Summary

In offshore deepwater field development, many inclined wells or horizontal wells are drilled. However, most sections and formations have varying degrees of anisotropy and the P- and S-wave velocities vary with propagation directions. In this study, we make use of a laboratory model made of an approximate transversely isotropic Phenolite to study acoustic logging in deviated wells. We drill holes at various deviations relative to the symmetry axis in the Phenolite block. Then we perform monopole and dipole sonic measurements in these holes and extract the qP, qSV, SH, and Stoneley wave velocities using the slowness-time domain semblance method. The velocities measured using monopole and dipole loggings vary with borehole deviations. We also measure the qP, qSV, and SH wave velocities using body waves at the same angles as the well deviations. We then compute the theoretical qP, qSV, SH, and Stoneley wave velocities based on an equivalent transverse isotropic (with a vertical symmetry axis) model of the Phenolite. We find the qP, qSV , and SH wave velocities obtained using the body wave measurement and acoustic logging method agree with the theoretical predictions. The Stoneley wave velocities predicted by the theory also agree reasonably well with the logging measurements.

Introduction

In traditional reservoir development, the wells penetrate the horizontal formations vertically. Sonic well logging measures the P- and S-wave velocities of the formation in the vertical direction. Deepwater reservoir development requires many deviated wells to be drilled off one platform. Logs acquired in these deviated wells can be significantly different from those in vertical wells since most marine formations show strong anisotropy. Similarly, in fractured reservoirs, formations also show fracture-induced anisotropy. Therefore, it is very important for us to understand what the sonic logging tools measure in these deviated or horizontal wells. Anisotropy correction to well logs is often necessary for constructing correct velocity models for the section. Thus velocities are important both for formation evaluation and for establishing ties to seismic imaging. White et al. (1983) and Issac and Lawton (1999) performed laboratory measurements of the anisotropy in TI media. Zhu et al. (1995) also studied sonic logging in azimuthally anisotropic formations. Hornby (2002) showed significantly improved seismic well ties using anisotropy-corrected sonic logs in deviated wells. Tang and Patterson (2005) demonstrated the apparent anisotropy measured using crossdipole sonic data in deviated wells could be different from the true formation anisotropy. Chi and Tang (2006) derived the Stoneley wave velocity in deviated well penetrating anisotropic formations. Therefore, understanding sonic measurements in deviated boreholes drilled in anisotropic formation can provide important insight on applications of sonic logs in offshore fields and fractured reservoirs development. In this study, we use an anisotropic Phenolite as the laboratory material. We first measure the qP, SH, and qSV wave velocities using body waves. We then drill boreholes at different angles with respect to the slowest P-wave principle axis. We record the sonic waveforms measured using a monopole and dipole wireline acoustic logging system in the deviated boreholes.;

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