Evaluation of reservoir crack based on equivalent effect of scattering waves due to crack-propagation

Abstract

The purpose of this study is to determine if borehole stress concentrations produce compressional velocity variations that are measureable from within the borehole. Experimentally, an azimuthal microsonic measurement technique is used to measure the azimuthally resolved, axially propagating, refracted compressional wave in the 50–100 kHz band and to detect the stress-induced damage around a 10 cm borehole in a 0.5 m cube of Massillon sandstone under uniaxial stresses from 0–21 MPa. An ultrasonic pulse.echo technique is used to record the deformation both parallel and perpendicular to the uniaxial stress. The compressional velocity distribution around a stressed borehole for waves propagating parallel to the borehole is modeled using acoustoelastic theory. For static deformations around the borehole, a finite.element formulation of the elasto-plastic model is used. The key results are: 1) the theoretical and experimental compressional velocity variations around the borehole map the near wellbore stress concentrations due to the far-field applied stress; 2) the acoustic results are measureable even in the absence of breakouts; and 3) the mechanical state of the rock (either elastic or elastic/plastic) can be detected by utilizing azimuthal variations in velocity. Potential applications include damaged rock identification in both the oilfield and mining and stress direction determination for fracturing and perforation in the oilfield.