The effect of frequency bandwidth on DSI anisotropy evaluation

Abstract

Anisotropy evaluation in carbonate rocks plays a pivotal role in their reservoir characterization and modeling. This study demonstrates the benefits of advanced interpretation using the Dipole Shear Sonic Imager (DSI) log data for anisotropy evaluations of porous media. Splitting the shear wave into fast and slow components, with different polarizations and velocities, provides useful indications of formation anisotropy. Fast shear waves travel faster in directions parallel to layering and fracturing while slow shear waves travel more slowly perpendicular to layering and fracture networks. Quantifying the difference between fast and slow shear velocities provides a measure of acoustic anisotropy in the medium penetrated.The purpose of the research is to evaluate the effects of frequency bandwidth on the depth of the wave propagation rates and slowness values for anisotropy analysis, thus four bandwidth frequencies: 1–2 kHz, 2–4 kHz, 4–6 kHz, and 1–5 kHz are selected and examined in a vertical drilled well in the gas reservoirs of Upper Dalan Member, and Kangan Formations in the South Pars Gas-Condensate Field.The frequency filtering, semblance analysis, and slowness picking are done on waveforms for frequency-slowness dispersion analysis and slowness correction. The dispersion correction for bandwidth 4–6 kHz isn't performed completely in good hole condition intervals, because high- frequencies around 6 kHz are absorption due to the intrinsic attenuation of the rocks. Therefore, the considered bandwidth is decreased to 4–5.5 kHz. In the following, Alford rotation, align waveforms, and calculation and evaluation of derived fast and slow components are done for deriving the azimuth of the fast shear wave for each bandwidth of waveforms. The dominant trend of the azimuth of anisotropy is that aligned with the azimuth of the fast shear wave.The azimuth of the fast shear wave as azimuth of anisotropy and its corresponding intervals are compared with the results of image log data for the same well interval. This made it possible to reject or confirm the findings of the DSI acoustic anisotropy analysis for each bandwidth considered.In the low bandwidth frequency (1–2 kHz), the differences in slowness, energy, travel time, and penetration depth are more extreme than those recorded by the other bandwidths considered. The azimuth of anisotropy is NW-SE with a slight E-W trend. However, the azimuth of anisotropy was found to be contrary to the image log interpretation where the dominant trend is NE-SW.The dominant trend of the azimuth of anisotropy for bandwidth 2–4 kHz is NE-SW with a slight E-W trend. The anisotropy directions and zones are consistent with those identified from the wellbore image log results. Moreover, the penetration depth is meaningful and far away from the borehole drilling and mud effects. These results suggest that the bandwidth (2–4 kHz) is best suited for acoustic anisotropy analysis of wellbore sections using Dipole Shear Sonic Imager (DSI) recorded data in this well.The trends of the azimuth of anisotropy for bandwidth 4–5.5 kHz are NE-SW, and NW-SE, with expanded tolerance azimuth of anisotropy. The results that have been achieved in a small penetration depth and close to the borehole wall are affected by signal absorption and attenuation in flushed and transition areas. This leads to some errors and inconsistencies with the image log interpretation.The trends of the azimuth of anisotropy for bandwidth 1–5 kHz are NE-SW, NW-SE, and E-W. The outcomes are the consequence of the variety of directions and zones associated with this wider bandwidth, together with the penetration depths of the waveforms in the diverse zones considered. However, the azimuth of anisotropy is inconsistent with the image log interpretation.