Determination of dipole shear anisotropy of earth formations

Details Determination of dipole shear anisotropy of earth formations Determination of dipole shear anisotropy of earth formations

2002-10-31

2004-04-06

McElheny, Jr., Donald E. (Department: 2857)

Data processing: measuring, calibrating, or testing

Measurement system in a specific environment

Earth science

C702S018000, C367S075000

Reexamination Certificate

active

06718266

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to investigation of earth formations and, more particularly, to a method and apparatus for obtaining properties of earth formations using sonic logging and determining dipole shear anisotropy and related characteristics of the earth formations.
BACKGROUND OF THE INVENTION
It is well known that mechanical disturbances can be used to establish acoustic waves in earth formations surrounding a borehole, and the properties of these waves can be measured to obtain important information about the formations through which the waves have propagated. Parameters of compressional, shear, and Stoneley waves, such as their velocity (or its reciprocal, slowness) in the formation and in the borehole, can be indicators of formation characteristics that help in evaluation of the location and/or producibility of hydrocarbon resources.
An example of a logging device that has been used to obtain and analyze sonic logging measurements of formations surrounding an earth borehole is called a Dipole Shear Sonic Imager (“DSI”—trademark of Schlumberger), and is of the general type described in Harrison et al., “Acquisition and Analysis of Sonic Waveforms From a Borehole Monopole And Dipole Source For The Determination Of Compressional Arid Shear Speeds And Their Relation To Rock Mechanical Properties And Surface Seismic Data”, Society of Petroleum Engineers, SPE 20557, 1990. In conventional use of the DSI logging tool, one can present compressional slowness, &Dgr;t
c
, shear slowness, &Dgr;t
s
, and Stoneley slowness, &Dgr;t
st
, each as a function of depth, z. [Slowness is the reciprocal of velocity and corresponds to the interval transit time typically measured by sonic logging tools.]
An acoustic source in a fluid-filled borehole generates headwaves as well as relatively stronger borehole-guided modes. A standard sonic measurement system consists of placing a piezoelectric source and an hydrpohone receivers inside a fluid-filled borehole. The piezoelectric source is configured in the form of either a monopole or a dipole source. The source bandwidth typically ranges from a 0.5 to 20 kHz. A monopole source generates primarily the lowest-order axisymmetric mode, also referred to as the Stoneley mode, together with compressional and shear headwaves. It can also excite pseudo-Rayleigh mode in fast formations. A pseudeo-Rayleigh mode asymptotes to the formation shear slowness at low (cut-in) frequencies and to the Scholte waves at high frequencies. All monopole measurements provide formation properties averaged in the plane perpendicular to the borehole axis.
A dipole source excites compressional headwaves at high frequencies and dispersive flexural mode at relatively lower frequencies in both the fast and slow formations. It can also excite shear headwaves in fast formations and leaky compressional modes in slow formations. The lowest-order leaky compressional mode asymptotes to the formation compressional slowness at low frequencies and to the borehole fluid compressional slowness at high frequencies. All dipole measurements have some azimuthal resolution in the measurement of formation properties. The degree of azimuthal resolution depends on the angular spectra of the transmitter and receivers.
The radial depth of investigation in the case of headwave logging is dependent on the transmitter to receiver spacing and any in-situ radial variations in formation properties that might be present. The radial depth of investigation in the case of modal logging is well characterized and it extends to about a wavelength. This implies that the low- and high-frequencies probe deep and shallow into the formation, respectively.
Most formations exhibit some degree of anisotropy. The formation shear anisotropy can be caused by aligned fractures, thin beddings or microlayering in shales. This type of anisotropy is called formation instrinsic anisotropy. Non-hydrostatic prestresses in a formation introduce stress-induced anisotropy. A dipole dispersion crossover is an indicator of stress-induced anisotropy dominating any instrinsic anisotropy that may also be present
A present technique for measuring dipole shear anisotropy incudes recording the inline and crossline receiver waveforms from both the upper and lower dipole sources (C. Esmersoy, K. Koster, M. Williams, A. Boyd and M. Kane, Dipole Shear Anisotropy Logigng, 64
th
. Ann. Internat. Mtg., Soc. Expl. Geophys, Expanded Abstracts, 1139-1142, 1994). These sources are orthogonal to each other and spaces apart, for example by 6 inches. The inline and crossline receivers are oriented parallel and perpendicular to the dipole transmitter direction, respectively. A total of four sets of waveforms u
xx
, u
xy
, u
yy
, and u
yx
) are recorded at a given depth in a borehole. The first and second subscripts X and Y denote the dipole source and receiver orientations, respectively. The four sets of recorded waveforms are low-pass filtered and time-windowed and then subjected to the so-called Alford rotation algorithm that yields the fast or slow shear directions with respect to a reference dipole source direction. The recorded waveforms are then rotated by the aforementioned angle. The rotated waveforms correspond to the fast and slow flexural waveforms. These waveforms can then be processed by a known algorithm that yields the fast and slow shear slownesses, respectively. (See C. V. Kimball and T. L. Marzetta, Semblance Processing Of Borehole Acoustic Array Data, Geophysics, (49); 274-281, 1986; C. V. Kimball, Shear Slowness Measurement By Dispersive Processing Of The Borehole Flexural Mode, Geophysics, (63), 337-344, 1998.)
The described present technique for estimating formation fast shear azimuth is based on the following assumptions (see C. Esmersoy, K. Koster, M. Williams, A. Boyd and M. Kane, Dipole Shear Anisotropy Logigng, 64
th
. Ann. Internat. Mtg., Soc. Expl. Geophys, Expanded Abstracts, 1139-1142, 1994.)
a. It is assumed that the upper and lower dipole radiation characteristics are identically the same and that both the inline and crossline receivers are well matched.
b. The low-pass filtering of the recorded waveforms retains essentially the nondispersive part of the borehole flexural wave.
c. A depth matching of the recorded waveforms from the upper and lower dipole sources reduces the number of processed 8 waveforms to 7.
Since the Alford rotation algorithm assumes a nondispersive shear wave propagation, it is necessary to low-pass filter the recorded waveforms. Low-pass filtering of the waveforms also ensures removal of any headwave arrivals that might interfere with the shear slowness and the fast-shear direction processing of the dipole logs.
A processing time window is also selected extending over a couple of cycles at the beginning of the waveform. The placement of the processing window attempts to isolate a single flexural arrival from other possible arrivals in the waveforms.
The existing techniques have certain limitations and/or drawbacks, and it is among the objects of the present invention to provide improved technique and apparatus for determining shear slowness and directionality of anisotropic formations surrounding an earth borehole.
SUMMARY OF THE INVENTION
In accordance with a form of the invention, a system and technique are set forth that require recording of the inline and crossline receiver waveforms from only one dipole source. This eliminates the need for orthogonal source dipoles to have identical radiation characteristics. Further advantages of this approach include reduction in the amount of data and logging time to half of the referenced current procedure that requires four-component acquisition from the two orthogonal dipole sources.
As was noted above, in the existing technique, since the Alford rotation algorithm assumes a nondispersive shear wave propagation, it is necessary to low-pass filter the received waveforms. However, the more energetic part of the flexural waveforms is at somewhat higher frequencies.
As was also noted above, in the existing technique the placement o

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