Systematic hybrid modelling using fracture subset upscaling

Abstract

Fractured reservoirs often exhibit multiple length scales and are best modelled using hybrid methods that partition fractures into a subset to be upscaled and another to be represented explicitly. To address the open question of how partitioning can be done, we propose a single porosity hybrid modelling workflow that makes use of fracture subset upscaling to identify poorly connected small fractures for upscaling. Fracture subset upscaling was performed numerically on three datasets (two real, one synthetic); results showed that the smallest, most numerous fractures are the least connected. Semi-analytical fracture subset upscaling was also performed for the synthetic dataset, showing that this process can be accelerated with the aid of analytical upscaling tools. To test the proposed workflow, hybrid models were created with different partitioning sizes and compared against full Discrete Fracture Networks (DFN) using single phase pressure drawdown as a test problem. It is observed that as the upscaled small fractures begin to connect, deviations in flow response will start to grow. In some cases, the flow regime in the model will change entirely. Overall, the results justify the proposed workflow as a promising means for systematic construction of hybrid models with minimal guesswork.