A Comparison of Various Laboratory Techniques to Measure Thermodynamic Asphaltene Instability

Abstract

Asphaltenes are the n-pentane or n-heptane insoluble fractions of crude oil that remain in solution under reservoir temperature and pressure conditions. They are destabilized and start to precipitate when the pressure, temperature and/or composition changes occur during primary production. The precipitated asphaltene particles will then grow in size and may start to deposit onto the production string and/or flowlines, causing operational problems. In this paper, our emphasis is to identify the first pressure and/or temperature conditions at which the asphaltene will start to precipitate for two reservoir oils. Four different laboratory techniques were independently used to define the onset of the asphaltene precipitation envelope. These methods aregravimetric,acoustic resonance,light scattering, andfiltration. The gravimetric method was found to be precise and within the accuracy of the analytical methods. However, the method was time consuming. The acoustic resonance technique (ART) was fast and less subjective, but it did not define the lower asphaltene boundary. The interpretation of the onset pressure from the near-infrared (NIR) light-scattering technique (LST) was subjective to a degree. However, the NIR response defined the upper and lower boundaries of the asphaltene envelope and the bubblepoint pressure, as did the gravimetric technique. In a way similar to those of the gravimetric technique and LST, the filtration technique can also define the upper and lower asphaltene phase boundaries in addition to the bubblepoint pressure. The filtration technique is fast compared to gravimetric technique, but takes more time than the ART and LST methods. Introduction Asphaltenes remain in solution under reservoir temperature and pressure conditions. They start to precipitate when the stability of the colloidal dispersion is disturbed. This disturbance can be caused by changes in pressure, temperature, and/or composition of the oil. Precipitation and deposition of asphaltenes have reportedly caused operational problems ranging from plugging of tubulars and flowlines1–3 to clogging of production separators.4 Leontaritis and Mansoori present a comprehensive description of field problems caused by asphaltene deposition.5Fig. 1 schematically presents the asphaltene-related problems that may occur in the field. Asphaltene precipitation problems can be categorized as follows:Precipitation can be caused by the changes in temperature and/or pressure during primary depletion.Precipitation can be caused by blending or commingling of two noncompatible reservoir fluid streams (i.e., subsea completions), acid stimulation and/or enhanced recovery injection gases (CO2, H2S or rich gas). The correct operating procedure to minimize the asphaltene problem is not well understood. We believe a better understanding of the fundamental processes leading to solids precipitation is a prerequisite to management and prevention of production problems.

Prediction of the Gas Injection Effect on the Asphaltene Phase Envelope

“…The de Boer plot was developed based on field observations (de Boer et al, 1995), Figure 1 indicates that the reservoir fluid is in a moderate problem region. Moreover, the Asphaltene-Resin ratio screening method (Jamaluddin et al, 2001), suggests that the asphaltenes are unstable in the given fluid (Fig. 2).…”

Ready for Gas Injection: Asphaltene Risk Evaluation by Mathematical Modeling of Asphaltene-Precipitation Envelope (APE) With Integration of All Laboratory Deliverables

“…AOP is an important parameter in understanding asphaltene-precipitating behavior. As summarized by Jamaluddin et al (2001), various laboratory techniques to measure AOP [i.e., gravimetric, acoustic resonance, light-scattering technique using near infrared (NIR), and fi ltration techniques] have been developed and improved. During the long history of the fi eld, the latest techniques at each study campaign were adopted for the AOP measurement.…”

Estimating Onset of Precipitation of Dissolved Asphaltene in the Solution of Solvent + Precipitant Using Artificial Neural Network Technique

“…The results show less than 12% absolute deviation and the average absolute deviation (AAD) among all the experimental and estimated data is less than 2 %. The deviations may be attributed to unreliability of some experimental data [14], as it is known that measuring onset of asphaltene precipitation, especially in dilute systems is one of the most difficult problems of oil analyses [32]. It should finally be mentioned that in the ANN model developed in this work, three parameters of the four parameters approach zero indicating that this ANN model is approximately equivalent to a linear function.…”