Part 4 the Tools

Basic Principles

ELIAS ABLLAH

OCTOBER 2011

UKM

The Tools

Physical Properties

Velocity

Density

Clay content

Water Saturation

Porosity

Fluid Type

Fracture

Dipping

Lithology

Boundary

Probe / Sonde Types

Electric Logs

Caliper Log

Radioactivity

Sonic Acoustic Log

Probe / Sonde Types

Electric Logs

Caliper Log

Radioactivity

Sonic Acoustic Log

Self Potential; SP Resistivity Log Normal Laterolog Induction

Probe / Sonde Types

Electric Logs

Caliper Log

Radioactivity

Sonic Acoustic Log

Gamma Ray Total Spectrometry U, Th, K Neutron Log Density Log Lithodensity

Probe / Sonde Types

Electric Logs

Caliper Log

Radioactivity

Sonic Acoustic Log

Caliper

Measure the borehole diameter

Borehole diameter changes due to

Drill bit shakes

Cave in

3 arms @ 6 arms

Profile Log

Caliper

Gamma Ray Logs measures both

natural & induced radioactivity characteristic of the formations U, Th. Po, Total

Gamma

Geiger Muller counter @ scintillation detector

Stand alone @ combination with others (sonic, neutron, Density, Induction)

1 or more receiver

Profile Log

GR Application

Defining Bed boundary

Shale indicator

Correlation for open hole & cased hole

Gamma Ray Logs

Another common log

Records radioactivity of a formation

Shales have high gamma radioactive response

Gamma ray logs infer grain size (and so subsequently inferred depositional energy)

Gamma ray logs are most commonly used logs for sequence stratigraphic analysis

Electric Logs

Measure the resistance of the earth layer

I

V

L

A

Resistance / Conductivity

Dry Solid rock = 0

No salt No conductivity High Resistivity


Porous rock oil

Resistance / Conductivity (same porosity; diff. fluid)


Porous rock Fresh water

Porous rock Salty water

salt present high conductivity low Resistivity

Resistance / Conductivity (same porosity; diff. salinity)

salt present moderate conductivity

Porous rock High salt

Porous rock Moderate salt

salt present High conductivity

Resistance / Conductivity (diff. porosity; same fluid)

Small porosity Salty water

Highly Porous rock Salty water

salt present high conductivity low Resistivity

salt present small conductivity high Resistivity

Resistance / Conductivity (same porosity; diff. fluid ratio)

Low conductivity High Resistivity

Porous rock 20% Salt water

80% oil

high conductivity low Resistivity

Porous rock 80% Salt water

20% oil

Borehole condition

Borehole condition

Rm

Rmc Rxo

Rt

Rm > Rmc > Rxo > Rt

To determine Rt accurately, tools were designed with different depths of investigation

Shallow investigation tools

Microresistivity resistiity of invaded zone

Deep investigation tools

Laterolog, Inductions resistivity of uninvaded zone

Resistivity Logs

The most commonly used logs

Measures resistance of flow of electric current

Is function of porosity & pore fluid in rock

Frequently used to identify lithology

SP Logs

SP curves are caused by electromotive force in the formation

Electro Chemical Membrane

potential

Electro Kinetic

of the movement

1 or more receiver

Profile Log

Tr

Re

Re

SP Logs

Difference in salinity creates an electrical potential

The magnitude of deflection indicate the difference in salinity between the drilling fluid and the formation water.

- sand

Shale +

High NaCl Contentration

Impervious to Cl-

+ Salty water

less Salty - water

SP Logs

The value is also influenced by the thickness of the beds, the shaliness of the permeable beds, and others

Shale base line

Mud filled hole

SP Logs

Salinity of mud > salinity of

formation water

- +

Salinity of mud = salinity of

formation water

Salinity of mud < salinity of

formation water

NORMAL SP

SUPPRESSED SP

REVERSE SP

deep shallow intermediate

SP Application

Define Boundary

Detection of permeable beds

Correlation

Evaluate formation water resistivity

Bed shaliness

Spontaneous Potential (SP) Logs

Next most common log

Measures electrical current in well

Result of salinity differences between formation water and the borehole mud

Separates bed boundaries of permeable sands & impermeable shales.

Neutron Logs

Radioactive tool (2) Radioactive

source bombards the rock around well bore

Neutron bombardment causes rocks to emit gamma rays in proportion to their hydrogen content.

The gamma ray will be detected by the sonde

Hydrogen exist in all formation fluids (oil, gas, water), but not in the minerals

Thus indirect indicator on the Porosity

Problem shale (bounded water)

1 or more receiver

Profile Log

R

R

S

Neutron Logs

Another common log

Measures porosity of formation

Uses quantity of hydrogen present

Measures lithology when used with Density Log

Density Logs

Radioactive tool (3)

Radioactive source emit gamma radiation & records the gamma returning from the formations

Gamma-gamma tools

1 or more receiver

Profile Log

R

R

S

Density Logs

A common log

Measures formations bulk density

Used as a porosity measure

Differentiates lithologies with Neutron Log

Used with Sonic Logs to generate synthetic seismic traces to match to seismic lines

Common Density g/cm3

Sandstone 2.65

Limestone 2.71

Dolomite 2.87

Shale 1.9 2.7

Acoustic Logs

Measure time taken for sound wave to travel through different material

Acoustic velocity depends on

Rock type &

Porosity

1 or more receiver

Profile Log

Tr

Re

Re

Sonic (Acoustic) Logs

Another common log

Measures of speed of sound in formation

Tied to porosity and lithology

Used with Density Logs to generate Synthetic Seismic traces to match to Seismic traces

Lithology

(porosity = 0)

velocity

ft/s m/s

Sandstone 18,000-

21,000

5,400 6,300

Limestone 21,000-

23,000

6,300 7,000

Dolomite 23,000 7,000

Anhydrite 20,000 6,000

Halite 15,000 4,500

Fluid

(fresh water / oil)

5,300 1,500

Composite Log

Other Logs

Temperature

Pressure

Televiewer

Image

NMR

Dip Meter

Dip meter

Measure the formation dips

3 arms @ 4 arms

Profile Log (tadpole)

Natural Radioactivity Principles Tools Application Environmental

Corrections Evaluation Technique

Natural Gamma Ray

Natural Gamma Ray Natural Radioactivity

In the early 1900s, many atomic particles were discovered. These i n c l u d e d :

Electron: An elementary particle consisting a negative charge

Proton: An elementary particle that is identical to the nucleus of the hydrogen atom and is a constituent of all other atomic nuclei. The proton carries a positive charge numerically equal to the charge of an electron and is nearly 2000 times heavier than an electron.

Neutron: An unchanged elementary particle that has a mass nearly equal to the proton. The neutron is present in all known atomic nuclei, except the hydrogen nucleus.


Beta particles: either negative or positively charged particles with the same mass and charge as an electron. Beta particles are easily stopped by athin sheet of metal.

Gamma rays: electromagnetic waves traveling at the speed of light having discrete energy levels. Gamma rays penetrate farther than most particles, mainly because they lack charge.

During the same period it was also discovered that, as the atomic nuclei of some elements disintegrated, they spontaneously emitted:

Alpha particles: positively charged particles that are made up of two neutrons and two protons, making it identical to the nucleus of a helium atom. Alpha particles are easily stopped by a thick cloth.


Of the three particles generated during natural radioactive decay, the gamma ray is the only one that can penetrate a rock formation for any appreciable distance and as a result it is the only one that can be measured.

In nature, potassium (K40), thorium (Th232) and uranium (U238) are the three main radioactive elements.

Each element is capable of producing Gamma rays that can be measured.

The figure shows the different energies of the Gamma rays produced by these radioactive materials.

Natural Gamma Ray Principles

To measure the natural Gamma rays emitted from the formation, the Gamma ray (GR) tool is lowered in the borehole.

The GR tool consist of a detector and associated electronics to measure the gamma radiation originating in the volume of formation near the tool.


The most commonly used detector for Gamma rays is the scintillation detector. There are three main components of a scintillation detector:

Crystal Scintillator: convert the Gamma ray energy into visible light flash.

Photomultiplier: converts the individual light flashes into electrons, which are amplified to generate a detectable electrical pulse..

Amplifier-Discriminator Circuit: differentiates between pulses caused by Gamma rays from the formation and pulses caused by background electrons.


The standard GR tool measures the total number of gamma rays coming to the detector, irrespective of the energy of the Gamma ray.

The Natural Gamma Ray Spectrometry (NGT) tool measures both the number and the energy level of Gamma rays.

This permits the determination of the concentrations of radioactive K40, Th232 and U238 in the formation rocks.

Natural Gamma Ray Tools

Wireline: GR tools are available in various sizes and ratings for various applications.

LWD: The GR sensor is part of the Resistivity Tool. The GR sensor is also available as a part of the MWD tool. The specifications of the Schlumbergers MWD Slim-1 system is listed here as an example.


Gamma Ray Tools

Depth of Investigation

10 15 (varies with deformation density)

Vertical Resolution

24 (varies with logging speed)


The radioactive decay is a random process.

Because of the random nature of the process, it is important to log at speeds slow enough that averaging functions can reduce these fluctuations.

Schlumberger has set logging speeds for tools so that accuracy is maintained and logging speed is maximized.

The appropriate logging speed can be found in the Log Quality Control manual.


The American Petroleum Institute (API) has created a primary standard that defines the Gamma ray measurement units known as GAPI.

This unit has become the standard throughout the world.

The GAPI represent 1/200 of the difference between zones of high and low radiation of the Gamma ray calibration pit of the University of Houston.


Since a tool cannot be run this calibration pit each time it needs to be used, the calibration is reproduced in the field using a secondary standard, which is a wrap-around blanket containing radioactive monozite sand with a predefined radioactivity.

Natural Gamma Ray Applications

Gamma Ray Applications

Correlation

Well to well correlation

Depth matching between separate trips in the well

Positioning of open-hole sampling tools

Providing the depth control needed for cased hole perforation

General Lithology indicator

Discriminate between reservoir & non reservoir (Net/Gross)

Quantitative shaliness evaluation of the reservoir rock

Natural Gamma Ray Applications

Natural Gamma Ray Spectrometry Applications

Lithology identification

Study of depositional environments

Investigation of shale types

Correction of the GR for clay content evaluation

Identification of organic material and source rocks

Fracture identification

Geochemical logging

Study of rocks diagenetic history

Natural Gamma Ray Environmental corrections

Corrections to GR logs

Hole size

Stand-off of the tool from the bore hole wall

Barite content of the mud

Potassium content of the mud (only NGS data can be corrected)

Cased hole operations


Schlumberger Charts GR-1, GR-2 & GR-3 contain the corrections for the log acquired using the Wireline tools under various open-hole and cased hole conditions.

These corrections are usually not a part of the standard log data and have to be applied before using it formation evaluation.


The Schlumberger Chart GR-4 represents the correction for the LWD-GR log.

The corrections illustrated by this chart are routinely applied to the LWD data before delivery and therefore care should be taken not to duplicate the correction.

Natural Gamma Ray Evaluation Technique

For the evaluation of hydrocarbon in place, it is very important to be able to discriminate between the reservoir and non reservoir interval to compute the N/G.

The GR log is often used as a lithology indicator to achieve this objective


In sedimentary formations, radioactive elements tend to concentrate in shales, causing a high GR log reading.

Clean formations, such as sandstones or limestones, usually have a very low level of radioactivity and consequently, a low GR log reading.

Thus, the GR log reflects the shale content.


A GR level in thick shale beds is identified. This reading is assumed to represent 100% shale and is called shale-line.

A sand line is constructed by reading the average GR level of thick clean sands (sands with the lowest GR)

A vertical line in the middle of the shale line and the sand line is constructed for an initial quick-look (cut-off line).

All intervals where the GR log is on the left of this cut-off line are assumed to be reservoir.

The actual GR level within the reservoir interval is the measure of shaliness

To discriminate reservoir and non-reservoir rock:


The actual cut-off level for the reservoir might be different from 50%.

Hence, using the 50% level can prevent certain shalier zones, which are good reservoir sections, from being included as part of the reservoir.

This figure shows the effect of changing the cut-off level.


Sometimes sands themselves contain radioactive minerals, like uranium.

Using the GR log radioactive sands will be misinterpreted as a shaly-sand.

In such cases, the Natural Gamma Ray Spectrometry Log (NGS) is used.


The NGS log provides the concentrations of K40, Th232 and U238 in the formation and the total formation GR (SGR).

Uranium can be present in both clean and shaly formations.

Thus, a corrected Gamma Ray (CGR) curve is also provided which is SGR with the effect of uranium removed.

This curve should be used for identifying reservoir and non-reservoir rock and for Vsh computation in the presence of radioactive sands.


Unless there is a complex mixture of radioactive minerals in the formation, the Schlumberger Chart CP-19 can be used to identify the common minerals.

As an example a sandstone reservoir with varying amounts of shaliness, with illite as the principal clay mineral, usually plots in the illite segment of the chart with Th/K between 2.0 and 2.5.

Less shaly parts of the reservoir plot closer to the origin, and more shaly parts plot closer to t h e 7 0 % i l l i t e a r e a .

Principles

Applications

Environmental Corrections

Evaluation Technique

S p o n t a n e o u s P o t e n t i a l

Spontaneous Potential Principles

The SP currents are developed from interactions which are Electrochemical or Electrokinetic in nature.

The Electrokinetic potential exists due to the flow of a saline mud through the mud-cake.

This flow exists because of the differential pressure between the mud column and the formation.

Electrokinetic potential is normally very small and will stop as soon as the mud-cake becomes permeable.


The electrochemical component can be broken into two components-the Membrane Potential and the Liquid Junction Potential.

The Membrane Potential exists because shales behave as ion selective membranes.

In this case, the shale is permeable to the Na+ ions.

These ions move through the shale from the higher salinity formation water in the sand to the lower salinity mud.

This results in a flow current through the shale as indicated, inducing a potential.

The flow is reversed if Rmf < RW (salt mud)


Liquid Junction Potential exists due to a difference in salinity between two fluids that are in contact with each other.

The mobility difference between the Cl- ions from the Na+ ions results in a net migration of Cl- ions from higher salinity fluids to lower salinity fluids.

In this example, the invaded zone salinity is lower than the virgin zone salinity.

This results in a positive charge in the virgin zone and a negative charge in the invaded zone.

The potential is reversed when RXO < Rt (salt mud)


The SP log is recorded by placing a movable electrode in the borehole and measuring the difference between the electrical potential of this movable electrode and the electrical potential of a fixed surface electrode.

Spontaneous Potential Applications

The Static SP (SSP) is defined as the sum of the Membrane Potential and the Liquid Junction Potential.

The SP log measures only the potential drop from the SP currents in the borehole fluid, which may not represent the total SP because there are also potential drops in the formation.

In practice, the recorded SP log approaches the SSP value only in thick permeable beds.


Spontaneous Potential

Objective

To explain how SP is created in the formation and how it is measured



Correlation

Depth matching between separate trips in the well

Positioning of open-hole sampling tools

Differentiate potentially porous and permeable reservoir rocks from impermeable clays.

Quantitative shaliness evaluation of the reservoir rock

Determine RW in both salt and fresh mud

(SP can only be acquired in open hole, conductive mud environment with Rmf RW)

Spontaneous Potential Environmental Corrections

Corrections to SP log

Bed Thickness

Resistivity of invaded zone

Diameter of invasion

Resistivity of adjacent shale beds

Resistivity of mud and borehole diameter

Spontaneous Potential Environmental Corrections

The Schlumberger Chart SP-3 provides the corrections charts for the SP log.

Charts SP-4 and SP-4m provide an empirical correction to the SP log.

Spontaneous Potential Evaluation Technique

We want to determine the SSP.

First the SP level in thick shale beds is identified.

This reading is assumed to represent 100% shale.

Similarly, a sand line is constructed by reading the lowest SP level in thick clean sands.

The SSP is the deflection seen on the SP log from the Shale Base Line to the Sand Line.


SP can provide anomalous responses under various circumstances.

Highly resistive formations interbedded between shales and permeable beds significantly alter the distribution of SP currents and change the expected shape of the curve, making it difficult to define bed boundaries in its vicinity.


A shift in shale baseline can occur when the formation waters of different salinities are separated by shale beds thet are not a perfect cationic membrane.

This figure shows the SP log recorded in a series on sandstones (B,D,F,H) separated by thin shales or shaly sandstones (A,C,E,G)

It is difficult to define the shale baseline for the determination of SSP in such cases.


The SP log can be affected by a number of surface effects as it relies on a surface electrode to be the reference for the measurement.

Power lines, electric trains, electric welding and radio transmitters can create ground currents which can induce noise on this reference, resulting in a poor, sometimes useless, log.


Other Limitations

No SP development for Rmf = RW

No SP development in non-conductive mud

Cannot be recorded in cased hole

Resolution of SP log varies with Rt/ Rm

Sonic:

Principles Tools Applications Evaluation Technique

POROSITY

Sonic Principles

Acoustic waves are pressure waves that propagate through the earth in a manner and velocity that is dependent upon the caharacteristics and geometry of the formation.

Acoustic waves move through a medium in wavefronts.

Sonic Principles

The wavefronts are classified by how they move in raletion to the particle movement. There are two types of wave fronts:

Compressional wavefronts (P waves) move in the direction of particle displacement.

Shear wavefronts (S waves) move in a direction perpendicular to the direction of particle displacement.

Shear waves can only exist in a medium that has elastic properties such as solids or highly viscous fluids.

Shear wavefronts are slower than compressional wavefronts, sometimes only half as fast as the compressional wave.

Sonic Principles

The typical sonic logging tool will consist of transmitter and receivers placed in the wellbore.

The transmitter generates a pressure pulse in the borehole fluid.

When this pulse reaches the borehole wall, P & S wavefronts are generated in the formation.

As the waves travel away from the source in the formation, the portions near the wellbore create pressure disturbances in the borehole fluid.

These fluid waves are called headwaves.

The headwaves move at the same velocity as the wavefronts that created them.

It is these headwaves that are recorded by sonic logging tools.

Sonic Principles

Acoustic Wave Propagation

Objective

To illustrate the modes of acoustic wave propagation and how these waves

propagate in a borehole environment.

Sonic Principles

In the single receiver and single transmitter sonic tool, the acoustic pulse is generated by the transmitter.

The pulse then travels through the mud (ray a). It is refracted along the borehole wall (ray b) and is refracted back through the mud (ray c)

The sonic waveform received by the receiver is analyzed to detect only the time of the first negative arrival which represents compressional transit time.

But, the transit time by itself does not provide the formation velocity or the slowness.

Sonic Principles

The next configuration is the Single Transmitter and Two Receivers configuration. The effects of the mud are cancelled from the measurement by placing two receivers close together.

This configuration works for situations where the tool is parallel with the borehole wall.

When the tool is tilted in the borehole, the travel times ofc at R1 and R2 are no longer equal.

Sonic Principles

Adding an additional transmitter solves the tilt problem but the paths that the waves take to each receiver are different.

Therefore, the formation being measured is not the same for each transmitter and receiver pair combination.

Sonic Principles

The next configuration is Two Transmitter and Four Receivers.

The zones investigated by the Two transmitter and receiver pairs are the same and the measured slowness is independent of changes in borehole diameter such as wash-outs or bit size changes.

The measurement is known as Borehole Compensated Sonic (BHC)

Sonic Principles

Sonic Principles

The previously discussed tools record can only extract the compressional slowness.

The Array Sonic tool consists of a transmitter and eight receivers which record the complete waveforms to extract the compressional, shear and Stoneley slowness.

The slowness-time-coherence processing is used to extract the various formation slowness values.

Sonic Tools

Sonic Tools

The Schlumbergers ISONIC tool incorporates a sonic transmitter and a 2 ft array of receivers in a drill collar.

During drilling, the transmitter is fired and acoustic waves are propagated through the mud and formation to four receivers.

The compressional transit time of the formation is extracted from the waveforms recorded by the electronics section of the tool.

The waveforms are recorded in down-hole memory, and the transit times are sent up-hole in real time.

The tool can support a rate of penetration of up to 350 ft/ht with 6 sampling

Sonic Applications

Sonic Applications

This figure outlines the relationship of the various elastic constants to the compressional slowness, shear slowness and the b

Sonic Evaluation Technique

The sonic tools only measure primary porosity and they do not see vugs or fractures.

M.R.J. Wyllie proposed a Time-Average relationship between porosity and interval transit time for clean and consolidated formations with uniformly distributed small pores.


Wyllies equation gives values which are too high in unconsolidated and under-compacted sands in geologically young formations, particularly at shallower depths.

Under-compaction should be suspected where the adjacent shales have slowness greater than 100 s/ft.

An empirical correction factor needs to be applied in such cases.


At high porosities, Wyllies equation, even with a compaction factor, has problems.

This lead L.L. Raymer, E.R. Hunt and J.S. Gardner to propose an alternative equation


The Schlumberger Chart Por-3 converts the sonic log interval transit time into using Wyllies equation (Time-Average) or the Raymer-Hunt equation (Field Observation)


When the sonic tool is run centralized in large holes, the mud signal may arrive at the receiver before the formation signal.

Such a log can not be recovered, even if the waveforms have been recorded.


In unconsolidated shaly formations, the formation has been invaded so radically that measurements made with the short spacing tool cannot accurately estimate the formation slowness.

To properly calculate the slowness of the formation, long spacing sonic must be used for the acoustic wave to propagate deeper into the formation before arriving at the receiver.

Density:

Gamma Ray Interactions Principles Tools Applications Environmental Corrections Evaluation Technique

POROSITY

Density Gamma Ray Interactions


Compton scattering occurs when a -ray collides with an electron, the electron is ejected from its orbit, and the -ray loses part of its energy.

Compton scattering predominates in the 75 keV to 10 MeV energy range.


The conversion of a -ray into an electron and positron as they enter the strong electric field near an atoms nucleus is called pair production (predominates at -ray energy levels > 10 MeV)

The disappearance of a low energy -ray as it collides with an atom, causing the ejection of an orbital electron, is called photoelectric absorption (predominates at energies < 75 keV).

Density Principles

The more electrons there are in the formation, the more likely -rays will undergo photoelectric absorption & Compton scattering and therefore lose energy.

By measuring the number of -rays and their energy levels at a given distance from the source, the electron density of the formation can be predicted.

Understanding the relationship between electron density and bulk density is an essential part of the density measurement.

Density Principles

Density Principles

A tool with a chemical -ray source (662 KeV) and -ray detectors is placed in front of the formation.

The -rays emitted from the source interact with the formation and are scattered back to detectors.

Density Principles

The scattered -rays are detected by a scintillation detector which convert the -rays into electrical signals.

The electrical signal is proportional to the energy of the detected -ray.

Density Principles

The electrical pulses are analyzed and converted to the Gamma ray count rate versus their energy.

The number of Gamma rays in the region:

A is related to the amount of photoelectric absorption

B is related to the amount of Compton scattering

taking place in the formation

Density Principles

As the density of the formation increases, the counts across the whole spectrum decrease.

Thus, the density tool utilizes the Gamma rays spectrum and an algorithm to produce the apparent bulk density output , a

Density Principles

The relationship between e, b, a for common elements and formations is listed here.

Density Tools

Ever since density tools were developed, they have been an integral part of Wireline logging.

Almost every well evaluated today is logged with some kind of formation density tool.

The FDC tool is an older generation tool with no spectrum analyzer and relied on total -ray counts for density determination.

The LDT and the TLD tools have a spectrum analyzer which can determine the energy of the detected -ray to compute the PEF of the formation.

Density Tools

Two commonly used wireline tools:

The LDT has a radioactive cesium Gamma ray source and two scintillation detectors.

The LDT is part of the Schlumberger PEx tool string and has an additional detector (back-scatter detector) placed closer to the source, producing higher count rates and yielding improved statistical variation. Both the tools are pad type tools and are run with the pad touching the borehole walls.

Density Tools

The source-to-detector spacing must be great enough to allow -rays to have multiple interactions with the formation electrons and must not be so great that all the GR lose their energy prior to reaching the detector.

The tool incorporates more than one detector.

Each detector spacing results in a different depth of investigation and enables compensation for the effects of mud-cake.

The LDT has only two detectors while the TLD has an additional back scatter detector very close to the source which improves the mud compensation.

Density Tools

Density Tools

This figure compares a log from a well where the LDT was run at 18000 ft/hr and the TLD was run at 3600 ft/hr.

Density Tools

Primary calibration standards for density tools are limestone formations of high purity with an accurately known density and filled with fresh water.

Field calibration is done with large aluminum and magnesium blocks manufactured to consistent purity and shape.

Density Tools

The Schlumbergers AND tool houses a cesium source and two detectors.

Since the tool is a part of the rotating drilling string, it allows the recording in all quadrants of the borehole while it is being drilled.

Density Tools

Density Applications

Density Environmental Corrections

The algorithms used to produce the a output from the tool is not perfect.

A small correction is needed between the tools bulk density output (a) and the true formation bulk density (b). a = b only when the formation is water-filled limestone.


The LDT tool has two detectors measuring the same density.

If there is no mud cake, both will read the same, if there is mud cake, there will be a slight difference which can be computed and hence the measurement corrected.

The Spine and Ribs plot is the graphical representation of the method used.

In the case of the TLD, a forward modeling algorithm is used to compute the formation and mud cake densities and the mud-cake thickness.

The provided density output is corrected for the mud-cake effect.


The measuring pad curvature is designed for an optimum borehole fit in an 8 hole.

In a borehole with a significantly different diameter, an additional correction may be needed which is provided by Schlumberger Chart Por-15a

Density Evaluation Technique

The density tools measure primary as well as secondary porosity.

In a clean formation of known ma and f, the is defined by the given equation.


The density tool has a very shallow depth of investigation and primarily measures the invaded zone.

The saturating fluid will be a mixture of mud filtrate and unmoved hydrocarbons.


The Schlumberger Chart Por-5 converts the density log reading into porosity for various matrix densities and average densities of the saturating fluids.


A rugose borehole makes the detectors see a volume of mud which cannot be corrected easily.

The rugosity effects can usually be seen on the (erratic and off scale) DRHO curve rather than the caliper.

Gamma Ray Interactions Principles Tools Applications Evaluation Technique

D e n s i t y - P E F

Density - PEF Gamma Ray Interactions

When the Gamma () Rays pass through matter, they experience a loss of energy due to collisions with other atomic particles which can be divided into three basic categories:

Pair production

Compton scattering

Photoelectric absorption

Fcr the determination of lithology the photoelectric absorption is the interaction of interest to us.

Density - PEF Gamma Ray Interactions

Photoelectric absorption is the disappearance of a low energy -ray as it collides with an atom, causing the ejection of an orbital electron

This interaction is a factor at -ray energies below 100 keV and predominates at energies below 75 keV

Density - PEF Principles

A tool with a chemical Gamma ray source (662 KeV) and Gamma ray detectors is placed in front of the formation.

The Gamma rays emitted from the source interact (photoelectric absorption and Compton scattering) with the formation and are scattered back to detectors.

The probability of absorption occurring is known as the photoelectric absorption cross section of the target atom.


The scattered Gamma rays are detected by scintillation detector which converts the Gamma rays into electrical signals.

The electrical signal is proportional to the energy of detected Gamma Ray.


The electrical pulses are analyzed and converted to the Gamma ray count rate versus their energy.

The number of Gamma rays in the region:

A is related to the amount of photoelectric absorption

B is related to the amount of Compton scattering

taking place in the formation


With the density held constant as the PEF is increased, the spectrum shows a decrease only in the low energy area.

This shows that the PEF is inversely proportional to the number of Gamma rays in the lower energy area.


Plot-1 shows the effect of changing bulk density on the spectrum.

As the density is increased, the spectrum drops in both the window.

Plot-2 shows the effect of changing the lithology on the spectrum.

Here the effects of changing the lithology can be seen only in the window A changes in both cases.

As a result of this, the window A must be normalized against the window B to obtain the density independent PEF.

Density - PEF Tools

Two commonly used wireline tools:

The LDT has a radioactive cesium Gamma ray source and two scintillation detectors.

The LDT is part of the Schlumberger PEx tool string and has an additional detector (back-scatter detector) placed closer to the source, producing higher count rates and yielding improved statistical variation. Three detectors allow for a new signal processing method that results in a better PEF measurement.

Both the tools are pad type tools and are run with the pad touching the borehole walls.

Density - PEF Tools

The source-to-detector spacing must be great enough to allow Gamma rays to have multiple interactions with the formation electrons and must not be so great that all the GR lose their energy prior to reaching the detector.

The tool incorporates more than one detector.

Each detector spacing results in a different depth of investigation and enables compensation for the effects of mud-cake.

The LDT has only two detectors while the TLD has an additional back scatter detector very close to the source which improves the mud compensation.

Density - PEF Tools

Density - PEF Tools

This figure compares a log from a well where the LDT was run at 1800 ft/hr and the TLD was run at 3600 ft/hr.

Density - PEF Tools

The Schlumbergers AND tool houses a cesium source and two detectors.

Since the tool is a part of the rotating drilling string, it allows the recording in all quadrants of the borehole while it is being drilled.

Density - PEF Tools

Density - PEF Applications

PEF Applications

Lithology Indicator for:

Mono-mineral simple matrix (alone)

2-mineral matrices (in combination with density)

3-mineral matrices (in combination with density & neutron log)

Clay mineral identification (in combination with NGS log)

Density - PEF Evaluation Technique

The PEF is a good matrix indicator.

As can be seen from this figure, the PEF responds mainly to the lithology and has very little effect due to changes in or fluid content.

Hence a safe interpretation of matrix lithology can be made dealing with simple lithologies.

Lithology identification using PEF


The Schlumberger Chart CP-21 (Lithology identification Plot), identifies rock mineralogy through comparison of the maa (chart CP-14) and Umaa (chart CP-20).

The lithology identification plot will help determine the percentage of the three minerals present in the formation.

This technique is the basis of the three mineral model available for processing at the well-site acquisition unit.


Limitations

Presence of Barite in the mud

Presence of thick mud-cake

Poor quality data in rugose holes


This example from a well drilled in Texas with heavy barite-weighted mud illustrates the improved compensation for the PEF measurement from the TLD.

There is a thick build-up of heavy mud-cake in front of the premeable zones at X70-X895, X905-X930 and X995-X020 ft.

The PEF recorded by the LDT reads too high in these zones and is too erratic to be used quantitatively.

The TLD-PEF is much more stable and can be used for interpretation.

Neutron:

Neutron Interactions Principles Tools Applications Environmental Corrections Evaluation Technique

POROSITY

Neutron Neutron Interactions

Neutrons have no electric charge and their mass is similar to that of the proton. This lack of charge allows the neutron to penetrate into the formation and makes it ideal for logging applications.

Neutron interact with matter in a wide variety of ways. There are four important interactions between a bombarding neutron and a target nucleus:

Neutron absorption is said to occur when it strikes the nucleus and is absorbed. Depending on the energy of the incident neutron, the interaction can be described as thermal or fast.

Neutron scattering is said to occur when it interacts with a nucleus, but both particles reappear after the interaction. In elastic scattering, the total kinetic energy of the two colliding particles is conserved but redistributed. In elasting scattering, part of the kinetic energy from the neutron is transferred to the nucleus as excitation energy.


The neutron energy loss for any particular collision depends upon the mass of the neutron and the mass of the element of or particle being struck.

The maximum loss energy occurs when the neutron collides with formation nuclei with nearly the same mass.

Neutron Principles

This figure shows that the neutron slows down to a thermal energy level at a fairly quick rate

The similarity between the neutron and hydrogen masses means that hydrogen is the most effective element in the slowing down.

The length of time that a neutron stays at the thermal energy level is determined by the capture cross section of the formation.

The population of epithermal or thermal neutrons at a certain distance from the source is determined by the quantity of hydrogen atoms.

Neutron Principles

The quantity of hydrogen atoms per unit volume of the formation is known as the Hydrogen Index (HI).

Since hydrogen atoms are primarily present in the fluids in the pore spaces, formation porosity can be determined using the measured HI.

The HI of fresh water = 1. Gas has very low HI as it has far fewer hydrogen atoms per unit volume than oil or water.

It is important to remember that neutrons will be affected by the hydrogen in both the formation fluids and the formation, even thogugh hydrogen is more commonly found in the fluids.

Neutron Principles

Neutron Principles

During neutron logging the number of thermal neutrons in the formation is about 10 times greater than the number of epithermal neutrons.

This gives thermal neutron detectors higher count rates and therefore better counting statistics than epithermal detectors.

The chemical neutron source is used in standard neutron porosity tools which detect thermal neutrons.

The tools which rely on detecting epithermal neutrons use a mintron as it can produce many more neutrons than a chemical source.

Neutron Principles

Neutron porosity tools use He3 gas proportional detectors.

Depending on the desired output and application, He3 detectors can be set up for either thermal or epithermal neutron detection.

Neutron Principles

Neutron Tools

Neutron Tools

The primary calibration standard for the neutron logging tool is a series of water-filled laboratory formations with accurately known porosities.

The secondary standard is a water-filled calibrating tank of precisely defined geometry

Neutron Tools

NPHI is the classic neutron porosity output.

The near-to-far detector count rate ratio is taken, borehole size correction applied and appropriate algorithm for cased-hole or open-hole used to compute NPHI.

The output is often presented assuming the matrix to be limestone, but a different matrix can be used (logging constant MATR)

Neutron Tools

The standard neutron porosity logging tools have source and detector spacing such that the resolution and depth of measurement of the two detectors are different.

NPHI: The two detectors count rates are not matched.

TNPH: The detectors counts are corrected and calibrated, matched for depth and resolution.

Environmental corrections can also be applied.

NPOR: Computed using Alpha processing

Neutron Tools

It is important to note that the porosity is computed for the specified matrix type.

Typically the matrix type is limestone, which means that the presented porosity is correct for a pure water-filled limestone

Neutron Tools

Neutron Applications

Neutron Environmental Corrections


The effects of the borehole are numerous but well known and characterized.

The Schlumberger Chart POR-14c can be used to apply the environmental corrections to the thermal neutron porosity uncorrected data.


Schlumberger Chart Por-14a provides for the corrections to the cased hole thermal neutron porosity for borehole diameter, casing and cement thickness and Chart Por-14c for the borehole salinity, mud weight, borehole temperature and pressure and formation salinity.

Neutron Evaluation Technique

Nuclear Magnetic Resonance:

Principles

Tools

Applications


POROSITY

Nuclear Magnetic Resonance Principles

Nuclear Magnetic Resonance, NMR refers to a physical principle of the response of atomic nuclei to magnetic fields, and to the techniques for measuring and interpreting those responses.

The Nuclear Magnetic Resonance measurement is obtained by manipulating the hydrogen nuclei with the magnetic field.


A permanent magnet mounted on the tools applies a strong magnetic field-B0


The B0 field aligns all the nuclei with magnetic moment including the hydrogen protons present in the pore fluid.

This is known as polarization or longitudinal relaxation.

This process occurs in an exponential manner with time constant T1 the longitudinal relaxation time constant/


An oscillating magnetic field B1 is now applied which is at 90 to B0.

This causes only the hydrogen protons in the pore fluids to tip and precess about the axis of B1 and at the same frequency as B1. This frequency is known as the Larmor frequency.


This decay is reversed by applying a 180o oscillating magnetic field. The precessing hydrogen protons change phase so that the fastest proton, which had precessed the farthest has the farthest to return. When the protons re-phase, they again induce a signal at the receiver called echo/


The echo disappears quickly and the hydrogen protons are re-phased again by the same technique.

This sequence is repeated thousands of times producing thousands of echoes.

With time, the protons lose energy and permanently de-phase causing the echo signals to decrease exponentially.

The pulse sequence is referred to as a CPMG, after Carr, Purcell. Meiboom and Gill, who discovered it.

This permanent de-phasing known as transverse relaxation, is due to the formation properties and is defined by transverse relaxation time constant-T2.


Three independent relaxation mechanism cause the hydrogen proton to relax:

Surface relaxation

Bulk Fluid relaxation

Diffusion relaxation

T1 is only affected by bulk and surface relaxation mechanism.


The echo signal measured by the tool is the sum of the contributions of all pores in the formation and the initial signal amplitude will represent the formation porosity.

Converting it to the T2 distribution curve will give three peaks with the sum of the peaks representing the formation porosity and the T2 values representing the pore sizes.


In reality, the volume measured by an NMR tool contains millions of pores with a distribution of T2 values.

The area under the T2 distribution is proportional to formation porosity and is independent of matrix, since only the formation fluids are measured.

The shape of the T2 distribution can be related to pore size distribution if the surface relaxation is dominant.

Nuclear Magnetic Resonance Tools


The cross section of the CMR shows two powerful magnets which create a field so that 90% of the signal comes from the sensed region.

This region is 0.75 to 1.25 from the tool face and 6 along the tool length.

The blind zone is 0.5, making the tool insensitive to mud cake and some levels of rugosity.

Nuclear Magnetic Resonance Applications

Nuclear Magnetic Resonance Applications

Porosity Tool Response

Objective

To illustrate the origins of different porosity types, their petrophysical

descriptions and logging tool response to each

Nuclear Magnetic Resonance Evaluation Technique


The value of the T2 free fluid cut-off can be found by measuring the T2 relaxation distribution on water saturated cores before and after they have been centrifuged in air to expel the producible water.

Before centrifuging, the relaxation distribution corresponds to all pore sizes.

After centrifuging, the relaxation distribution corresponds to the water that could not be expelled, indicating the non-producible fluids.

The T2 free fluid cut-off is the value below which pores contain only non-producible fluids.


Observations of many sandstones samples showed that a cut-off time of 33 ms for T2 distributions would distinguish between free-fluid porosity and bound-fluid porosity.

For carbonates, relaxation times tend to be three times longer and a cut-off of 100m is used.

However, both these values will vary if reservoir capillary pressure differs from the 100PSI used on the centrifuged samples.


Borehole NMR measurement made with the CMR tool have been successfully compared with conventional core measurements and with NMR measurements performed on cores in the laboratory to verify their accuracy


This example shows that the CMR tool provides a lithology-independent porosity useful in complex lithologies.

The lower half of the log is predominantly limestone, and density porosity on a limestone matrix overlays CMR porosity.

At 935 feet the reservoir changes to dolomite and the density porosity on a dolomite matrix overlays CMR porosity.


Here, CMR data are used to calculate the SWirr.

The conventional logs shows SW ranging from 60% to 90%.

The CMR measurement shows this rock to have very small pore sizes containing a high volume of irreducible water, so the predominant movable fluid is the hydrocarbons.

After hydraulic fracturing and completion, this well is producing gas, oil and a small fraction of water.

Nuclear Magnetic Resonance:

Principles

Tools

Applications


P E R M E A B I L I T Y


Theoretical studies of NMR phenomena predict that T2 in water saturated rocks is closely related to pore size.

The plot of the Berea sandstone shows that the T2 and the V/S are directly related by the 2 of the sandstone.

Thus the T2 distribution can be related to the pore size distribution with the short T2 times indicating small pores while the longer T2 times indicate larger pores


The algorithm to estimate permeability from the pore size distribution is based on the assumption that larger pores flow fluid more easily than smaller pores.

This figure shows two rock samples that have about the same porosity and thus have the same area under the T2 distribution curve.

But there is a considerable difference in the T2 distribution which clearly identifies the sample with higher permeability.


The cross section of the CMR shows two powerful magnets which create a field so that90% of the signal comes from the sensed region.

This region is 0.75 to 1.25 from the tool face and 6 along the tool length.

The blind zone is 0.5, making the tool insensitive to mud-cake and some levels of rugosity.

Nuclear Magnetic Resonance Application


The permeability calculation is based on the fact that permeability generally increases with both pore size and porosity.

NMR brine permeability measurements on core samples have resulted in several empirical correlations.

Two commonly used equations are presented here. The constant in these equation vary with the lithology and the field and should be verified for each lithology and field.

The constant are determined by calibrating the NMR permeability to the laboratory-measured core brine permeability in a few wells of a field.


Permeability calculated from NMR lab spectrometer measurements are compared with the conventional core brine permeability of sandstone core samples from two wells.

The excellent correlation between the conventional brine permeability measurements and the NMR-derived permeability measurements shows that good permeability values can be computed from NMR measurements.


This example is over a productive shaly sand sequence where sand lenses of equivalent porosity can have large changes in permeability.

A full diameter core was taken over this interval to identify the permeable sections.

The CMR tool was also run over this section to obtain a continuous permeability for comparison to conventional core permeability.

The constant c of the permeability equation had to be adjusted over a few samples to match the core-derived and CMR-derived permeability.

Sonic:

Principles Tools Applications Evaluation Technique

PERMEABILITY

Sonic Principles

Permeability is one of the most difficult measurement to acquire. Some measurements provide only a few points along the well, as is the case with well testing, wireline testers and core measurements.

The evaluation from the Stoneley wave and nuclear magnetic resonance can provide a continuous measurement of permeability along the well.

Sonic Principles

At low frequency the Stoneley mode becomes the tube wave and propagates as a piston like compression of the borehole fluid in the borehole.

When the borehole crosses permeable zones or permeable fractures, some fluid movement occurs between the borehole and the formation.

This results in some energy loss, hence attenuation, and a slowing down of the wave, hence increased Stoneley wave slowness.

In effect, the parameter measured by the Stoneley wave is not exactly the formation permeability, but rather the fluid mobility (the ratio of permeability to fluid viscosity, k/).

The energy of the Stoneley wave peaks at around 500Hz and gradually decreases at higher frequencies, providing a signal up to about 4 KHz.

Sonic Tools

Sonic Applications


The Stoneley slowness alone is inadequate to evaluate fluid mobility.

This figure shows the effect of permeability on slowness and attenuation as a function of frequency. Increased permeability leads to increased dispersion and increased attenuation.

The effect is larger on slowness at low frequencies, whereas it is larger on attenuation at high frequencies.

Thus, the Stoneley slowness and attenuation together will provide a better method to evaluate fluid mobility.


The mud-cake flexibility is modeled by adding a membrane stiffness on the borehole wall.

It is characterized by a membrane impedance


Obtaining fluid mobility from Stoneley complex phase slowness is an inverse problem.

The process described in the figure uses inversion and modeling to estimate the fluid mobility using slowness and attenuation over a wide frequency band.


The Stoneley modeling and inversion technique provides quantitative determination of the pore fluid mobility and does not require calibration from external information.

The model is suitable for distributed permeability in clastic type rocks.

In fractured reservoirs, other techniques are more appropriate.

Resistivity

H Y D R O C A R B O N

S A T U R A T I O N

Resistivity

Resistivity

The resistivity tools are designed to read at various depths of investigation.

The shallow reading is used to measure the SXO. The deep reading is affected by the invasion and is often not the Rt.

The medium reading, together with the shallow reading, is used to correct the deep reading to obtain the Rt.

Resistivity

Micro Resistivity:

Principles Tools Applications Environmental Corrections Evaluation Technique


S A T U R A T I O N

Micro-Resistivity Principles

Micro-Resistivity Tools

Micro-Resistivity Applications

Micro-Resistivity Environmental Corrections

The Schlumberger Chart Rxo-3 provides the mud-cake thickness and resistivity corrections to the MSFL.

Micro-Resistivity Evaluation Technique

Induction:



S A T U R A T I O N

Induction Principles



Objective

To understand the application of physical principles used in induction measurement


Born Function

Objective

To illustrate how the induction response of a formation is transformed to its wire-

frame using a Born function

Induction Tools

Induction Applications

Induction Environmental Corrections

Induction Evaluation Technique

Laterolog:



S A T U R A T I O N

Laterolog Principles


Resistivity and Geometrical Factor

Objective

To understand that mostly tools read resistance and compute formation resistivity

using their geometrical (k) factor



Objective

To illustrate how electrode and current flow can be configured to selectively measure volumes of

formation and to demonstrate the concept of equipotential

Laterolog Tools

Laterolog Applications

Laterolog Environmental Corrections

Laterolog Evaluation Technique

Documents

Part 4 the Tools