US 5,398,215 A · Identification of stress induced anisotropy in formations

Abstract

Low and high frequency flexural waves or their equivalents are generated with dipole or other source transducers. From measurements made at receiving transducers which are oriented at two orthogonal directions in a horizontal plane normal to the borehole axis, and via known processing techniques, the received signals are transformed into arrivals as a function of frequency such that the principal polarization directions and the magnitudes of the maximum and minimum wave velocities at those directions are determined at different frequencies. If the maximum velocity of the relatively low frequency flexural waves are in a first principal polarization direction, and the maximum velocity of the relatively high frequency flexural waves are in a second principal polarization direction which is substantially normal to the first principal direction, uniaxial stress in the formation is attributed to stress induced azimuthal anisotropy as opposed to an instrinsic anistropy in the formation. Then, the low frequency information can be utilized to obtain a value for a formation shear stress parameter such as c.sub.456 T.sub.23.sup.max /c.sub.66, where c.sub.456 is a third order elastic constant of the formation in Voigt notation c.sub.66 is the formation shear modulus and T.sub.23.sup.max is the magnitude of the formation shear stress in a direction forty-five degrees from the maximum far-field compressive stress direction.

Claims26

Dependent claims

A method for associating formation anisotropy with stress in the formation by utilizing a borehole tool with at least one sonic source and at least one sonic detector in a borehole traversing the formation, the method comprising:

a) generating with the at least one sonic source a relatively low frequency signal and a relatively high frequency signal, where the relatively low frequency signal is sensitive to the formation distant from the borehole, and the relatively high frequency signal is sensitive to the formation adjacent the borehole;
b) detecting at the at least one sonic detector the slow and fast wave arrivals for the relatively low frequency signal and the relatively high frequency signal;
c) determining the azimuthal orientation of the maximum velocity of at least one of the slow and fast wave arrivals for at least one of the relatively low frequency signal and the relatively high frequency signal;
d) determining the velocities of the slow and fast waves for the relatively low frequency signal and for the relatively high frequency signal at the determined azimuthal orientation and at an orientation normal thereto;
and
e) associating formation anisotropy with stress in the formation if, at the determined azimuthal orientation, the velocity of the fast wave for one of the relatively high frequency signal and the relatively low frequency signal is greater than the velocity of the slow wave, and at the orientation normal to the determined azimuthal orientation, for the other of the relatively high frequency signal and relatively low frequency signal, the velocity of the fast wave is greater than the velocity of the slow wave.

3 more elements

a) generating with the at least one sonic source a relatively low frequency signal and a relatively high frequency signal, where the relatively low frequency signal is sensitive to the formation distant from the borehole, and the relatively high frequency signal is sensitive to the formation adjacent the borehole;
b) detecting at the at least one sonic detector the slow and fast wave arrivals for the relatively low frequency signal and the relatively high frequency signal;
c) determining the azimuthal orientations of the maximum velocities for the slow and fast wave arrivals for both the relatively low frequency signal and the relatively high frequency signal;
and
d) determining whether the azimuthal orientation of the maximum velocity for one of the fast wave and slow wave arrivals for the relatively low frequency signal is substantially orthogonal to the azimuthal orientation of the maximum velocity for the same one of the fast wave and slow arrival for the relatively high frequency signal, and if so, associating formation anisotropy with stress in the formation.

2 more elements

An apparatus for associating anisotropy of a formation traversed by a borehole with stress in the formation, comprising:

a) a borehole tool having a sonic source means and a sonic detector means, said sonic source means for generating a relatively low frequency wave signal and a relatively high frequency wave signal, with said relatively low frequency wave signal being sensitive to the formation distant from the borehole, and said relatively high frequency wave signal being sensitive to the formation adjacent the borehole, and said sonic detector means for detecting said relatively low frequency wave signal as affected by the formation and said relatively high frequency wave signal as affected by the formation and generating outputs in response thereto;
b) processing means coupled to said sonic source means for transforming said output signals into orthogonal slow and fast wave arrivals as a function of frequency such that the principal polarization directions and the magnitudes of the maximum and minimum wave velocities at those directions are determined at different frequencies;
and
c) means for indicating that the maximum velocity of the received relatively low frequency waves are in a first principal polarization direction, and the maximum velocity of the received relatively high frequency waves are in a second principal polarization direction which is substantially normal to the first principal direction, from which a determination is made that formation anisotropy is due to uniaxial stress in the formation.

1 more element

Dependencies are inferred from the claim text.

Identification of stress induced anisotropy in formations

Life of this patent

Identification of stress induced anisotropy in formations

Life of this patent

Abstract

Claims26