Modeling high-pressure wax formation in petroleum fluids

A procedure is presented for the simultaneous prediction of fluid-fluid and solid-fluid equilibrium of light gases/heavy hydrocarbons systems under pressure. The objective of this work is to simplify the predictive approach previously proposed by the authors, making it more useful and accessible to users at the petroleum industry. The main modification with respect to the complex LCVM (linear combination of Huron-Vial and Michelsen) mixing rule that was previously used for the attractive term "a" of the equation of state (EOS). To avoid the use of a group contribution method, which is difficult to bring into play with real fluids, the LCVM mixing rule is here replaced by the classical quadratic van der Walls one-fluid mixing rule. The deviation that using this mixing rule introduces into the gamma-phi approach is cancelled by a new parameter, introduced in the calculation of the solid-phase activity coefficient. This procedure was tested for a large range of multicomponent systems where the parameter was fitted to match the wax appearance temperature (WAT) at atmospheric pressure. From the values obtained a correlation relating the parameter to the carbon number in the heavy fraction was developed, allowing a prediction of wax formation in petroleum fluids. It is shown that this approach can successfully be applied to the prediction of wax formation at high pressure for live oils. (c) 2005 American Institute of Chemical Engineers.