This paper presents a 3-dimensional numerical simulation approach and derivation of a simple analytical expression to track the fluid fronts using pressure transient analysis in long horizontal wells. In general, fluid front tracking studies are rarely used for horizontal wells. In the past, analytical models, such as radial composite model, have been used to estimate fluid front distance, which do not account for changed mobility due to different fluid saturations and relative permeabilities. The main assumption for an analytical model is to consider constant mobility for flooded and un-flooded reservoir regions. Hence, an appropriate methodology for fluid front estimation would be numerical analysis of pressure transients.

The study has been conducted for a homogeneous and low permeability artificial reservoir with long horizontal wells. The results show that changes in fluid properties at flood front cause a slope change in pressure response. An equation has been derived to calculate the distance to the flood front (diagnosed as a slope change), which follows the form of the standard equation for radius of investigation. The equation includes a constant ‘A' which is possibly dependent on permeability of the reservoir and relative permeabilities of the fluids. Various sensitivities have been performed to understand the dependency of the constant ‘A' on reservoir and fluid properties. In addition, it is shown that the constant ‘A' does not vary significantly with porosity, viscosity, reservoir thickness, well length and total compressibility of the system.

With a properly designed program for acquiring pressure transient data combined with the presented methodology, distance to fluid fronts can be estimated with reasonable accuracy.

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