oa Fracture characterization using borehole acoustic reflection: theoretical modeling and field data applications

Characterization of borehole fractures is important because they provide conduits for reservoir fluid flow. Borehole Stoneley waves have been used as a means for fracture analyses, which however, are a difficult task to interpret because borehole changes (e.g. washout) and bed boundaries also cause reflections. We used theoretical modeling to characterize fracture-induced reflections. Fractures were modeled as localized, highly permeable structures embedded in a varying borehole environment (e.g. washout). The modeling led to an important result; namely that permeable fractures/structures generally cause Stoneley wave reflectivity to increase towards low frequencies. This result was used to provide a dual-frequency Stoneley reflection method for fracture characterization. The reflection data were processed in two different frequency ranges (e.g., 0–1,000 Hertz and 1,000–2,000 Hertz). Fracture-induced reflections were characterized by low-frequency reflectivity, which is significantly higher than high-frequency reflectively. This method has been applied to fractured reservoirs with promising results. The fractures measured by the acoustic method agreed well with those from borehole image data. The presentation proposes a refined acoustic reflection method for fracture characterization. It demonstrates the practicality and robustness of the proposed technique in field data applications.