PREDICTION OF CONDENSATE FLOW RATES IN LARGE DIAMETER HIGH PRESSURE WET GAS PIPELINES

Abstract

Esso Australia Ltd. operates two offshore gas platforms for Esso Exploration and Production Australia Inc. and Hematite Petroleum Pty. Ltd. in the Gippsland Basin. Gas and condensate from the Marlin platform flow to the gas plant near Sale, Victoria through a 67 mile, 20 inch pipeline. Gas and condensate from the Barracouta platform flow to the plant through a 30 mile, 18 inch pipeline. Average flowing pressure is 1300 psig. Condensate: gas ratios are 65 bbl/MMscf for Marlin and 15 bbl/MMscf for Barracouta.

As these platforms are the only source of supply for the city of Melbourne, gas rates are changed to match gas demand. Changes in gas rate are accompanied by changes in condensate flow. From consideration of liquid holdup and liquid residence time, a method of predicting the condensate flow rate resulting from gas rate change was developed.

A controlled run was made to test the prediction. After holding the Marlin gas rate steady at 150 MMscfd for 50 hours to reach equilibrium holdup conditions, the rate was increased to 250 MMscfd and held at this rate for 26 hours to reach equilibrium conditions again. The condensate flow rate out of the pipeline was monitored continually.

The Marlin pipeline test demonstrated that changes in condensate flow rate resulting from changes in gas rate in high pressure wet gas pipelines can be predicted within 15 per cent of actual rates using liquid holdup and liquid residence time as input data. In the absence of holdup data from pipeline pigging, Eaton's correlation will provide good values for holdup for wet gas pipelines with operating pressure up to 1500 psig and which traverse relatively flat topography.

This work has application in the sizing of liquid surge capacity required to receive condensate from high pressure wet gas pipelines. In many cases, investment in slug catcher facilities can be greatly reduced without risk of overfilling with liquid.