'Faculty of Mining, Geology and Petroleum Engineering'
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