stag wireline manual

8/18/2019 STAG Wireline Manual

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technical training 2007

Module FE1

Wireline Logs

&

LWD Interpretation

Stag Geological Services Ltd.

Reading

UK

Revision J

February 2007


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technical training 2007

WIreline Logs & LWD Interpretation

Chapter 1 Introduction

Chapter 2 Spontaneous Potential Logs

Chapter 3 Gamma Ray Logs

Chapter 4 Resistivity Logs

Chapter 5 Bulk Density Logs

Chapter 6 Neutron Porosity Logs

Chapter 7 Sonic Logs

Chapter 8 Lithology Determination

Chapter 9 Reservoir Evaluation

Chapter 10 Shaly Sand Analysis

Chapter 11 MWD Overview

Chapter 12 LWD Imaging Logs

Chapter 13 Log Witnessing

Appendix A Vendor Brochures

Appendix B Log Interpretation Charts

Figure : Table of Contents


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Introduction

Wireline Logs LWD Interpretation 1-1

Introduction

Locating the presence of oil and gas deposits underground is a complex process

spanning many months of preliminary research followed by exploration and

development drilling. Potential sites for exploration are identified from seismic

studies but full evaluation can only be made by drilling wells to see what is

actually there.

Advances in seismic data collection and interpretation techniques are leading to

less uncertainty and greater chances of locating commercial reserves, but the

results of the drilling process are ultimately only as good as the interpretation

techniques used in the evaluation process.

Formation Evaluation can be grouped into four major categories:

• Before Drilling

Seismic Interpretation

Offset Data

• During Drilling

Mud Logs and Wellsite Geology

Measurement While Drilling (MWD)

Coring

• Post - Drilling

Wireline Logs

Production Tests

Whilst advances in seismic processing have been remarkable in recent times the

process is still best suited to large scale exploration and field evaluation. Wellsite

geology and mudlogging provide geological data while drilling the well but the

drilling process and the inefficiences of the hole cleaning process only allows for

a largely qualitative and subjective approach. Coring does produce whole rock

from which detailed petrophysical analysis and quantitative measurements of porosity, permeability, fluid saturation may be made but cores are normally only

taken over short intervals in reservoir rocks leaving the majority of the section

un-sampled.

Petrophysical logging enables large sections of exposed (and sometimes cased)

hole to be scanned and variety of geological and reservoir data to be obtained;

quantitative analysis can be performed on the data to supplement other informa-

tion. Historically, petrophysical logging has been called “Wireline Logging”, or

even “Electric Logging” but neither of theses terms adequately describe the

current range of logging tools or conveyance methods.


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Introduction

Wireline Logs LWD Interpretation-2

Formation Evaluation

The objectives of logging are multiple and varied; depending on the type of well

being drilled and the information required. However, we might try and list some

of the required information as follows:

• Geological Correlation

Identification of lithology for correlation between wells or to assist

general geological evaluation in the current well. Different logging

runs taken over the same interval need to be depth matched in order

to ensure that we are comparing like with like. Perforating, taking

sidewall cores or obtaining pressure information and fluid samples

all require accurate internal depth correlation using logs.

• Petrology

Logs can help to identify lithology, mineral assemblages and pick

out features such as bedding, lamination, porosity, permeability, ce-

mentation, fractures and facies and depositional environments.

• Reservoir Parameters

Logs can identify permeable zones, measure porosity and permeabil-

ity, identify fluid types and provide information to calculate satura-

tion levels, differentiate between water, oil and gas and determinefluid contact points. Reservoir pressure can be measured and fluids

obtained for analysis.

• Rock Mechanics

Rock strength and the tectonic forces acting upon rocks at depth can

be evaluated from logging tools and the information used to help un-

derstand drilling and borehole problems.

• Geosteering Applications

When MWD and LWD tools are used the information obtained, at

the time of drilling, may be used to help drill the well to the required geological target and indeed navigate the reservoir.

Wireline Logs

In September 1927, Marcel and Conrad Schlumberger, with Henri Doll, recorded

the first electrical resistivity log at Pechelbronn, France. This log was actually

called a “carottage electrique” or electrical core since it was a quantitative

recording of rock properties. The log was hand plotted from point-by-point resis-

tivity measurements. Since then, more than fifty geophysical-type well logs have


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Introduction


been introduced to record the various electrical, nuclear, acoustical, thermal,chemical and mechanical properties of the earth.

Figure 1: First “Electric” Log


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Introduction


Figure 2: First Schlumberger Log


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Without interpretation, the measurements provided by the various logs are not particularly useful. It takes time, knowledge, and experience to convert the raw

data into meaningful and practical information often using sophisticated

computer software; the input data consisting of raw well log data, and the output

being porosity, hydrocarbon type, fluid saturations, and lithology.

Logging tools are conveyed into and out of the borehole in a number of ways.

Traditionally wireline conveyed tools log boreholes after they have been drilled;

the wireline not only conveying the instruments but also providing the means of

data transmission from the tool to the surface equipment.

However, borehole conditions often make the use of wireline tools very difficult.High inclinations, high pressures and temperature and unstable borehole condi-

tions can provide severe limitations on the use of wireline tools. Attaching the

instruments to jointed drillpipe or tubing can overcome some of these issues and

the hole is logged whilst tripping the pipe to the surface. A cable attached to the

logging tool is strapped to the pipe and reeled in as the string is tripped. Whilst

this process does allow high angle and unstable boreholes to be logged the

process is very time consuming and, therefore, expensive. The use of coiled

tubing can significantly reduce costs as tripping speeds are much higher and the

conductive cable can be threaded internally through the coiled tubing eliminating

handling time.

The use of MWD and LWD logging tools overcomes many of these issues and

also enables the hole to be logged very shortly after drilling minimising invasion

and other interpretation issues.

The Wireline Logging Process

The logging company provides the tools, surface equipment and a team of expe-

rienced engineers to perform the logging operation, which may take anything

from a few hours to many days, depending on the nature of the work. The surface

logging unit comprises the control functions, surface computer systems, cable

drum and winch. The logging tools, which may be up to 30m long are attached to the cable, which is used both for suspension and data transfer, and lowered to

the bottom of the borehole.

The cable is then pulled out of the hole and the various rock properties are con-

tinuously measured. Pulling speeds are dependent on the type of tool being run

but are typically around 1800 feet per hour (600m/hr) when radioactive tools

such as a gamma ray log are present and can be as much as 6000 ft/hr (1800m/

hr). During the logging process the data is recorded at surface, correlated for

depth and corrected for borehole and mud conditions.


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Figure 3: Wireline Logging Schematic

Surface Data

Acquis it ion System

Mechanical

Winching

Drum

Digital

Data

Transmission

Loggingcable

Downhole

Logging Tool


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Introduction


Logging Runs

A logging run is typically made at the end of each drilled section, immediately

prior to casing being installed. Whilst some tools can make measurements

through steel it is beneficial to record basic information over the open-hole

section in order to maximise data quality and minimise interpretation difficulties.

Figure 4: Wireline Unit


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Each logging run is identified by a suitable alpha-numeric system to record thetype of instrument being used and the actual tools that were run. This is important

for calibration and cost management reasons.

Data Interpretation

Data processing is almost always done by computer, typically in town but

increasingly using modern high powered computers at the wellsite. Basic infor-

mation can be derived by hand using Quick-Look or Shaly Sand methods or by

using relatively simple spreadsheets or other processing software.

Types of Logs

Many different types of logs, measuring various rock properties may be run at

each casing point. Generally the first and intermediate logging runs are per-

formed for lithological evaluation and stratigraphic correlation purposes. Minor

hydrocarbon bearing zones may also be identified, together with possible source

rock information.

Over the main reservoir section the amount of information required is much

greater and a full suite of logs covering lithology, porosity, permeability and

fluid saturations are required.

Additionally there are many other types of tools available for specific purposes,and of helping with the evaluation of cement jobs and other completion opera-

tions. The major logs used for routine evaluation of open hole sections are:

• Lithology Logs

Gamma Ray

Spontaneous Potential

• Resistivity (Saturation) Logs

Laterologs

Induction LogsWave Propagation Logs

• Porosity Logs

Formation Density Log

Neutron Porosity Log

Sonic Log

• Miscellaneous

Caliper

Dipmeter


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Repeat Formation Tester Sidewall Cores

Cement Bond Logs

Measurement While Drilling

Measurement while drilling services have been available since the early 1980s

and provide a means of obtaining petrophysical data in real time during the

course of drilling the well. This can be of significant benefit when compared to

wireline data which is often only available weeks after drilling a particular

section. MWD data is very useful in providing additional geological information

for the wellsite geologist and helping with geosteering applications in particular.

Figure 5: Example Log

Gamma Ray

Caliper

IN10 20

Bit Size

IN10 20

Gamma Ray

API0 150

FEET Resistivity

Induction Deep

OHMM0.2 200

Induction Medium

OHMM0.2 200

Sonic

Sonic Transit Time

US/F140 40

Porosity

DRHO

G/C3-0.75 0.25

Neutron Porosity

PU0.45 -0.15

PEF

0 20

Bulk Density

G/C31.95 2.95

5600

5700

5800

5900


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The logging tools are installed inside special drill collar sections located in theBHA. Powered by downhole turbines or batteries they measure rock properties

whilst the well is being drilled and transmit the data to surface by mud pulse

telemetry. This data is decoded and interpreted at surface on the wellsite and is

available to the drilling engineers and geologists at the same time (and often

earlier) as other drill returns logging information.

The range of MWD applications has been significantly extended and enhanced

over the years and now includes:

• Gamma Ray

• Resistivity

• Density

• Neutron Porosity

• Sonic

In addition MWD tools also provide real time directional survey data and drillingdynamics information, both of which can be vitally important to the successful

drilling of the well.

Borehole Environment

Both Wireline Logging Operations and MWD tools have to be able to work

under a wide range of physical and chemical conditions in and around the bore-

hole. The depth of the hole, bit diameter, borehole erosion, hole deviation, for-

mation temperature, mud weight and type and formation pressures each cause

particular problems to the performance of logging tools. Calibration and correc-

tion for borehole environment variables must be carried out both during and after logging runs in order to ensure that the interpreted results are as accurate as pos-

sible. In most cases it is necessary to make multiple measurements with different

tools and cross-plot the results to try and minimise the various effects on partic-

ular tool response. Once allowance has been made for factors such as borehole

temperature and pressure, the key environment effects controlling interpretation

are:

• Drilling Mud Type

• Mud Invasion Profile


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• Relationship of Pore Water to Mud Filtrate

• Borehole Erosion

• Tool Depth of Investigation

Porosity

One of the most important pieces of reservoir information is porosity. That is, the

amount of void space present in the rock expressed as a percentage of total rock

volume.

N. B. When used in Quick Look calculations, porosity is expressed

as a number between 0 and 1.

For example:

Porosity (φ) = 20% use 0.20

= 8% use 0.08

Effective Porosity is the amount of porosity able to transmit fluid, and is of vital

importance in reservoir evaluation.

Maximum porosity of 48% is obtained in granular sedimentary rocks when per-

fectly spherical grains of the same grain size are packed in cubic mode. With

compaction due to burial grain packing becomes closer and porosities will bereduced to less than 30% in most cases. Where there is significant variation in

grain size and with the addition of matrix or cement, porosity values can be

further reduced.

Permeability

Permeability is the ability of the rock to transmit fluid. It is measured in darcy's

and usually given the notation k. One darcy is the permeability when a fluid of

viscosity 1 centipoise is passed through a 1 cm cube with a differential pressure

Porosity % =Pore Volume

Total Rock Volume-----------------------------------------------

⎝ ⎠⎛ ⎞ 100×


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of 1 atmosphere. Since this is a relatively large unit of permeability most oil field reservoir permeability is expressed in millidarcy's (one thousandth of a darcy).

For granular clastic rocks, grain size is also a key variable in determining rock

permeability along with grain shape and sorting. Larger pore throats will allow

fluid to pass more easily than smaller sized throats.

Both porosity and permeability in carbonates (limestones and dolomites) are less

uniform than in granular clastic rocks, being less to do with transportation and

grain erosion, and more a product of original sedimentary features (grain type

and matrix) and subsequent (often post-depositional) diagenesis. Dolomites are

formed by post-depositional percolation of magnesium bearing fluids whichcauses original calcite (CaCO3) to re-crystallise as dolomite [(Ca.Mg (CaCO3)].

This process normally results in enhanced porosity and is a key factor in the pro-

duction of carbonate reservoirs.

The other major control on porosity in carbonates is fracturing, particularly in

Chalks. Whilst primary porosity of Chalks may be very high, being composed

mainly of highly spherical calcareous grains, (microscopic coccoliths), permea-

bilities may be almost zero because of the very small pore throats. Enhancement

of both porosity and permeability is required for these rocks to become potential

reservoirs. This can be a problem for wireline and MWD interpretation since the

resulting secondary porosity may be too large to be evaluated by the logging tool.

The main controls on porosity in clastic rocks are:

• Size of available pores

• Connecting passages between them

Definitions of Permeability

Absolute Permeability

When the rock is 100% saturated with one fluid

Effective Permeability

The ability to transmit a fluid in the presence of another fluid when the two are

immiscible.

Relative Permeability

The ratio of effective to absolute permeability.


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Permeability from Log Data

Reservoir permeability is not normally available form direct measurement, either

from wireline or MWD tools. Values are computed using mathematical models

which use porosity and irreducible water saturation as a means of deriving the

permeability. Irreducible water saturation is the amount of porosity that remains

containing water in a hydrocarbon bearing zone. Such water is present in isolated

pores not connected to the main permeable flow paths, or left adhered to grains

by capillary action and is not able to be removed from the rock. In certain cases

permeability may be estimated from imaging tools such as NUMAR’s NMRIL,

(Nuclear Magnetic Imaging Log).

Permeability is usually defined from the Darcy formula:

Where:

Q = 1cc volumetric flowrate

µ = 1 centipoise viscosity of flowing fluid

A = 1cm2 cross-sectional area

∆ p = 1 atmosphere/cm pressure gradient

L = 1 cm length of section

A permeability of one darcy is usually much higher than that commonly found;

consequently, a more common unit is the millidarcy, where: 1 darcy = 1000 mil-

lidarcy's

A practical oil field rule of thumb for classifying permeability is:

• poor to fair = 1.0 to 15 md

• moderate = 15 to 50 md

• good = 50 to 250 md

• very good = 250 to 1000 md

p A

LQk

∆×××

=µ


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• excellent = 1 darcy

Reservoir permeability is a directional property. Horizontal permeability (kH) is

measured parallel to bedding planes. Vertical permeability (kV) across bedding

planes is usually lower than horizontal. The ratio kH/kV normally ranges from

1.5 to 3.

When only a single fluid flows through the rock, the term absolute permeability

is used. However, since petroleum reservoirs contain gas and/ or oil and water,

the effective permeability for given fluids in the presence of others must be con-

sidered. It should be noted that the sum of effective permeabilities will always beless than the absolute permeability. This is due to the mutual interference of the

several flowing fluids.

Reservoir Permeability from Log Data

Timur Equation

Morris and Biggs

Where C is a constant as follows:

Gas: 80

Oil: 250

2

441360

irr

md Sw

k .

. φ =

2

3

irr

md Sw

C k

φ =


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Irreducible Water Saturation

This state is reached in hydrocarbon bearing zones when the reservoir will not

produce any water. It depends upon the Bulk Volume Water (BVW) which is cal-

culated from water saturation and porosity:

BVW = Sw x φ

When a zone’s bulk volume water values are constant, then the zone is at Swirr .

This is normally computed from cross-plotting Sw and Porosity on charts which

have hyperbolic lines indicating constant BVW values.

Water Saturation

The fraction of the pore space containing water is known as the water saturation,

and is given the notation Sw. The remaining fraction that contains oil or gas is

known as the hydrocarbon saturation, Sh, and is determined by 1- Sw, where 1 =

100% f.

Sw can be calculated from log interpretation, normally using a combination of

resistivity and porosity data.

Figure 6: Bulk Volume Water


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Formation Temperature

The resistivity of saline solutions is affected by temperature, so that corrections

must be made to raw data whenever the temperature has varied between data col-

lection points. This is particularly true when using Rmf or Rm information in sat-

uration or Rw calculations.

In order to determine formation temperature at any point the Geothermal

Gradient must be known. Unless known to be otherwise, this gradient is

normally assumed to be linear, and is computed from knowledge of Surface

Temperature and Bottom Hole Temperature as recorded from the logging tools.

Surface Temperature

This is an estimated value from offset data or general knowledge of the area. The

following rules of thumb can be applied in the absence of better data:

Offshore (1m beneath sea bed): 35°F: (1.5° C )

Onshore (3m deep): 50°F: (10° C )

Bottom Hole Temperature

BHT is calculated from the results of maximum temperature data obtained

during the logging runs. Since the actual formation temperature is disturbed by

the drilling process and the invasion of mud filtrate into the rock pores, the

maximum measured values may not be accurate. Over time, however, the mud

in the borehole and the invaded zone will tend to equalise to true formation tem-

perature. If this increase in temperature can be measured, (by looking at BHT

values obtained from successive logging runs), the rate of change of temperature

with time can be extrapolated to infinite time and an interpreted true BHT value

can be obtained.

There are many mathematical models available for this interpretation but the

most widely used method is an adaptation of the Horner Plot which was devel-

oped to interpret pressure buildup during formation testing operations.

Geothermal Gradient

Once estimates of Surface Temperature and Bottom Hole temperature have been

made, a geothermal gradient can be established as follows:

BHT Ts –

TVD S – ----------------------- 100× °F /100ft=


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Where:

BHT = Bottom Hole Temperature °F

Ts = Surface temperature °F

TVD = True vertical depth

S = Surface Depth

Invasion Effects

During drilling the mud pressure in the annulus is maintained at a higher level tothe pore fluid pressure in order to prevent fluid incursions and wellbore instabil-

ity. When drilling through permeable formations this means that, with water-

based muds, liquid from the mud passes into the formation displacing original

pore fluids. The solid particles in the mud are left behind and eventually form an

impermeable mud cake which seals the rock and prevents further invasion. The

amount of fluid invasion that occurs is dependent on many factors including mud

properties and rheology, flow rates, differential pressure and rock permeability.

The net result though is to produce an annulus in the rock around the borehole

which contains predominately mud filtrate rather than original pore fluids.

Log interpretation techniques must take this invasion into account, particularlywhen using resistivity tools to locate hydrocarbon bearing zones. If the tool does

not penetrate deeply enough into the rock only mud filtrate may be seen and sub-

stantial hydrocarbon reservoirs may not be recognised. MWD tools can have a

significant advantage in this respect since they log the formation very shortly

after it has been drilled and before invasion has fully developed, whereas

wireline tools may be run weeks after drilling, allowing invasion to run its full

course.

Proceeding outwards from the borehole the following profile is normally estab-

lished:

• Flushed Zone

Formation pore space has been predominately flushed by mud fil-

trate. Irreducible water or hydrocarbons remain in isolated pores or

by capillary action. Water displaces medium gravity oil quite well,

but low gravity oil or light gas quite poorly. In gas reservoirs there-

fore, residual hydrocarbon content in the flushed zone can be quite

high.


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• Transition ZoneSome of the original pore water and hydrocarbons, if present, have

been replaced by mud filtrate but significant quantities remain. The

ratio of mud filtrate to original fluids decreases away from the bore-

hole.

• Uninvaded Zone

This zone is furthest from the borehole and remains undisturbed by

mud filtrate invasion. Pore fluids are 100% original water or hydro-

carbons.

Resistivity Log Profiles

Resistivity Logs with multiple depths of investigation such as Dual Laterologs

or Dual Induction Logs will show variable resistivity profiles across the flushed and invaded zones depending on the relationship of mud water (Rmf) to pore

water (Rw) resistivity.

• Where Rw is greater than Rmf

(salty mud and fresh water pore fluids) the flushed zone will show

lower resistivity values than the invaded and uninvaded zones when

no hydrocarbons are present.

• Where Rw is less than Rmf

Figure 7: Invasion Profiles

R e s i s t i v i t y

Borehole Wall

Rxo

Ro

Step Profi le

dj

Distance

R e s

i s t i v i t y

Borehole Wall

Rxo

Ro

Transition Profile

dj

Distance


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(fresh mud and salty formation water), the flushed zone will showhigher resistivities than the invaded and uninvaded zones when no

hydrocarbons are present.

This invasion profile is normally considered to be a simple step profile for quick

look analysis, but in reality is more complex since the three zones will have tran-

sitional not sharp boundaries. However, assuming a step profile means that three

tools with different depths of investigation are required for full evaluation, in

order to identify and make corrections for the mud filtrate invasion. Figure 1-4

shows different Resistivity Log profiles and also includes the Annulus Profile

which may occur for a short time when hydrocarbons are present. In this casewater may be flushed more easily than the oil or gas and subsequently dumped

ahead of them as a ring or annulus of low resistivity, between the flushed and

uninvaded zones. If present this phenomenon is short lived and the fluids quickly

find equilibrium.

Log Presentation

Wireline Log data is presented as a series of curves representing the continuous

measurement of various parameters. Logs are usually presented as a combinationof several individual tools. Traditional logs might be, for example:

• ISF - Sonic:

Gamma Ray

Deep Induction Resistivity

Spherically Focused Resistivity

Sonic

• Dual Laterolog:

Gamma Ray

Deep Laterolog Resistivity

Shallow Laterolog Resistivity

Micro Spherically Focused Resistivity


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Log Types

There are two major types of logs:

• Acquisition Logs

These logs contain the raw data as measured by the tool. It is often

referred to as the "Field Print " and is an unmodified wellsite log.

• Processed LogsThese are edited logs, subjected to computer processing to correct

for borehole conditions, invasion etc., and may contain the results of

Quick Look Interpretation.

Figure 8: ISF-Sonic Log


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Figure 9: High Resolution Laterolog


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API Presentation

The traditional API presentation of field prints has three tracks separated by a

depth column.

Track 1, to the left, is linear and normally contains Gamma ray, S.P. and Caliper

log data.

Track 2, to the right of the depth column, is usually a 3 or 4-cycle logarithmic

scale used for plotting resistivity data. This might cover the complete width of

the sheet or their may be a third track.

Track 3, on the right, is usually a linear scale and is used for porosity, sonic and

density data.

Log Heading

A log heading is attached to the top of each paper log or film. It includes infor-

mation about the location, rig type, mud properties, calibration and tool type.

Depth Scales

Logs are plotted according to customer requirements and to maintain compati-

bility with other data. Typically they are plotted on a 1:500 or 1:1000 scale,

although this can be varied and detailed sections may be required at scales of 1:200. Indeed with modern computer processing it is possible to generate any

scale for any section of log very easily.

Logging Speeds

The ultimate quality of log data is very much related to logging speed. This is

particularly true for nuclear devices where statistical data is used. If the tool is

pulled too fast not enough data will be recorded to provide accurate information,

especially for thin beds. Normal logging speeds for tools containing nuclear

devices are around 1800 ft/hour (600m/hr).


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Figure 10: Log Header


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Logging Tool Combinations

Early logging tools were required to be run independently and, of course, during

the 1920s and 1930s there were fewer of them to run. By the 1950s and 1960s

the Gamma Ray, S.P. and basic resistivity tools were being supplemented by

Induction and Laterolog devices, sonic, density and neutron porosity tools. Still,

however, only certain combinations were possible and into the 1970s it was usual

to run at least two suites of logs, (resistivity and porosity) to obtain the basic

information followed by sidewall coring and pressure testing and fluid samplingtools.

With the development of Schlumberger’s triple combination tool, and similar

devices from the other leading service providers, it became possible to obtain

resistivity, porosity and gamma ray data from one logging run. The triple combo

tool though, at 90ft long and weighing around 1200 lbs was somewhat unwieldy

and less useful in tough logging conditions of high borehole inclination, severe

doglegs and sticky holes.

Figure 11: Log Presentation


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In the early 1990s Schlumberger developed the Platform Express Service which provides, in a tool only 38ft long, all the data from the old triple combo but using

better, modern sensors and electronics.

The following is a summary of the Baker Atlas and Schlumberger combination

tools:

Baker Atlas

FOCUS , from Baker Atlas, is the latest in high efficiency premium open hole

logging systems. All of the downhole instruments have been redesigned, incor-

porating advanced downhole sensor technology, into shorter, lighter, more

reliable logging instruments, capable of providing formation evaluation meas-

urements with the same precision and accuracy as the industry’s highest quality

sensors, at much higher logging speeds. Logging speeds are up to twice the

speed of conventional triple-combo and quad combo logging tool strings. The

logging system consists of the four standard major open hole measurements

(resistivity, density, neutron, acoustic) plus auxiliary services.

Service Application

• Array Resistivity (FOCUS HDIL) - includes real time 1-D radial inversion

processing for more accurate measurements of Rxo and Rt.

• Nuclear Porosity (FOCUS ZDL & FOCUS CN) - design changes improved

detector response and efficiency at high logging speeds of conventional

instruments, and enable production of a real time nuclear porosity cross-

plot log.

• Acoustic Slowness (FOCUS DAL) - offers an improved monopole signal

resulting in accurate compressional slowness values (Delta t) using a depth

derived borehole compensation technique.

• Auxiliary Measurements - Correlation Gamma Ray (GR), Borehole Temper-

ature, Downhole Tension, Mud Resistivity, Accelerometer (TTRmA), Two

Arm Caliper (TAC).


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Schlumberger

The Platform Express system is less than half as long as a triple-combo and

weighs about half as much, yet it gives you better, quicker and more accurate

answers—in real time. The use of integrated sensors, flex joints that improve pad

contact and other innovative technologies upgrade and expand traditional resis-

tivity and porosity measurements to include high-resolution micro-resistivity

and imaging measurements, plus tool movement measurements for speed correc-

tion and depth matching.

Figure 12: Baker Atlas Focus Log


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Introduction


Resistivity measurements are made with either the AIT* Array Induction Imager Tool or the High- Resolution Azimuthal Laterolog Sonde (HALS), both with a 12-

in. maximum vertical resolution.

Sensors for the Three-Detector Lithology Density (TLD) and Micro- Cylindri-

cally Focused Log (MCFL) measurements are integrated in the single pad of the

High-Resolution Mechanical Sonde (HRMS), which presses against the forma-

tion. The TLD log is a backscatter-type density measurement with 16-, 8- or 2-

in. vertical resolution. The MCFL Micro-resistivity measurement, which investi-

gates the same volume of the formation as the density measurement, has 2-in.

vertical resolution. Flex joints greatly improve pad application in rough holes.

The Highly Integrated Gamma Ray Neutron Sonde (HGNS) provides gamma ray

and neutron porosity measurements with a standard vertical resolution of 24 in.

Alpha processing is available to achieve 12-in. vertical resolution of the neutron

log.

Real-time speed correction and automatic depth matching of all measurements

are provided by an accelerometer for much faster turnaround on wellsite

processing.


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Figure 13: Platform Express


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Spontaneous Potential (S.P.) Logs


Introduction

The S.P. Log is a measurement of the electrical potential difference between a

moveable electrode in the borehole and a fixed electrode at the surface. It is used

to identify permeable zones and can be a very useful geological correlation tool

under the right conditions. To obtain meaningful results, the log must be run in

a water based mud borehole with a significant variation in mud filtrate and pore

water resistivity. The moveable electrode is attached to the cable, lowered to the

bottom of the borehole and pulled to the surface. Where there is no permeability,

no electrical potential exists between the rock and borehole and nothing is meas-

ured.

Origin of the S.P. Curve

At the bed boundary between a permeable and an impermeable rock, the mud

water and pore water are in contact via two interfaces. Along the permeable bed

the two waters are in direct contact. If there is a difference in salinity between the

two fluids chemical diffusion can take place across the interface. This is the dif-

fusion potential. If the mud water is less saline than the pore water then the +ve

sodium ions will tend to flow more freely to the higher concentration pore fluid

Figure 1: S.P Log


34/310



from the mud, leaving a greater concentration of -ve chlorine ions behind in the borehole.

Across the bed boundary alongside the impermeable shale the shale potential is

effective. Here the chlorine ions are more mobile through the semi-permeable

membrane and tend to leave a higher concentration of sodium ions behind in the

mud. Figure 3-1 illustrates this process, and shows that in this case there are four

quadrants around the bed boundary having opposite electrical charges which

creates the potential for an electrical current to flow. Note that the electrical

potential only exists at the bed boundary and that the current is focussed at the

bed junction. The same situation exists in reverse at the base of the permeable

bed.

The electrical potential at the boundaries between permeable and non-permeable

beds is measured on a millivolt gauge. If the mud water is less saline than the

pore water the reading will be a negative value on the millivolt gauge, and the

deflection across the bed boundary will show as a movement to the left on the

log. If the mud water is more saline than the pore fluid then the movement will

be to the right on the log, indicating a positive deflection. Where the two fluids

have the same salinity, no electrical potential will be measured and no deflection

will be seen on the log curves.

The value in millivolts has no absolute meaning but merely represents a changein electrical potential across the bed boundary. The logging engineer sets the

shale baseline either to the right or to the left of the track depending on the

relative salinities of the mud filtrate and formation water.

Log Presentation

The S.P. data is normally recorded on Track 1 of the log. The track is scaled in

millivolts, usually shown as mv/chart division. Sometimes there may be a full

scale shown such as -140 to +60. In this case there are 200 mv across the full

scale. Movement to the left from the shale baseline is a -ve movement, and

movement to the right is +ve.

Any deflection of the curve away from the shale baseline indicates rock perme-

ability. It is not possible to calculate the actual amount of permeability in Darcys,

nor does the S.P. deflection indicate the amount of permeability. However the

log will show interbedded sections of permeable and impermeable rocks, pick

out bed boundaries and formation tops and enable calculations of bed thickness

to be made.


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Corrections

Corrections need to made to raw data before any quantitative interpretation of

S.P. data is done. In particular thin beds and the presence of hydrocarbons will

cause the S.P. deflection to be under-developed. Also, since a current flows

around the bed boundary, the amount of energy stored in the system is dimin-

ished, resulting in lower S.P. deflections than might otherwise be the case. In practice, corrections to bed thickness should be made for sections less than 10ft

(3m) thick.

Log Characteristics

The ideal response would be a sharp, histogram type, curve as the change from

permeable to non-permeable beds was recognised. However, the tool is moving

and a current is flowing, both of which contributing to a spreading of the current

patterns and a diffusion of the curve. Bed boundaries are normally attributed to

Figure 2: Origins of the S.P curve

+++

+++------

Relative excesscharge

+

Formationwater

Mudfiltrate

Lower salinityHigher salinity

Shale

Sandstone

Formation Borehole

millivolts- +

S.P. Log

-


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the inflexion point of the curve. That is the straight part of the curve as the con-cavity changes direction.

The amount of deflection is reduced from its ideal response, the Static Spontane-

ous Potential (or S.S.P.), by the current flow and also in thin beds less than about

10ft (3m) thick. Algorithms and charts are available to make corrections for these

effects when performing quantitative analysis. The presence of hydrocarbons

will also reduce the current potential.

Quantitative Analysis

The S.P. Log is mainly used for qualitative interpretation of geology and for

inter-well correlation. The curves are generally very repeatable across the same

sequence and provide a tool similar in scope to the Gamma Ray Log. One major

quantitative use however, is in the calculation of Rw (formation water resistiv-

ity). This value must be known in order to make saturation calculations. It can be

measured from RFT samples or calculated from log analysis. The S.P. data

provides a means of performing this calculation, and can act as useful back-up

data if other methods are not available.

The amount of movement, in mv, of S.P. deflection away from the shale baselineis directly related to the difference in resistivity between the mud filtrate and the

pore water. Since the deflection can be read from the log and a value for Rmf,

(resistivity of mud filtrate) can be measured from a mud filtrate sample, the cor-

responding value of Rw can be calculated. Calculations of Rw are made in a zone

100% saturated with water, i.e where Sw = 1.0, as near as possible to the hydro-

carbon bearing zone being investigated. Rw is assumed to be constant through-

out the reservoir section.


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Figure 3: Schlumberger Chart SP-1


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Figure 4: Schlumberger Chart SP-2


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Gamma Ray Logs


Gamma Ray Log

The gamma ray log is a measurement of the natural radioactivity of a formation,

and is most often used as a shale indicator and for general geological correlation.

It is also used for depth matching of different suites of logs run at one casing

point.

The spectral gamma is used to provide more petrological information including

mineral suites, radioactive volumes and depositional environments.

On typical field prints, the Gamma Ray curve is located in Track #1, with scale

deflections in standard API units on a linear grid.

Most vendors use the mnemonic GR to represent the standard tool, though with

some variation.

Schlumberger:

• NGT: Natural Gamma Ray Tool

• NGS: Natural Gamma Ray Spectrometry

• HGNS: Highly Integrated Gamma Neutron Sonde (Platform Express)

Halliburton:

• HNGR: Hostile environment Natural Gamma Ray

• CSNG: Compensated Spectral Gamma

• PSG: Pulsed Spectral Gamma Ray

Baker Atlas:

• GR: Gamma Ray

• Focus-GR: Focus service Gamma Ray

Natural Gamma Ray

This log measures and records the natural radioactivity within a formation. Some

rocks are naturally radioactive because of the unstable elements contained in the

formation. Generally, three elements contribute the major portion of the radia-

tion observed in sedimentary rocks: the uranium series, the thorium series and

the potassium-40 isotope. The Gamma Ray log usually reflects the clay content

of sedimentary formations. Clean sands and carbonates normally exhibit a low

level of natural radioactivity, while shales tend show higher radioactivity.

However, not all shales are radioactive and not all radioactivity represents shales.


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Gamma Ray Logs


Figure 1: Gamma Ray Log (Reeves Wireline)


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Gamma Ray Logs


Natural Gamma Ray Spectral Log

The spectral log breaks the natural radioactivity of the formation into the differ-

ent types of radioactive material: thorium, potassium or uranium.

This can be used for stratigraphic correlation, facies identification, reservoir sha-

liness determination and sometimes for fracture identification.

Advantages of the Gamma Ray Log

• It is useful as a correlation tool

• It is used for depth control

• The major tool used for shale content calculations

• It may be run in casing, empty holes and in all kinds of drilling fluids.

Figure 2: Spectral Gamma ray Log

(Reeves Wireline)


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Gamma Ray Logs


Limitations of the Gamma Ray Log

• Traditionally the GR tool must be logged at relatively low speeds

(1800 ft/hr) to give accurate bed definitions. Some newer tools are

extending this to nearer 3600 ft/hr.

Radioactivity

Radioactivity is a spontaneous disintegration of atomic nuclei by the emission of

subatomic particles:

• alpha particles

• beta particles

or of electromagnetic rays

• X rays

• Gamma rays

Gamma Rays

The phenomenon was discovered in 1896 by the French physicist Antoine Henri

Becquerel when he observed that the element uranium can blacken a photo-

graphic plate, although separated from it by glass or black paper.

In 1898 the French chemists Marie Curie and Pierre Curie deduced that radioac-

tivity is a phenomenon associated with atoms, independent of their physical or

chemical state.

The Curies measured the heat associated with the decay of radium and estab-

lished that:

• 1 g (0.035 oz) of radium gives off about 100 cal of energy

every hour

This heating effect continues hour after hour and year after year. The complete

combustion of one gram of coal results in the production of a total of only about

8000 cal of energy.

Embedded in a nucleus, a neutron is usually stable—that is, it will not decay into

a proton and an electron. The nucleus itself is then stable. However, if the nuclear

conditions are not optimal, for example if the nucleus has too many neutrons, one

or more of the neutrons may decay to produce gamma rays.


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Gamma Ray Logs


Carbon 14

Every carbon atom contains six positively charged particles, (protons), in its

nucleus and six or more neutral particles, (neutrons).

The carbon atom's nucleus is surrounded by six negatively charged electrons.

The number of neutrons in a carbon atom's nucleus determines its isotope: atoms

of the same element that have different numbers of neutrons in the nucleus.

Carbon Dating

Three different isotopes of carbon exist naturally:

• Carbon-12 contains six protons and six neutrons and represents98.89% of all carbon

• Carbon-13 contains six protons and seven neutrons and represents

1.11% of all carbon

• Carbon-14 contains six protons and eight neutrons and represents a

negligibleamount of all carbon.

Carbon-14 is in a constant state of decay but, as long as an organism is alive,

ingesting more carbon, the balance between carbon-12 and carbon-14 remains

stable. When the organism dies, however, new carbon is not being taken in, and

so, as the carbon-14 decays, the ratio of carbon-12 to carbon-14 changes. Thehalf-life of carbon-14 is 5,730 years. This means that, after 5,730 years, half of

the carbon-14 will have gone. Therefore, the year of death of an organism can be

calculated from the proportion of carbon-14 left in a sample taken from its

remains. Although the proportion of carbon-14 has varied significantly during

the history of the Earth, correction tables have been developed to compensate for

this. In samples older than about 50,000 years, there will be insufficient carbon-

14 left to provide reliable results, and, conversely, recent samples will show too

little decay to provide reliable results.

Sources of Gamma radiation

As mentioned above, natural radiation from rocks comes from three sources, K,

U and Th. Whilst potassium (K) is the most abundant of the three elements in

rocks it produces less radiation than U or Th which, in relation to their weights,

produce more.

Gamma emission is usually found in association with alpha and beta emission.

Gamma rays possess no charge or mass, thus emission of gamma rays by a

nucleus does not result in a change in chemical properties of the nucleus but

merely in the loss of a certain amount of radiant energy. The emission of gamma


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Gamma Ray Logs


rays is a compensation by the atomic nucleus for the unstable state that followsalpha and beta processes in the nucleus.

The energy emissions occur in the range of 0-3 MeV, and the elemental origins

are determined by their peak frequencies within this range. The radiation from40K is distinct at 1.46 MeV. Thorium and Uranium produce radiation over a

wider spectrum but Th has a distinct peak at 2.62 MeV and U at 1.7 MeV. This

is the methodology used in spectral analysis to identify the source of radiations.

Radiation Detectors

The Gamma Ray Tool, which was introduced into the oil field in 1939, measuresnatural radioactivity of formations penetrated by the wellbore. Detection is

accomplished by the ability of gamma rays to produce tiny flashes of light in

certain crystals, which are then converted into electrical pulses. The pulse size is

dependent on amount of energy absorbed from the gamma ray.

The main types of detector are:

• Ionization Chamber

• Geiger-Mueller Tube

• Scintillation Counter

Ionization Chamber

This is a gas filled chamber with an anode maintained at approximately 100 volts

positive with respect to the housing. The case is filled with high pressured gas.

An incoming gamma ray interacts with the detector wall material and/or gas

which releases an electron. The freed electron moves toward the anode through

the dense gas. Electron interactions with gas atoms release additional electrons

(the ionization process). As the free electrons are drawn to the anode, a minute

current is produced, making the gamma ray influx into the borehole proportional

to the amount and magnitude of current pulses produced at the anode.

Geiger-Mueller Tube

The Geiger-Mueller counter is similar to the ionization chamber, but has much

higher voltages and a lower gas pressure. The initial reaction is much the same

as that of the ionization chamber; however, the high positive voltage (1,000

volts) at the anode causes the free electron to be fast moving as it collides with a

gas atom, discharging additional electrons. The secondary electrons are drawn

rapidly toward the positive wire which causes additional collisions resulting in

many more electrons reaching the anode in pulses which are more easily


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Gamma Ray Logs


detected. This ionization must be stopped or quenched because the cumulativeelectron showers can damage the detector. Quenching is achieved by lowering

the anode voltage.

Scintillation Counter

The most modern logging detector is the scintillation counter. It has two basic

components, a scintillating crystal and a photo multiplier tube. The transparentsodium-iodide crystal (NaI) will give off a minute burst of light when struck by

a gamma ray. The light energy strikes a photo sensitive cell or cathode which

causes electron emission. The electrons so produced are drawn to an anode

which, upon impact, releases additional electrons which are directed to another

anode. There are several stages of such amplification which finally give a suffi-

cient flow of electrons to be easily measured and recorded as an indication of the

gamma radiation penetrating the detector.

Figure 3: Geiger-Mueller Tube


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Gamma Ray Logs


Radius of Investigation

Ninety percent of the measured gamma rays originate with the first six inches of

the formation being investigated. The addition of another medium (i.e., cement

or casing) reduces the total quantity of gamma rays, but does not detract from the

usable information. With the proper speed and time constants, adequate resolu-

tion can be achieved in formations as little as three feet thick.

Formation boundaries are located at the mid-point of the recorded curve.

Units of Measurement

Gamma radiation is measured from the various detectors as discreet pulses of

electricity representing individual gamma ray “hits”. These are counted and

Figure 4: Scintillation Counter


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Gamma Ray Logs


averaged over a time period and may be reported in a number of ways includingBecquerels and Curies. However in borehole logging API Gamma Ray Units are

mostly used.

This relates to a test borehole at the University of Houston, Texas. The well is

surrounded by special high and low radioactive concrete. One API unit is 1/200th

of the difference in radioactivity measured in the two sections of concrete. “Reg-

ular” shales having a radioactive content of about 2.7% will exhibit values of

around 100 API units assuming the same operating conditions, (8½” hole, water

based mud etc.) are used. Obviously this varies with changing tool and borehole

environmental conditions and formation mineralogy.

Uses of the Gamma Ray Log

As discussed above, the gamma ray tool is used to:

• Identifying lithologies

• For correlation and depth matching

• For calculating shale volume

Lithology Determination

Radioactive isotopes of K, Th and U are the source of the gamma rays. These are

present in various minerals, particularly clay minerals. However, some evapor-

ites, for example, are also rich in K, and igneous and metamorphic rocks are very

radioactive. For Th and U content. Sands and carbonates whilst lacking radioac-

tive minerals in their pure forms can have significant amounts of associated

gamma producing minerals.

The heavy radioactive elements tend to concentrate in clays and shales. Gamma

rays (bursts of high energy, electromagnetic waves) are statistical in nature. This

means that the number of gamma rays received by the detector will fluctuate,

even when the instrument is stationary in the hole. These statistical variations are

averaged out.

Occurrence of Potassium (K)

Clay Minerals:

Illite 5.20%

Glauconite 4.5%

Kaolinite 0.63%

Smectite 0.225%


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Gamma Ray Logs


Evaporites:Sylvite 52.5%

Carnallite 14.1%

Polyhalite 12.90%

Muscovite Mica

Biotite Mica

Orthoclase Feldspar

Occurrence of Uranium

Origin: Acid Igneous Rocks

Preserved in: Reducing Conditions

Black Shales

Distribution: Erratic Peaks

Occurrence of Thorium

Origin: Acid and Intermediate Igneous Rocks

Preserved as: Detrital Grains

Zircon, Thorite, Epidote

Clay Minerals:Bauxite, Kaolinite, Illite, Smectite

The contribution to the overall radioactivity of the three elements is fundamen-

tally the same although, because of the variation in energy, a small quantity of

uranium has a large effect and a large quantity of potassium has a small effect.

The radiation from 40K has a single energy value of 1.46 MeV. Uranium and

thorium emit radiations over a wide spectrum but with some distinct peaks; 2.62

MeV for thorium and 1.7 MeV for uranium. As the gamma rays pass through the

formation, drilling mud and steel of the tool before hitting the detector their

energy levels will be degraded by Compton Scattering; however, the three peak


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Gamma Ray Logs


values noted above are usually distinct and form the basis of the spectral gammaray detector.

Quantitative Interpretation of Gamma Ray Logs

The Gamma Ray Log can be sued to give a quantitative assessment of clay

content of sandstone reservoirs in order to aid porosity and saturation calcula-

tions. Neutron log porosity values will be incorrect where there is significant clay

content in a sandstone because of the contained hydrogen within some clay

minerals such as Smectite.

Shale volume (Vsh) calculations begin with determining the Gamma Ray Index(IGR ).

where:

IGR = Gamma Ray Index (dimensionless)

GR = Gamma Ray Reading of Formation

GRmin = Minimum Gamma Ray (clean sand or carbonate)

GRmax = Maximum Gamma Ray (shale)

The calculated IGR is then used on the appropriate chart or determined

mathematically using:

Consolidated - Older rocks

Unconsolidated - Tertiary Rocks

IGR GR GRmi n –

G Rm ax G Rm in – -------------------------------------------=

V Sh

0.33 22 I GR×( )

1.0 – [ ]=

V Sh

0.083 23.7 I

GR×( )

1.0 – [ ]=


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Gamma Ray Logs



51/310

Resistivity Logs


Introduction

Resistivity logs were the first tools to be developed for wireline logging opera-

tions, and remain amongst the most important. They are also referred to as

Saturation Logs since their primary aim is to help with hydrocarbon evaluation.

The main uses of resistivity logs are:

• Identification of Hydrocarbon Bearing Zones

• Quantification of Hydrocarbon Saturation

• Identification of Permeable Zones

• Calculation of Diameter of Invasion

• Calculation of Porosity

Where Sw = 1.0

Resistivity tools measure how easy it is for an electrical signal to pass through

the formation. Rock grains and hydrocarbons are both insulators so the only

conductive part of the formation is salty water in the pore space. Hence, a porous

rock saturated with salty water will have low resistivity while the same rock

containing hydrocarbons will have a higher resistivity. High resistivity may also

indicate a low porosity rock, even if water saturated.

The log may also be used for geological correlation and, in association with other

petrophysical data, to help with lithological identification, environments of

deposition, facies analysis and overpressure detection.

Types of Resistivity Tools

The major types of resistivity tools are:

• Electrode Logs (conductive drilling fluids)

Normal Devices

Lateral Devices

Laterologs

Spherically Focused Logs

• Induction Logs (non-conductive drilling fluids)

• Micro Resistivity Logs

• Electromagnetic Wave Propagation LWD Tools


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Resistivity Logs


Log Presentation

Most modern Resistivity Logs are plotted in track 2 on a typical field print under

a logarithmic scale. The units of measurement of resistance are ohms. Resistivity

is measured in ohm-m2/m (ohm-m). In order to accommodate a sufficient range

of values a logarithmic scale of 0.2 - 2000 ohm-m is normally used, with a back

up scale of x10, (2 - 20000). Older, Normal or Lateral Logs were displayed on a

linear scale plot.

Figure 1: Dual Laterolog


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Resistivity Logs


Electrode Logs

Normal Tools

The first electric logs were called Normal Tools. A current is passed between two

electrodes (A & M) on the logging tool and the potential drop between them

indicates the resistivity.

Tool depth of investigation is a function of the distance between the two elec-

trodes on the tool. The larger the distance between electrodes, the deeper the

depth of investigation. Thus typical configurations were the 16" Short Normal

and the 24" Normal. The 16" Short normal was the basic tool and allowed inves-tigation of the invaded zone around the borehole.

Figure 2: Normal Electrode Logging Tool


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Resistivity Logs


To penetrate deeper into the formation and have a greater chance of measuringthe true, undisturbed formation resistivity (Rt), the 18’ 8" (5.68m) Lateral Log

was used. This large distance between electrodes was achieved by varying the

position of them and providing guard, or bucking, electrodes to focus the current

and force it to travel laterally from the tool rather than in a spherical nature,

resulting an a far deeper depth of investigation.

Using two tools with different depths of investigation enables evaluation of the

invaded zone to determine the extent of mud filtrate invasion and its affect on

formation resistivity. If three tools are used with different depths of investigation

then the diameter of invasion can be determined and corrections made for calcu-

lating true formation resistivity, which may still not be measured correctly by thedeepest reading tool where the amount of flushing is very large.

Figure 3: Lateral Electrode Logging Tool


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Resistivity Logs


Laterolog

The modern electrode log is called the Laterolog and is a refinement of the long

spaced Lateral log described earlier. It attempts to do the same job by further

refining the focused current with the use of even stronger guard electrodes to

ensure that the current is emitted laterally from the tool and penetrates far into

the formation.

One of the main reasons for the development of the Laterolog was to produce a

tool capable of giving good results in very saline water based systems. Obviously

in this case, the easiest route for the emitted current to take is to travel straight up

the borehole through the conductive drilling mud. No formation resistivity meas-

urements would be obtainable. The Laterolog minimises this process and results

in formation measurements being made.

Typically two Laterologs with different depths of investigation have been run

alongside each other. The LLD is a long spaced tool for measuring Rt, or close

to it depending on the extent of invasion. LLS is a medium spaced tool which

measures the resistivity of the invaded or transitional zone. These Dual Later-

ologs (DLL) are combined with a short spaced tool (Micro Resistivity) for

measuring the flushed zone. When the three readings are combined, full evalua-

tion may be made of the extent of fluid invasion and calculations made for

Diameter of Invasion and a correction factor for estimation of true formation

resistivity, Rt.

Modern laterolog tools have multiple transmitters and receivers to produce an

array of resistivity measurements with different depths of investigation and

vertical resolution.

Deeper investigating devices are usually centred in the borehole while the shal-

lowest reading tools designed to measure Rxo are mounted on a pad forced up to

and touching the borehole wall.


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Resistivity Logs


Figure 4: Laterolog Tool

A2

A1

M2

M1 A0

M'1M'2 A'1

A'2

Rxo pad

28ft


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Resistivity Logs


Baker Atlas HDLL

The Baker Atlas High Definition Laterolog provides up to eight resisitivity

measurements from 10”- 50” depth of investigation. This provides:

• More accurate formation resistivity, water saturation, and reserves estimates

• Better determination of movable fluids and recovery factor

• Improved evaluation of thinly bedded reservoirs

• Superior measurements in deeply invaded formations

• Detailed evaluation of the drilling fluid invasion profile

Schlumberger HRLA

The Schlumberger High Resolution Laterolog Array Tool provides five resisi-

tivity measurements together with a Micro-Cylindrically Focused Log (MCFL)

for flushed zone resistivity, Rxo for invasion profiling and Rt determination.

Figure 5: Schlumberger HRLA Log


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Resistivity Logs


Induction Logs

Induction logs were developed to obtain readings in non-conductive drilling

fluids, such as fresh water or oil based muds.

Transmitter coils produce magnetic fields by passing an AC current around

them. These magnetic fields induce electrical currents to flow in the formation

which in turn produce secondary magnetic fields. These are detected by the

receiver coils, their strength being proportional to the induced current flowing in

the formation. In this way the non-conductive fluid is by-passed and normal

resistivity measurements can be made. In fact the primary measurement made by

the tool is conductivity, which is converted to resistivity for log presentation.

This does mean that in heterogeneous formations the tool tends to give a slightly

low apparent resistivity value since the induced current swill be travelling

through the most conductive part of the rock.

Several transmitter and receiver coils are used to focus the current and to provide

multiple depth of investigation curves. These are given notations such as 6FF40,

which refers to 6 coils and an effective tool spacing of 40". As with Laterologs,

the longer the spacing the deeper the depth of investigation.

In general, induction logs tend to saturate out at lower resistivity values than

laterologs so are less happy in high resistivity environments but tend to give

better estimates or Rt with deep invasion.

Figure 6: Induction Tool


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Resistivity Logs


Schlumberger Array Induction Tool (AIT)

The Schlumberger AIT uses eight induction coil arrays operating at multiple

frequencies to produce a set if five resistivity logs with 1ft vertical resolution and

progressive radial investigations from 10”-90”.

Baker Atlas Focus High Definition Induction Log (HDIL)

The Baker Atlas Focus High Definition Induction Log also provides a set of five

resisitivity logs from 10”-90” depth of investigation, running at frequencies from

10-150 kHz.

Micro Resistivity Logs

Micro Resistivity Logs are special tools developed to measure the resistivity of

the flushed zone. They consequently have a very small depth of investigation,

usually a matter of centimetres, which is achieved by having very short spacing

between the electrodes. There are a number of different types of Micro Resis-

tivity tools; their use is dependent on the type of information required and their

compatibility with other tools. The following is a list of the most common types

of Micro Resistivity Logs although with modern tools such as the Schlumberger

Platform Xpress and Baker Atlas Focus service these are normally integrated

into the main suite of tools.

• Microlog (ML)

• Microlaterolog (MLL)

• Proximity Log (PL)

• Micro Spherically Focused Log (MSFL)

All of these logs have very short spaced electrodes for evaluation of the flushed

zone, but they are arranged in a slightly different manner. All of the micro logs

are pad mounted devices, which means that the array of electrodes are mounted

on a pad which is forced up to the side of the borehole by a spring loaded arm,making direct contact with the mud cake or borehole wall.

Microlog

The Microlog is unique in that it produces two curves which, whilst both only

penetrating the flushed zone, have slightly different depths of investigation. The

three electrodes are arranged so that there are two sets of spacing, a 1" and a 2"

set. The longer set enables a deeper penetration than the shorter.


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Resistivity Logs


The two curves are called the micro normal (2") and the micro inverse (1"). Sinceone curve penetrates deeper in to the flushed zone than the other it is less affected

by the resistivity of the mud cake than the other.

The overall effect is that, in the presence of mud cake, the two curves show

different values of resistivity and the traces on the log move apart. Where there

is no mud cake present, the two curves will show the same values and overlay

each other.

Separation of the curves will always indicate the presence of rock permeability

since no mud cake build up will be seen alongside impermeable rocks and

therefore the two curves will overlay each other.

The Microlog is a very old tool however, and seldom run in modern applications.

Its primary use was in evaluating very thin interbedded sand/shale sequences

where the sand laminations and thin beds could be quantitatively measured from

the nature of the Microlog. The sand count is the overall amount of sand in the

reservoir section being evaluated. Most of this application was relevant to certain

plays in the Gulf Coast area of the USA.

Micro Spherically Focused Log

The MSFL is the only micro resistivity log that may be combined with other

resistivity tools and run at the same time. The other micro logs need to be run asindependent logs and are thus very expensive. The MSFL is usually the only

micro log that is used in modern logging operations.

Embedded in an articulated neoprene pad, pushed up against the borehole wall

by a spring loaded arm, are a series of concentric metal rings containing the elec-

trodes. The arrangement is similar to the Laterolog but the focusing ensures that

only a few cms depth of investigation is achieved. By comparing the MSFL with

the shallow and deep Laterolog or induction log the diameter of invasion can be

calculated and a correction factor for Rt established. Because of the influence of

the mud cake on the Microlog readings, true resistivity of the flushed zone (Rxo)

can only be obtained after mathematical correction for the effect the resistivityof the mud cake.


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Resistivity Logs


Logging While Drilling Tools (LWD)

The earliest LWD tools used version of the traditional 16” Short-Normal tool for

resistivity measurements. This was a simple, tried and trusted tool with a shallow

depth of investigation. Since LWD tools log the well within minutes of being

drilled it was thought that invasion would not be a significant factor and therefore

a deep reading tool would not necessarily be required. However, being an

electrode type device it will only work in conductive, salty water drilling fluids.

In actual fact invasion can be an issue even with LWD tools since invasion can

happen ahead of the bit even before the section has been drilled and, with resis-

tivity tools often many metres behind the bit, slow drilling can result in signifi-

cant invasion. Additionally the need to run LWD tools with oil based mud precludes the use of short-normal devices.

In order to overcome these issues LWD Electromagnetic Wave Propagation

resistivity tools have been developed. These are similar to the Induction tools

used in wireline logging but work at higher frequencies and are able to offer

multiple depths of investigation, including deep reading devices for estimates of

Rt and better vertical resolution. Typical wireline induction tools work at 20

kHz, for example.

Figure 7: Micro Spherically Focused Log


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Resistivity Logs


The tool broadcasts a constant frequency propagation signal (either 2 MHz or 400 kHz) from the transmitting antennas into the formation. The signal travels

through the formation and is picked up by the receiving antennas. The resistivity

of the formation produces changes in the electromagnetic wave form: the wave

amplitude is attending and the phase is shifted as it passes through the rock. The

receiving antennas are able to measure these changes and the formation resis-

tivity is determined from both effects. EMR tools are able to work in all mud

types.

Wave propagation tools therefore provide, as a minimum, two resistivity curves:

• Amplitude Attenuation (Deep)

• Phase Shift (Shallow)

Figure 8: EMR Theory of Operation


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Resistivity Logs


Some generalities regarding EMR measurements are:

• Tools measure more accurately in conductive media

• Improved vertical resolution in conductive media

• Depth of investigation increases with increasing formation

resistivity

• Depth of investigation is deeper for the 400 kHz resistivities

than the 2 MHz resistivities

• Depth of investigation for attenuation resistivities is deeperthan phase difference resistivities

• Depth of investigation for long spaced resistivities is deeper

than for short spaced resistivities

• Depth of investigation for ratio and difference resistivities

is deeper than for raw measurements

• Depth of investigation order is as follows:

400 kHz >Rat 2> MHz >Rat 400 kHz> Rpd > 2 MHz Rpd

long spaced > short spaced attenuation > far amplitude > near amplitude

phase difference > far phase > near phase

• • Vertical resolution is better for 2 MHz resistivities than

for 400 kHz resistivities.

• Vertical resolution is better for phase difference

resistivities than attenuation resistivities.


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Resistivity Logs


Figure 9: 2 mHz Radial Response


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Resistivity Logs


Interpretation Concepts

Lithology Determination

By themselves resistivity tools are unlikely to define lithology directly. However

the resistivity response can indicate certain features and curve styles can help

with facies and environmental analysis.

Shales tend to have low - medium resistivity values (depending on clay miner-

alogy), perhaps around 1-2 ohm-m. Non-porous rocks such as coal and evapor-

ites will have high resistivities.

Deeper reading tools have large spacing between the transmitters and receivers

and will only pick out gross formation characteristics. Shallower reading tools

and micro-resistivity devices will show more detail in finely bedded shaly sand

sections and may pick out other texture-related features.

Separation of array resistivity tools will indicate invasion and, therefore, perme-

ability. Non-separation of curves may indicate that the rock is tight or it may be

porous and have been invaded with similar fluid. For example in a water

saturated zone when Rw is similar to Rmf.

Figure 10: 400kHz radial Response


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Resistivity Logs


Curve behaviour and trends may be useful for identifying grain size or clay-content variations where, again, the micro-tools will give more detail.

Fluid Saturation

In order to use the results of resistivity logs for quantitative saturation calcula-

tions the data must be combined with porosity and lithology information, since

this will also affect resistivity. Areas of high resistivity are possible hydrocarbon

bearing zones because oil and gas are effective insulators of electrical activity;

but only if the rock is porous.

If an increase in resistivity is caused only because of an increase in hydrocarbon

saturation then the amount of resistivity change can be used to estimate fluid

saturation. This is the basis of the quantitative analysis first proposed by Archie

in 1942 and used, albeit with modifications and enhancements, since.

Definitions

The overall, bulk rock, resistivity in the uninvaded zone is called Rt. It is

produced by the passive rock framework mineral and grain structure and by the

resistive or conductive pore fluids. Rt is derived from the deepest reading resis-

tivity tools but the apparent Rt values read directly from the log will often need correction for the effects of deep invasion by conductive drilling fluids.

The same bulk rock resistivity of the flushed zone is called Rxo and is measured

directly by the micro-resistivity tools.

The resistivity of the natural, or connate, water in a porous formation is called

Rw. This is determined by direct measurement of fluid samples obtained from

testing or by calculation from resistivity and porosity data.

The invaded zone primarily contains water from the drilling fluid, called mud

filtrate, and the resistivity of the zone is called Rmf .

When a formation is 100% water saturated with water of resistivity Rw its resis-

tivity, Rt , is termed Ro. The ratio of Ro/Rw is called the Formation Resistivity

Factor, F. The value of F, in water saturated formations, is independent of the

resistivity of the water with which it is saturated and varies only with porosity.

The value of Ro can be determined from:

Ro = F x Rw


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Resistivity Logs


Geosteering Applications

Logging While Drilling (LWD) resistivity tools can be very useful in geosteering

applications. Near-bit resistivity measurements, such as the Schlumberger RAB

tool, can indicate lithology and fluid changes whilst the variable depths of inves-

tigation of MPR tools can indicate distance to bed or distance to fluid contacts

when drilling ERD or horizontal wells. Drilling pilot holes and detailing

modelling of expected resistivity responses will need to be done to make best use

of the technology.

Figure 11: EMR Log


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Density Logs


Introduction

The Formation Density Log provides information on bulk formation density,

(ρ b). The Litho-Density Log with additional photo-electric absorption curvegives information about matrix type which can be a valuable aid in geological

interpretation and correlation.

The log is used quantitatively as a porosity tool, but is also useful in formation

pressure evaluation and rock mechanics work. It can also provide, indirectly,

information about hydrocarbon density.

Principle of Operation

This nuclear device measures electron density from which bulk density is

derived. The data is plotted on a linear scale as gm/cc, with each chart division

normally representing 0.05 gm/cc.

Collimated Gamma Rays

Collimated Gamma Rays are emitted from a chemical source such as Caesium-

137, with a high energy level of around 1.5 Curie. This is one of the radioisotopes

of caesium with an atomic mass of 137 and a half-life of around 30 years. It is an

artificial radionuclide which was released into the stratosphere by the above

ground testing of thermo-nuclear weapons in the 1950s and 1960s and deposited

as fallout.

The emitted particles are interfered with by electrons in the formation and

gradually lose energy. The rate of energy loss is an indication of electron density,

and can be measured at different energy levels. After initial pair production,

Compton Scattering is the dominant energy reducing process. This is similar to

the interaction of snooker or pool balls colliding sequentially and losing energy

as they do so and represents the mid-range energy levels.

Eventually, at very low energy levels, remaining gamma rays are absorbed by

mineral particles in a process called Photo-electric Absorption, (Pe). Pe ismeasured in barns/electron and each mineral has a particular Pe coefficient,

which is very nearly unique. Analysis of Pe values, which are recorded on the

Litho-Density Log, can help in identification of rock matrix when cross-plotted

against sonic, density or neutron porosity data.

Compton Scattering

Some energy from the gamma ray is imparted to an orbital electron of the target

atom resulting in a freed electron and a gamma ray of reduced energy and change

of direction.


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Density Logs


The number of scattered gammas available for detection depends on the electrondensity, ρe, of the material through which they have passed and the ability of anatom to scatter gamma rays increases as the number of electrons in its orbital

shells (i.e. atomic number Z) increases. Since Z/A approximates to 1/2 for most

materials the electron density ρe can be estimated as:

The normal calibration standard is done using limestone and fresh water filled

porosity which then means that the estima

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