(65d) Development of Experimental and Modeling Methods to Understand Asphaltene Precipitation

Asphaltene Precipitation and subsequent deposition during oil production is a potential problem in many oilfields around the world, especially for light crude oils with relatively low asphaltene content, or in oilfields that are treated with miscible gas injection.

Different experimental procedures and modeling methods have been proposed by several research groups trying to understand the mechanisms of asphaltene precipitation and deposition. However, because of the complexity of these phenomena, there is still active discussion about the theories governing asphaltene stability.

In this work we present experimental evidence that supports the theory that precipitation and aggregation of asphaltenes is a multi-step process, where the former is driven primarily by thermodynamics whereas the latter is driven by kinetics. Under this multi-step mechanism, asphaltene precipitation, which leads to the formation of an impure asphaltene rich phase in liquid-liquid equilibrium with an asphaltene-lean phase, is a fully reversible process. On the other hand, from the precipitated phase, subsequent aggregation and aging leads to the formation of more solid-like structures. Furthermore, we will present experimental results that suggest that the currently available commercial technologies to detect asphaltene precipitation (i.e. NIR spectroscopy and High Pressure Microscopy) might not be appropriate to detect the exact point of asphaltene precipitation, but instead they give a combined reading of precipitation plus aggregation. For this reason, the results obtained using standard Solid-Detection-System are extremely sensitive to the depressurization rates.

These experiments are in conceptual agreement with the modeling approach for asphaltene stability based on the PC-SAFT equation of State.  It was already proven that the PC-SAFT EOS can provide very accurate predictions of the phase stability of asphaltenes, bubble point of the oil, density of gas and liquid phases, GOR, composition of liberated gas and even asphaltene compositional grading that can induce in extreme cases the formation of tar-mats. In this work, we will present the progress of an enhanced characterization method that has been applied to various wells from three different fields in the Middle East. We found that it is possible to reduce from three to one the number of adjustable parameters for asphaltenes and we are still able to get comparable results for the phase stability of asphaltenes as well as the other properties mentioned above. The new method simplifies the fitting process and reduces its computational time significantly, without sacrificing the accuracy of the predictions.

With these results we aim to contribute to the understanding of the complex phenomena associated to the asphaltene phase behavior and the prediction of the occurrence and the magnitude of asphaltene precipitation and deposition.