Next-Generation Modeling of a Middle Eastern Multireservoir Complex

Abstract

An elusive goal of reservoir simulation has been the ability to accurately model multiple reservoirs producing through a common surface facility. In the past, loosely coupled techniques have often been used which did not fully converge the overall solution of the simulation. This led to instabilities at worst or to inaccuracies in the solution at best because it did not properly account for the complete interaction of the reservoir. In the following paper, we discuss the application of a fully implicit, tightly coupled surfacesubsurface simulation with a next-generation reservoir simulation of the north Kuwait Raudhatain multireservoir complex with common surface facilities using actual field data. In addition to surfacesubsurface simulation, the reservoir simulator provides a parallel unstructured grid capability and significant computational efficiency from a improved model formulation. For the simulations of this study, four stacked reservoir horizons form the subsurface portion of the model. The surface facilities consist of the production gathering centers, and gas lift and water injection capabilities. A unique feature of the model included the capability to automatically switch the flow lines for the more than 500 wells among six different separator trains at a gathering center, depending on the wellhead pressures and producing water cuts. The resultant 600,000 cell multireservoir model was first validated by comparing fifty years of historical performance for the individual reservoirs from the original simulation models with the nextgeneration model. In general, the matches between the original simulations and those generated with the next generation simulator were extremely close for each of the four reservoirs. With the validation completed, a simulation surface network was constructed that attempted to capture all of the salient features of the current and future surface facilities for the field by including, for example, actual flowline lengths in the model. In particular, future facilities expansions were included with gas lift, water injection, and the multiple header switching of wells at the gathering center. The complete model, including all four reservoirs and the surface facilities, was run for a prediction period of over sixty years. The resulting predictions provided, for the first time, solutions that show the interaction of the reservoirs with the surface facilities, including reallocation of production and injection based on facilities constraints. The multireservoir model forms the basis of a field planning and optimization tool whose forecasts can be used with greater confidence because of the inclusion of the comprehensive physics of the field production. A comparison of the simulation output with a spreadsheet field planning model shows interesting results which should form the basis of future work.