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APPLICATIONS OF SONIC LOGPresented by Badal Dutt Mathur 104100075th Year Integrated M.tech Geological TechnologyA 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 LogFig 1.1 Well LogThe sonic log measures interval transit time (t) of a compressional sound wave traveling through one foot of formation.The units are micro seconds/ft, which is the inverse of velocity.

Sonic LogThe 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

ReceiverTransmitter1.Early Tool2.Dual Receiver Tool3.Borehole Compensated Sonic(BHC) Tool

Working ToolsTxRxABCEarly tools had one Tx and one Rx.

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. t=A+B+CEarly ToolFig 1.2 Early Sonic Toolsbecause 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 dependingupon the formation altered the critical refraction angle.

6Rx2ABCRx1DETxThese 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 (t)

TRx1= A+B+CTRx2= A+B+D+Et=(A+B+D+E)-(A+B+C)t=D ( If tool is axial in borehole C=E)

Dual Receiver ToolFig 1.3 Dual receiver sonic tools in correct configurationRxRxTxRxRxTxIf the tool is tilted in the hole, or the hole size changes (Fig 3)Then CEThe two Rx system fails to work.

Problem with Dual Arrangement

Fig 1.4 Dual receiver sonic tools in incorrect configurationCDABTxRxTxRxRxRx 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 t values are measured from alternate pairs of receivers (Fig.1.5)

These two values of t are then averaged to compensate for tool misalignment

t=A+B/2

Borehole Compensated Tool

Fig1.5 Borehole compensated sonic toolsPorosity DeterminationSecondary and Fracture PorosityStratigraphic CorrelationCompactionOverpressureSynthetic SiesmogramIdentification 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:As a quality check on the FDC and CNL log determinations.As a robust method in boreholes of variable size (since the sonic log is relatively insensitive to caving and wash-outs etc.).To calculate secondary porosity in carbonates.To calculate fracture porosity.Porosity DeterminationThe velocity of elastic waves through a given lithology is a function of porosity

sonic = sonic derived porosity in clean formationt = interval transit time of formationtma = interval transit time of the matrix (sandstone=55.5,limestone=47.6,dolomite=43.5,anhydrite=50)tp = 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 rocks12Wyllie 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 c= /Cp Cp=c* tsh/100 (tsh= 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 byOIL: corr= c*0.9GAS: corr= c*0.7

The Wyllie Time Average EquationThe sonic log is sensitive only to the primary intergranular porosityThe sonic pulse will follow the fastest path to the receiver and this will avoid fracturesComparing sonic porosity to a global porosity (density log, neutron log)should indicate zone of fracture.2 = (N , D ) - SSecondary 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 analysisStratigraphic 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 estimatedCompactionFigure 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 wellbecause it has lower interval transit times (is more compact) but occurs at a shallower depth.16

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 seismogramacoustic impedence

reflectioncoefficient reflection coeffiecientwithtransmission 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 interestSynthetic SeismogramsFig 1.10 The construction of a synthetic seismogram.SONIC-NEUTRON CROSSPLOTSDeveloped 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 identificationIdentification of Lithologies

110Shale - NE regionField observation100Fracture - South90Gas-NW8070Trona605040010203040(lspu)Apparent neutron porosityt, Sonic transit time (s/ft)Time aFieldverageSyiviteTrSONIC-NEUTRON PLOTS

ExampleTwo types of data is takenGamma ray > 80

Gamma ray < 30

ShaleSandstoneSerra, O. (1988) Fundamentals of well-log interpretation. 3rd ed. New York: Elselvier science publishers B.V.: 261-262Rider, 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

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