Alain Brie

A new method for fracture identification using array sonic tools

The development of array sonic tools has enabled the recording of an array of sonic waveforms at each depth location with several receivers. In this paper, the authors develop a new method to study the sonic amplitude/energy by taking advantage of the redundant information in these waveforms. First, they identify the wave components of interest. Second, they apply windowing procedures to extract the desired wave components. Third, they calculate the energy of the wave component at each receiver depth location. Finally, the calculated energies are displayed as an energy log or a variable-density image as a function of depth. Open fractures are detected when the method is applied to the low-frequency Stoneley wave. Reductions of the Stoneley wave energy caused by open fractures appear as a characteristic pattern on the output. The vertical extent of the pattern provides an estimate of the fracture dip angle, and the magnitude of the energy reduction is related to the hydraulic conductivity of the fractures.

Radial variations in cross-dipole shear slownesses in a limestone reservoir

Summary The Backus-Gilbert technique is used for the inversion of cross-dipole dispersions for radial variations in formation shear slownesses. We have applied this technique to eld data from a horizontal well in a limestone reservoir. Most of the sections of this well exhibit shear isotropy in a plane perpendicular to the wellbore trajectory. The cross-dipole measured dispersions coincide with each other and are rather close to the predictions from a classical model for an equivalent homogeneous and isotropic formation. The radial variation in inverted shear slowness is essentially uniform and agrees well with the measured shear slowness. In two sections of this horizontal well, we have observed dipole dispersion crossovers indicating stress-induced shear anisotropy dominating the measurement. Inversions of the fast and slow flexural dispersions in these anisotropic sections yield radial variations in shear slowness parallel and perpendicular to the maximum stress direction. The two radial variations in shear slownesses show a crossover conrming near-wellbore stress concentrations caused by biaxial stresses in a plane perpendicular to the wellbore trajectory. Dierences in the shear slownesses between the near-wellbore and far-eld is in the range of 5 to 7%. The shear slowness radial proles indicate the stress-induced perturbed zone to extend to about three times the borehole radius. The far-eld shear slowness is consistent with the processed shear slowness log.

Practical dipole sonic dispersion correction

Frequency dispersion refers to the variation of sound velocity and of its inverse, the slowness, with frequency. The slowness of compressional and shear body waves in a homogeneous medium do not change with frequency. However, modes linked to the presence of the borehole are dispersive and change with frequency. In monopole propagation the Stoneley and pseudo-Rayleigh waves are dispersive. In dipole propagation the flexural wave, which is linked to the borehole motion, is also dispersive. Dispersion makes dipole shear slowness determination more difficult. It also introduces an error, or bias, in the slowness determination which needs to be corrected. Various approaches are possible to avoid this problem:

Multiple-shot processing in slowness and time domain of array sonic waveforms

Summary In this paper, we present an improved multiple-shot processing for array sonic waveforms. The objective of multiple-shot processing is to improve both the vertical resolution of the slowness logs and the accuracy of the slowness estimation. The existing multiple-shot algorithm projects the slowness-time (ST) plane computed from subarrays on to the slowness axis prior to stacking them. Projection eliminates the time shift uncertainty among the subarrays from the motion of the tool. However, this operation loses the time resolution of ST planes, and as a result, a weak arrival can be masked by a stronger arrival propagating at a similar speed. This can cause difficulties when building a continuous slowness curve later. The algorithm presented here allows stacking of the ST planes in the slowness-time domain directly without using the projections. This allows better separation of different arrivals with similar slownesses. We demonstrate the effectiveness of this algorithm on real field data. The methodology is implemented in a complete slowness evaluation product.