Triaxial induction tools were initially designed to measure anisotropic resistivity and the dip of formations. Recent application of the high-resolution zero-D dip provides excellent structural dip information by involving mathematical inversions of the data. A new application using triaxial induction measurements for fracture characterization is based on algorithms using the 3×3 apparent conductivity tensor measurements directly to detect the existence of fractures and estimate the fracture orientation.
Extensive modeling demonstrates that the important fracture subclass of near-vertical fractures can be robustly detected where there is significant resistivity contrast between the fracture and background formation. The vertical fracture indicator (VFIND) and fracture orientation indicator (FOI) are derived from the triaxial induction measurement tensor. VFIND is a qualitative indicator of the existence of near-vertical fractures, representing the cumulated effects of all fractures within a 3- to 4-ft diameter of the wellbore. FOI is the estimated fracture strike in the fracture zones delineated by VFIND. In unconventional shales VFIND identifies zones of high fracture intensity for effective completion design.
For resistive fractures, such as open fractures filled with oil-based mud or healed fractures filled with high-resistivity cement, VFIND is relatively independent of the fracture aperture. A 0.1-in. and 1-in. aperture fracture have almost the same response. For conductive fractures, VFIND is sensitive to fracture aperture. Over zones containing multiple conductive fractures, using VFIND we can invert for an effective fracture aperture through a simple model to represent the fracture systems near the wellbore. The deep depth of investigation of triaxial induction measurements means VFIND preferentially responds to large-extent fractures, including those that do not intersect the borehole. For deep drilling-induced fractures, the FOI is used to deduce the far-field maximum horizontal stress direction for planning hydraulic fracturing programs to optimally drain the reservoir.
Modeling results and field examples are presented for the fracture characterization work flow. Field examples show a high correlation between the VFIND-indicated fracture zones and fracture locations identified from borehole microresistivity images. FOI over fracture zones also matches well with fracture strike from borehole images. The strike of a deep induced fracture indicates the far-field maximum horizontal stress direction. VFIND also delineated zones of high fracture intensity in the Haynesville/Bossier shale.