Print this pageStatistical Analysis of Natural Fracture Networks: Implications to Reservoir Permeability
MALMANGER, EVA M., and LAWRENCE W. TEUFEL, New Mexico Institute of Mining and Technology, Socorro, NM
Fluid flow through a naturally fractured reservoir depends on the fracture spacing, length, aperture and morphology. Optimum characterization of these fracture parameters requires an integrated approach using core, log, and outcrop studies. Outcrop studies provide essential information on the spatial distribution of fractures and fracture length. A geostatistical analysis of fracture network maps and scanlines of sandstone outcrops in Wyoming, Colorado, and Texas shows that fracture spacing and length can be approximated by a log-normal distribution and that the fracture population is dominated by short unconnected fractures. In general fracture spacing increases with increasing fracture length. Average fracture spacing ranges from about 1 m for fractures less than 10 m in length to 4 m for fractures of lengths of 30-40 m. The relationship between fracture aperture and fracture length has also been examined using three statistical methods: 1) constant aperture, 2) linear relationship between fracture length and aperture, and 3) a natural-log-relationship. Fracture orientation and length have been combined with these fracture aperture/length relationships to calculate magnitude and orientation of a permeability tensor for the same total porosity. These simulations show that the natural log relationship gives the lowest permeability and the linear relationship gives the highest permeability by an order of magnitude. Simulations using only fractures with lengths greater than 20 m (less than 10% of the fracture population) show little reduction in bulk permeability and suggests that the fluid flow through a fractured, tight reservoir rock is dominated by the longest fractures, which have the widest spacing and fewest number.