ESTIMATING FORMATION STRESSES USING RADIAL PROFILES OF THREE SHEAR MODULI

摘要

Maximum and minimum horizontal stresses, and horizontal to overburden stress ratio, are estimated using radial profiles of shear moduli. Inversion enables estimation of maximum and minimum horizontal stresses using radial profiles of three shear moduli associated with an orthogonal set of axes defined by the three principal stress directions. Differences in the far-field shear moduli are inverted together with two difference equations obtained from the radial profiles of the dipole shear moduli C44 and C55, and borehole stresses in the near-wellbore region. The horizontal to overburden stress ratio is estimated using differences in the compressional, dipole shear, and Stoneley shear slownesses at two depths in the same lithology interval where the formation exhibits azimuthal isotropy in cross-dipole dispersions, implying that horizontal stresses are nearly the same at all azimuths. The overburden to horizontal stress ratio in a formation with axial heterogeneity may also be estimated using the far-field Stoneley shear modulus C66 and dipole shear modulus C55 together with the radial variation of the dipole shear modulus C55 caused by near-wellbore stress concentrations.

主权项

1. A method for estimating stress ratio in a formation in which a borehole is present comprising:
determining radial profiles of Stoneley slowness for the formation and dipole shear slownesses for the formation; calculating at least one of dipole shear moduli C44 and C55 using the dipole shear slownesses; calculating modulus C66 using the Stoneley slowness; estimating horizontal to overburden stress ratio by inverting (i) a difference equation obtained from far-field shear modulus C66 and at least one of moduli C44 and C55, (ii) a difference equation obtained from a radial profile of at least one of dipole shear moduli C44 and C55 at two or more different radial positions within the formation, and (iii) a relationship between stresses within the formation proximate to the borehole and stresses in the far-field of the formation; and producing an indication of the horizontal to overburden stress ratio.