The need for shear-wave velocity for geophysical, reservoir, and drilling applications has led to the development of dipole-shear wave logging, which is now a mature technology for the oil and gas industry. Recent advancement of the technology has been the measurement of formation anisotropy using a cross-dipole system. Despite the technological achievement in this field, it is still quite common in the industry to treat the dipole logging as a scalar measurement with a "single value" at each depth. However, the complexity of the real world requires more than a single measurement to be made to ensure proper characterization of the formation shear-wave properties.
The dipole is a dispersive, guided wave mode in the borehole. The actual borehole conditions, such as borehole size, shape, tool position/orientation, and formation type, etc., must be considered in interpretating the measured data. An important aspect of the dipole is its directionality, which, in the presence of formation azimuthal shear-wave anisotropy, causes measurement discrepancy depending on the orientation of the dipole system in the measurement.
Added to the discrepancy are the effects of near borehole stress concentrations in formations with stress-sensitive rocks. The effects cause significant radial variations in the shear velocity field around the borehole. Consequently, shear measurement using dipole flexural waves may differ from that using monopole refracted shear waves because different rock volumes, or different velocity values, are sensed by the waves of different frequencies. In deviated wells another typical source of uncertainty is the anisotropy (commonly regarded as TI anisotropy) effect in shales where the dipole response depends greatly on the angle of the well relative to the formation beds. To resolve the TI effects in deviated wells, additional acoustic measurements such as the monopole Stoneley wave logging can be integrated with the cross-dipole measurement. Because anisotropy has a direct impact on seismic migration and on AVO modeling, the anisotropy measurement from acoustic logging provides useful information.
The discussions and results of this paper demonstrate that the simplistic treatment of the dipole measurement as a "single scalar value" at each depth can have many pitfalls and may lead to problems. To understand and communicate both the application(s) and the borehole environment is important to properly conduct the necessary acoustic measurements so that the required information can be obtained. Failure to do so can result in the lost opportunity to obtain the needed information and the inability to resolve the apparent measurement discrepancies that contain the information.
The use of borehole shear measurements from acoustic logging is an immense value in many geoscience disciplines. The shear measurement results are used in seismic modeling and data interpretation applications for geophysicists, in formation strength and borehole stability analyses for drilling engineers, and in hydrocarbon production optimization for reservoir engineers.
Dipole acoustic technology was first developed by Zemanek et al. (1984) of then Mobil Research in the 1980s, and became a commercial application to the industry in 1990.