Formation Evaluation

Lecture 1

Introduction to Well Logging

R.Evans, Department of Petroleum Engineering, Curtin University

1
Introduction to Well Logging
Introduction
Objectives of this course

• The objectives of this course are for students to become familiar with well
logs, well log interpretation and its application in the petroleum industry.

• This lecture provides an introduction to well logging, covering well logs,

the borehole environment, Archie's equations and mud logging. The
following lecture covers the storage properties of reservoir rocks.

• Subsequent lectures deal with the common tools used in formation

evaluation, covering the theory of how tools work, what they measure and
how the logs are interpreted.

• The final part of the course deals with well log interpretation and the
integration of core and log data for reservoir evaluation, and shows the
various techniques used in formation evaluation for determining the type
and quantity of fluid (hydrocarbons or water) in a reservoir .
R.Evans, Department of Petroleum Engineering, Curtin University
2
Introduction to Well Logging
Introduction to the Well
The Well

• There is much to be said about the well, whether it is

• drilling a well,
• logging a well, or
• understanding the borehole environment.

• These will be covered in enough detail to provide an introduction to

formation evaluation and the topics covered in subsequent lectures

• Mud logging will also be covered in this lecture

.
• Conventional cores and sidewall cores will also be discussed

R.Evans, Department of Petroleum Engineering, Curtin University

3
Introduction to Well Logging
Introduction to the Well
History of Well Logging

• Well logging traces its origins to 1927 when the Schlumberger brothers
introduced their ‘electric coring’ method in the Pechelbronn Field in
France.

• This technique was rapidly adopted in California (1932) and the Gulf Coast
(1933). Acoustic velocity logging (sonic logging) was developed in the
1940s and 1950s, initially as a support to seismic reflection surveying, but
was found to be a very effective wireline tool in its own right.

• Radioactivity logging was well established by the 1950s in oil field practice.
It is used in discriminating lithological types, as well as in formation density
determination.

R.Evans, Department of Petroleum Engineering, Curtin University

4
Introduction to Well Logging
Introduction to the Well
History of Well Logging

• A large variety of wireline logging tools are available to the explorer today,
and many provide very sophisticated interpretations of the formation,
providing details of formation lithology, porosity and fluid content.

• This course is an introduction to the wireline techniques used and the

interpretation of those techniques.

• Nowadays, many of the interpretations are performed by computer, and

presented to the end user as a graphical output of lithology, porosity, fluid
type and saturation, etc. Often the end user does not see how these
interpretations were made.

• This course emphasises the techniques of interpretation of the wireline

data to obtain useful geological and petrophysical information.

R.Evans, Department of Petroleum Engineering, Curtin University

5
Introduction to Well Logging
Introduction to the Well
Gathering Well Log Data

• Well log data is obtained by two different techniques

•(a) wireline logging, and

•(b) Measurement while drilling (MWD) logs.

R.Evans, Department of Petroleum Engineering, Curtin University

6
Introduction to Well Logging
Introduction to the Well
Wireline Logging

• The traditional technique is called wireline logging, and this involves

recording the well log data shortly after the well has been drilled.

• When a log is recorded it is called a ‘run’. There may be several runs of

the same or different tools over the lifetime of a well. A log run is typically
made at the end of each drilling phase, before casing is run in the hole.

• These logs are called ‘open hole’ logs (which will be the primary focus of
this course), as opposed to ‘cased hole’ logs. Usually, there are several
runs carried out over the levels of interest.

• Conversely, there may not be any other tools such as the sonic, density, or
neutron logs run in shallow overburden rocks, which are considered to be
to be non-reservoir, perhaps with the exception of a gamma ray.

R.Evans, Department of Petroleum Engineering, Curtin University

7
 Figure 1.1. Schematic illustration of the
basic wireline logging operation, conducted
onshore from a logging truck. The tool is
Introduction to Well Logging hoisted up the well on a wire cable shortly
after the well has been drilled.
Introduction to the Well
Wireline Logging Rig
Hoisting Winch Logging truck
• Wireline logs are pulled to the
surface by means of a hoisting
winch.

• Measurements are made by the

detector on the tool, and recorded
digitally by computers in the Wireline
(string)
logging hut.
(not to scale)
• Readings are made approximately
every 15 cm (0.5 ft).
Sonde (log tool)
• To run wireline logs the hole is
cleaned and stabilised, then the Rock
tool is lowered into the hole. Formations

R.Evans, Department of Petroleum Engineering, Curtin University

8
 Figure 1.2. Offshore logging
operations are conducted from a
logging cabin, where data is recorded
Introduction to Well Logging and monitored.

Introduction to the Well

Wireline Logging

Offshore Logging Cabin

Inside the Logging Cabin

R.Evans, Department of Petroleum Engineering, Curtin University

9
Introduction to Well Logging
Introduction to the Well
MWD Logs

• Logs measured while drilling are termed MWD, or LWD logs.

• These tools are sometimes risky and expensive to run.

• However, these tools theoretically have the advantage of measuring formation

properties before drilling fluids get the opportunity to invade deeply.

• Moreover, conventional wireline tools can be extremely difficult or impossible

to run in highly deviated wells.

• The measurements of MWD and wireline logs are quite similar, but MWD logs
frequently do not have the same range of tools.

R.Evans, Department of Petroleum Engineering, Curtin University

10
 Figure 1.3. Measurement while drilling
(MWD) allows instant measurement of the
reservoir. The example above illustrates a
Introduction to Well Logging long reach horizontal well (not to scale).
Figures taken from Baker Hughes Inteq
Introduction to the Well website.

MWD Logs

This figure illustrates an

MWD tool used to drill a
long reach horizontal well

A wireline log tool reading

is superimposed on the
well trajectory

(Diagram not to scale).

Figures taken from Baker

Hughes Inteq website.

R.Evans, Department of Petroleum Engineering, Curtin University

11
Introduction to Well Logging
Introduction to the Well
Logging Tools

• Well logging tools are designed to withstand extreme conditions encountered

in the borehole.

• These are essentially electrical tools, which also carry nuclear recording and
induction devices.

• Recordings are transmitted back to the surface via the logging cable. Logging
tools are made of galvanised steel which protects the measuring devices.

• Tools come in a variety of combinations, which are usually interchangeable.

Specialised tools have also been developed for slim holes.

R.Evans, Department of Petroleum Engineering, Curtin University

12
 Figure 1.4. Example of drilling and
logging tools.

Introduction to Well Logging

Introduction to the Well
Logging Tools
•Logging Tools are multifunctional, with several different tool varieties attached, and
can be tens of metres long.
B
A

(a) A rough neck

lines up the
drilling tool on
the drill floor.

(b) Example of
an array of
compact open
hole tools.

R.Evans, Department of Petroleum Engineering, Curtin University

13
Introduction to Well Logging
Introduction to the Well
Tool Types

The fundamental well logging tool types covered in this course are as follows:

• The caliper tool measures the diameter of a borehole, usually in inches, and is
useful for calibrating hole condition.

• The SP tool measures the potential differences between solutions of different

salinity in a borehole (drilling mud and formation). The units are millivolts. This
tool is useful for identifying permeable zones.

• The sonic tool measures the speed of sound in a well bore. The units are in
microseconds per foot (μsec/ft). The sonic tool is used to calibrate the well data
with seismic data, and can be used to calculate porosity.

R.Evans, Department of Petroleum Engineering, Curtin University

14
Introduction to Well Logging
Introduction to the Well
Tool Types

• The gamma ray tool measures the natural radiation of a formation in API units.
This tool is useful for lithology identification.

• The density tool measures the bulk density of a formation (matrix and fluid in
pore space together). The units are g/cm3 and this tool is frequently used to
measure the porosity of a formation.

• The neutron tool uses neutron bombardment to measure the amount of

hydrogen in a formation. The measurements are displayed in neutron porosity
units, or limestone porosity units, as a decimal or percent. This tool is used to
measure porosity and identify hydrocarbon zones. The neutron log is often used
in combination with the density log to determine lithology and fluid type.

R.Evans, Department of Petroleum Engineering, Curtin University

15
Introduction to Well Logging
Introduction to the Well
Tool Types

• Resistivity tools are electrical logging tools that measure the resistivity of a
formation. A wide variety of tools exists to measure resistivities deep, medium or
shallow distances into the formation. The units are ohm-metres. Resistivity tools
were developed to find hydrocarbons but can also be used to identify lithology
and for geological correlation.

• Dipmeter and borehole image logs are briefly covered. Dipmeter tools are
used to record geological information on the change in dip of beds. Borehole
image logs are electrical or sonic scans of the borehole wall that can identify
bedding, fractures and borehole damage.

• All of these tools are used in combination to evaluate a reservoir in an attempt to

determine the existence and quantity of hydrocarbons.

R.Evans, Department of Petroleum Engineering, Curtin University

16
Introduction to Well Logging
Introduction to the Well
Display of Log Data

• A good start to understanding well log data is being familiar with the display
of well logs and with the accompanying nomenclature.

• You should be familiar with:

•Log Tracks
•Log Headers
•Log Scales
•Log Depth
•Log Curves
•Log Units
•Linear and Logarithmic Scales

• The majority of logs are displayed with linear scales, but resistivity logs are
displayed on logarithmic scales.

R.Evans, Department of Petroleum Engineering, Curtin University

17
 Figure 1.5. Example of a typical well
log demonstrating the standard
nomenclature used on well log
Introduction to Well Logging displays.

Tops & Reservoir

Log Headers Log Units Log Scales
Zones

SP PEF DRHO SFLU

-20 MV 20 0 B/E 10 -0.4 G/C3 0.2 0.2 OHMM 2000

CALI TOPS RHOB ILM MSFL

2 IN 12 Depth 1.95 G/C3 2.95 0.2 OHMM 2000 0.2 OHMM 2000
METRES
SGR CNL ILD LLD DT
0 GAPI 100 0.45 V/V -0.15 0.2 OHMM 2000 0.2 OHMM 2000 140 US/F 40

20

40
Zone 1

Depth Group of Log Curves

Logs Log Tracks
Track (e.g. Resistivity)
R.Evans, Department of Petroleum Engineering, Curtin University
18
Introduction to Well Logging
Introduction to the Well
Depth Displays

• Note- You must be careful to understand the depth displayed on a well log.
Not all wells are vertical wells, and logs can be displayed in vertical depth, or
in measured depth, sometimes with the well deviation graphically displayed.

• The common terminology for different depth displays is as follows:

•measured depth (MD),
•true vertical depth (TVD)
•true vertical depth subsea (TVDSS)
•true vertical subsea (TVSS)

• True vertical depth subsea (TVDSS) and true vertical subsea (TVSS) depths
are referenced to the sea level datum. If the true vertical depth and the kelly
bushing (KB) or rotary table (RT) are known, then the TVDSS can be
calculated by subtracting the KB or RT from the TVD.

• TD is the total depth of the well.

R.Evans, Department of Petroleum Engineering, Curtin University
19
Introduction to Well Logging
Introduction to the Well
Depth Displays
ONSHORE OFFSHORE
RIG RIG
KB
KB

DATUM (e.g. SL) KB

Datum =Mean Sea Level
TVD

TVD
TVDSS

TVSS
MD

Or
MD
MD
MD
A B
R.Evans, Department of Petroleum Engineering, Curtin University
20
Introduction to Well Logging
Introduction to the Well Log Depth
Cores and Core Depths

• For proper geological and petrophysical analysis,

core depth has to be matched to the well logs.

• This process can sometimes be quite difficult, Core

especially if there are several cores, or some of the Depth
core is damaged or missing.
Core
• This is further complicated if the lithology is Matched
repetitive, with similar bed thickness, or if the to Log
lithology is homogeneous. Depth

• Incorrect depth matching can lead to significant

errors in well log interpretation.

R.Evans, Department of Petroleum Engineering, Curtin University

21
Introduction to Well Logging
Introduction to the Well
Well Deviation Surveys

• Well Deviation Surveys define the sub-surface path of the borehole. They
typically consist of a table that contains X and Y distance values from the well
surface location (KB).

• Surveys include two depth columns (TVD and MD), and possibly the borehole
trajectory deviation (DEVI) and Azimuth (AZIM).

• The deviation data is also known as inclination data. On deviated well paths,
the inclination and azimuth data are required to plot a well path correctly, and to
calculate TVDSS.

• Mistakes can be made during data loading so that wells are sometimes loaded
with the wrong co-ordinates. Well trajectories should therefore be checked
before log analysis.

R.Evans, Department of Petroleum Engineering, Curtin University

22
Introduction to Well Logging
The Borehole Environment
Borehole Environment

• The borehole environment is influenced by pressure, temperature and salinity

changes, both from the formation and the drilling fluid.

• Tools are designed to withstand the extreme conditions of elevated pressure

and temperature that can be encountered during drilling.

• Logging tools attempt to measure the virgin formation, but their readings are to
a greater or lesser degree influenced by mud invasion and the condition of the
hole.

R.Evans, Department of Petroleum Engineering, Curtin University

23
Introduction to Well Logging
The Borehole Environment
The Drilling Mud

• The main purpose of drilling fluid is to remove cuttings from the hole, but also to
exert sufficient pressure on the formation to prevent any hydrocarbons from
escaping into the borehole.

• The drilling mud also cools the drill bit and lubricates the drill pipe.

• The drilling mud is characterised by:

•(1) its density,

•(2) its viscosity and
•(3) its filtration rate.

• Another important property of the drilling mud that must always be measured is
its resistivity at a given temperature. This is in order to apply corrections to the
resistivity log to obtain true formation resistivity.

R.Evans, Department of Petroleum Engineering, Curtin University

24
Introduction to Well Logging
The Borehole Environment
The Drilling Mud

• There are three main categories of drilling fluid:

•(1) single-phase fluids, such as water-clay or oil base muds;

•(2) two-phase fluids, which have solid additives, and
•(3) air or gas.

• 1- Water-clay muds generally consist of a bentonite clay and water mix. Also
Lime Muds (used to drill through hydratable clays), Gypsum Muds (used to drill
through anhydrite sections) and Oil-Base Muds (to prevent clay swelling and
loss of permeability of shaly sandstones).

• 2- Two-phase drilling fluids, or Emulsion Muds, are used to improve the

lubricating properties of the mud. This gives longer bit life and decreases the
chance of the drill pipe sticking in the hole.

• 3- The last group of fluids is used when drilling low productivity targets.
R.Evans, Department of Petroleum Engineering, Curtin University
25
Introduction to Well Logging
The Borehole Environment
The Drilling Mud

• The resistivity of the mud can vary considerably, but is commonly in the range
of 0.1 to 1.0 ohm-metres at surface temperatures.

• Another quantity that is important in the interpretation of the electric log

response is the resistivity of the ‘mud filtrate’.

• The mud filtrate is the clay-free part of the drilling mud that enters the rock
formation during drilling.

• This mud filtrate solution usually displaces the formation fluid for a small
distance around the drill hole.

R.Evans, Department of Petroleum Engineering, Curtin University

26
Introduction to Well Logging
The Borehole Environment
Pressure

• The pressure environment during drilling, and during logging, is influenced by

the formation pressure and drilling-mud column pressure.

• Formation pressure is the pressure under which water, oil and gas are confined
in the subsurface.

• Formation pressures are usually somewhere between normal, hydrostatic

pressure and moderate overpressure.

• Hydrostatic pressure is simply due to weight of the fluid above the formation, so
that the framework grains of the formations and the overburden are self-
supporting.

R.Evans, Department of Petroleum Engineering, Curtin University

27
Introduction to Well Logging
The Borehole Environment
Pressure

• Overpressure exists for a number of reasons, but in all cases it means that the
formation fluids are being squeezed by the surrounding rocks.

• This is essentially related to the burial and compaction of sediments. If a

sediment compacts and the fluids are unable to escape from the porespace,
the pressure will increase and that formation becomes overpressured.

• The pressure of the drilling mud is hydrostatic and depends only on the depth
of the well, and the mud density.

• Mud pressures are generally maintained at slightly higher pressures than the
formation. Basically, the more the mud pressure exceeds the formation
pressure, the deeper the penetration of the mud filtrate into a permeable
reservoir.

R.Evans, Department of Petroleum Engineering, Curtin University

28
Introduction to Well Logging
The Borehole Environment
Pressure

• Under- and over-balance refers to the pressure balance relative to bottomhole

circulating pressure.

• Underbalanced drilling has become popular because it is cost effective, due to

lower bit and mud costs, increased rate of penetration, real-time formation
evaluation, and environmental/ economic benefits.

• However, the reservoir and its associated heterogeneities need to be well

understood.

• Drilling practice can vary from controlled overbalanced, to balanced (for

unconsolidated reservoirs) to controlled underbalance (for fractured reservoirs,
low pressure or low permeability gas reservoirs).

R.Evans, Department of Petroleum Engineering, Curtin University

29
Introduction to Well Logging
The Borehole Environment
Temperature

• Normal sedimentary basins show a general increase in temperature with depth.

• However, temperature increase is not linear as might be expected, but varies

according to the lithology and the thermal conductivity of the rocks.

• However, there is an overall persistent increase in temperature with depth. This

temperature increase is often expressed as a gradient, which is referred to as
the geothermal gradient. Typical ranges for geothermal gradients are between
20°C and 30°C per kilometre.

• The temperature of the borehole environment is influenced by the drilling fluid,

which is relatively cold compared to the formation.

R.Evans, Department of Petroleum Engineering, Curtin University

30
Introduction to Well Logging
The Borehole Environment
Salinity, and resistivity change with temperature

• The resistivity of most porous rocks changes with temperature.

• Ignoring salinity changes, resistivity change is due mostly to the way in which
the fluid resistivity in the rock’s pores changes with temperature.

• The two principal fluids, of which we should know the temperature and salinity,
are the mud filtrate and the formation water.

• These measurements are needed to calculate the resistivity of the formation

water and the mud filtrate. However, salinity of the pore water is probably the
major factor affecting a formation’s resistivity.

• The behaviour of water resistivity with salinity (NaCl concentration) is well

known. The salinity is often expressed in terms of ‘NaCl Equivalent’, which is
the case where a solution has ions other than Na+ and Cl-.

R.Evans, Department of Petroleum Engineering, Curtin University

31
Introduction to Well Logging
Drilling Fluids, Invasion and Formation Damage
Drilling Fluids and Mud Weight

• Rotary rigs require drilling fluid to cool and lubricate the bit, and to circulate the
drilled cuttings back to the surface.

• From the engineering standpoint, it is usually important to maintain a pressure

in the borehole slightly above the pressures in the formations to be penetrated.
This is a safety factor to avoid well blowouts.

• In many situations, water is the fluid used, and the hydrostatic head is sufficient
to provide a slight overbalance.

R.Evans, Department of Petroleum Engineering, Curtin University

32
Introduction to Well Logging
Drilling Fluids, Invasion and Formation Damage
Drilling Fluids and Mud Weight

• Some situations require an increase in mud weight because of formation

pressures known to be abnormally high.

• In the case of an exploration wildcat drilled in a remote area, the drilling

contractor and oil company may choose to increase the mud weight as a
precautionary measure.

• Different units of mud weight are used in different operating areas. Drilling with
overbalanced pressure can be damaging to reservoir properties immediately
adjacent to the borehole.

R.Evans, Department of Petroleum Engineering, Curtin University

33
Introduction to Well Logging
Drilling Fluids, Invasion and Formation Damage
Formation Damage

• With sufficient well pressure, the drilling fluid can invade deeply into the
reservoir or deep enough to cause clay swelling or other problems that hinder
or forbid drainage access for hydrocarbons when the well is put on production.

• Freshwater-based muds often cause a chemical reaction with clay materials

disseminated in the formation, which results in clay swelling or clay removal
and transport to other areas of the pore throats.

• Either circumstance can result in effectively eliminating permeable pathways.

Mud solids or particles injected into the pore throats with the invading fluids
cause a similar damaging effect.

• Formation damage immediately surrounding the borehole can severely diminish

the formation's ability to produce into the wellbore. The downhole behaviour of
drilling fluids has therefore become an important speciality area in the
petroleum industry.
R.Evans, Department of Petroleum Engineering, Curtin University
34
Introduction to Well Logging
Drilling Fluids, Invasion and Formation Damage
Formation Damage

• There are special circumstances where the fluid is maintained in an

underbalanced pressure condition, particularly when formation damage can be
expected from invasion of drilling fluids.

• Many wells are drilled underbalanced to keep water off swelling clays as much
as possible, i.e. clay swelling would inhibit production from the reservoirs.

• Most formation damage occurs near the borehole and creates a permeable
barrier between the gas or oil and the perforated entries into the well bore.

• Drilling with underbalanced pressure to avoid formation damage can

inadvertently affect log responses if gas bleeds from the formation into the
borehole fluid.

• Measurements most affected are the SP, neutron, and acoustic logs.

R.Evans, Department of Petroleum Engineering, Curtin University

35
Introduction to Well Logging
Drilling Fluids, Invasion and Formation Damage
Invasion

• Borehole tool measurements are often influenced by the drilling fluid, which
enters the formation and contaminates it in a process called invasion.

• The degree of invasion will generally be larger (and deeper) in porous and
permeable rocks, and with high mud weights.

• The amount of invasion gradually decreases away from the wellbore into the
formation. This invasion profile is represented in the following figure.

R.Evans, Department of Petroleum Engineering, Curtin University

36
Introduction to Well Logging
Invasion
• A thin mudcake lines the
borehole wall.

• The flushed zone is the

formation immediately adjacent
to the well bore, where the
formation fluid has been
displaced by the mud filtrate.

• The zone of transition is

between the flushed zone and
the uninvaded zone.

• The uninvaded zone represents

the formation with the original
formation fluid, unaffected by
the invasion of the drilling mud.

R.Evans, Department of Petroleum Engineering, Curtin University

37
Introduction to Well Logging
Invasion

• R stands for resistivity, and

S stands for water saturation.

• There are three basic

categories of readings in each
of the zones.

• 1) The resistivity of the zone

(square).

• 2) The resistivity of the water in

the zone (circle).

• 3) The water saturation in the

zone (triangle).

R.Evans, Department of Petroleum Engineering, Curtin University

38
Introduction to Well Logging
Invasion
Rt Resistivity of uninvaded
zone (true resistivity)

Rw Resistivity of formation water

Sw Water saturation of
uninvaded zone

Rxo Resistivity of flushed zone

Rmf Resistivity of mud filtrate

Sxo Water saturation of

flushed zone

Rmc Resistivity of mud cake

R.Evans, Department of Petroleum Engineering, Curtin University

39
Introduction to Well Logging
Drilling Fluids, Invasion and Formation Damage
Invasion

• Resistivity tools have been designed that attempt to measure each of these
zones, with short, medium and long reaching tools.

• The long reaching tools attempt to read the undisturbed formation, in the
uninvaded, or unflushed zone. These tools will be covered later in the course.

R.Evans, Department of Petroleum Engineering, Curtin University

40
Introduction to Well Logging
Archie’s Equations
• Gus Archie (1942) derived several equations on water saturation and resistivity,
which are used as the basis for modern quantitative log interpretation.

Formation Factor and Porosity

• Archie’s experiments showed that the resistivity of a 100% water-filled

formation (Ro), filled with a water having a resistivity of Rw can be related by
means of a formation resistivity factor (F):

Ro = F Rw , or

F = Ro / Rw

R.Evans, Department of Petroleum Engineering, Curtin University

41
Introduction to Well Logging
Archie’s Equations
• Formation Factor and Porosity

• The formation factor can also be related to porosity (ø) by the following formula,
where m equals the cementation exponent:

F=1/ø m

• The value of m varies with grain size, grain size distribution, and the complexity
of the paths between the pore space (tortuosity). The higher the value of
tortuosity, the higher the value of m.

R.Evans, Department of Petroleum Engineering, Curtin University

42
Introduction to Well Logging
Archie’s Equations
• Water Saturation

• Water saturation (Sw) is the percentage of pore volume in a rock which is

occupied by formation water.

• Water Saturation is measured in percent and is calculated as follows.

Water saturation (Sw) = Formation water occupying pores

Total pore space in the rock

R.Evans, Department of Petroleum Engineering, Curtin University

43
Introduction to Well Logging
Archie’s Equations
• Water Saturation

• Water saturation (Sw) is determined from the water-filled resistivity (Ro) and the
formation resistivity (Rt) by the following relationship, where n equals the
saturation exponent.

Sw = (Ro / Rt) 1/n

• The value of n varies from 1.8 to 2.5, but is commonly 2.

R.Evans, Department of Petroleum Engineering, Curtin University

44
Introduction to Well Logging
Archie’s Equations
• Water Saturation

• By combining the formulae, Ro = F Rw , and Sw = (Ro / Rt) 1/n, the water

saturation can be re-written in the following form:

Sw = (F Rw / Rt) 1/n
• This is the formula that is most commonly referred to as the Archie equation for
water saturation (Sw). All the present methods for interpretation involving
resistivity curves are derived from this equation.

• We will return to invasion later in the course to examine the impact of mud
filtrate on the displacement of formation water and hydrocarbons and its impact
on tool readings, especially the resistivity logs.

R.Evans, Department of Petroleum Engineering, Curtin University

45
Introduction to Well Logging
Mud Logging
• Introduction

• A mud log consists of the continuous monitoring of the drilling operation,

including the drilling mud and cuttings returns, and includes a wide variety of
data.

• The data collected, including computed parameters, are usually presented in

analogue (or digital) form versus depth.

• The final log output is quite variable in content and format and could probably
be more descriptively entitled a drilling operations log.

• Drilling data have been recorded as a function of depth since the infancy of the
oil industry, with published references to drilling time logs available from the
1880s. From these early beginnings, the sophisticated and indispensable
drilling operations log (mud log) has developed.

R.Evans, Department of Petroleum Engineering, Curtin University

46
Introduction to Well Logging
Mud Logging
• Introduction

• Today's logs are generally the continuous presentation of data from three
primary sources:

•(1) drilling operations

•(2) formation cuttings

•(3) mud data.

• The principal advantage of the mud log is that it is the first log available (except
for MWD or LWD logs). It is, therefore, a necessity on an exploratory well, but it
does have other applications.

R.Evans, Department of Petroleum Engineering, Curtin University

47
Introduction to Well Logging
Mud Logging
• Introduction

• This information should be an integral consideration in the formation evaluation

program. Therefore, the log analyst should be directly concerned (and involved)
with the drilling program, because it will affect his analysis.

• This is particularly important when cuttings and/or mud analysis are planned as
part of the drilling program.

• It is surprising how often cuttings and mud analysis logs are planned (at
considerable expense) without considering the influence of the mud program on
these analyses.

• Clearly, the log analyst should assist in the design of the mud program in order
to optimise the quality of the cuttings and mud analysis data.

R.Evans, Department of Petroleum Engineering, Curtin University

48
Introduction to Well Logging
Mud Logging
• Mud System

• Mud systems are usually designed by drilling fluid engineers whose primary
goal is to optimise the drilling operation. The drilling fluid program is designed to
allow optimised drilling rate at minimum cost. In some cases, the mud costs
may exceed 10% of the total well costs.

• In designing an effective mud system, there are at least ten important design
criteria that must be met:

• (1) Removing the cuttings from the bottom of the hole and carrying them to the
surface.
• (2) Cooling and lubricating the bit and drill string.
• (3) Sealing porous and permeable zones with an impermeable cake.
• (4) Controlling subsurface pressures.
• (5) Holding cuttings and weight material in suspension when circulation is
interrupted.

R.Evans, Department of Petroleum Engineering, Curtin University

49
Introduction to Well Logging
Mud Logging
• Mud System

• In designing an effective mud system, there are at least ten important design
criteria that must be met (continued):

• (6) Releasing sand and cuttings at the surface.

• (7) Supporting part of the weight of drill pipe and casing.
• (8) Reducing to a minimum any adverse effects upon the formation adjacent to
the hole.
• (9) Ensuring maximum information about the formations penetrated.
• (10) Transmitting hydraulic horsepower to the bit.

• A deficiency in performing some of these functions properly could have a

serious adverse effect on the quality of the data obtained at the surface and,
therefore, on the ability of the log analyst to analyse this data.

R.Evans, Department of Petroleum Engineering, Curtin University

50
Introduction to Well Logging
Mud Logging
• Mud Properties

• Important mud properties recorded by the mud engineer include:

•Viscosity
•pH
•Mud Weight
•Rheology
•Gel Structure
•Mud Salinity
•Fluid Loss

R.Evans, Department of Petroleum Engineering, Curtin University

51
Introduction to Well Logging
Mud Logging
• Mud Properties

• Viscosity is a measure of a fluid's resistance to flow. The viscosity of drilling

fluid is typically reported on wireline log headers. Viscosity typically varies from
the low to mid 40s to the mid to high 50s, but much higher viscosities are
occasionally encountered.

• pH values are a measure of the acid or alkaline condition of a substance. Neutral

solutions have a pH of 7, acid solutions are <7, and basic or alkaline solutions
are >7. pH is an important parameter in mud quality, and occasionally for
particular rock or casing cement cuttings that need to be investigated by the well-
site geologist.

• Mud Weight. It is essential that the differential between mud hydrostatic

pressure and formation pressure be kept as low as possible to maximise
penetration rate. Equalising the hydraulic pressures around the cuttings at the
instant of fracture prevents chip hold-down. Drilling rates decrease from air or
gas, to clear water, to mud, and get progressively worse as mud solids increase.
R.Evans, Department of Petroleum Engineering, Curtin University
52
Introduction to Well Logging
Mud Logging
• Mud Properties

• Rheology. To obtain best bottom-hole cleaning and the transport of chips to

the surface, rheology should be maintained to deliver maximum hydraulic
energy at the formation face.

• Gel Structure. A preferred fluid should possess a gel structure that forms
quickly, but to a fragile gel of limited total strength.

• Mud Salinity. Mud salinity plays an important part as different salts that make
up the salinity interact differently with formations. For instance, NaCl causes
swelling of certain shales and this in turn can cause pipes to get stuck and
cause drilling problems. KCl base muds are preferred under such
circumstances but this affects the gamma ray tool.

R.Evans, Department of Petroleum Engineering, Curtin University

53
Introduction to Well Logging
Mud Logging
• Mud Properties

• Fluid Loss. Water loss plays a very important part in obtaining accurate log
data that are critical to an analysis.

• High-water-loss muds tend to invade and flush deeper into the formation, which
influences log measurements used to evaluate the types and amounts of fluid
or gas saturation.

• High-water-loss muds effectively flush and invade different reservoirs with

filtrate of different salinities. Most flushing takes place as a spurt with the initial
penetration of the bit.

R.Evans, Department of Petroleum Engineering, Curtin University

54
Introduction to Well Logging
Mud Logging
• Mud Properties

• Fluid Loss. Massive loss of mud into the formation is rare because the mud
solids are filtered out onto the borehole wall.

• Muds should be treated to keep cake permeability as low as possible, thereby

maintaining a stable borehole and minimising filtrate invasion and possible
formation damage of potential reservoirs.

• High-mud-cake permeability results in thick filter cake that reduces borehole

diameter, causes several potential problems for the driller, and also inhibits
many log measurements.

R.Evans, Department of Petroleum Engineering, Curtin University

55
Introduction to Well Logging
Mud Logging
• Mud Properties

• Fluid Loss. Differential pipe sticking should be expected to be most prevalent

in recently drilled formations before a stable, mature matrix membrane has
formed.

• Shales do not possess permeability, therefore shale problems occur as a result

of wellbore stresses within shale or wetting by the liquid mud phase. Neither of
these conditions will be affected by fluid loss.

• If recurrent shale instability is encountered, which is not an abnormal pressure

phenomenon, a change in the mud system should be considered rather than
filtration lowering.

• To control these and other properties, a variety of additives are commercially

available, but care should be taken as they could have a detrimental effect on
the quality of the data being logged.

R.Evans, Department of Petroleum Engineering, Curtin University

56
Introduction to Well Logging
Drilling Fluid Types
Fresh water-based drilling fluids

• The most common drilling fluid is water-based, usually somewhat less saline
than the connate formation waters. The types of mud additives are widely
varied and are used for different purposes.

• Gels were introduced to eliminate or at least reduce caving and form mud
cakes to prevent formation damage.

• Barite is the most common weighting material.

• Fresh muds are generally those having a resistivity (at comparable temperature
to the formation temperature) of >3.5 times that of the formation water.

• The presence in the formation of illite, smectite, and mixed layer illite/smectite
clays, causes problems when water-based drilling fluids are used.

R.Evans, Department of Petroleum Engineering, Curtin University

57
Introduction to Well Logging
Drilling Fluid Types
Oil-based drilling fluids

• Numerous types of drilling fluids have been called oil-based, but variations in oil
percentages and other fluid additives exist, as well as their use globally.

• Due to environmental concerns a reduction in the use of Oil Base Mud systems
was seen in the late 90s. New muds using synthetic or natural oils are still
being developed.

• Today's oil-based fluids are made up of very expensive, specially refined oils
that are less toxic (<5% aromatics) than the diesel oils formerly used.

• Oil-based muds are used to achieve better borehole conditions (avoid

excessive washouts, possibly keep water away from swelling materials) and
also for their reusable qualities.

R.Evans, Department of Petroleum Engineering, Curtin University

58
Introduction to Well Logging
Drilling Fluid Types
Oil-based drilling fluids

• The application of oil-base mud is economically limited, but may have special
advantages and could be used for the following conditions:

(1) Drilling troublesome shales.

(2) Drilling deep, hot holes.
(3) Drilling and coring pay zones.
(4) Drilling salt, anhydrite, carnallite, and potash zones.
(5) As a directional drilling fluid.
(6) As a slim hole drilling fluid.
(7) In drilling hydrogen sulphide and carbon dioxide bearing formations.
(8) As a perforating and completion fluid.
(9) As a spotting fluid to free stuck pipe.
(10) As a packer fluid.
(11) As a workover fluid.
(12) For corrosion control.

R.Evans, Department of Petroleum Engineering, Curtin University

59
Introduction to Well Logging
Drilling Fluid Types
Salt-based drilling fluids

• Salt muds are commonly used in salt basins, which are geological basins having
thick beds of salt and other evaporites, because fresh water leaches the salt
beds, creating enormous washouts or cavities that create major problems. Most
of these problems are alleviated if high-salinity drilling fluids are used.

• However, high-salinity drilling fluids also cause excessive problems for certain
wireline measurements, as the fluid is exceptionally conductive and the borehole
signal from any logging device influenced by conductivity will be severely
affected.

• Bentonite is the most practical material for improving the viscosity and wall-
building properties of freshwater muds but it does not work effectively in salt
water.

R.Evans, Department of Petroleum Engineering, Curtin University

60
Introduction to Well Logging
Drilling Fluid Types
Potassium Chloride (KCL) drilling fluids

• Muds containing potassium chloride (KCI) and a suitable polymer are often
used to improve borehole stability.

• The potassium ion replaces the commonly used sodium or calcium ions to
inhibit clay swelling in the shales.

• Oil based muds sometime curtail formation evaluation efforts, and KCI muds
can be substituted for oil-based muds if other circumstances permit.

• However, KCL causes problems for the gamma ray log.

R.Evans, Department of Petroleum Engineering, Curtin University

61
Introduction to Well Logging
Drilling Fluid Types
Air or Gas-Drilled Holes

• In areas where air drilling occurs, the producing horizons are typically low-
porosity, low-permeability reservoirs.

• To avoid clay swelling or other formation damage immediately adjacent to the

borehole, operators may choose to drill with air.

• Many of the gas fields today in Appalachia and Arkansas (U.S.A.) are drilled
with air. Reefs in southwest Ontario (Canada) and shallow production wells in
Michigan (U.S.A.) are often drilled with cable tools.

• Economics and formation properties are the primary factors that determine the
drilling technique.

R.Evans, Department of Petroleum Engineering, Curtin University

62
Introduction to Well Logging
Drilling Fluid Types
Air or Gas-Drilled Holes

• Air-drilled holes also limit logging capabilities. Induction tools perform in the air-
filled borehole, as do density, gamma ray, and neutron devices.

• Electrode resistivity, SP, and acoustic measurements cannot be recorded.

Sidewall neutron devices are preferred.

• Production from the tight reservoirs is usually gas, and temperature surveys are
often a component of logging programs.

• Modifications to the air-drilling process are also attempted by using foam

agents or aerated muds.

R.Evans, Department of Petroleum Engineering, Curtin University

63
Introduction to Well Logging
Mud Logging
Cutting Analysis

• Cuttings samples are examined for lithology, porosity indications, and

hydrocarbon shows as a function of depth.

• The geologist's sample log finds its greatest application where sediments
contain many limestones, dolomites, and anhydrites in addition to sands and
shales (generally pre-Cretaceous).

• In these areas the drilling rate is slower, and as a result the cuttings are larger
(small chips), and the cuttings’ depth can be more readily identified.

• In ‘soft’ rock areas, typically sand/shale sequences where the potential pay
zones contain unconsolidated or poorly consolidated sands, the drilling rate is
much faster, resulting in poor cuttings returns (often only sand grains), which
are difficult to classify with respect to depth.

R.Evans, Department of Petroleum Engineering, Curtin University

64
Introduction to Well Logging
Mud Logging
Cutting Analysis

• Samples must be depth-correlated because there is a lag time from the time the
sample is cut until it reaches the surface.

• One method is to use a tracer material that is timed for the round trip after being
added at the surface. By subtracting the time from surface to bit, the lag time
can be determined.

• Sometimes lag time can be determined by using the drilling time log. When a
drilling break occurs, drilling can be halted. The time required for the samples to
be circulated out is a good measure of sample lag.

R.Evans, Department of Petroleum Engineering, Curtin University

65
Introduction to Well Logging
Mud Logging
Cutting Analysis

• Some causes of poor samples include:

•contamination (i.e. shale cavings),
•insufficient mud gel strength to lift the cuttings
•lost circulation problems where the samples are unrepresentative

• The greatest problem in poor sample quality, however, may be caused by

carelessness or human errors.

• Evaluation of the recovered samples includes the geologic analysis of rock

type, analyses for oil and gas shows, and analysis of rock properties such as
shale density. The quality of this information depends on the geologist and his
local experience.

R.Evans, Department of Petroleum Engineering, Curtin University

66
Introduction to Well Logging
Mud Logging
Cutting Analysis

• The samples are microscopically examined in either the dry or wet state. Wet
examination can assist in defining crystal formation and oolitic structure by
differential refraction.

• Two techniques are practiced:.

• Interpretive method in which the geologist selects representative cuttings,
and
• Percentage method in which the geologist uses all the cuttings in the
sample (except cavings) and describes the zone in a composite manner.

• In addition to the graphic presentation, a detailed written description of the

samples is also included, which describes the rock properties.

R.Evans, Department of Petroleum Engineering, Curtin University

67
Introduction to Well Logging
Mud Logging
Cutting Analysis

• Sample analysis, with respect to hydrocarbon shows, is another important part

of sample description.

• A number of tests are used to indicate hydrocarbon presence in samples

including:

• (1) oil staining,

• (2) hydrocarbon odour,
• (3) oil fluorescence (percent, intensity, and colour),
• (4) cut (visible cut and cut fluorescence),
• (5) cuttings gas analysis and
• (6) acid test in carbonates and calcareous sands.

• These different tests are covered in more detail in the notes.

R.Evans, Department of Petroleum Engineering, Curtin University

68
Introduction to Well Logging
Interpretation of a Mud Log

The following is a brief outline of the basic procedure for the construction and
interpretation of a mud log.

Lithology

• The most important consideration is to pin point the first occurrence of sand
(out of casing).

• This is achieved by examining the cuttings.

• An indication of lithology and permeability can be derived from looking at the

drilling rate.

• More permeable material will be friable and easier to drill through.

R.Evans, Department of Petroleum Engineering, Curtin University

69
Introduction to Well Logging
Interpretation of a Mud Log
Fluorescence

• The first occurrence of fluorescence is the key thing to look for.

• The cuttings may show an increase in fluorescence, a consistent fluorescence,

or a decrease in fluorescence.

• The fluorescence will be at a maximum in the main oil zone, and will be reduced
in the gas zone.

• Therefore, a consistent fluorescence indicates that you are still in the

hydrocarbon zone, and a second increase may indicate that you have
encountered gas, then oil.

• A decrease in fluorescence will indicate that you have left the hydrocarbon zone.

R.Evans, Department of Petroleum Engineering, Curtin University

70
Introduction to Well Logging
Interpretation of a Mud Log
Gas Shows

• This is a deviation in gas amount or composition from the gas background.

There should be a significant increase on the detector, but this does not
necessarily indicate a significant quantity of gas. Chromatographic analysis will
differentiate the different carbon components in their parts per million (ppm).

• C1 = Methane (CH4)
• C2 = Ethane (C2H6)
• C3 = Propane (C3H8)
• C4 = Butane (C4H10)
• C5 = Pentane (C5H12)

R.Evans, Department of Petroleum Engineering, Curtin University

71
Introduction to Well Logging
Interpretation of a Mud Log
Gas Shows

• Many seal rocks will generally have a high methane (C1) content, with C2 and
possibly C3.

• In the gas zone, C1 and C2 will increase dramatically, with a significant change
over background.

• In the oil zone, there will be an increase over background, but the compounds
should be heavier.

• Gas chromatographs are also used to indicate the wetness of a gas to quantify
the amount of condensate, or to indicate the presence of oil. Local knowledge
and experience can sometimes be used to predict the type of oil from this data.

• The above information is used to evaluate the mud log. This information is
useful when used in combination with the wireline log, as it can give a good
indication of lithology and fluid content.
R.Evans, Department of Petroleum Engineering, Curtin University
72
Introduction to Well Logging
Mud Logging
Example Mud Log

• This is an example of a mudlog from the

notes

• The various components of the mud log

are labelled from A - L

• These will be examined briefly

R.Evans, Department of Petroleum Engineering, Curtin University

73
Introduction to Well Logging

R.Evans, Department of Petroleum Engineering, Curtin University

74
Introduction to Well Logging

R.Evans, Department of Petroleum Engineering, Curtin University

75
Introduction to Well Logging

R.Evans, Department of Petroleum Engineering, Curtin University

76
 From DBS Core Services

Cores, core catcher and

coring bit

R.Evans, Department of Petroleum Engineering, Curtin University

77
 Diagram of conventional coring operation with steel barrel

R.Evans, Department of Petroleum Engineering, Curtin University

78
From DBS Core Services
 Diagram of conventional
coring operation using
fibreglass barrel

R.Evans, Department of Petroleum Engineering, Curtin University From DBS Core Services 79
 From DBS Core Services

Oriented core showing

scribe lines on core
spaced for
identification and to
indicate way up.
Orientation of primary
groove measured by
compass on core
barrel.

R.Evans, Department of Petroleum Engineering, Curtin University

80
CoreDrill can
produce 2 inch
diameter core
4.5 to 9 metres
long recovered
by wireline
without pulling
out of hole

From Baker Hughes INTEQ

R.Evans, Department of Petroleum Engineering, Curtin University
81
 Comparison between
changes in core fluid
saturation from downhole
to surface in conventional
core and sponge core for
water-based mud

R.Evans, Department of Petroleum Engineering, Curtin University From DBS Core Services82
 Core and sponge liner in plain and ultraviolet light

From DBS Core Services

R.Evans, Department of Petroleum Engineering, Curtin University
83