Fracability evaluation of unconventional oil/gas reservoirs has become a general practice during the hydraulic fracturing job design to identify the best candidate pay zone. The brittleness of the reservoir rock was first used to characterize the capability of the reservoir rock to form a complex fracture network. Later, other factors like fracture toughness, in-situ stress, and natural fracture parameters were introduced in combination with brittleness to more comprehensively describe the fracability of the reservoir. However, most of the existing fracability evaluation has been performed for single wells. In contrast, the three-dimensional distribution of fracability within reservoirs was less pursued, though it might be beneficial for guiding the well placement.
This study developed an integrated multi-scale fracability evaluation method for tight sandstone reservoirs based on three-dimensional geomechanical analysis. Factors like brittleness, fracture toughness, in-situ stress contrast that influence the hydraulic fracture initiation and propagation at different scales were first identified and analyzed. Then methods of deriving these quantities from well logs and three-dimensional seismic data were summarized. Finally, an integrated index incorporating these quantities was defined to differentiate the so-called engineering sweet-spot zones favorable for stimulation. A typical application of this method is the integrated multi-scale fracability evaluation method applied to a tight sandstone reservoir located in China's south sea oil field, which demonstrated the effectiveness of the method. In addition, some implications for improving fracturing operations of tight sandstone reservoirs have been provided.
The concept of fracability was originally proposed for shale reservoirs, and it is a quantitative indicator that intuitively describes whether or not the reservoir is favorable for fracturing. Early studies simply assumed that the more brittle the rock was, the easier it was to fracture. Therefore, only the brittleness index was used to measure the fracability of the reservoir rock. Different researchers put forward different definitions of brittleness. There are about a dozen brittleness calculation methods that are widely used, among which the two most common are: (1) The Mechanical Brittleness Index calculated by elastic parameters such as Young's modulus and Poisson ratio; and (2) The Mineral Brittleness Index defined by the percentage of brittle minerals.
