Quantifying Natural Fracture Spatial Organization: Application in Shales
Natural fracture systems are important for production in shale-gas reservoirs; they may reactivate during hydraulic fracture treatments or they may be partly open, contributing to permeability without reactivation. One of the more elusive parameters in fracture characterization is spatial organization. Fractures may be evenly spaced, but more commonly fractures occur in complex and, in some cases, fractal arrays of clusters. Here we test a method to quantify and predict the spatial organization of natural fractures at a range of scales relevant to shale gas wells.
The method uses a correlation count method to rigorously identify different types of spatial arrangement, including periodic, random, and clustered. Our correlation count method provides a measure of the preferred spacing at different length scales relative to what would be expected from a random ordering of spacings. Input data are the fracture spacings and kinematic apertures measured normal to the fracture planes for a single set. We tested the method on fractures in outcrops, in an image log of a horizontal well in the Marcellus Shale, and in image logs from other shale gas wells. Results from the Marcellus data sets suggest natural fractures have preferred spacing at several different length scales; 0.2 m, 2 to 4 m and 12.5 to 14 m. This may reflect various scales of fracture organization relating to characteristic spacing for composite fractures (0.2 m) and different mechanical layer thicknesses (3.5 m and 14 m). Analysis of fracture data in image logs from other shale wells reveal a power law decrease in positive correlation (preferred spacing) at length scales from 0.1 m up to 100 m, with some indication of periodic signals beyond this.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013