Modelling Fluid Flow in Grosmont C Carbonates Using Fractals - A Concept

Abstract

Bitumen saturated carbonates of the Grosmont C member of the Grosmont Formation in the Saleski area are intensely fractured. The fractures connect the vuggy porosity and are the reason for in situ permeabilities that exceed 10 D. These carbonate rocks are more hydrophobic with high bitumen saturations. Most existing pilot data suggest the reservoir is in general "leaky" in comparison with siliclastics, such as the McMurray or Clearwater Formations. Furthermore, it is also observed that during CSS operations, especially in the earlier cycles, oil cuts are initially high and decrease with time. This character differs from CSS in siliclastics where initial oil cuts are low and increase with time. To fully understand these observed phenomena, we believe that common knowledge gained in studying siliclastics may not be enough. Geological studies have indicated that the Grosmont carbonate fracture system may not be a well-defined dual scale fracture-matrix system. It may be better defined with a multiple scale system. Geological models using the concept of fractals having selfsimilarity and fractional dimension have been proposed by Wagner et al.. In this presentation, we further postulate that the fractal nature of the Grosmont carbonates offers explanations to the above-mentioned observations. Using the published data of pilot tests in Grosmont, we demonstrate that it is possible to attribute "the loss of pressure" to very large total contact areas between fractures/vugs and the rock matrix in a fractal system, although the matrix permeability can be very low. The high oil cuts were interpreted as the result of considering high permeability channels with significant volume. Numerical simulations confirmed these assertions. This fractal point of view sheds some lights on our journey of eventually understanding and mastering the fluid flow behaviours in Grosmont carbonates.