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Method for estimating formation in-situ stress magnitudes using a sonic borehole tool

  • US 5,838,633 A
  • Filed: 01/27/1997
  • Issued: 11/17/1998
  • Est. Priority Date: 01/27/1997
  • Status: Expired due to Term
First Claim
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1. A method of estimating in-situ stress magnitude of an earth formation traversed by a borehole containing borehole fluid by utilizing a sonic logging tool having a monopole source, a plurality of dipole sources, and a plurality of sonic detectors, said method comprising:

  • a) generating a relatively low frequency signal and a relatively high frequency signal with at least one source;

    b) detecting the slow flexural, fast flexural, and Stoneley wave arrivals at the detectors for the relatively low frequency signal and the relatively high frequency signal;

    c) determining the Stoneley wave velocity as a function of frequency;

    d) determining the azimuthal orientation of the maximum velocity of at least one of the slow and fast flexural wave arrivals for at least one of the relatively low frequency signal and the relatively high frequency signal;

    e) determining the velocities of the slow and fast flexural waves, as a function of frequency, for the relatively low frequency signal and for the relatively high frequency signal at the determined azimuthal orientation and at an orientation normal thereto;

    f) associating formation anisotropy with stress in the formation if, at the determined azimuthal orientation, the velocity of the fast flexural wave for one of the relatively high frequency signal and the relatively low frequency signal is greater than the velocity of the slow flexural 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 flexural wave is greater than the velocity of the slow flexural wave; and

    , if formation anisotropy is associated with stress in the formation;

    g) selecting a reference state velocity;

    h) determining a velocity change, B, due to uniaxial stress, at a plurality of discrete frequencies; and

    i) determining the stress magnitude and a plurality of non-linear formation parameters according to the equation;

    
    
    space="preserve" listing-type="equation">A.sub.1 S+A.sub.2 Sc.sub.111 /c.sub.66 +A.sub.3 Sc.sub.112 /c.sub.66 +A.sub.4 Sc.sub.123 /c.sub.66 =B,where A1, A2, A3, A4, and B are functions of frequency, c66 is the formation shear modulus in the reference state, and A1, A2, A3, and A4 are frequency dependent sensitivities of flexural wave velocities to S, Sc111 /c66, Sc112 /c66, and Sc123 /c66, respectively.

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