Use and Application of 'Coupled' Reservoir Simulator in Gas Field Management - A Case Study
Authors: Tripathy Brun , Dobson Jeff , Van Appelen Richard 
Published: December 2007

Introduction
The paper describes the use of a ‘coupled’ surface network and 3-D 3-phase reservoir simulator in the reservoir management decisions of the Hanlan Gas Field in Western Canada. The black-oil and surface facility models were independently developed but were combined to produce the prediction forecast. The paper emphasizes:the importance of the dynamic exchange of data between the surface network and reservoir simulator, andthe creation of a reliable forecast for effective gas field management.

Field History & Model Development
The Hanlan Swan Hills Sour (H2s=10%) gas reservoir is a dolomitized devonian reef margin at a depth of 15400 Ft. The reservoir is in contact with an aquifer system in two directions. The OGIP of the field is estimated to be 1.6 tcf of raw gas. The field went on production in 1983. Currently there are 12 Swan Hills unitized (no. 1) wells and 3 non-unit wells including a horizontal well. Figure 1 displays the field outline and gathering system network. The 6000 grid block 3-D model for Swan Hills was history matched for gas production, gas/water ratios, and pressures for 13 years. It was used to evaluate aquifer activity and potential impact on gas recovery. The reservoir model was coupled to the surface facility network model and calibrated for production and pressure. The surface facility model provided a detailed description of pipeline network, facility constraints, contractual rates, swing factors and fuel usage.

Working of Coupled Simulator
The surface facility and reservoir simulators were coupled through an ‘Interface Subroutine’. In its working, the reservoir simulator first reads its input data file and determines the grid block pressures, gas/water ratios and well status. Next, the surface facility model reads its input data file and retrieves grid block pressures, and well status from the interface routine. The surface model computes sandface inflow performance for each well. Multi-phase pressure loss correlations are used to calculate wellbore pressure losses to compute wellhead deliverability. Multi-phase pressure losses in the gathering system are computed using user specified correlations. The flow rate for each well is calculated after accounting for contract limitations. A summary of calculated pressure and flow rates is then written to output file. The reservoir model retrieves this information from the interface routine to perform material balance. The surface model recomputes the system deliverability using the revised pressures and water/gas ratios.
There are two limitations to the coupled simulator:the surface model cannot handle oil-gas system, andthe black oil model does not handle compositional changes in the reservoir and thus cannot be used for gas cycling or gas condensate reservoirs.

Applications
The combined Hanlan Model was developed for the following applications. Numerous sensitivity runs were planned for each application:Optimize reservoir depletion plan by evaluating timing, number and configuration of additional drilling, additional compression requirements and potential modifications to gathering Systems;Evaluate impact of increasing water/gas ratio on the gathering system;Evaluate impact of competitive drainage;Increase understanding of interaction of multiple reservoirs connected to the limited surface facility and impact of gathering system on ultimate gas recovery.
Based upon simulation runs, a horizontal section of 1026 feet was re-entered at one of the existing vertical wells and is being evaluated at this time.
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