Crude oil fingerprint heterogeneity assessment by investigation of asphaltene subfractions: Implementation for reservoir continuity evaluation

Abstract

Asphaltenes are being a proxy of kerogen in the reservoir oil, which they have structural similarity. This characteristic provides an invaluable tool for obtaining oil fingerprints from the structural composition of asphaltenes. Since these compounds are classically defined as a solubility class, they constitute a large number of compound types with similar solubility features, instead of chemical characteristics. An effective strategy for the comprehensive characterization of these complex compounds is the fractionation of asphaltenes into several solubility subfractions. In this paper, asphaltenes were fractionated into three subfractions via adsorption of asphaltene onto CaCO3. The three obtained subfractions were entirely distinguishable, in which the FR-3 has the highest aromatic content, lowest aliphatic content, and the highest polarity among three subfractions. The FTIR spectroscopy was used for achieving structural composition information from each subfraction. Ninety peak ratios (10 × 9) based on ten characteristic bands were calculated for each subfraction, and a total of 270 peak ratios (90 × 3) were generated for each asphaltene sample. The modified moving window correlation coefficient technique was employed on generated peak ratios to obtain oil fingerprints for each sample and then the pairwise comparison between different samples. As exemplified, ten samples from the Asmari and Bangestan reservoirs in the Rag-e Sefid field were studied by the proposed procedure. The thresholds of MMWR and AMWR were determined as 0.9494 and 0.9894, respectively, in the Rag-e Sefid field. The similarity matrix was constructed based on a comparison of calculated MMWR and AMWR for all pairs of samples and thresholds to distinguish homogeneous and heterogenous oil fingerprints. However, the Bangestan reservoir has fingerprint homogeneity, some heterogeneities were observed in the Asmari reservoir. The investigation of migration pathways revealed initial heterogeneity in the reservoir, in which these fingerprints dissimilarities were removed in connected wells during geological times. The examination of heterogeneities revealed three compartmentalized regions in the Asmari reservoir.