Download - Sonic log and its applications
APPLICATIONS OF SONIC LOG
Presented by Badal Dutt Mathur 104100075th Year Integrated M.tech Geological Technology
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
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.
Sonic Log
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.
Principles of measurements
Receiver
Transmitter
1.Early Tool
2.Dual Receiver Tool
3.Borehole Compensated Sonic(BHC) Tool
Working Tools
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
Rx2
A
B
C
Rx1
D
E
Tx
• 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)
Dual Receiver Tool
Fig 1.3 Dual receiver sonic tools in correct configuration
Rx
Rx
Tx
Rx
Rx
Tx 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.
Problem with Dual Arrangement
Fig 1.4 Dual receiver sonic tools in incorrect configuration
C
D
AB
Tx
Rx
Tx
Rx
Rx
Rx
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
Porosity Determination
Secondary and Fracture Porosity
Stratigraphic Correlation
Compaction
Overpressure
Synthetic Siesmogram
Identification of Lithology
Applications
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 Determinati
on
The velocity of elastic waves through a given lithology is a function of porosity
Øsonic = sonic derived porosity in clean formationΔt = interval transit time of formationΔtma = 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
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
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
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
Fig 1.8 Uplift and erosion from compaction trends.
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
Compaction
Fig 1.9 An overpressured zone distinguished from sonic log data.
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 highertransit times with no change in lithology
Indicates the top of anoverpressured zone.
Overpressure
synthetic seismogram
acoustic impeden
ce
reflectioncoefficient
reflection coeffiecient
withtransmission
losses
sonic velocity Represents the seismic trace that should be
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 Seismogram
s
Fig 1.10 The construction of a synthetic seismogram.
SONIC-NEUTRON CROSSPLOTS
Developed for clean, liquid-saturated formations
Boreholes filled with water or water-base muds
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
Identification of Lithologies
110• Shale - NE region
Field observation
100Fracture - South 90
Gas - NW80
70Trona
60
50
400 10 20 30
40
(lspu)Apparent neutron porosity
t, S
on
ic t
ran
sit
tim
e (
μs/f
t)
Time a
Field
verage
Syi
vite
Tr
SONIC-NEUTRON PLOTS
Example
Two types of data is taken
Gamma ray > 80
Gamma ray < 30
Shale
Sandstone
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
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