A typical fractured carbonate reservoir introduces a tremendous challenge to upscaling of single- and multi-phase flow. The complexity comes from both heterogeneous matrix and fracture systems where separation of scales is very difficult, if not impossible. The mathematical upscaling techniques, based on representative elementary volume, must therefore be replaced by a more realistic geology-based approach. In the case of multi-phase flow, an evaluation of the main forces acting during oil recovery must also be performed.
A matrix sector model from a highly heterogeneous carbonate reservoir is linked to different fracture realizations in dual continuum simulations. An integrated iterative workflow between the geology based static modelling and dynamic simulations is used to investigate the effect of fracture heterogeneity on multi-phase fluid flow. Heterogeneities at various scales, from diffuse fractures to sub-seismic faults, as well as fracture barriers, are considered. The diffuse fracture model is built based on facies and porosity from the matrix model together with core data, image log data and data from outcrop analogues. The sub-seismic fault model is conceptual and based on analysis of outcrop analogue data. Fluid flow simulations are run for both single-phase and multi-phase flow, gas and water injection.
Better understanding of fractured reservoirs behaviour is achieved by incorporating realistic fracture heterogeneity into the geological model and analysing the dynamic impact of fractures at various scales. In the case of diffuse fractures, the heterogeneity effect can be captured in the upscaled model. The sub-seismic faults, however, must be explicitly represented, unless the sigma factor is included in the upscaling process. A local grid refinement approach is applied to demonstrate explicit fractures in large scale simulation grid. This study provides guidelines to how to effectively upscale a heterogeneous fracture model and still capture the heterogeneous flow behaviour.