Abstract

Asphaltene data for 36 reservoir fluids from different geographical regions has been used to evaluate and compare the ability of four commonly used screening methods and three equations of state to identify fluids likely to present asphaltene problems during production. The screening methods are the de Boer method and three SARA based methods. The data covers asphaltene onset pressure data with and without gas injection at a wide range of temperatures. The evaluation showed that the Asphaltene Stability Index method is the most reliable of the tested screening methods, but all screening methods may fail to identify fluids with asphaltene problems. Analyzing asphaltene onset pressure data, it was seen that fluids precipitating asphaltenes by depressurization consistently seem to be causing asphaltene problems during production. The three equations of state tested were the SRK equation, the CPA equation and the PC-SAFT equation. None of the equations was able to predict asphaltene onset pressures in the absence of measured data, but all three equations could be tuned to match the development in asphaltene onset pressure with temperature and the impact of gas injection on the asphaltene onset pressure. The more complex CPA and PC-SAFT equations showed no advantage over a classical cubic equation of state. A newly developed algorithm for generating complete asphaltene phase diagrams helped explain why different trends in asphaltene onset pressure with temperature may be observed.

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