Logging while drilling shear sonic logging in very large boreholes

As LWD sonic tools have become more advanced, they have become more routine in use and are now commonly used in multiple hole sections, with the results having applications across a wide range of problems in well construction. The acquisition of quadrupole shear measurements in large boreholes and slow formations is highly challenging however, due to the borehole conditions, the very slow formation shear values and the effects of the drilling fluid. This paper will discuss the limits of the measurement under these conditions, using both modeled data and real well examples and the effect on measurement quality as a result of these conditions will be shown. The modeling studies shown indicate the importance of understanding the drilling mud properties (slowness and density) was well as the formation and borehole properties when attempting to derive robust shear information under these conditions. The results show that, although the acquisition of reliable shear data in these conditions is challenging, it is not impossible, provided that the LWD tool is correctly designed, the physics are understood and suitable processing applied.