This paper describes the procedures used in a joint venture by two software vendors to combine an existing reservoir simulator and an existing surface facilities model into a single forecasting tool. Relatively small changes were made to each program. In the new model, the black oil reservoir simulator provides the formation pressure and water to gas ratio for each well The surface facilities model then calculates the multiphase flow pressure losses in the wellbore and gathering system, plus the corresponding flow rates for each well. The actual production required from each well to satisfy the pipeline contractual requirements, over each time step, is computed by the surface facilities model and relayed back to the reservoir simulator. The time step is determined dynamically according to the requirement of each program. The performance and results from the coupled model are compared to that of running each model separately for a gas storage field in the U.SA and for a gas production field with bottom-water. It is shown that running each model separately does not account for all the factors affecting the forecast.
To determine if a gas contract can be satisfied now and in the future, it is necessary to forecast the performance of the gas reservoir, the gas inflow into the sandface, the multiphase pressure losses in the wellbore and gathering system and the field facilities. Surface production models which rigorously model from the sandface to the plant gate are available. However, these surface packages model reservoirs simply, in most cases as tank-type reservoirs. Comprehensive 3 dimensional reservoir simulators are available, but typically only include simple surface networks which don't adequately model multiphase flow in complex gathering systems1,2,3.
A different approach has been taken in this paper. Two existing commercial models, a 3 dimensional black oil simulator and a multiphase surface facilities model, from different vendors were coupled into one comprehensive model. Ihis work required only a small fraction of the development time and cost which would have been equired to "add on " a surface network to a reservoir simulator. No simplifications of either component were required and each component was supported by experts in that field. Existing data files and documentation were used irectly. New developments can be added to either model without adversely affecting the interface.
The black oil simulator, IMEX, from the Computer Modelling Group was chosen as the reservoir simulator. FORGAS, from Neotecnology Consultants Ltd. was selected as the surface model. Both models were linked into one executable to save execution time. Two new subroutines were created to pass data between each model. Each routine is responsible for accessing the required data and then converting the data into the acceptable format (e.g. SI to field units). The other model then calls this subroutine whenever it needs the information. The name of each well is used to link the models. A new mainline routine was created which calls each model (as a subroutine) sequentially in a loop.