Due to its complex nature, long term development scenarios have been modeled using a subsurface reservoir centric model, with representative surface constraints such as well head pressure. Near term operating plans, on the other hand, have been based on a detailed surface network model that included a comprehensive representation of plant, flowlines and operating conditions. Each of these models have been calibrated and their accuracy were verified independently. We have recently implemented a fully coupled reservoir-surface network model consisting of complex surface network, including three processing plants, and two subsurface reservoir models. The model is run using a controller which manages the surface network model running on a PC and the reservoir model running on a Linux cluster. The coupling modeling approach in long term forecasting becomes essential when the field deliverability is impacted by the dynamic conditions in the surface facilities. The coupled model provides critical insights when major changes are introduced throughout field life, such as major surface facility expansion, surface network depressurizing, differing regional depletion rate in the reservoirs. This paper presents the advantages and challenges of applying the coupled model in a complex surface gathering system network being fed by several subsurface reservoirs with different pressures. The model provided insight on the detailed and complex interaction between the subsurface reservoir and the surface network, which cannot be achieved using either standalone model (surface or subsurface). The information enabled us to identify opportunities for debottlenecking and optimizing production through management of back pressure in the system.

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