Method for detecting, locating, and characterizing single and multiple fluid-filled fractures in fractured formations

1. A method to detect a fluid-filled fracture in a formation surrounding a borehole, comprising the steps of:

generating acoustic waves using a dipole source in a fluid-filled borehole intersecting or near vertical fluid-filled fractures in the formation surrounding the borehole; and

analyzing dual flexure waves and a leaky fracture mode resulting from the acoustic waves.

2. The method of claim 1 , wherein the dual flexure waves and leaky fracture mode are further used to determine the distance between the borehole and the fracture in the formation surrounding the borehole.

3. The method of claim 1 , wherein the dual flexure waves and leaky fracture mode are further used to determine the position and orientation of the fracture in the formation surrounding the borehole.

4. The method of claim 1 , wherein the dual flexure waves and leaky fracture mode are further used to determine the relative fracture aperture of multiple fractures in the formation surrounding the borehole.

5. The method of claim 1 , wherein the analyzing step is performed as follows: (a) the first mode of the dual flexure wave is shifted to a lower frequency and has smaller amplitude and significantly faster group velocity than the flexure mode excited in a borehole surrounded by a uniform earth medium without a fracture; (b) the second mode of the dual flexure wave is shifted to a higher frequency and has larger amplitude, a moderately higher group velocity, and longer duration than the fundamental flexure mode that is excited in the borehole in the absence of the fracture; (c) the modes defined in (a) and (b) are seen in the amplitude spectrum as peaks; and (d) the leaky fracture mode is represented by a sharp minimum in the amplitude spectrum. At the frequency of the minimum, a considerable amount of energy leaks to the fracture. This leakage reaches its full capacity when the fracture intersects the borehole.

6. A method to determine the distance, d, between a borehole and a fluid-filled fracture in the formation surrounding the borehole, having a radius, a, comprising the steps of:

generating acoustic waves using a dipole source in a fluid-filled borehole intersecting or near vertical fluid-filled fractures in the formation surrounding the borehole;

and analyzing dual flexure waves and a leaky fracture mode resulting from the acoustic waves;

wherein the analyzing step is performed by recognizing that a fracture within about half the wavelength or 3 times the borehole diameter from the borehole center has a significant effect on the dipole acoustic wave; that for d>a, the separation of the dual flexural waves increases with d, but the amplitude of the first flexural wave decreases with d The leaky fracture mode is well developed only for detectors at large offset; and that for d<a, the dual flexural waves are mixed, and the leaky fracture mode is well developed for all detectors.

7. A method to determine the position and orientation of a fluid-filled fracture in a formation surrounding a borehole, comprising the steps of:

generating acoustic waves using a dipole source in a fluid-filled borehole intersecting or near vertical fluid-filled fractures in the formation surrounding the borehole;

and analyzing dual flexure waves and a leaky fracture mode resulting from the acoustic waves;

wherein the analyzing step is performed by recognizing that the orientation of the fracture can be easily determined from the distinct properties of borehole acoustic waves for dipoles oriented perpendicular to the fracture (0°/0°) and parallel to the fracture (90°/90°); that the (0°/0°) case has a slower direct S wave with no amplitude loss, while in the (90°/90°) case, the direct S wave maintains its velocity but often with notable amplitude loss; that more significant differences are in the flexural waves; and that the dual flexural waves and leaky fracture mode reach their maximum at the orientation (0°/0°) and vanish at the orientation (90°/90°).

8. A method to determine the relative fracture aperture of multiple fluid-filled fractures in a formation surrounding a borehole, comprising the steps of:

generating acoustic waves using a dipole source in a fluid-filled borehole intersecting or near vertical fluid-filled fractures in the formation surrounding the borehole;

and analyzing dual flexure waves and a leaky fracture mode resulting from the acoustic waves;

wherein the analyzing step is performed by recognizing that in the spectra, a higher peak in the higher frequency regime is an indicator of a smaller fracture aperture, such that if multiple fractures are detected, separately or simultaneously, in the formation surrounding the borehole, the relative fracture aperture can be determined by comparing their spectral responses.