Asphaltenes are analyzed with two advanced chemical analyses capable of measuring various aspects of their chemical composition: sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy, which measures the relative abundance of different sulfur species; and surfaceassisted laser desorption ionization (SALDI) mass spectrometry, which measures the mass of asphaltene nanoaggregates. The results of these analyses indicate that the detailed chemical composition of asphaltenes is not graded, validating the assumption of the FHZ EoS and suggesting that it is appropriate for compositional-gradient analysis even in reservoirs with large asphaltene gradients. On the other hand, the cubic EoS, which requires large variation in asphaltene properties to model such gradients, appears to be inappropriate.