Types of sonic logging tools are explained briefly with help of animation and what are the application of these tools in determining the formation properties.
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Sonic log and its applications
1. Presented by
Badal Dutt Mathur
10410007
5th Year Integrated M.tech Geological Technology
2. A well log is a continuous record of some property
of the formation penetrated by borehole with
respect to the borehole depth
There are many logs and corresponding logging
tools for different objectives
Well Log
Fig 1.1 Well Log
3. Sonic Log
The sonic log measures interval transit time (Δt) of a compressional
sound wave traveling through one foot of formation.
The units are micro seconds/ft, which is the inverse of velocity.
4. Principles of measurements
The tool measures the time it takes for a pulse of “sound” (i.e., and
elastic wave) to travel from a transmitter to a receiver, which are both
mounted on the tool. The transmitted pulse is very short and of high
amplitude vice versa.
Receiver
Transmitter
6. Tx
Rx
A
B
C
1. Early tools had one Tx and one Rx.
2. The body of the tool was made from rubber
(low velocity and high attenuation material) to
stop wave travelling preferentially down the
tool to the Rx.
There was main problems with this tool.
The measured travel time was always too long.
Δt=A+B+C
Early Tool
Fig 1.2 Early Sonic Tools
7. Rx2
A
B
C
Rx1 D
E
Tx
Dual Receiver Tool
• These tools were designed to overcome the
problems in the early tools.
• They use two receivers a few feet apart, and
measure the difference in times of arrival of
elastic waves at each Receiver from a given pulse
from the Transmitter
• This time is called the sonic interval transit time
(Δt)
TRx1= A+B+C
TRx2= A+B+D+E
Δt=(A+B+D+E)-(A+B+C)
Δt=D ( If tool is axial in borehole C=E)
Fig 1.3 Dual receiver sonic tools in correct configuration
8. Tx
Rx
Rx
Problem with Dual
If the tool is tilted in the hole, or the hole size
changes (Fig 3)
Then C≠E
The two Rx system fails to work.
Arrangement
C
D
Fig 1.4 Dual receiver sonic tools in incorrect configuration
9. B A
Automatically compensates for borehole
effects and sonde tilt
It has two transmitters and four receivers,
arranged in two dual receiver sets, but with
one set inverted
Each of the transmitters is pulsed
alternately, and Δt values are measured
from alternate pairs of receivers (Fig.1.5)
These two values of Δt are then averaged
to compensate for tool misalignment
Δt=A+B/2
Borehole
Compensated Tool
Fig1.5 Borehole compensated sonic tools
11. The sonic log is commonly used to calculate the porosity of formations,
however the values from the FDC and CNL logs are superior.
It is useful in the following ways:
1. As a quality check on the FDC and CNL log determinations.
2. As a robust method in boreholes of variable size (since the sonic log is
relatively insensitive to caving and wash-outs etc.).
3. To calculate secondary porosity in carbonates.
4. To calculate fracture porosity.
Porosity
Determination
12. The velocity of elastic waves through a given lithology is a function of
porosity
Øsonic = sonic derived porosity in clean formation
Δt = interval transit time of formation
Δtma = interval transit time of the matrix
(sandstone=55.5,limestone=47.6,dolomite=43.5,anhydrite=50)
Δtp = interval transit time of the pore fluid in the well bore
(fresh mud = 189; salt mud = 185)
Unit=microsecond per feet
The Wyllie Time
Average Equation
Fig 1.6 The wave path through porous fluid saturated rocks
13. Wyllie Time Average Equation is valid only for
For clean and consolidated sandstones
Uniformly distributed small pores
Correction: Observed transit times are greater in uncompacted sands; thus apply
empirical correction factor, Cp
Фc= Ф/Cp
Cp=c* Δtsh/100 (Δtsh= Interval transit time for the adjacent shale)
C=shale compaction coefficient (ranges from 0.8 < c < 1.3)
Fluid Effect in high porosity formations with high HC saturation.
Correct by
OIL: Фcorr= Фc*0.9
GAS: Фcorr= Фc*0.7
The Wyllie Time
Average Equation
14. The sonic log is sensitive only to the primary intergranular porosity
The sonic pulse will follow the fastest path to the receiver and this will
avoid fractures
Comparing sonic porosity to a global porosity (density log, neutron
log)should indicate zone of fracture.
Ф2 = (ФN , ФD ) - ФS
Secondary and
Fracture Porosity
15. Fig 1.7 Subtle textural and structural variations in deep sea
turbidite sands shown on the sonic log (after Rider).
The sonic log is sensitive to small changes in grain
size, texture, mineralogy, carbonate content,
quartz content as well as porosity
This makes it a very useful log for using for
correlation and facies analysis
Stratigraphic
Correlation
16. Fig 1.8 Uplift and erosion from compaction trends.
Compaction
As a sediment becomes compacted, the velocity
of elastic waves through it increases
If one plots the interval transit time on a
logarithmic scale against depth on a linear scale,
a straight line relationship emerges
Compaction trends are constructed for single
lithologies, comparing the same stratigraphic
interval at different depths
Compaction is generally accompanied by
diagenetic changes which do not alter after uplift
Amount of erosion at unconformities or the
amount of uplift from these trends can be
estimated
17. Fig 1.9 An overpressured zone
distinguished from sonic log data.
Overpressure
An increase in pore pressures is shown on the
sonic log by a drop in sonic velocity or an
increase in sonic travel time
Break in the compaction trend with depth to higher
transit times with no change in lithology
Indicates the top of an
overpressured zone.
18. synthetic
seismogram
acoustic
impedenc
e
reflection
coefficient
reflection
coeffiecient
with
transmission
losses
sonic
Represents the seismic trace that should be velocity
observed with the seismic method at the
well location
Improve the picking of seismic horizons
Improve the accuracy and resolution of
formations of interest
Synthetic
Seismograms
Fig 1.10 The construction of a synthetic seismogram.
19. Identification of
Lithologies
The velocity or interval travel time is rarely
diagnostic of a particular rock type
The sonic log data is diagnostic for coals, which
have very low velocities, and evaporites, which
have a constant, well recognized velocity and
transit time
Sonic log best work with other logs (neutron or
density) for lithological identification
SONIC-NEUTRON CROSSPLOTS
Developed for clean, liquid-saturated formations
Boreholes filled with water or water-base muds
20. 110
• Shale - NE region
Field
observation
100
Fracture -
South 90
Gas - NW
80
70
Trona
60
50
40
0 10 20 30
40
Apparent neutron porosity (lspu)
t, Sonic transit time (μs/ft)
Time a
verage
Syivite
Tr
SONIC-NEUTRON
PLOTS
21. Example
Two types of data is taken
Gamma ray > 80
Gamma ray < 30
Shale
Sandstone
22. Serra, O. (1988) Fundamentals of well-log interpretation. 3rd ed. New
York: Elselvier science publishers B.V.: 261-262
Rider, M. (2002) The geological interpretation of well logs. 2nd ed.
Scotland: Rider French consulting Ltd.: 26-32.
Neuendorf, et al. (2005) Glossary of Geology. 5th ed. Virginia: American
geological institute: 90, 379, 742.
Schlumberger (1989) Log interpretation principles/applications.
Schlumberger,Houston, TX
Refrences
Editor's Notes
because the time taken for the elastic waves to pass through the mud was included in the measurement.
because changes to the velocity of the wave depending
upon the formation altered the critical refraction angle.
Figure 1.7 compares the compaction trend for the same lithology in the same stratigraphic interval in one well with that in another well. The data from the well represented by the circles shows the interval to have been uplifted by 900 m relative to the other well
because it has lower interval transit times (is more compact) but occurs at a shallower depth.