Nanoaggregates of Asphaltenes in a Reservoir Crude Oil and Reservoir Connectivity

Abstract

Recently, asphaltenes have been shown to form nanoaggregates in toluene at very low concentrations (10−4 mass fraction). Subsequently, in situ analysis of a 3000 ft vertical column of crude oil by downhole fluid analysis (DFA) indicated that the asphaltenes in a black crude oil exhibit gravitational sedimentation according to the Boltzmann distribution and that the asphaltene colloidal size is ∼2 nm. Here, we perform a follow-up study of a reservoir black oil from a different field. The black oil in a 658 ft vertical column is analyzed by DFA and advanced laboratory analytical chemical methods. An asphaltene colloidal particle size is found to be ∼2 nm according to the Archimedes buoyancy term in the Boltzmann distribution. In addition, an equation of state (EoS) approach based on literature critical constants and molecular weights for asphaltenes gives an aggregation number of ∼8. Molecular compositional similarities between different oil samples were established with comprehensive two-dimensional gas chromatography (GC × GC). Likewise, results from electrospray ionization Fourier transform ion cyclotron resonance mass spectroscopy (ESI FT-ICR MS) of the samples are consistent with the oils being from the same equilibrium column of oil. The results herein support a growing body of literature indicating that asphaltenes in black oils form relatively tightly bonded nanoaggregates of a single size range. The similarity of results between asphaltenes in crude oil and asphaltenes in toluene points to a very limited role of resins in these nanoaggregates, in contrast to much speculation. The implications of this work on the determination of reservoir connectivity are discussed.