Energy Reports (Nov 2021)
An investigation of factors influencing carbonate rock wettability
Abstract
The presence of asphaltenes and polar compounds in a crude oil plays a major role in dictating the type of wettability it displays with oil reservoir rocks. The main objective of this study was to investigate the effect of the asphaltenes structure and other polar compounds in altering rock wettability. Five, natural crude oil samples were used with varying asphaltene (6.4 to 11.6), total nitrogen (0.029 to 0.097) and sulfur (2.259 to 4.234) contents, all in mass%. Crude oil characterization showed the asphaltene samples to consist of condensed hydrocarbon and sulfur aromatic molecules with one or two sulfur atoms per molecule. The weight-average numbers of fused aromatic rings for hydrocarbon molecules were 6 to 7 and for sulfur-containing molecules was an average of 8 aromatic rings, where the aromatic systems were substituted with short alkyl chains of less than 20 carbon atoms. Based on mass spectral evidence, the average empirical molecular structures of the asphaltenes were constructed and they were all found to be relatively similar. Entrained maltene species with 2 to 5 aromatic rings with longer alkyl chains were also found. Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) was used to characterize each crudes’ asphaltenes fractions. Three rock samples with mineral compositions of 100% dolomite (D100), 100% calcite (C100), and 63% calcite + 37% dolomite (C63D37) were used. Variation of the contact angle with time was measured using brine, as the external phase, and the five crudes on the rock samples. The interfacial tension (IFT) between brine and the five crudes were also measured. Among the five crude types, the three rocks show the strongest wetting tendency when contacted by type V oil. These were 129.2°, 109.2°, and 135.1° for D100, C100 and C63D37, respectively. Interestingly, wettability was not linearly correlated with asphaltene contents for all crude types. Wettability varied with the rock sample, nitrogen and sulfur contents, and crude’s non-asphaltenic composition. As a result, there is no strong evidence suggesting that the content nor the structure of the asphaltenes alone has a large effect on altering rock wettability.
