WELL LOGGING

BBy LYOIDAH KICONCO

(GEOLOGIST)(GEOLOGIST)BSc., MSc., MSc., Dip.Mngt

SONIC OR ACAUSTIC LOGS

The sonic log provides theformation’s interval transit timedesignated as ∆t (delta t thedesignated as ∆t (delta t, thereciprocal of the velocity).

It is a measure of theformation’s capacity to transmitsound waves.

Geologically, this capacity varieswith lithology and rock texturewith lithology and rock texture,notably porosity.

Some typical sonic log responses

SINIC LOGS‐ Principles of measurement

The conventional, general purpose sonic tools measure the time it takesfor a sound pulse to travel between a transmitter and a receiver, mounteda set distance away along the logging tool.y g gg g

The pulse measured is that of the compressional or ‘P’ wave the fastest or‘first arrival’ in which particles vibrate in the direction of the sense ofmovement. The shear or Stoneley waves are ignored by the conventionaltools but can be fully measured by modern array acaustic tools.

The full acaustic wave h b d d ithat  may be recorded in a borehole.

SINIC LOGS‐ TOOLS

Modern sonic tools consist of bothtransmitters and receivers, the actualarrangement depending on the tool type.

The borehole‐compensated (BHC) sonic toolgives a reliable measure of formation valuessince is allows unwanted borehole and toolsince is allows unwanted borehole and tooleffects to be eliminated. BHC has twotransmitter receiver groups (one inverted),each group consisting of a transmittercoupled with a near receiver and a farcoupled with a near receiver and a farreceiver. It has transmitter‐receiverdistances of three feet and five feet withtwo feet between the two receivers. TheBHC has a static compensation.BHC has a static compensation.

The long spaced sonic (LSS) has tworeceivers two feet apart separate by eightf t f t t itt l t f t BHC gives instantaneous readings withfeet from two transmitters also two feetapart and has a dynamic compensationsysyem

BHC gives instantaneous readings with an inverted receiver‐transmitter arrayLSS gives long and short spaced readings 

using a time delay system

SINIC LOGS‐ Presentation, scales and units

Sonic values are given in microseconds (µs) per foot. 1microsecond = 1x10‐6 seconds )

The value is called interval transit time and is symbolised as∆t.

The most common ∆t fall between 40µs and 140µs, thearithmetic chosen scale for the logarithmetic chosen scale for the log.

The sonic tool is frequently run in combination with theresisitivity logs eg Schlumberger ISF‐Sonic tool or Atlaswireline Acaustilog –Resisitivity tool

SONIC LOGS‐ Depth of investigation

The path of compressional waves detected by sonic tools isessentially along the borehole wall with very littlepenetration generally between about 2 5cm and 25cm frompenetration, generally between about 2.5cm and 25cm fromthe borehole wall. The penetration is independent of thereceiver separation and depends on the signal wavelength;th t th l th th t th t ti Fthe greater the wavelength, the greater the penetration. For aparticular frequency therefore, penetration is greater inhigher velocity formations.

Wall damage can create low velocity zone around theborehole. The compressional wave penetration can bep pincreased by increasing the transmitter‐receiver distance i asonic tool.

SONIC LOGS‐ Bed resolution

The vertical resolution of the sonic is the span betweenreceivers for the BHC tools and should be similar for the longspacing This is frequently two feet (61cm) Beds of less thanspacing. This is frequently two feet (61cm). Beds of less than60cm thickness will be registered on the sonic log but a truevelocity will not be recorded. Specialist tools now exist withmuch higher resolutions ( ie the array sonic of Schlumbergerin certain modes, the digital array of Atlas wireline.

Unwanted effectsCycle skiping occurs in extremely poor holes; this happens

when the first compressional arrival is too weak to activatethe receiver, which is only tripped by subsequent arrival,y pp y qleading to too large transit time.In hard formations such as limestones, noise triggering

occurs where noise signals trip a receiver, causing noise spikeson the log.

SONIC LOGS‐ Unwanted effectsThe BHC is very robust even in poor and oversized holes due to theThe BHC is very robust even in poor and oversized holes due to theeffectiveness of the compensated system. However,

Cycle skipping occurs in extremely poor holes; this happens whenCycle skipping occurs in extremely poor holes; this happens whenthe first compressional arrival is too weak to activate the receiver,which is only tripped by subsequent arrival, leading to too large transittimetime.In hard formations such as limestones, noise triggering occurs where

noise signals trip a receiver, causing noise spikes on the log.

Whereas the BHC is very robust, the long spaced tools have twoweaknesses;

Si l tt ti hi h lt i i l b i t k t t iSignal attenuation which results in a signal being too weak to triggerthe receiver and causes cycle skipping.In the dynamic compensation system, each transmitter‐receiverdi i d t i d f th i ht direading is used twice and an error on any one of the eight readings

comprising a full sequence, causes paired errors on the log.

SONIC LOGS‐ Unwanted effects

Unwanted environmental effects on the sonic log. a) BHC tool cycle skipping; b)BHC tool noise spikes; c) Long spacing tool paired aberrations.

PRINCIPAL USES OF SONIC LOGSQuantitative usesQuantitative uses

to evaluate porosity on liquid‐filled holes.As an aid to seismic interpretation, it can be used to give intervalvelocities and velocity profiles and can be calibrated with theseismic sections.

Qualitative usesIdentify lithologyIndicate source rocks, normal compaction, overpressure,fracturesCorrelation

PRINCIPAL USES OF SONIC LOGS

QUANTITATIVE USES OF SONIC LOGSThe sonic log is used to calculate porosity. However, it is usually inferior toThe sonic log is used to calculate porosity. However, it is usually inferior toneutron or density log calculated values. The assumption is that theformation has, on average, a uniform distribution of small pores and whensubjected to confining pressure, there is a simple relationship betweenporosity and pressureporosity and pressure.

1/V =Φ /VL +( 1‐ Φ) /Vma ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐(1)

This van be re‐written as

∆ Φ ∆ (1 Φ) ∆ (2)∆t = Φ ∆tL + (1‐ Φ) ∆tma‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐(2)Where V = tool‐measured velocity∆t = tool‐measured transit timeΦ = porosityVL = velocity of the interstitial fluidVma = velocity of the matrix material∆tma = transit time of the matrix material∆tL = transit time of the interstitial fluid

QUANTITATIVE USES OF SONIC LOGSEquation (2) means that the transit time measured by the tool is theEquation (2) means that the transit time measured by the tool is thesum of time spent in the solid matrix and the time in the fluid; calledthe time average relationship and is a function of the matrix velocityand constituent timesand constituent times.

QUALITATIVE USES OF SONIC LOGSLithology identificationLithology identificationThe velocity of common sedimentaryrock types is rarely diagnostic of lithologyd t t h i ti ithi hdue to too much variation within eachtype and too much overlap between thetypes. However, carbonates are likely toh hi h l iti iddl l iti ihave high velocities, middle velocities insands and low velocities in shales/clays.Coals have unusually low velocities (highi t l t it ti ) d th l itiinterval transit times) and thus velocitiesare diagnostic of them.

Sonic log in sand shaleSonic log in sand‐shale sequences

Distinctive sonic response in coals

QUALITATIVE USES OF SONIC LOGSTextureSonic log response is sensitive to subtle rock textural changes. The way inwhich sound travels through a formation is intimately associated with texture.

Sonic responds well to bedding and texture because the detected signalsphysically travel up and down through the formation.

CorrelationAlthough sonic character is notdiagnostic of lithology, slightg gy, gchanges of the sonic character insome formations indicate somesubtle formation changes.

This characteristic makes the soniclog excellent for correlation andeven for identifying specificeven for identifying specificstratigraphic intervals

The sonic log character used for correlation

QUALITATIVE USES OF SONIC LOGS

Compaction ‐ There is a regular increase in velocity downwards due tocompaction

High pressure identification‐ An increase in pore pressure or overpressure is indicated by a drop on sonic velocity, other factorsconstant.

Borehole damage – sonic velocities can be affected by mechanical orchemical damage around the borehole.g

Source rock identification‐ The presence of organic matter, especiallyin shales, lowers the sonic velocities, apparently in direct relationshipin shales, lowers the sonic velocities, apparently in direct relationshipto abundance and when combined with the resisitivity log value thevelocity is a good qualitative and possibly quantitative indicator ofsource rocks.