Systematic hybrid modelling using fracture subset upscaling

Abstract

Multi-scale fractured reservoirs can be modelled effectively using hybrid methods that partition fractures into two subsets: one where fractures are upscaled and another one where fractures are represented explicitly. Existing partitioning methods are qualitative or empirical. In this paper, we present a novel and quantitative partitioning approach based on a single-porosity hybrid modelling workflow that uses numerical (Embedded Discrete Fracture Methods - EDFM) and semi-analytical (Effective Medium Theory - EMT) methods for fracture subset upscaling. We demonstrate this workflow using synthetic fracture data and realistic data sourced from outcrops of the Jandaira Carbonate Formation in the Potiguar Basin, Brazil. Fracture subset upscaling with EDFM and EMT using three datasets (two real, one synthetic) shows that the smallest, most numerous fractures are poorly connected. The ability of fracture subset upscaling to identify these fractures is essential to the hybrid modelling workflow. EDFM and EMT methods give nearly identical results, but EMT enables us to greatly accelerate the calculations. To validate our workflow, hybrid models were created with different partitioning sizes and compared against EDFM simulations where all fractures are represented explicitly. A single-phase pressure drawdown was used a test problem. The simulation results show that once the upscaled fractures begin to connect, deviations in flow response start to grow because single-porosity representations are inadequate to capture the separation of timescales between flow in a well-connected fracture subset and flow in the matrix. In some cases, the flow regime in the model were observed to change entirely. Overall, the results justify the proposed workflow as a means for systematic and quantitative construction of hybrid models.