Fracture Density Index Delivers Effective Perforation Cluster Picks for Hydraulic Fracturing of Naturally Fractured, Tight Sand Reservoirs

In unconventional and tight reservoirs, it is essential to drill horizontal wells and hydraulically fracture them to ensure optimum performance. With a myriad of subsurface settings such as rock quality, in-situ stress distributions, natural fractures, reservoir heterogeneity and a well trajectory that may not intersect all these attributes uniformly in the entire wellbore poses difficulties in properly exploiting these types of reservoirs. Some of the common, persisting challenges that are often faced are high formation breakdown pressures and low fluid injectivity resulting in inadequate fracture placement and poor well productivity. In such situations it becomes essential to have a good methodology to choose the best areas to place the fractures and commercially produce the well. The distribution of fluid flow paths is a critical component in naturally fractured reservoirs. The main element that can help identify the fluid flow paths is critical stress analysis of the reservoir system. The in-situ stress regime model (in terms of magnitude and direction) and the spatial distributions of natural fractures that create fluid-flow paths are exhibited through Fracture Density Index (FDI) models. The critical stress analysis calculates the normal effective and shear stress in each fracture plane revealing conditions for creation of critical fractures that provide dominant fluid-flow paths. In this paper the concepts of critically stressed fractures and fracture apertures are used to choose the most attractive fracturing stage intervals. The successful perforation picks are further validated by physically identifying fluid contributions behind pipe through the predicted fractures using diagnostic logs. This process offers a robust procedure to ensure optimum contribution from the perforated intervals, eliminates wasted perforations, and helps reduce breakdown pressures, or inconsistent breakdown and treating pressures. Fracturing sweet spots minimizes proppant screen-out situations and avoids wasting money fracturing non-productive rock.