Simulating Naturally Fractured Reservoirs: Comparing Discrete Fracture Network Models to the Upscaled Equivalents

Huabing Wang, Craig Forster, and Milind Deo
University of Utah, Salt Lake City, UT

Naturally-fractured reservoirs are an important, but difficult to manage, worldwide reservoir type.Complex, difficult-to-define, fracture networks yield complex reservoir systems with significant uncertainty regarding their ability to aid, or impair production.Despite the key role of fracture networks in production performance, reservoir simulations typically use equivalent, porous medium properties to represent the aggregate impact of fracture networks. A series of simulations performed for two idealized, fractured basement reservoirs provide a basis for comparing results obtained using: 1) a discrete 3-D fracture network (DFN) simulator, and 2) two different equivalent porous media simulators. The two reservoir cases illustrate the possible impact of geologic uncertainty in assessing the characteristics of subsurface fracture networks. Results obtained using both single- and dual- porosity simulation methods are also compared.

Three-phase, black-oil simulators used in this study include the conventional reservoir simulators ECLIPSE and IMEX and the upstream transmissibility weighted control volume finite element DFN simulator developed at the U of Utah. The geometry-based Oda method is used to upscale permeability tensors initially defined in the discrete fracture network. Volumetric fracture intensity is calculated in each grid block to represent the upscaled porosity. Upscaling with a series of different grid block sizes(ranging from 10 to 200 ft cubes) in a 1000 by 1000 by 200 ft reservoir volume reveals that the upscaled results depend strongly on the relationship between grid block size, fracture network geometry and simulator type. A portfolio of comparative simulation results are helping us to better understand the level of uncertainty that might be introduced when using equivalent property, multi-phase simulators to represent fractured reservoir systems.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas