acoustic imaging log

8/2/2019 Acoustic Imaging Log

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Since the mid-eighties an explosive development comesin imaging technique

Active field

New fundamental concept introduced to logging field

Not a picture like core photo which is taken in the visiblelight

Computer created image based on geophysicalmeasurements of acoustic reflectivity or of electrical

conductivity. Image represent formation response and provide

continues record of the entire borehole.

Still evolving rapidly and affecting the entire logging field


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Introduced by Mobil in1960s

Low detailed

Sampled every 15cm (6)

One measurement perdepth point

Introduced bySchlumberger in 1980s

High detailed

Sample every 0.25cm(0.1)

250 measurements per

depth point.


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Generally called BHTV

Used detailed acoustic

response of formation tocreate an image

Provide full coveragearound the borehole 360degree.

Function in holes filled withany type of liquid, freshwater, water-based baritemud and oil-based mud.

Evolved from Dipmetertechnology.

Use detailed electricalresponse of theformation to create theirimage

Provide partial boreholecoverage

Water-based only


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The tool use a rapidly rotating pulsed soundsource, which is a piezoelectric transducer,which send and receive the sound signal, that

is in a pulse echo mode. It sweeps the entire borehole circumferences

several times a second making over 200measurements of amplitude and travel timeduring each revolution.


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The amplitude varies with the acousticimpedance of the reflecting borehole wall due

to lithology and physical features. As the tool is pulled up the hole with rotating

transducer a very dense matrix of paired datapoint is collected from around the borehole wall

which is processed into an image.


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Company Symbol Name Description

BHTV Borehole Televiewer General Name

Western Atlas CBIL

Circumferential

Borehole ImagingTool

6 revolutions/sec250 samples/rev

Schlumberger UBI Ultrasonic BoreholeImager

7.5 revolutions/sec180 samples/rev

Halliburton CAST CircumferentialAcoustic ScanningTool


BPN AST Acoustic ScanningTool



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Western Atlas (Company)

12m long with rotatingtransducer.

Bowstring keep this part oftool centered.

Orientation , telemetry

electronics and spectralgamma ray sonde is placedabove the bowstring.

Bowstrings


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When tool is logging, the transducer In pulseecho mode ,

6 revolutions per seconds

Taking 250 digitized samples on each

revolution Logging speed 3m per minute

Tool scan each 0.83cm borehole

circumference of depth Acquiring a matrix of 30,000 sample points of

paired data readings (30,000 readings ofamplitude & 30,000 readings of travel time


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Transducer is piezoelectric, activated by anelectric pulse.

Normally made from a thin circular disc, size(1-2) in diameter.

Transducer is hemispherical and has aconcave surface facing outwards which hasthe effect of collimating (focusing) the soundpulse.

Transducer produce an ultrasonic pulse1500 times per second with frequency of250 KHz.


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The tool can only be used in holes withlower density muds.

CBIL uses a lower frequency signal which

improves operating ranges (distinctpenetration), so that it can be used with muddensities up to 1.7-1.9

Other companies frequency ranges: Mobil ----------------------- 2MHz

Amoco --------------------- 1.3 MHz

LowPenetration


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Raw acoustic travel time and amplitude data aregenerally processed to color image or into greyscale and presented in the unwrapped boreholeformat.

In the processing the measurements compared andplaced side by side to construct an image

Each sample is represented by one pixel

Pixel matrix is built up from 250 measurements

The actual area of borehole wall represented by apixel will depend on borehole size(diameter) and thelogging speed.

Unwanted effects are removed by filtering,

equalization, edge detection and other techniques


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Resolution is the ability to separate

two objects. Features resolution is controlled by

transducer size and tool electronics

(beam characteristics)


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Borehole geometry and tool position

Mud weight

Acoustic impedance contrast

Borehole surface


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Thejuxtaposition of two acoustic

image log, the reflected amplitudeand the travel time from borehole

wall is an aid to interpretation


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Dip and azimuth

Investigate fractures Borehole breakouts

Lithological boundaries

Texture Some sedimentary features


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When two log processing (amplitude & travel

time) are displayed for comparing: Amplitude scale use lighter color or

shades for log amplitude and black color

for zero amplitude The travel time scale for near reflections

darker shade and for far reflection non atall (light only)

Amplitude log gives lithological informationand travel time log gives borehole geometry(ovality and breakouts).


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The mutual interpretation of both logsprovide information about fractures isopen or closed

Open fracture: loss of signal in bothcases

Closed fracture: filled fracture provideimage on the amplitude log


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Book: The geological interpretation of well logs by Malcolm Rider

Acoustic and optical borehole-wall imaging for fractured-rock aquifer

studies John H. Williams, Carole D. Johnson


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acoustic imaging log

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