Discrete Fracture Network Model for Numerical Analysis of Tight Unconventional Reservoirs

• Author / Creator

  Pedro Abdiel Mateo Villanueva

• Fracking, or hydraulic fracturing, is a process where high permeability fractures are induced in low permeability reservoir rocks in order to create a more conductive pathway for oil production. Production from shale and other low permeability reservoirs is a complex multi-physics problem. The interaction between the natural fractures already present in the formation and the fractures induced from the fracking process, coupled with the uncertain spatial distribution of the heterogeneous features, introduce even more uncertainty to the production estimates. Numerical modelling of tight and shale formations is, for this reason, still challenging.

  Various numerical methods have been used to study flow and immiscible displacement through fractured reservoirs. While all numerical methods make compromises due to the highly complex nature of oil reservoirs, the discrete fracture (DF) method based on the Mixed Finite Element (MFE) discretization scheme proves to be a superior alternative for computational analysis of fractured media. While the numerical accuracy of the three most common schemes to study transport in oil formation; the finite difference (FD), finite element (FE), and finite volume (FV) method, strongly rely on the grid size of the matrix cells, the MFE formulation can be frame din such a way to relax this dependency. Additionally, while local mass continuity cannot be ensured in the conventional finite element method, it is naturally incorporated in the mixed finite element formulation. A novel modelling framework is proposed where point-source well models are incorporated in order to simulate production from tight reservoirs with natural and hydraulic fractures. To eliminate the need for local grid refinement when implementing the discrete fracture model, both natural and hydraulic fractures are represented physically as lower-dimensional features in the numerical domain, making our method a computationally efficient approach.

  The proposed model is suitable to account for reservoir heterogeneities and is also able to handle irregular reservoir geometries and arbitrary fracture orientation; which are some of the major drawback of many existing numerical algorithms used to study tight reservoirs. We modelled single phase flow moving inside a fractured reservoir, using the Discrete Fracture Network approach where fractures are represented by lower dimensional spatial features in the computational domain to avoid grid refinement. The model introduced a point source well model based on Darcy's law applied inside the fracture network. The results of this thesis demonstrated that model accuracy of the MFE-based numerical scheme is achieved using a much lower mesh density when comparing to conventional Finite Elements-based commercial software. Additionally, the developed model, coupled with the point source well model predicted very similar production profiles to popular reservoir simulators, assuming orthogonal fractures. When studying non-orthogonal fractures the model indicated that uniformly distributed, well connected fracture networks will impact positively the oil production, and that for the cases where the hydraulic fractures are well connected, regardless of fracture orientation, the fracture pressure is almost constant.

  Although the MFE formulation has been implemented to model fractured reservoirs in the past, an implementation used to study the production/extraction process from unconventional formations by means of a well model using a natural MFE scheme is novel.

• Subjects / Keywords

  • Porous Media Flow
  • Discrete Fracture Network
  • Mixed Finite Element

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-8crm-w730

• License

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