REVISED
© 2004 – 2009 WELLOG
BOREHOLE
ACOUSTICS:
In acoustic logging,
acoustic (sound) energy is transmitted by an acoustic transmitter. The transmitted
acoustic energy is received by an acoustic receiver. Acoustic transducers that
convert electrical energy into mechanical energy and mechanical energy back
into electrical energy are used for this purpose. Acoustic transducers in
current use are piezoelectric and magneto-strictive
types.
The acoustic
transmitter is briefly pulsed by a pulse of electrical energy and in turn,
creates an acoustic impulse. The pulsing continues at rate called the
repetition rate or “rep” rate. Repetition rates may be once or twice per
second. A transmitted acoustic waveform travels in all directions from the
transmitter. After a short period of time related to the formation materials
and distance traveled, the acoustic waveform arrives at a receiver. The
receiver converts the arriving acoustic (mechanical) energy into electrical
energy that is measured within the probe or transmitted electrically up hole to
the surface electronics for processing.
METHODS
USED:
Two primary acoustic
methods are in use today. The two methods are referred to as the “reflection
method” and the “transmission method”.
Reflection method:
Systems that use the
reflection method direct acoustic energy horizontally toward the borehole wall.
The reflected signal is measured by the same transducer. Borehole Acoustic Televiewer systems fall into this category.
Transmission method:
Systems using the
transmission method transmit acoustic energy in all directions into the
surrounding formation. One or more receivers at fixed distances and acoustically
isolated from the transmitter(s) measure the arriving acoustic signal.
Note: Acoustic
isolation is used in the probe design to prevent coupling of acoustic energy
through the tool.
Several types of
probes are in use today. The most basic
is a single transmitter, single receiver probe. Increasing complexity is seen
in the single transmitter, dual receiver probe and finally, the dual
transmitter, dual receiver probe.
Single transmitter,
single receiver probe: View
probe
This probe has a
single receiver at 3 ft. or 5 ft. spacing from the transmitter. The single receiver
probe has a disadvantage in rugose holes or when not
properly centralized in the borehole. It is often used in amplitude measurement
in cement bond logging operations.
Single transmitter,
dual receiver probe: View
probe
The dual receiver
probe offers an advantage over the single receiver probe in that formation
velocities are more accurately obtained. Accuracy is increased by virtue of the
two receiver velocities or delta-time measurements when subtracted reveal the
actual formation travel time. Given the time for acoustic energy to travel from
transmitter thru borehole fluid, to the far receiver including travel thru the
borehole fluid a second time and a second shorter path to the near receiver,
when the two travel times are subtracted, the result is the actual formation
travel time over the distance between the two receivers. The borehole travel
times are cancelled arithmetically by subtraction.
Dual transmitter,
dual receiver probe: View
probe
A further
improvement involves adding an additional transmitter below the two receivers.
Measuring and canceling borehole effects in both directions and having an
average of two measurements helps eliminate errors due to borehole rugosity and
tool eccentricity in the borehole. This system is referred to as Borehole
Compensated (BHC) in the literature.
WAVES OF
INTEREST:
Transmitter pulse:
The transmitter
transducer generates an alternating expansion and contraction that propagates
into the surrounding borehole and formation in all directions. The propagated
waveform is predominately a 20 KHz harmonic wave.
Formation wave: Formation
waveforms
As the waveform
travels into the surrounding formation, the compression and shear components
traveling at different velocities are changed in relative time. The compression
(P) waves move ahead in advance of the shear (S) waves and later (L) borehole
and mud waves. Ultimately these waveform components arrive at the receiver and
are measured.
Composite wave: Composite
waveform
A composite waveform
representing the “acoustic wave train” arrives at the receiver. The first arrival of the Compression wave (P)
arrives followed by the remaining compression waves. Before the end of the compression wave
arrivals, the shear waves arrive. Overlapping occurs resulting in interference.
Finally the later (L) waves arrive before the end of the shear wave arrivals
again causing interference.
Casing waveform: Casing
waveform
When logging in free
casing, the transmitted pulse causes ringing of the pipe resulting in a
relatively high amplitude persistent dampened waveform.
CASING BOND LOG:
When used for casing
bond logging operations, a time gate is generated that tracks the amplitude of
the first arrival of the casing
waveform. The amplitude is much greater in free casing. This amplitude establishes
a reference for no cement bond referred to a zero percent bond. The tool is
lowered to a position in the well where 100 percent bond is assured and the
lower amplitude is referenced as indicating 100 percent bond
ACOUSTIC
LOGGING SYSTEMS:
MEASURING DELTA-T:
Using any of the
tools mentioned above, a timing sequence is initiated by a transmitter pulse.
Time is measured using electronic circuitry from the transmitter pulse
(referred to as T0) to the “first arrival” of an acoustic waveform at the receiver. The first arrival is usually considered to be
the first negative half-cycle of the receiver waveform. Delta-t is determined by division of the
elapsed time by the number of feet from transmitter to receiver. Borehole compensated systems use two transmitters
and two receivers that perform a subtraction of delta-t that cancels the effect
of borehole size and rugosity and results in formation travel time only.
INTEGRATED TRAVEL
TIME:
Delta-t measurements
can be integrated (added with depth) to provide geologists with cumulative
formation travel times. Integrated travel time data can be used to improve the
accuracy of seismic systems.
MEASUREMENT OF
AMPLITUDE:
Again, using any of
the acoustic tools mentioned previously, a sequence of transmitting and
receiving an acoustic signal is initiated by an acoustic pulse
transmission. Amplitude of a desired
portion of the acoustic waveform is measured using a timing gate that is placed
at the appropriate point in time during the received waveform. Applications of
amplitude measurement include cement bond logging in cased holes.
RADIAL BOND TOOLS:
Bond logging tools
that have segmented receivers arranged around the radius of the tool give
improved accuracy. A tool having eight receivers can measure the signals
reflecting from 45 degree radial arcs. This increases the accuracy of cement
bond logging.
ULTRASONIC IMAGER
TOOL (USIT):
One service company
provides a tool having a 200 – 700 KHz Ultrasonic capability. The receiver
sensor is rotated 360 degrees as tool is pulled uphole.
A log of reflected acoustic energy is created that measures the quality of
casing to cement bond, and casing condition.