Estimating shear wave in shale based on the variable critical porosity model

With abundant organic matter, shale and mudstone have emerged as key targets of hydrocarbon exploration and development in recent years. An accurate shear-wave velocity, one of cardinal data for drilling-seismic integration, prestack inversion, brittleness calculation and stress assessment, plays an indispensable role in identifying the most interesting shale. Actually, few shear data may be derived from logging. For the purpose, it is imperative to prepare one methodology to estimate shear-wave velocity peculiar to shale pay zones. Most theoretic models in early discussions were created for sandy conglomerate pay zones rather than shale ones. This study witnesses that the critical porosity model proposed by Amos Nur is appropriate in the shale zones. Thus, the Nur model is creatively introduced. The Nur’s critical porosity is adjusted to an adaptive variable porosity in an effort to improve it for the shale. Moreover, an inversion restrained by compressional-wave time difference is done after figuring out mineral content. The variable critical porosity is calculated adopting genetic algorithms with settled-down populations. Finally, the shale zones can get better shear prediction under the novel methodology whose application to the Sichuan Basin shows that, the average absolute error ranges from 0 µs/ft to 5 µs/ft and the relative error lower than 10% between the two shear-wave time differences separately from Schlumberger’s dipole shear sonic imager and the methodology. The latter may achieve more accurate results than Krief and Nur models. In brief, stemming from mineral content and the improved Nur model, the new methodology (namely the variable critical porosity model) has its strengths in shear prediction, such as higher accuracy, better flexibility and excellent application effects.