Asphaltene Stability in Crude Oils
- E. Rogel (PDVSA-INTEVEP) | O. Leon (PDVSA-INTEVEP) | Y. Espidel (PDVSA-INTEVEP) | Y. Gonzalez (U. Metropolitana)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- May 2001
- Document Type
- Journal Paper
- 84 - 88
- 2001. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 5.1.5 Geologic Modeling, 1.8 Formation Damage, 4.3.3 Aspaltenes, 4.1.2 Separation and Treating, 5.2.1 Phase Behavior and PVT Measurements
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The influence of different factors on the asphaltene stability in crude oils was evaluated. Compositional studies and structural characterization of resins and asphaltenes were carried out to study a possible relationship between these properties and asphaltene deposition behavior. Asphaltenes were obtained by precipitation with n-heptane and characterized by elemental analysis and spectroscopic techniques. Low hydrogen to carbon ratios, high aromaticities, and high condensation of aromatic rings were the main characteristics of the asphaltenes from unstable crude oils. According to these results, the stability behavior of the asphaltenes was influenced strongly by their structural characteristics. However, it was also found that stabilization by means of a commercial inhibitor was affected by the composition of the crude oil. In particular, a high content of basic functionalities in the crude oil can be related to the decrease of the inhibitor's effectiveness (dodecylbenzene sulphonic acid). Based on these findings, some useful suggestions applicable to the oilfield operations are made.
Asphaltene deposition is a well-known problem that generates a large cost increase in the petroleum industry.1 This phenomenon seriously affects the production and refining operations at different stages.2 Minimizing asphaltene precipitation is a major goal for many crude oil corporations; however, the main causes of asphaltene deposition have not been completely understood at present.
Asphaltenes, together with resins, comprise the disperse phase of the crude oils, while maltenes comprise the continuous phase.3 The precipitation of asphaltenes depends on the colloidal stability of these complex systems.4 Composition plays a major role among the causes that originate asphaltene precipitation. In general, the presence of similar weight percentages of saturates, aromatics, and asphaltenes are considered signs of similar asphaltene stability,5 but this is not always the case. Recent studies have pointed out that the nature of asphaltenes together with the nature of the dispersion medium are also important factors that determine the relative stability of crude oils and related materials.6-8
A possible way of avoiding asphaltene precipitation is by adding an asphaltene stabilizer, i.e., an amphiphile. Asphaltene stabilizers or inhibitors act in a way similar to resins, peptizing the asphaltenes and keeping them in solution.9 The effectiveness of an inhibitor is controlled primarily by its chemical and structural characteristics;10,11 however, its ability to stabilize asphaltenes depends also on the solvent or dispersion medium.10 For this last reason, commercial products must be tested before they can be used. Their effectiveness can be very different depending on the characteristics of the crude oil.
In this work, the interest is focused on three different aspects related to the asphaltene deposition problem and how to avoid it: composition of crude oils and its effect on asphaltene stability; chemical and structural characterization of asphaltenes and resins and their relation to asphaltene stability; and the effect of the composition of crude oils on the effectiveness of a commercial inhibitor.
Various crude oils with and without deposition problems are studied in this work. Their compositions and general properties were determined by standard techniques. Asphaltene and resin fractions from some selected crude oils were examined in detail using structural analysis. One of the main objectives of this work is to explore the relation between crude oil stability and the characteristics of the crude oil. For this reason, the crude oil stability was estimated through the determination of flocculation onsets adding a nonsolvent. The ability of a commercial inhibitor to stabilize asphaltenes was tested at different concentrations for the crude oils studied and its effectiveness was related to the chemical characteristics of the crude oils.
The main objective of this work is to improve the understanding of the relation between the stability of the crude oils and their chemical characteristics, as a first step towards the development of new tools for the prediction and prevention of asphaltene deposition.
Crude Oil Characterization.
Of the eight crude oils studied for the present work, six were stable crude oils and two were unstable crude oils. The unstable crude oils, NM1 and NM2, have shown operational problems during oil extraction, mainly clogging of the wellbore tubing due to asphaltene deposition. To establish the relationship between the composition and the stability behavior of the crude oils, the main constituents of the crude oils, saturates, aromatics, resins, and asphaltenes (SARA)12 were determined using Iatroscan thin layer chromatography with a flame ionization detector (TLC-FID). Other standard measurements carried out were total nitrogen by chemiluminescence detection (ASTM D-4629), basic nitrogen (UOP-269), acid number (ASTM D-664), and base number by potentiometric titration (ASTM D-2896).
Preparation and Characterization of Asphaltenes.
Four crude oils were selected to characterize their asphaltenes and resins. Two of these crude oils are classified as stable materials; the other two are unstable materials presenting asphaltene deposition problems. Asphaltenes were extracted from crude oils according to the method described in IP143/90.* Resins were obtained from maltenes using high-performance liquid chromatography.13 Elemental compositions were determined on a Leco CHNS 244 elemental analyzer model, and the average number of molecular weights was obtained on a Knauer vapor pressure osmometer in CH2Cl2 at 25°C.
Nuclear magnetic resonance (NMR) spectra for asphaltenes and resins were obtained on a Bruker ACP-400 spectrometer, at a resonance frequency of 400 MHz for protons. A flip angle of 45° was used, with a repetition rate of 3 seconds, spectral width of 12 ppm, and the chemical shift was referenced relative to TMS. 25 mg samples were dissolved in 1 ml of dichloromethane, and 5% by weight hexamethyl cyclosiloxane was added as an internal standard to be used in normalization. Average molecular parameter and molecular weights (MW) were calculated according to León14 for the studied samples using the NMR integral, the weight of the sample/internal standard system, and elemental analysis (%C, %H) of the sample. The average molecular models were constructed using a method developed by Carbognani et al.15
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