This paper describes the application of a front tracking simulator for studying flow in a 2D cross-section of a braided river system. The internal sand body geometries were based on a braided river outcrop from the USA. Rock properties were derived from a North-Sea oil field. Front tracking represents a direct, accurate and fast method to simulate oil and water flow in highly hetergeneous reservoirs. In the front tracking simulator the saturation solution is decoupled from the grid and its accuracy is to a large extent independent of grid block sizes end geometry. This makes the reservoir simulator well suited for modelling of complex geological architectures. The simulator has been compared with analytical methods and traditional finite difference simulators. Results from a water-oil displacement process in a braided river system are presented, showing fingering of the saturation fronts due to par-ability contrasts. For coarse grid systems. the front tracking simulator proved superior to the finite difference simulator with respect to the numerical solution. An automatic grid fitting option that utilizes triangular grid cells enabled an easy and fast construction of the complex geological model. Introduction The increasing use of statistical geological models implies a more detailed geological description and necessitates an increased number of geological realizations. This in turn calls for simulators which are able to correctly describe detailed deterministic models without an excessive CPU-consumption. It is also required that the construction of new realisations can be carried out in an easy and time efficient manner. The use of standard five-point finite difference simulators has practical limitations. A detailed geological description demands in itself a large number of grid blocks and to avoid inaccuracies in the numerical solution, a dense grid system is required. The front tracking simulator as presented in this paper fully meets the above requirements, both in terms of CPU- and man-time efficiency, as well as accuracy in the numerical solution. To demonstrate the capabilities of the front tracking simulator, water flooding in a cross-section from a braided river system has been simulated. THEORETICAL ASPECTS The reservoir simulators most commonly used today are based on finite difference methods to approximate the partial differential equations that describe fluid flow in porous media. Since both the pressure and the saturation equation are solved iteratively, and stable solutions require restricted time step times. such models can be CPU-demanding, particularly when simulating larger grid systems. Usually. a five-point difference scheme is used and unwanted grid effects will occur in case of skewed grid cells. Hence, such simulators have their limitations when modelling irregular reservoir geometries. Also. numerical dispersion can be significant when large grid blocks are used. This creates a need for larger grid systems to obtain reliable simulation results. The front tracking simulator is based on a different numerical method compared with traditional finite difference simulator, but the mathematical foundation is the same.

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