A Unified Approach to Asphaltene Precipitation: Laboratory Measurement and Modeling

Abstract

Summary A unified approach to evaluating asphaltene precipitation based on laboratory measurement and modeling is presented. This approach used an organic deposition cell for measuring asphaltene drop out onset conditions. Asphaltene precipitation was detected by changes in optical fluorescence, electrical conductance, and visual observation. A series of experiments measured the effects of changing pressure, temperature and composition on asphaltene precipitation. A fully-compositional V-L-S mathematical model completed the analysis by matching the experimental results. The model was then used to forecast asphaltene precipitation under a variety of production scenarios including response to gas-lift operations, and to evaluate the possible location of a tar-mat. Introduction An organic deposition cell (ODC) was developed to experimentally detect asphaltene drop out onset. Two different detection techniques were incorporated into the cell. Fluorescence spectrometry has been used to study aromatic compounds. Fiber optics allowed application of this basic technique to asphaltene detection in the ODC. Conductance measurements were also incorporated into the ODC. Thermodynamic models have been used to predict asphaltene precipitation from reservoir crudes under a variety of conditions. Burke et al and others applied polymer solution theory to the prediction of asphaltene precipitation. While this technique has been very successful, it is limited in its capability to predict the amount of asphaltene precipitate and varying precipitate composition. It was therefore decided to use the more rigorous approach presented by Thomas et al to model asphaltene precipitation. This method is based on an earlier wax deposition model which allows a fully compositional representation of the asphaltene phase. EXPERIMENTAL TECHNIQUES The high-pressure (6000 psig) I high-temperature (250 F) ODC employed a piston to allow sample or chemical introduction and pressure/volume control(Figure 1). The piston was designed to minimize dead volume so that flashing during sample introduction was reduced. Heating elements and cooling coils were installed for temperature control. The sapphire window allowed limited visual inspection and the use of a flexible fiber optics probe for luminescence detection. The cell was rocked to allow sample mixing and could be inverted so the window was on top. Electrical conductance was measured between the cellwall and the conducting probe. The conductance values obtained included a high background contribution from the equipment.