Improved simulation of naturally fractured reservoirs using unstructured grids and multi-rate dual-porosity models

Abstract

Naturally Fractured Reservoirs (NFR) hold about half of the world’s remaining oil reserves and are typically very heterogeneous. NFR are also important for many other subsurface engineering applications including (nuclear) waste storage, CO2 sequestration, groundwater aquifers, and geothermal energy extraction. They contain faults, fracture corridors, large fractures but also many small-scale fractures as well as a heterogeneous rock matrix. Multi-phase flow in NFR is strongly influenced by this multi-scale heterogeneity. Therefore, accurate conceptual models that reliably quantify fluid flow in NFR are needed. In this thesis, three important contributions are made towards an improved simulation of multi-phase flow processes in NFR. First, the Implicit Pressure Implicit Saturation (IMPIS) method using unstructured grids was implemented to numerically simulate two-phase flow in a Discrete Fracture and Matrix (DFM) model. Second, a Multi-Rate Dual-Porosity (MRDP) model was developed including fracture-matrix transfer functions that are based on analytical solutions for spontaneous imbibition and gravity drainage. Finally, the two approaches were combined to a DFM-MRDP model. This model represents the multi-scale heterogeneity inherent to NFR more accurately by resolving fluid-flow processes in large-scale fractures directly using the DFM model while accounting for complex matrix heterogeneities when modelling fluid exchange between small-scale fractures and rock matrix using the MRDP model.