An integrated solution was required to optimise the use of gas in a complex network installed for gas lifting operations, gas export and reservoir re-injection. The solution was required because the compression facilities represent a major element of the cost of operating the field and therefore it is important that the high pressure gas is used as efficiently as possible. It is also important that recommendations from the optimiser are implemented with minimal delay and therefore closed-loop optimisation was selected, which requires that the optimiser is integrated with the field's SCADA system.

The complexity of the system to be optimised presented a unique challenge. Traditional nodal analysis modelling techniques are generally not robust when applied to complex, non-hierarchical networks. Consequently, the Sequential Linear Programming (SLP) optimisation technique is applied to solve the computer model of the low pressure gas gathering system, multi-stage compression facilities and high pressure gas distribution system. The novel implementation of this technique simultaneously simulates and optimises since it is able to select the best combination of pressures and flowrates from the set of all combinations which will satisfy the problem constraints.

This paper describes the development of the optimisation application, the network and well models and the underlying mathematical techniques. The integration with SCADA and other databases is described, along with a discussion of how the form of the integration reflects the desired workflow for operating and optimising the field. Finally, the paper provides a quantitative assessment of the benefits realised by applying the optimisation application in the field. These benefits are obtained through increased oil production, reduced compressor fuel gas consumption and reduced venting of low pressure gas.

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