Cross-Dipole Acoustic Log
Exploration Technique: Cross-Dipole Acoustic Log
|Exploration Technique Information|
|Exploration Group:||Downhole Techniques|
|Exploration Sub Group:||Well Log Techniques|
|Parent Exploration Technique:||Acoustic Logs|
|Information Provided by Technique|
|Stratigraphic/Structural:||Rock stress and fracture analysis|
|Hydrological:||Use for fracture identification in open and cased holes. Also used for evaluating hydro fracturing/well stimulation effectiveness.|
Acoustic sources can be monopole, dipole, or quadrupole; traditionally instruments were monopole, but dipole sources and receivers are a newer and improved technology. Quadrupole instruments are not commonly used and cannot be found commercially available. A dipole source emits acoustic energy in a single direction rather than radially as a monopole does. The advantage of a dipole source is that it generates strong shear waves in both fast and slow formations, the shear waves are also called flexural or bender waves and they travel along the borehole wall. Modern sonic logging tools usually have both monopole and dipole sources and receivers, so that shear waves and compressional waves can be recorded in fast and slow formations. Some of the most modern acoustic logging tools have two sets of dipole sources set orthogonally, these are called crossed-dipole tools. With this setup shear data can be recorded in two directions and measurements can be translated into minimum and maximum travel times also called slowness of sound in rock. The ratio of travel times in rock is called acoustic anisotropy and is an important property in tectonic studies, stress analysis, fractured reservoir description, and hydraulic fracture design.
An instrument designed by Schlumberger called the DSI Dipole Shear Sonic Imager uses a combined monopole and dipole system and can reliably measure compressional, shear and Stoneley Slownesses. For Stoneley wave measurements a specific low frequency pulse is used. Slowness measurements can be made from behind well casing. The instrument has a cross dipole mode called both cross receivers (BCR) and is used for anisotropy analysis. Weatherford also designs a very similar instrument called the Cross-Dipole Sonic (CXD).
-The instrument does not operate properly in air, so the well must be filled with some type of fluid.
- Crain, P. Eng. Crain’s Petrophysical Handbook [Internet]. 2013. SONIC TRAVEL TIME (SLOWNESS) LOGS. [cited 2013/10/01]. Available from: http://www.spec2000.net/07-soniclog.htm
- Schlumberger. DSI Dipole Shear Sonic Imager [Internet]. 2013. [cited 2013/10/10]. Available from: http://www.slb.com/services/characterization/geomechanics/wireline/dipole_shear_sonic_imager.aspx
- Weatherford. Compact Cross-Dipole Sonic (CXD) [Internet]. 2010. [cited 2013/10/10]. Available from: http://www.weatherford.com/weatherford/groups/web/documents/weatherfordcorp/www018914.pdf
|Page||Area||Activity Start Date||Activity End Date||Reference Material|
|Cross-Dipole Acoustic Log At Alum Area (Moos & Ronne, 2010)||Alum Geothermal Area|