North Sea Field and Well Allocation Using CHARM

Determining well potentials for a complex allocation regime

Our client had a complex set of older offshore assets and, over time as modifications had been made, the allocation arrangements had become increasingly complex and difficult to manage. The issues included:

• Multiple methods for generating well potentials as new tie-ins had been brought on-line;

• Ongoing offline effort and costs to calculate well characterisations. The purpose of these characterisations was to generate well specific polynomial equations to estimate factors that compensated for the process effects on measured oil and gas flows from well tests to export conditions and generate well potentials. Obtaining reliable results was often difficult due to the changes in the way the assets were operating;

• The polynomial equations were applicable for one well routing through the process;

• The equations had to be reviewed and updated at intervals to reflect changes in the wellstream compositions or well routing;

• The allocation rules constrained the way the assets could be operated. The operations team were looking for an approach which allowed increased process flexibility;

• The effect of Gas Lift was not included in the calculations;

• Significant manual input which can result in transposition errors.

By implementing CHARM, offline stand-alone calculations were replaced by an online process simulation model for the whole system.  CHARM uses the well-recognised Peng Robinson Equation of State (EOS) to model the behaviour of the hydrocarbons through the process. The advantages of this approach were:

• Standard recovery factor approach throughout the whole system increasing transparency of the allocation calculations;

• No requirement for characterisation studies reducing ongoing costs;

• Considerably less data handling;

• CHARM uses the same EOS as used in other commercial simulation packages such as HYSYS. However, CHARM is built specifically to integrate smoothly and robustly with allocation software and run routinely as part of the daily allocation work processes without assistance from process engineering personnel;

• More accurate and robust: the method calculates well potentials on a commingled basis taking account of the presence of other well fluids. The method is also be used to adjust wellstream composition to account for changes in GOR and also accounts properly for the presence of Lift Gas in the well test;

• More flexible: The implementation of this process model involves generation of a process configuration which defines how all the vessels in the process are linked together. These routings were implemented in such a way that routings can be modified as required in the future without modifications to the rules or to the allocation calculations within the IT system. This provides much more flexibility in the use of the asset.

As part of the study and presentation to partners, the results were validated against HYSYS and acceptable results were achieved. 

CHARM is called directly from the allocation calculations.  XML files are passed to the cloud-based service containing the asset configuration applicable on that day, the well test data and the required measured data.  

In this implementation, the CHARM calculations are called multiple times per well to determine well test input data and then as a main process is run to calculate oil and gas potentials across the asset. 

Typically, the full set of CHARM runs takes less than 10 seconds.  Results are passed back via a set of XML files with a full log available for review if required.

The CHARM service is managed and supported by the Accord support team.  It has proven to be a remarkably stable service with more than 99.99% availability.