Cross-Dipole Acoustic Log

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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
Lithology:
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.
Thermal:
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Cross-Dipole Acoustic Log:
An acoustic logging technique where the acoustic transmitter and receivers are lowered down hole and waveforms that travel through the well mud, borehole walls, and surrounding rock are measured to determine and compare sound travel times.
Other definitions:Wikipedia Reegle


 
Introduction
Acoustic logs are sometimes referred to as sonic logs, slowness logs, stoneley analysis, or velocity logs. Acoustic logs measure the time it takes for sound waves to travel through rock. The tool emits a pulse of sound waves from a transmitter and two or more detectors located roughly a meter from the transmitter receive the sound pulse. The time elapsed between the arrival of the sound pulse at two detectors is calculated as the desired travel time. The instrument can use the first arrival waves or a cross correlation of shear, Stoneley, compressional, and mud waveforms to determine the travel time.[1]

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.[1]

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.[2] Weatherford also designs a very similar instrument called the Cross-Dipole Sonic (CXD).[3]

 
Use in Geothermal Exploration
Well logging is standard practice as a wealth of information can be gained by sending various instruments down a well. This technique supports geothermal resource development and helps to characterize the structures and formations within the reservoir that each well penetrates. Acoustic logging is beneficial for learning about stress induced anisotropy, tensile wall fractures, and conductive fractures. This well logging technique can be especially useful for EGS projects. The data provided by this method would be beneficial before any hydraulic fracturing techniques are performed.

 
Field Procedures
The geophysical/well logging service company conducts the down hole logging operation and produces both digital and hard copy logs. The Drilling contractor trips the drill pipe and bit and conditions the well bore for logging.

Conceptual drawing of a sonic logging device and sonic log ray paths with recorded wave forms.[1]



 
Best Practices
-Sonic logs can be performed through cemented well casing.

-The instrument does not operate properly in air, so the well must be filled with some type of fluid.

 
Potential Pitfalls
An unstable well bore (sluffing, wash outs, etc) can be of concern in any well logging operation. In extreme condition, the loss of the logging tool down hole could possibly result in the loss of the hole and would require premature abandonment or the necessity to side track to complete the well drilling operation.


 
NEPA Analysis
Well logging is a standard operation associated with the drilling permit approval and is included in the down hole analysis of the drilling program.






 
References
  1. 1.0 1.1 1.2 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
  2. 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
  3. 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




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