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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