Challenge to reservoir management and production operations. The appropriate field operational procedures to minimize the impact of asphaltene deposition are not yet well understood. In this work, we present a comparison of the fixed wavelength near infrared (NIR) and the variable wavelength spectral analysis system (SAS) for assessing asphaltene properties and discuss the application of each technique. In addition, we discuss and present estimations of asphaltene particle size and initial growth kinetics from data collected on two separate samples. These properties are used to establish fundamental differences between fluid samples collected using differing techniques.
Asphaltenes are the most aromatic and heaviest fraction of a crude oil. They have a molecular weight between 500–1000 g/gmol and are generally believed to be suspended as a micro-colloid of approximately 3 nm particles within the crude oil.1 Each colloidal particle is believed to consist multiple "sheets" of asphaltene molecules along with associated resin molecules which act as surfactants to stabilize the colloidal suspension. Resins are the next heaviest crude oil fraction and are somewhat less aromatic than asphaltenes. Whereas, isolated asphaltenes are friable and powdery, isolated resins are quite tacky and viscous. This distinction can be important in the interpretation of laboratory and field asphaltene phenomenon.2
"Asphaltenes" are typically defined in the literature on the basis of a solubility class. For example, asphaltenes are defined as the n-alkane (nC7) insoluble fraction of crude oil that is also soluble in toluene. This process isolates the most aromatic portions of the oil and represents the isolated friable and powdery substance described above as asphaltene molecules. However, in the field asphaltenes can destabilize and start to flocculate from their micro-colloid suspension. In these circumstances, asphaltene deposits are likely to contain both asphaltene and resin molecules. Depending on the chemistry of this mixture, the tackiness or deposition tendency of the precipitate will vary.
The micro-colloidal suspension described above can become unstable as a result of a decrease in pressure, the commingling of produced fluids3, or as a result of fluid composition change from gas injection,4,5 artificial lift, well stimulation,6 etc. Of particular interest to this work are the effects of pressure on asphaltene destabilization and deposition. Generally, crude oils that are susceptible to pressure-induced asphaltene formation are highly undersaturated, have relatively low solubility class asphaltene concentrations (< 3 wt%) and are relatively compressible7. As a result, they experience a significant change in physical properties with pressure, resulting in a decrease in solvating power.8 It is also noted that the asphaltene particles formed during a pressure reduction will show a continuous change in properties as the solvating power of the fluid decreases and additional material precipitates. In this sense, the ability to detect the formation of asphaltenes becomes a function of equipment threshold and the relative sensitivity of the detection method employed.9
Many experimental techniques have been proposed for making measurements related to asphaltene and have different sensitivities.9 In this work two similar, however, fundamentally different techniques for assessing the conditions of asphaltene formation and deposition are considered.