Fracture Size and Intensity Scaling from Core to DFN Model


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Geomodeling of naturally fractured reservoirs involves scale-up challenges that are not faced in conventional reservoirs.Matrix properties can be reasonably well measured at the core scale and applied to the reservoir scale, whereas the distributions of the fracture properties observed in cored and imaged wells must be scaled to account for the larger features that dominate flow in fractured reservoirs.

Fractures described in core and those interpreted in image logs allow the identification of fracture sets and their properties.Often the most important flow features are too sparsely distributed to be sampled in a single well.Even if one of these larger features is intersected by a well, the size of the fracture cannot be determined.For reservoirs with mainly vertical fractures, deviated and horizontal wells can provide good estimates of fracture intensities after accounting for sampling bias.In wells oriented to cross fractures, the aperture is only fracture dimension that can be measured directly from core or estimated from wellbore images.Vertical wells have the possibility of drilling parallel to many fractures allowing fracture heights to be measured as well.

To build a Discrete Fracture Network model (DFN) both the fracture sizes and intensities must be scaled to obtain the truncated size distribution relevant to flow modelling.The extreme sample bias of a vertical well requires a robust correction to determine fracture intensities.It is helpful to have fracture measurements from inclined or horizontal wells to ensure that the fracture intensities obtained from vertical cores are reasonable.Core provides the highest resolution from the smallest sample allowing description of smaller fractures not resolved by the image log.Fracture observations suffer from two factors that limit the useful size distribution, truncation (minimum size detected or measured) and censoring (inability to adequately sample large features).Care must be taken to use the appropriate distribution function by fitting curves within the region that is not impacted by truncation or censoring.

This presentation describes the process used to scale vertical core observations to the inputs needed for a realistic DFN model of a shale reservoir.The impact of the fracture size-intensity scaling is demonstrated with model results that are able to match other observations.

Your Instructor

Doug Bearinger
Doug Bearinger

Doug Bearinger is a Geology Advisor with Nexen Energy ULC. Since 2000 he has developed, characterized and modelled fractured granite reservoirs in Yemen, Mannville Coal Bed methane in Alberta, Horn River Shales in NE British Columbia, and Carbonate reservoirs in Iraq. His areas of focus include natural fracture network description, stimulated fracture network characterization and modelling, fracture growth modelling, geochemical fluid behaviour and fluid clean-up modelling. He graduated in 1982 with a BSc in Applied Earth Science-Geotechnical Option from the University of Waterloo. Bearinger has served on SPE workshop committees for Hydraulic Fracture Flowback, Horizontal Completions in Shales, and Characterizing Unconventional Reservoirs Using In-Situ Tests.

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