Published by the DTI Oil & Gas Directorate for the reservoir
engineering and IOR community in the UK.
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by 15 August 2002.
Streamline Simulation Using Frontsim
Martin Crick, Geoquest, looks at the benefits of streamline simulation (martin.crick@slb.com)
What is Streamline Simulation? FrontSim is a reservoir simulator based on an IMPES (IMplicit Pressure Explicit Saturation) formulation and a streamline/front-tracking concept and is one of a number of such simulators available commercially.
The governing equations, that is Darcy's law and conservation of mass, are split into two equations, the pressure equation and the saturation equation. The pressure equation is solved implicitly in a similar manner to finite difference simulators, such as Eclipse.
Streamlines are then computed normal to the potential field. In 2D, the streamlines are therefore perpendicular to the pressure contours. Each streamline represents a constant volumetric rate and acts as a one-dimensional space for the saturation solver. The flow is then solved along each streamline using a front tracking method. Buckley-Leverett solved the front tracking problem for the first front in an oil water problem. Frontsim solves the more general problem of tracking each individual front along each streamline by solving the Riemann problem.
In reservoir engineering terms, the streamlines can be thought of as an adaptive gridding technique. Conventional gridding concentrates on the geological features of the reservoir. However, the flow direction depends not only on the geology, but also on the position of wells, the flow rate, aquifer influx, etc. Streamlines provide a means of discretising the flow equations around the flow direction. Because the streamlines are redrawn each time the pressure is recomputed, the discretisation of the flow solution adapts to the problem being studied.
Until recently, streamline techniques were limited to incompressible flow, making simulation of oil difficult and of gas impossible. Frontsim supports compressible oil and water problems, and recent development extends the technique to three phase problems.
What are the benefits? This technique confers several practical benefits on the engineer.
Speed: streamline simulation can be much faster than finite difference simulation. The reason is that in Frontsim the timestep length is limited only by the need to update the pressure calculation, whereas in finite difference simulators timesteps are limited by throughput. For large models (i.e. over 100,000 active cells) Frontsim can be many times faster.
Front tracking: by tracking the position of individual saturation fronts, numerical dispersion is almost eliminated. This allows much more accurate prediction of water breakthrough and water cut development.
Visualisation: graphics post processing using packages such as FloViz allows the engineer to see where the fluid is coming from and going to.
Applications of Streamline Simulation. The ability to simulate on much finer scale grids in practical timescales allows more account to be taken of the geology. Many modern 3D geocellular models can be modelled directly, without the need for up-scaling. Frontsim can be used as a screening tool, selecting the realisation from a geostatistical model that gives flow performance closest to the observed history. Frontsim can also be used to verify an up-scaling technique, prior to a study using the fuller physics modelled in a conventional finite difference simulator.
History matching a water flood becomes much easier when an over or under prediction of water production at a producer can be traced along the streamlines to a specific injector or aquifer. Reservoir properties along the streamlines such as permeability can be adjusted to control breakthrough. Again, for reservoir management, identifying which injector is contributing to excessive water production allows more precise injection management. Streamline visualisation also identifies areas of the reservoir bypassed by injection, making selection of infill drilling locations more straightforward.
Another feature unique to streamline simulation is the calculation of allocation factors, which attribute flow from a producing well to a specific injector, aquifer or boundary. Similarly, allocation factors show the breakdown of injection volume between each producer and loss to the aquifer.
SPE 54616, May 1999 and SPE 63152, Oct 2000, describe BP's experience at Prudhoe Bay. History matching using finite difference simulation was abandoned after 10 months when they estimated it would take 5 years to achieve a history match. Using Frontsim, they completed a history match in 6 months. They estimate that the changed injection management resulting from the study saved them several million dollars annually in lifting costs alone.
SPE 49000, Oct 1998 and SPE 63154, Oct 2000 describe Chevron's experience on several large fields. The compared results with finite difference simulators proving that results were virtually identical, but with speed ups of up to 23 times.
e-mail gqmktg@geoquest.slb.com for further information or to arrange a demonstration.
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