Principles of well logging and formation evaluation m.m.badawy

Principles of Well Loggingand Formation Evaluation

M.M.Badawy

Principles of Well Logging and FormationEvaluation

Mahmoud Mostafa Badawy

Lecturer Assistant of Geophysics, Faculty of Science, Alexandria Univ.Egypt.

Principles of Well Logging and Formation Evaluation

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Well Logging


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Well Logging is the continuous recording of a geophysical parameteralong a borehole produced a geophysical well log.The value of the measurement is plotted continuously against depth in the well.During drilling a liquid mixture containing clays and other natural materials, calledMud is pumped down the drill string forcing the rock cutting up to the surface anddecrease the heat from the interaction between the bit and the well wall rocks.Hydrostatic pressure of the mud column is usually greater than the pore pressure ofthe formation.This forces mud filtrate into the permeable formations and a mud cake on theborehole wall.This makes an establishment of:

Flushed Zone. Transition Zone. Uninvaded Zone.


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Cased Hole:

The portion of the wellbore which has a metal casing placed and cemented toprotect the open hole from fluids, pressure, wellbore instability or acombination of those. Logs are recorded in the uncased portion of the wellbore.

We Have Two General Types Of Log Mechanism:

Wireline log LWD (Log While Drilling)

Three General Types of Logs:

Electrical: Spontaneous Potential, Resistivity Nuclear: Gamma Ray, Density, Neutron Acoustic: Sonic

Importance of Geological Well Logging:

Zone correlation. Structure and isopach mapping. Defining physical rock characteristics (lithology, porosity, pore geometry,permeability). Identification of production zone (pay zone). Determination of depth and thickness of zones. Distinguish between oil, gas and water. Estimation of hydrocarbon reserves. Determination of facies relationships.


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Borehole Environment:

Rock fluid system is disturbed and altered during drilling Borehole diameter (dh) 7-12 inches determined by the bit size , but may belarger due to wash out, may be smaller due to buildup of mud cake)

Drilling Mud:

Remove cuttings. Lubricate and cool drill bit. Maintain excess borehole pressure over formation pressure.

Invaded Zone:

Zone around the borehole where the original fluid content has completely orpartially replaced by drilling mud. Invaded zone = flushed zone + transition zone Depth of invasion depends on mud cake; mud cake formation is more efficientin porous rocks, causing less deep penetration of mud filtrate in porous rocks. Flushed zone resistivity (Rxo)

Uninvaded Zone:

Pores uncontaminated by the mud filtrate. Pores contain formation water, oil/or gas. Hydrocarbon bearing reservoir always has some formation water on graincontacts. Uninveded zone resistivity (Rt). Ratio of Sw to Sxo is an index of HC move ability.


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Invasion Model:


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Temperature Log

Normal sedimentary basins show a more or less regular increase intemperature with depth. The increase is not a linear as frequently depicted; itvaries according to lithology depending principally on the letters thermalconductivity. A knowledge of borehole temperatures in important, it is required foraccurate log evaluation and is effective in the detection of fluid movement andsubsurface pressure.

Formation temperature is an important factor to know to detect the resistivity’s(Rm), (Rmf) and (Rw), because they are vary in different temperatures, and can becalculated by knowing: Formation depth. Bottom hole temperature. Total depth of the wall. Surface temperature.


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Geotemperature:

The temperature of the earth usually increases with depth and as a result wecan conclude that thermal energy flows from the earth’s interior to thesurface. A well drilled into the earth’s, therefore shows a persistent rise intemperature with depth.

This persistent rise is usually expressed in terms of a temperature gradient that is inC0 increase per Kilometer of depthIf: Ts= 150c Tf= 800c Depth= 3000 m


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conductivity (the efficiency with about the formation transmitsheat or in case of the earth permits heat loss).


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Shale like as a blanket is inefficient and it keeps heat in and has a low thermalconductivity. Salt conversely is very efficient and let’s heat escape rapidly and therefore hasa high thermal conductivity. Thus the real temperature gradient in a well is not a straight line but a

series of gradient related to the thermal conductivities of the variousdata.

(So the Gradient Varying Inversely To the Thermal Conductivity)


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Thermal Maturation of Organic Matter:

Perhaps the best known use of borehole temperature measurements, simplyas BHT values for calculating organic matter maturity. Landes (1967) prposedthat there was a fairly strict relationship between (thermal gradient, depthand hydrocarbon type). The modern view is to interpret this relationship in terms of hydrocarbongeneration and maturity. It is certain that simply plotting temperature against depth and pressure. Allow typical expected HC depth ranges to be defined.


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Caliper Log

Caliper tools measure the hole size and shape of the interior hole. The mechanical caliper measures variation in borehole diameter with depth. The measurements are made by two articulated arms pushed against theborehole wall. The arms are linked to the cursor of a variable resistance. Lateral movements of the arms is translated into movements of the cursoralong the resistance and hence variation in electrical output.


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Simple Two Arm Caliper Interpretation:Increase in Borehole:

A hole that has the same size (diameter) as the bit which drilled it is called OnGauge. On Gauge holes are the target for all drilling and essentially indicate a gooddrilling technique. Holes with a much larger diameter than the bit size are called Caved orWashed Out. That is during Deeping of the hole, the borehole walls cave in, are broken byturning drill pipe or are eroded away by the circulating borehole mud. This id typically of shales, especially when geologically young andunconsolidated. However, caving is also typical of certain specific lithology such as coals oreven organic shales.

Decrease in Borehole:

Caliper may show a hole diameter smaller than the bit size. If the log has a smooth profile a mud cake build up is indicated. This isextremely useful indicator of permeability; only permeable beds allow mudcake to form. The limits of mud cake indicate clearly the limits of the potential reservoir. Borehole with a smaller diameter than the bit size but rugose are probablysloughed.


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Log Presentation:

The caliper log is printed out simply as a continuous value of boreholediameter with depth. The curve is traditionally a dashed line and usually plotted in track. The horizontal scale may be inches of diameter or in the differential caliperexpressed as increase or decrease the hole diameter about a zero defined bythe bit size. The ordinary caliper log is accompanied by a reference line indicating bit size.


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Four Arm Caliper Interpretations: A great deal more information can be gained from dual caliper tool than fromthe simple caliper tool. Dual caliper information is generally taken from the four arm dipmeter tool. Data from a four arm caliper however enables the shape of a hole to be muchbetter defined. A hole can be seen to be On Gauge and round or Oval and Washed Out orEnlarged by a key seat.

We Have Three Main Types Of Elliptical Borehole Have Been Recognized:

a- In/On Gauge.b- Key seats.c- Wash Outd- Break Out.


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b- Key Seats:

Are asymmetric oval holes formed by wear against the string at points wherethe borehole inclination changes, (Dip Angle of the well).c- Wash Out:

Develops from general drilling wear, especially in shaly zones and dippingbeds. On the geometry logs, a washout has a considerable vertical extent and bothcalipers are larger than the drill bit size with one caliper being much largerthan the other.

d- Break Out:

Are recognized using the following:

The tool must stop rotating (ideally the tool should rotate before and aftera break out zone). Small brittle fractures (spalling) occur in the borehole around a rotatingbit. Natural and artificial fractures are most likely to be oriented in themaximum horizontal stress.


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Spontaneous Potential,

Self Potential (SP)

The Log:The SP log is a measurement of the natural potential differences of self potentialbetween an electrode in the borehole and a reference at the surface.[No artificial currents are applied]

We Can Use SP Log To:- Estimate the formation water resistivity (Rw)- Estimate the shale volume (Vsh) The scale of SP can be -] 20 [+ The unit is (Mill volt)

Principles Uses:The principles uses of the SP log are to locate and to calculate formation waterresistivity and to indicate permeability, it can also be used to estimate shale volume,to indicate facies and in some case correlation.


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Principles of Measurements:

Three Factors Are Necessary To Make An SP Current:

A conductive fluid in the borehole. A porous and permeable bed surrounded by an impermeable formation. A different in salinity or pressure between the borehole fluid and theformation fluid.

SP Currents Are Created When Two Solutions Of Different SalinityConcentration Are In Contact By:

Diffusion or (liquid junction potential). Shale potential.


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The Diffusion Potential (Liquid Junction):

Arise when solution of differing salinity are in contact through a porousmedium. Sodium chloride is the most common cause of oilfield salinity so that it iseffectively two solutions of sodium chloride of differing salinity. Mixing of the two solutions takes place by ionic diffusion the Cl- is bothsmaller and mobile than the larger Na+.

The Shale Potential:

Arises when the same two solutions are in contact across a semi-permeablemembrane. Clay minerals which form shales consist of layers with large negative surfacecharge. Because of charge similarity the negative chloride ions effectively cannot passthrough the negatively charged shale layers. While the positive sodium passes easily the shale acts as a selective barrier. If a bed is not permeable ions will not be able to move, they will be no currentflow and thus no potential change, (That means no SP)

Unwanted Logging Effect:- As SP to be effectively it must be connected to a surface earth.- For onshore wells these cause no problem.- Offshore, no such possibility exists, without an effective earth the SP will notbe recorded.


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Log Characteristics: (Bed Boundary Definition and Bed Resolution):

The Sharpness Of A Boundary Will Depend On The Shape And Extent Of TheSP Current Patterns. Generally when there is considerable difference between mud and formationresistivity, currents will be spread widely and the SP will deflect slowly. SP bed resolution is poor, for a full SP deflection (SSP) and proper bedresolution as a rule of thumbs a bed should be thicker than 20 times theborehole diameter. The exact minimum SP bed resolution will obviously depend on depth ofinvasion and salinity differences.


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Quantitative Used:

SP Values for Calculating Shale Baseline and Static SP (SSP):

With no absolute values the SP is treated quantitatively and qualitatively interms of deflection that is the amount of the curve moves to the left or theright of defined zero. The definition of the SP zero is made on thick shale intervals where the SPdoes not move, it is called the shale baseline, all values related to this line. The theoretical maximum deflection of the SP opposite permeable beds iscalled the static SP or SSP; it represents the SP value that would be observedin an ideal case with the permeable bed isolated electrically.

Water Resistivity (Rw):

The greater SP deflection, the greater salinity contrast between the mudfiltrate and the formation water. The SP is used to calculate formation water resistivity using the relationbetween resistivity and ionic activity. Ionic activity is the major contributing factor to the electrochemical SP. There is a direct relationship between ionic activity and the resistivity of asolution. The relation allows a mathematical expression of the amplitude of the SPdeflection to be expressed in terms of formation water resistivity.


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Shale Volume from SP (Pseudo-Static SP):

It is considered that the volume of shale on a water as a shaly sand stone.

PSP: Pseudo static (SP read in the water bearing shaly sand zone).

Qualitative Uses:

Permeable Recognition:

If there is even a slight deflection on the SP, the bed opposite the deflection ispermeable.Mineral Identification:

The rare exceptions when the SP will deflect and the formation is notpermeable are due to mineralization. Coal which are extremely reduced, given a large negative SP deflection.


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Resistivity & Conductivity

LogsThe Log:

Resistivity log is a measurement of a formation resistivity, which is itsresistance to the passage of an electric current; it is measured by resistivitytools.

Conductivity tools measure formation conductivity or its ability toconduct an electric current; it is measured by the induction tools.


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Most rocks materials are essentially insulators, while their enclosed fluidsare conductors. The hydrocarbons are the exception to fluid conductivity.

Principles of Uses:

The Resistivity logs were developed to find hydrocarbon, however a formationsresistivity is of its typical geophysical characteristics and such can contributeinformation on lithology, texture, facies, over pressure and some rock aspects.Limitations:

Resistivity tools can only function in boreholes containing conductive mud,which is mud mixed with salts. They cannot be run in oil based mud or fresh water mud.


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Rock Resistivity (Formation Resistivity Factor [FF]):

Only the formation waters that are conductive, the conductive of the rock ingeneral should be that of the solution it contains. But it is not, although the rock plays no active part, it plays an important

passive one. This passive role is basically dependent on rock texture or more especially onthe geometry of the pores and pore connection. The easier the path through the pores the more current that passes. The expression of this passive behavior of a rock is called [F] or [FF].


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Resistivity Tools:

Passing a current between two electrodes in the earth and measured thepotential drop between two other electrodes. Modern tools are considerably more complex than this, especially becauseemitted currents are focused by continuous guard currents. Resistivity tools exist with diverse capability as a result of the need tomeasure formation resistivity from anywhere between the immediate vicinityof the borehole wall and the flushed zone.

Induction Tools:

It was based on the design of a mine detector, a basic induction tool consists ofan emitting coil and a receiving separated along the length of the tool by anelectrically isolated section. A constant amplitude sinusoidal current is applied to the transmitted coil; thiscreates a magnetic field around the tool which in turn induces eddy currentsin the formations, curving a circular path around the tool. The eddy currents create their own magnetic field and induce an alternatingcurrent in the receiver coil.


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Source Rock Investigation:

The resistivity log may use both qualitatively and quantitatively to investigatesource rock. The effect a source rock has on the resistivity log depends on the maturity ofthe organic matter. It has a little effect when immature, but causes a large increase when it ismature. The reason seems to be that it is the pore fluid content that causes theincrease and not the solid matter. Typical shale which is not a source rock consists of a clay mineral matrix and acertain water filled porosity. If is source is immature [lower R] it contain water, it is mature [higher R]and it contain both water and hydrocarbon

Log Format and Scale:

The unit of resistivity log is [ohm.m]. Resistivity logs are plotted on a logarithmic scale, either in track two aloneor in track two and three. The values (scale) are usually [ 0.20 to 2000 ] ohm.m


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Quantitative Used:

The principle use of well logs is to detect oil resistivity logs are used to givethe volume of oil in a particular reservoir. In a petrophysical terms, to define water saturation [Sw].


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The Archie Equation:

n = saturation exponent [usually 2]

OR


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Gamma Ray & SpectralGamma Ray Logs

The log:

The gamma ray log is a record of formations radioactivity. The radiation emanates from naturally occurring Uranium, Thorium andPotassium. The simple gamma ray log gives the radioactivity of the three elementscombined. While the spectral gamma ray log shows the amount of each individualelements. Most of shale has a strongest GR, so we can call it [Shale Log].

The Principles Uses:

The gamma ray log is still principally used quantitatively to derive shalevolume. Qualitatively, in its simple form, it can be used to correlate, to suggest facies,sequence and to identify lithology [Shalness]. The spectral gamma ray can be used to derive a quantitative radioactivemineral volume and a more accurate shale volume.


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Natural Gamma Radiations:

Normal radiation in rocks comes essentially from only threeelemental sources:

The radiation radioactive element of the Uranium family Uranium-Radium family Radioactive isotope of Potassium 40K, Potassium is by far the most abundantof the three elements. The radiation from 40K is distinct with a single energy value of 1.46 MeV. Both Thorium and Uranium emit radiations with a whole range of energies.


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One of the characteristics of gamma rays is that [when they pass through anymaterial their energy is progressively aborted]. Compton scattering, due to the collision between gamma ray andelectrons this produces a degrading of energy. So, when the higher the common density through which the gamma rays pass,the move rapid the degradation or loss of energy, [in reality it depends on thematerials electron density].

Tools:

Simple Gamma Ray Tool:

The simple GR tool is a sensitive GR detector consisting of a scintillationcounter and photomultiplier. The scintillation counter is typically a sodium iodide crystal 2cm in diameterand 5cm long in the simple tool. When GR pass through the crystal they cause a flash, these are collected by thephotomultiplier and stored in the attached condenser over a set period oftime.

Spectral Gamma Ray Tool:

The spectral GR tool, like the simple tool, it consists of a scintillation counterand photomultiplier. However the sodium iodide crystal has a much greater volume, typically 5cmin diameter and 20cm long to give the tool much better [counting sensitivity].


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When a GR passes through a scintillation crystal it not only causes a flash, butthe intensity of that flash depends on the energy of the incident GR. These can be related to the abundant of each element by comparison withknown spectra.

Log Characteristics:

The accepted unit for radioactivity logging is API [American PetroleumInstitute]. Its scale can be from [1 – 200API] The simple GR log is usually recorded in track one along with the caliper. There are several common presentation formats for the GR spectral log, thesimplest and probably best to put a three arithmetic scales due to the Thoriumand Uranium are given in PPM [Part per Million], Potassium is given inPercent %. 1 ppm U = 8.09 API 1 ppm Th = 3.93 API 1 % K = 16.32 API

Depth of Investigation:

The depth from which radiation can be detected by the simple GR tool isgenerally small but difficult to be precise about. One experiment found that 75% of radiations detected came from a 14cmradius and 25cm vertically above and below the detector.


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Logging Speed:

Because gamma radiations are discrete events and as describe, are measuredin the gamma ray tools by counting, these are restrictions on logging speeds. Radiations are counted by the tool over a fixed period of time called timeconstant. Because the number of individual emissions is not high to have as large acount as possible, the time constant should be long.

Unwanted Borehole Effects:

Simple GR log is relatively unaffected by small scale borehole irregularities,but is affected by large caves. The effect is due to the increased volume of drilling mud between theformation and the gamma ray detector which causes increased of Comptonscattering. A quiet different effect is caused by the use of the radioactive mud additiveKCl, which make a constant absolute increase in the gamma ray. The spectral gamma ray log is run held near the borehole wall by a bow springto reduce the borehole effect which occurs when a tool is centered.


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Geochemical Behavior of Potassium, Thorium and Uranium:

The old GR log is a [shale log] was based on its use as a black box, with nounderstanding what was inside. Now we want to describe the natural occurrences’ of the radioactive mineralsand their geological significant.

Potassium:

Potassium is both chemically active and volumetrically common in naturallyoccurring rocks, because of it’s chemically activity. In clay minerals: clay silicatr structure. In evaporates: salt [sylisite] 52.5% KCl. In rock forming: feldspare. Illite contains by far the greatest amount of K.

Uranium:

Come from acid igneous rocks, if forms soluble form, especially in the uranyleform [U6+] as an oxide UO22+ [uranyle ion] is transported in river. From river or especially sea water, uranium passes into sediments in threeprinciple ways:1- Chemical precipitation in acid, reducing environment.2- Adsorption by organic matter, or living plants.3- Chemical reaction in phosphorites. Anoxic waters with a relatively slow rate of sediments deposition, whichtypically produces [Black shale]. Probably a more common way of introducing uranium into sediments is inassociation with organic matter. It has been established experimentally that carboniferous material can extracturanium from solution.


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In general, uranium behaves as an independent constituent, it is notchemically combined in the principle molecules of rocks like potassium, but isloosely associated with secondary components, for this reason it has a veryheterogeneity distribution in sediments.Thorium:

Like uranium, thorium has its origin principally in acid and intermediateigneous rocks. However it is extremely stable and unlike uranium will not go into solution. It’s generally transported to site of sediments deposition as clay fractiondetrital grains. These are of heavy minerals such as zircon, thorite, monazite and epidotewhich all very stable. Describe its lake of solubility, thorium is however widely and relatively evenlydistributed in sediments , so much so that in shales it is used as a base levelfrom which the relative abundant of the other radioactive elements.

Radioactivity of Shale and Clay:

In petroleum borehole logging the commonest natural radioactivity (byvolume) is found in shales (clays). A high gamma ray value frequently means shale. But the gamma ray log should not be used as a [black box] shale indicatorqualitatively or quantitatively as is commonly the case. However, potassium occurred in detrital minerals such as feldspars. Potassium may occur in both the shale and sand, but cannot alone be used as ashale indicator. Uranium distribution is very irregular as has been know, because its affinity isto secondary components and not the main the rock forming minerals. The behavior of thorium in shale is not fully understood, experience showsthat despite its varying content in clay mineral species.


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To Summarize:

As shale indicator, thorium may be used in most cased, potassium may beused in many cases, but uranium should not be used at all.Quantitative Uses of the Simple Gamma Ray Log:The most one is to calculate the volume of shale in each formation.

If GR Index = [IGR = Vsh]But if we have a younger and older rocks so we will have consolidated andunconsolidated rocks which IGR will not equal Vsh as a linear relation.


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Qualitative Use of the Simple Gamma Ray Log:

1-Lithology:

As a first indicator of lithology, the GR log is extremely useful as it suggestwhere shale be expected.2-Radioactivity of sandstones and other aranaces rocks.

3-Radioactivity in evaporates:

The most common evaporates, such as salt and anhydrite give extremely andabnormally low values on the gamma ray log, however the high radioactivityin some evaporates causes by potassium content.4-Radioactivity in carbonates:

Carbonates in their pure state are not radioactive and their aids theiridentification, in certain facies carbonates contain organic matter.5-Correlation:

The gamma ray log is one of the most frequently used for correlation.


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Sonic LogOR

Acoustic Log

The Log:

The sonic log provides a formation interval transit time [∧t]. It is a measure of the formation capacity to transmit second waves. Geologically this capacity varies with lithology and rock texture. The sonic log shows a formation ability to transmit second waves, it isexpressed as interval transit time [∧t] which can be [(1*106)/∧t] = Velocity[ft/sec].

Principal of Uses:

Quantitatively, the sonic log is used to evaluate porosity in liquid filled holesas an aid to seismic interpretation; it can be used to give interval velocities. And cross multiplied with density log, the sonic is used to produce theacoustic impedance log, which is the first step to make a syntheticseismogram. Qualitatively, for the geologist the sonic log is sensitive to subtle texturevitiations and it help to identify lithology and may help to indicate sourcerock.


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Principle of Measurement:

The convention, general purpose sonic tool, measure the time it takes for asound pulse to travel between a transmitter and a receiver, mounted a setdistance away along the logging tool. The pulse measured is that [compression wave P] followed by shear and stoneley waves which in theconvention tools, can be ignored but recorded in modern array ones.Transmitters [Transducers]:

Are either magneto astrictive or more commonly, piezoelectric and translatean electrical signal into an ultrasonic vibration. In piezoelectric transmitters, the application of an electrical change causes achange in volume which can be translated into a pressure pulse.

Receivers:

Are usually piezoelectric and convert pressure wave into electromagneticsignal which can be amplified to provide the logging signal.Log Presentation, Scale and Unit:

Sonic values are given in microsecond [µs] per foot. And between the common values 40µs to 140µs and in an arithmetic [normal]scale.If ∧t = 40µsThen the interval velocity = 1/∧t = 1/ (40*10-6) = 25.000 ft/sec The depth of investigation between 2.5cm to 25 cm.


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Quantitative Uses:

Porosity calculation:

Qualitative Uses:

Source rocks. Texture. Correlation. Fracture identification. Compaction.


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Density Log

The log:The density log is a continuous record of a formation bulk density [ρb], this is theoverall density of a rock including solid matrix and the fluid enclosed in the pores.Principle of Uses:

Quantitatively, the density log is used to calculate porosity and indirectlyhydrocarbon density; it is also used to calculate acoustic impedance. Qualitatively, it is useful in lithology indicator and can be used to identifycertain minerals, can help to detect source organic matter content,overpressure and fracture porosity.

Principles of Measurement:The logging technique of the density log tool is to subject the formation to abombardment of medium-high energy [0.2-2.0 MeV] collimated [focused] gammaray and to measure their attenuation [Compton scattering] between source anddetector.Log Scale and Unit:The density log is normally plotted on a linear scale between [1.95-2.95] g/cm3


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Quantitative Uses: Porosity Calculation:

Acoustic Impedance:

(ρ * v)

Qualitative Uses: Lithology identification. Shale age. Mineral identification. Evaporate identification.


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Neutron Log

The Log:The neutron log provides a continuous record of a formations reaction to fastneutron. Which are related to a formations hydrocarbon index.

Principle of Uses:

Quantitatively the neutron log is used to measure porosity. Qualitatively, it is an excellent discriminator between gas and oil, it can beused geologically to identify gross lithology, evaporates, hydrated mineral andvolcanic rocks. When combined with density log the can be a good lithology indicator.


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Principle of Measurement:

Neutrons are subatomic particles which have no electrical charge but whosemass is essentially equivalent to that of a hydrogen nucleus. The interact with matter is in two principle ways, by collision and absorption. Collisions are mainly at higher energy states, absorption occurs at lowerenergy.

Log Scale and Unit:Unit is [%] and from [ -15% to 45%]

Principles of well logging and formation evaluation m.m.badawy

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