Constraining the Spatial Distribution of Fracture Networks in Naturally Fractured Reservoirs Using Fracture Mechanics and Core Measurements

Abstract

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.

Natural fractures in the Barnett Shale and their importance for hydraulic fracture treatments

“…In tectonically stressed crustal rocks, subcritical crack growth can be significant, and studies have shown that fracture-spacing length distributions, connectivity, and fracture aperture can be controlled by this process (Olson et al, 2001). For low values of n (<20), computed natural fracture patterns exhibit small spacing relative to bed thickness.…”

Predicting and characterizing fractures in dolostone reservoirs: using the link between diagenesis and fracturing

“…The growth of a fracture modifies the surrounding stress field, and the interaction of neighbouring fractures controls the developing fracture pattern. Olson (1993) and Olson et al (2001) used boundary-element modelling to show that for constant strain, mechanical-layer thickness and other elastic rock properties, fracture patterns are dependent on the subcritical crack index. In particular, they showed that fracture clustering depends on the value of the subcritical crack index (Fig.…”

Modeling Coupled Fracture-Matrix Fluid Flow in Geomechanically Simulated Fracture Networks

“…In this paper, we use a mechanistic approach to generate the fracture patterns. 4 Attributes of the fracture network depend on the applied boundary conditions and material properties.…”

“…This variation in propagation velocity within the crack population ultimately controls the spatial distribution of the fracture network, influencing important parameters such as fracture clustering and length distributions. 4,6,[9][10][11] Modeling also indicates that the details of a final fracture pattern will depend on the applied strain, strain rate, mechanical layer thickness, critical stressintensity factor, and Young's modulus.…”