Quantifying the Fracture Mechanics Properties of Rock for Fractured Reservoir Characterization

Natural fractures are important conduits for fluid-flow and can control the deformational behavior of rock. The accurate evaluation of natural fracture geometry in the subsurface is difficult because of the sampling problems inherent to wellbores and indirect investigation methods such as seismic. Because of these limitations in observational capability, predictive models are needed to provide more constraint on natural fracture characteristics. One approach is to look at the mechanics of fracture propagation in sedimentary rock as controlled by subcritical crack growth (also known as stresscorrosion cracking). We have measured the subcritical fracture properties of numerous sedimentary rocks, including core samples from petroleum reservoirs, in conjunction with detailed petrographic analysis. Our preliminary results suggest that carbonates and sandstones tend to have very different subcritical fracture properties, and the variation in sandstone properties can be linked to grain and cement mineralogy and volume fractions. Introduction The direct characterization of natural fracture network attributes such as length, spacing, aperture, orientation and intensity in most reservoirs is very difficult, primarily because of the low probability of intersecting vertical fractures with vertical wellbores. Accordingly, various predictive schemes based on geostatistics, or geomechanical models are used to estimate subsurface fracture attributes. Statistical approaches fit distributions of fracture characteristics to available data. The statistical fit can treat different fracture attributes independently, or it can incorporate interdependences into the fit, but in either case, the result is dependent on available direct fracture observations. Alternatively, geomechanics-based simulations of fracture attributes combine a physical understanding of the fracturing process with measurements of a small set of rock properties and geologic boundary conditions to predict fracture network properties, even in the absence of direct observations. This approach provides for the determination of physically reasonable fracture attributes, and of relationships between different fracture attributes. The geomechanical model used here is based on subcritical crack growth. Although subcritical fracture velocities are several orders of magnitude slower than rupture velocities, natural fracture growth can be significant in tectonically strained crustal rocks. A number of studies have demonstrated subcritical crack growth controls of fracture spacing and length distributions, connectivity and fracture aperture. Subcritical crack growth can be described by the empirical relationship