Measurement of the P-Wave Velocity Anisotropy (WVA)

The aim of the WVA-investigation is, to determine the 3D-in situ rock stress state around a wellbore by means of directional and pressure dependent p-wave travel times in core sample. The orientation of the minimum p-wave velocity (Vpmin) coincides with the orientation of the maximum stress direction. The formation of tensional microcracks is related to the stress relief (anelastic strain recovery) of freshly drilled cores.

The sketch beside demonstrates the microcrack formation within a core after it is drilled out. Due to maximum anelastic strain recovery (e1) the microcrack formation takes place, perpendicular to the orientation of maximum stress (SH). Measuring the p-wave velocity in different directions in the core sample, then:

Parallel to maximum strain recovery (e1) runs the orientation of the minimum velocity (Vpmin) and maximum attenuation (Apmax) of p-wave signals.

Microcrack distribution as well as preferred orientation of fabric elements cause a considerable p-wave velocity anisotropy. The pressure dependent p-wave velocity anisotropy is used to separate crack caused anisotropy from anisotropy induced by texture.

The stress magnitudes are interpreted with respect to the crack closure pressure, which is determined by means of the pressure dependent p-wave velocity measurements.

The line drawing beside shows the horizontal (h), vertical (v) and conical (c) p-wave transmission of a cylindrical core sample, which is rotated in 10 degree increments about core axis. The plot (Vp-hysteresis) shows  furthermore the dependence on p-waves velocity from confining pressure. The strong increase of velocity in the region up to 100 MPa is caused by the closure of microcracks and pores.

The next two diagrams show the anisotropy of p-wave velocity and the attenuation of the ultrasonic signals in a core sample in dependence on azimuth and confining pressure (1, 10, 40 and 175 MPa). The orientation of the minimum p-wave velocity (Vpmin) runs perpendicular to the microcrack orientation. The orientation of the maximum signal attenuation (Ap max) is coincidental with the direction of minimum p-wave (Vp min) velocity.

The advantage of the WVA-method in comparison to the ASR-method is, that there is no dependence on time applying the WVA-measurements. This investigation can also be carried out on older cores.

3D-State of Rock Stress

Using the results of three different p-wave transmission directions (vertical, horizontal and conical), a reliable interpretation of the 3D-state of rock stress can be done. The next diagram shows the result of a 3D-evaluation (Schmidt net, lower hemisphere) of the in situ rock stress state around a well in a depth range of about 3500m.

Magnitudes and orientation (azimuth/dip) of principal stresses:

S3:  47±1,5 MPa; 66°/12°

S1 = maximum principal stress, S2 = intermediate principal stress, S3 = minimum principal stress, WA: wellbore axis

Download information leaflet:       WVA-info.doc

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Microcrack Formation

WVA-measuring technique

P-wave velocity anisotropy

Anisotropy of P-wave attenuation

3D-State of rock stress