Optimizing separator pressure in a multistage crude oil production plant

Abstract

This thesis contains a steady state simulation model of a multistage crude oil production plant. The unstable reservoir fluid is separated into crude oil and gas in a three stage gas oil separation plant. Peng Robinson Equation of State and successive substitution is used to model the separators. The liquid output for the third stage separators is stable crude oil and the crude oil is sold on the international market. The gas liberated in the separators is compressed by centrifugal compressors and injected into the reservoir. Several methods for modelling centrifugal compressors are discussed. The discussion concludes that Dimensionless Parameters is the best suited method for this case and this method is therefore used. The plant studied has three separation stages at decreasing pressure and the standard control method is to use fixed separator pressure. This is not optimal and brute force optimization is used to find the optimal pressure set-points. Adaptive pressure set-points will improve product separation and therefore increase the profit. The profit is calculated as the income from crude oil sale minus the energy cost. Using this new method, the optimum separator pressures for Snorre A are determined. As a result, the profit is increased with 0.17, 0.11 and 0.07 present for three different inlet conditions. For one year this small increase in profit gives an extra two million dollars in income. The adaptive pressure set-points give an extension of the plant s maximum capacity for a reservoir fluid with a high gas oil ratio.