Observations of natural fractures in core or image logs typically give limited information on orientation, aperture and intensity. Because of the sparseness of wellbore intersections of fractures, data analysis results in incomplete statistical characterization of the fracture population, leaving interwell characterization almost impossible. Using basic fracture mechanics models and a novel core-testing technique, we propose that the fundamental shape of fracture parameter distributions can be predicted, and that there is a characteristic, quantifiable relationship between fracture length, spacing and aperture. We have performed subcritical fracture growth tests on numerous core samples, using credit card sized specimens, demonstrating the ability to characterize fracture mechanics properties of rock on a bed by bed basis. Using the subcritical index, a parameter that quantifies the relationship between natural fracture propagation velocity and tip loading conditions, we can predict the degree of fracture spacing regularity or clustering for a given reservoir bed. This subcritical parameter, along with information on the number of initial natural flaws in a given rock type, allows us to quantify the expected length distribution of the fractures. Under many conditions, as verified from outcrop data, fracture length is theoretically expected to follow an exponential distribution. Since natural fracture length is typically unobservable in subsurface data, we derive relationships that relate fracture length to aperture and spacing, both more readily measurable quantities. With this information, matrix block size and fracture drainage continuity can be estimated for the purpose of flow simulation in a fractured reservoir.
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