Fracture evaluation using reflected Stoneley‐wave arrivals

Abstract

We use the low‐frequency reflected Stoneley‐wave mode to locate permeable fractures intersecting a borehole and to estimate their effective apertures. Assuming a model in which the average aperture of the fracture is roughly constant, theoretical work relates the magnitude of the Stoneley‐wave reflectivity to an effective fracture width, We treat both the case of a horizontal fracture and the case of a fracture crossing the borehole at an angle. Laboratory experiments verify the analytic solution for the case of a horizontal fracture. Full‐waveform array sonic data were also acquired in a wellbore with a long recording time (25.5 ms) in order to capture the late Stoneley‐wave arrivals. The data processing involves computation of the Stoneley‐wave reflectivity response using the measured direct and reflected Stoneley‐wave arrivals. A least‐squares fit to the arrival time of the reflected‐wave arrivals is used to estimate the locations of permeable fractures, and the effective width of the fractures is estimated by comparing the computed Stoneley‐wave reflectivity to the theoretical response from a parallel‐plate model. Test‐well results are consistent with a borehole televiewer analysis.