Demonstrating the Accuracy of a Novel Modification to Cubic EOS to Predict the JT Effect Using Field Data

Abstract

Abstract Joule-Thomson (JT) cooling plays an essential role in the flowline's design phase. It is one of the defining factors for pipeline minimum temperature. Therefore, it controls the pipe's alloy selection based on its minimum allowable temperature. Additionally, JT cooling impacts hydrate control. Common design models can overpredict the JT cooling value. This leads to extra CAPEX and OPEX costs. This paper will use data from nine fields to demonstrate the accuracy of cubic EOS's ability to predict JT cooling out of the box and with tuning. The cubic EOS was modified to shift the enthalpy curve to improve the JT cooling prediction across the choke. For this study, the measured pressures and temperatures before and after the choke for 28 wells located in 9 different fields across the world were analyzed. This data was fed into a thermodynamic simulator to calculate JT cooling for both the original and modified cubic EOS. A comparison between the JT cooling values calculated by both methods with the actual data from the field demonstrated the improved cubic EOS's ability to predict JT cooling accurately. It was noted that the JT predicted by the modified cubic EOS was much more accurate to the temperature drop across the choke in the field. The reason for inaccuracy in the original cubic EOS is that there are limited terms in the equation to predict the change in enthalpy due to pressure. This inaccuracy can be specially noted at higher pressures where the EOS predicts a lower density this can results in large errors in the JT prediction (typically too low). Using the improved cubic EOS can accurately predict JT cooling value and ultimately leads to improved predictions for online model. It also provides an idea of the margins which should be looked at for the design of areas where JT occurs in the field (choke and topside control valves).