Inter-well scale natural fracture geometry and permeability variations in low-deformation carbonate rocks. (April 2017)

• Record Type: Journal Article
• Authors: Bisdom, K.; Bertotti, G.; Bezerra, F.H.
• Is Part Of: Journal of structural geology. Volume 97(2017:Apr.)
• Page Start: 23
• Page End: 36
• Publication Date: 2017-04
• Subjects: Natural fractures -- Equivalent permeability -- Aperture -- Discrete fracture networks -- Potiguar basin -- Shallow-water carbonates; Geology, Structural -- Periodicals; Géomorphologie structurale -- Périodiques; Geology, Structural; Periodicals; 551.805
• DOI: 10.1016/j.jsg.2017.02.011
• Languages: English
• ISSNs: 0191-8141

Abstract

Abstract: Regional natural fracture networks often show variations on a scale below that captured by seismic reflection data. This variability is not considered in most reservoir models, but likely impacts uncertainties in permeability. We quantify this uncertainty using a database of 13, 000 fractures in nine outcrops digitised in the carbonate Jandaíra Formation (Potiguar basin, Brazil). Distance between outcrops is on average 11 km, with a minimum of 300 m, which is comparable to the distance between wells in naturally fractured reservoirs. In between outcrops, significant variations exist in orientation, intensity, length and topology. Using discrete fracture-matrix flow models, we model the permeability of each deterministic pattern and find that small changes in geometry and topology result in permeability variations that are not captured by connectivity-based analyses such as percolation probabilities, particularly when the matrix is permeable. The permeability variations associated with subseismic-scale fracture variability are not captured in conventional stochastic models, but can be captured using deterministic outcrop models with flow through discrete fractures. The deterministic models provide a permeability range associated with subseismic fracture variability, that can be assigned to grid cells of fractured reservoir flow models, as an alternative to assuming constant permeability in the absence of subseismic-scale deformation. Graphical abstract: Highlights: The geometry of fractures in the carbonate Jandaíra Formation in Brazil is mapped. Geometry is heterogeneous at scales finer than for seismic reflection resolution. Discrete fracture-matrix flow models quantify the heterogeneity impact on flow. Subseismic fracture variability leads to inter-well scale permeability variations. These permeability variations can be assigned to fractured reservoir flow models.