8/10/2019 Wireline Logging Full Report http://slidepdf.com/reader/full/wireline-logging-full-report 1/38 1. INTRODUCTION When deciding whether to develop a field, a company must estimate how much oil and gas will be recovered and how easily they will be produced. Although the volume of oil and gas in place can be estimated from the volume of the reservoir, its porosity, and the amount of oil or gas in the pore spaces, only a proportion of this amount will be recovered. This proportion is the recovery factor, and is determined by various factors such as reservoir dimensions, pressure, the nature of the hydrocarbon, and the development plan. The advantages of wireline logging are considerable. It allows the acquisition of valuable data at a fast rate and over a wide range of depths. This allows fast and accurate decisions to be made regarding drilling, based on the information obtained. Understanding the physical properties of an oil well is critical to properly managing it over its lifetime. Wireline logging makes that possible. In wireline logging time is a critical factor. The cost of running operations on an offshore drilling rig is very high: drilling a well might cost $1 –2 million per day of opera ons. In such opera ons, down me and logging - equipment failures are expensive. Well logging equipment costs are only a small part of the cost of drilling operations and generally a very small fraction of the hydrocarbon production costs. Modifications that improve the accuracy of logging without compromising reliability of the data are welcome in the industry even if they raise the cost. As a result, many techniques have been used for well logging. Several techniques are discussed in this report. Well loggers use combinations of both radiation-based and non-radiation-based tools (called nuclear and nonnuclear in this field) to examine the earth formations surrounding the well and sensors to detect the media’s response to interrogation tools. An analyst examines detector logs to look for some or all of the following parameters of the formation: formation water saturation, porosity, rock characteristics, carbon/oxygen ratio, and permeability. Because of the complexity of earth formations, only a combination of all the logs allows the log analyst to draw accurate conclusions for the formation parameters. For example, combining resistivity and nuclear logs, the log analyst can determine porosity, water content, and density (see fig(1.1)).
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When deciding whether to develop a field, a company must estimate how much oil and gas will be recoveredand how easily they will be produced. Although the volume of oil and gas in place can be estimated from the
volume of the reservoir, its porosity, and the amount of oil or gas in the pore spaces, only a proportion of
this amount will be recovered. This proportion is the recovery factor, and is determined by various factors
such as reservoir dimensions, pressure, the nature of the hydrocarbon, and the development plan.
The advantages of wireline logging are considerable. It allows the acquisition of valuable data at a fast rate
and over a wide range of depths. This allows fast and accurate decisions to be made regarding drilling, based
on the information obtained. Understanding the physical properties of an oil well is critical to properly
managing it over its lifetime. Wireline logging makes that possible.
In wireline logging time is a critical factor. The cost of running operations on an offshore drilling rig is very
high: drilling a well might cost $1 –2 million per day of opera ons. In such opera ons, down me and logging-
equipment failures are expensive. Well logging equipment costs are only a small part of the cost of drilling
operations and generally a very small fraction of the hydrocarbon production costs. Modifications that
improve the accuracy of logging without compromising reliability of the data are welcome in the industry
even if they raise the cost. As a result, many techniques have been used for well logging. Several techniques
are discussed in this report.
Well loggers use combinations of both radiation-based and non-radiation-based tools (called nuclear andnonnuclear in this field) to examine the earth formations surrounding the well and sensors to detect the
media’s response to interrogation tools. An analyst examines detector logs to look for some or all of the
following parameters of the formation: formation water saturation, porosity, rock characteristics,
carbon/oxygen ratio, and permeability.
Because of the complexity of earth formations, only a combination of all the logs allows the log analyst to
draw accurate conclusions for the formation parameters. For example, combining resistivity and nuclear
logs, the log analyst can determine porosity, water content, and density (see fig(1.1)).
Well logs result from a probe lowered into the borehole at the end of an insulated cable. The resultingmeasurements are recorded graphically or digitally as a function of depth. These records are known as
geophysical well logs, petrophysical logs, or more commonly well logs, or simply logs.
Wireline logging has a history that goes back just over 80 years to September 5th, 1927 when two brothers, Conrad and Marcel Schlumberger, ran what is considered to be the very first wireline log at
the Pechelbronn Oil Company oil field in France.
Their experimental logging attempt was a success and the brothers called their new technique an
electric survey. A few years later in the early 1930s in the USA the term "well log" was being used.
Wireline logging is so called because the logging tool is lowered through the oil well or borehole on
the end of a wireline.
1.2 Main principle:
The sensing element is fixed on the sonde, this element gathers information from the well and then entered
to a signal conditioning element to make the signal ready for transmission.The data from the sonde are
transmitted up the cable to instruments in the logging truck where they are recorded (field print). The data
are also processed later, and a cleaner log (final print) is made. The logging data are digitised (if was not
digital already), recorded on the hard drive, and sent to a logging company office (email), otherwise put on a
server or the Internet.
1.3 How to make a wireline well log:
To make a wireline well log after the well (a section) is drilled (and before setting casing), the hole is first
cleaned by the circulating drilling mud and then the drilling equipment is pulled from the well.
Then a sonde (probe) is lowered down the well (which is still filled with the drilling mud) on a logging cable.The logging cable is an armoured cable with steel cables surrounding conductor cables in insulation. It is
reeled out from the drum in the back of a logging truck.
The spontaneous potential log, commonly called the self-potential log or SP log, is a measurement
taken by oil industry well loggers to characterise rock formation properties. The log works bymeasuring small electric potentials (measured in millivolts) between depths in the borehole and a
grounded voltage at the surface.
It’s one of the first log measurements made. It was discovered as a potential that effected old electric
logs .It has been in use for over the past 50 years.
The change in voltage through the well bore is caused by a buildup of charge on the well bore walls.
Clays and shales (which are composed predominantly of clays) will generate one charge and
permeable formations such as sandstone will generate an opposite one. This build up of charge is, in
turn, caused by differences in the salt content of the well bore fluid (drilling mud) and the formation
water (connate water ). The potential opposite shales is called the baseline, and typically shifts only
slowly over the depth of the borehole. Whether the mud contains more or less salt than the connatewater will determine the which way the SP curve will deflect opposite a permeable formation. The
amplitudes of the line made by the changing SP will vary from formation to formation and will not
give a definitive answer to how permeable or the porosity of the formation that it is logging.
2.1 APPLICATIONS
The SP tool is one of the simplest tools and is generally run as standard when logging a hole, along
with the gamma ray. SP data can be used to find:
Correlation from well to well . Depth reference for all logging runs .
Detecting permeable beds (Where the permeable formations are ).
The boundaries of these formations Detecting bed boundaries .
Rw determination and the values for the formation-water resistivity .
The SP curve can be influenced by various factors both in the formation and introduced into the
wellbore by the drilling process. These factors can cause the SP curve to be muted or even inverted
depending on the situation.
Bed thickness (h), and true resistivity (Rt) of the permeable bed.
Invaded resistivity (Rxo) and the diameter of invasion (di)
Ratio of mud filtrate to formation water salinities - Rmf/Rw
A display of travel time of acoustic waves versus depth in a well. The term is commonly used as a synonym
for a sonic log. Some acoustic logs display velocity.
3.2 Formation evaluation:
A record of some acoustic property of the formation or borehole. The term is sometimes used to refer
specifically to the sonic log, in the sense of the formation compressional slowness. However, it may alsorefer to any other sonic measurement, for example shear, flexural and Stoneley slownesses or amplitudes,
or to ultrasonic measurements such as the borehole televiewer and other pulse-echo devices, and even to
noise logs.
3.3 Introduction
The sonic or acoustic log measures the travel time of an elastic wave through the formation. This
information can also be used to derive the velocity of elastic waves through the formation. Its main use is to
provide information to support and calibrate seismic data and to derive the porosity
of a formation. The main uses are:
Provision of a record of “seismic” velocity and travel time throughout a borehole. This information can
be used to calibrate a seismic data set (i.e., tie it in to measured values of seismic velocity).
Provision of “seismic” data for the use in creating synthetic seismograms.
Determination of porosity (together with the FDC and CNL tools).
The tool works at a higher frequency than seismic waves, therefore one must be careful with the direct
comparison and application of sonic log data with seismic data.
3.4 Wave Types
The tool measures the time it takes for a pulse of “sound” (i.e., and elastic wave) to travel from a transmitter
to a receiver, which are both mounted on the tool. The transmitted pulse is very short and of high
amplitude. This travels through the rock in various different forms while undergoing dispersion (spreading of
the wave energy in time and space) and attenuation (loss of energy through absorption of energy by the
formations). When the sound energy arrives at the receiver, having passed through the rock, it does so at
different times in the form of different types of wave. This is because the different types of wave travel with
different veloci es in the rock or take different pathways to the receiver. Figure 16.1 shows a typical
received train of waves. The transmitter fires at t = 0. It is not shown in the figure because it is masked from
the received information by switching the receiver off for the short duration during which the pulse is
transmitted. This is done to ensure that the received information is not too complicated, and to protect the
sensitive receiver from the high amplitude pulse. After some time the first type of wave arrives. This is the
compressional or longitudinal or pressure wave (P-wave). It is usually the fastest wave, and has a small
amplitude. The next wave, usually, to arrive is the transverse or shear wave (S- wave). This is slower than the
P-wave, but usually has a higher amplitude. The shear wave cannot propagate in fluids, as fluids do not
behave elastically under shear deformation. These are the most important two waves. After them comeRayleigh waves, Stoneley waves, and mud waves. The first two of these waves are associated with energy
moving along the borehole wall, and the last is a pressure wave that travels through the mud in the
borehole. They can be high amplitude, but always arrive after the main waves have arrived and are usually
masked out of the data. There may also be unwanted Pwaves and S-waves that travel through the body of
the tool, but these are minimized by good tool design by (i) reducing their received amplitude by arranging
damping along the tool, and (ii) delaying their arrival until the P-wave and S-wave have arrived by ensuring
that the pathway along the tool is a long and complex one. The data of interest is the time taken for the P-
wave to travel from the transmitter to the receiver. This is measured by circuitry that starts timing at the
pulse transmission and has a threshold on the receiver. When the first P-wave arrival appears the threshold
is exceeded and the timer stops. Clearly the threshold needs to be high enough so that random noise in the
signal dies not trigger the circuit, but low enough to ensure that the P-wave arrival is accurately timed.
Fig (3.1) The geophysical wavetrain received by a sonic log.
There are complex tools that make use of both P-waves and S-waves, and some that record the full wavetrain ( full waveform logs). However, for the simple sonic log that we are interested in, only the first arrival of
the P-wave is of interest. The time between the transmission of the pulse and the reception of the first
arrival P-wave is the one-way time between the transmitter and the receiver. If one knows the distance
between the transmitter (Tx) and the receiver (Rx), the velocity of the wave in the formation opposite to the
tool can be found.
In practice the sonic log data is not presented as a travel time, because different tools have different Tx-Rx
spacings, so there would be an ambiguity. Nor is the data presented as a velocity. The data is presented as a
slowness or the travel time per foot traveled through the formation, which is called delta t (t or T), and is
usually measured in s/ft. Hence we can write a conversion equation between velocity and slowness:
where the slowness, t is in microseconds per foot, and the velocity, V is in feet per second.
The velocity of the compressional wave depends upon the elastic properties of the rock (matrix plus fluid),
so the measured slowness varies depending upon the composition and microstructure of the matrix, the
type and distribution of the pore fluid and the porosity of the rock. The velocity of a Pwave in a material is
directly proportional to the strength of the material and inversely proportional to the density of the
material. Hence, the slowness of a P-wave in a material is inversely proportional to the strength of the
material and directly proportional to the density of the material, i.e.;
It was recognized that in some logging conditions a longer Tx-Rx
distance could help. Hence Schlumberger developed the long spacing
sonic (LSS), which has two Tx two feet apart, and two Tx also two feet
apart but separated from the Tx by 8 feet. This tool gives two
readings; a near reading with a 8-10 . spacing, and a far reading with
a 10-12 . spacing.
Fig(3.7) Long spacing sonic tools.
3.7 Calibration
The tool is calibrated inside the borehole opposite beds of pure and known lithology, such as anhydrite (50.0s/ .), salt (66.7s/ .), or inside the casing (57.1s/ft.).
3.8 Depth of Investigation
This is complex and will not be covered in great detail here. In theory, the refracted wave travels along the
borehole wall, and hence the depth of penetra on is small (2.5 to 25 cm). It is independent of Tx- Rx spacing,
but depends upon the wavelength of the elastic wave, with larger wavelengths giving larger penetrations. As
wavelength l = V / f (i.e., velocity divided by frequency), for any given tool frequency, the higher the velocitythe formation has, the larger the wavelength and the deeper the penetration.
1. Total count probes ( measures the concentration of Gamma rays).
2. Spectral probes ( measures the energy of each gamma ray).
4.5 Method of operation:
Natural gamma-ray tools are designed to measure naturally occurring gamma radiation in the earth
caused by the disintegration due to Potassium, Uranium, and Thorium. Unlike nuclear tools, these
natural gamma ray tools do not emit any radiation.
Natural gamma ray tools employ a radioactive sensor, which is usually a scintillation crystal thatemits a light pulse proportional to the strength of the gamma ray pulse incident on it. This light pulse
is then converted to a current pulse by means of a photo multiplier tube PMT where the current is
amplified about 1x106 times. From the photo multiplier tube, the current pulse goes to the tool's
electronics for further processing and ultimately to the surface system for recording. The data then
can be converted to energy spectra which can be easily read to find information about the well. The
strength of the received gamma rays is dependent on the source emitting gamma rays, the density of
the formation, and the distance between the source and the tool detector.
4.6 Main difference between neutron method and gamma ray method :
The natural gamma-ray tool has no source and detects the natural gamma rays that are present in therock formation outside the borehole.
A datasheet of a gamma probe is given at appendix A.
· Shale volume determination, in combination with the density tool
· Lithology indication, again in combination with the density log and/or sonic log
· Formation fluid type.
Depending on the device, these applications may be made in either open or cased holes.Additionally, because neutrons are able to penetrate steel casing and cement, these logs can be used
for depth tie-in as well as providing information on porosity and hydrocarbon saturations in cased
holes
An example of such a tool is API string tool from schlumberger (down, right) ,you can find more in
Electrical resistivity is a fundamental geophysical method used in both SURFACE and
SUBSURFACE geophysics. The method is legendary among Geophysical methods for exploration,
development and definition of existing targets .
Electrical resistivity is popular because it is a simple, low cost and efficient method. It is without
doubt the most practical, cost-effective logging method available today.
Most rock materials are essentially insulators, while their enclosed fluids are conductors.
Hydrocarbon fluids are an exception, because they are almost infinitely resistive. When a formation
is porous and contains salty water, the overall resistivity will be low. When the formation contains
hydrocarbon, or contains very low porosity, its resistivity will be high. High resistivity values may
indicate a hydrocarbon bearing formation.
A log of the resistivity of the formation, expressed in ohm-m. The resistivity can take a wide range of
values, and, therefore, for convenience is usually presented on a logarithmic scale from, for example,
0.2 to 2000 ohm-m. The resistivity log is fundamental in formation evaluation because hydrocarbons
do not conduct electricity while all formation waters do. Therefore a large difference exists between
the resistivity of rocks filled with hydrocarbons and those filled with formation water . Clay minerals
and a few other minerals, such as pyrite, also conduct electricity, and reduce the difference. Some
measurement devices, such as induction and propagation resistivity logs, may respond more directly
to conductivity, but are presented in resistivity.
6.2 Definition
By definition, resistivity is a function of the dimensions of the material being measured; therefore, itis an intrinsic property of that material. Resistivity is defined by the formula:
Where Electrical resistivity ρ is defined by:
Where Fig(6.1)
ρ is the static resistivity (measured in volt-metres per ampere, Vm/A);
E is the magnitude of the electric field (measured in volts per metre, V/m);
J is the magnitude of the current density (measured in amperes per square metre, A/m²).
The electrical resistivity ρ (rho) can also be given by,
where
ρ is the static resistivity (measured in ohm-metres, Ωm);
R is the electrical resistance of a uniform specimen of the material (measured in ohms, Ω);
is the length of the piece of material (measured in metres, m);
A is the cross-sectional area of the specimen (measured in square metres, m²).
Finally, electrical resistivity is also defined as the inverse of the conductivity σ (sigma), of thematerial, or:
6.3 Method of operation
Resistivity logs measure some aspect of the specific resistance of the geologic formation. There are
about 17 types of resistivity logs, but they all have the same purpose which is to measure the electric
conductivity fluid in the rock. Electrical resistivity (also known as specific electrical resistance or
volume resistivity) is a measure of how strongly a material opposes the flow of electric current. A
low resistivity indicates a material that readily allows the movement of electrical charge.
In these logs, resistivity is measured using 4 electrical probes to eliminate the resistance of thecontact leads with 2 current electrodes and 2 measurement electrodes. The log must run in holes
containing electrically conductive mud or water .
6.4 Basic Principle:
The principles of measuring resistivity are illustrated in fig (6.2). If 1 amp of current from a 10-V
battery is passed through a 1-m3 block of material, and the drop in potential is 10 V, the resistivity of
that material is 10 Wm. The current is passed between electrodes A and B, and the voltage drop is
measured between potential electrodes M and N, which, in the example, are located 0.1 m apart-, so
that 1 V is measured rather than 10 V. The current is maintained constant, so that the higher theresistivity between M and N, the greater the voltage drop will be. A commutated DC current is used
to avoid polarization of the electrodes that would be caused by the use of direct current.
Fig (6.2). Principles of measuring resistivity in Ohm-meter. Example is 10 Ohm-meter.
There are 3 different configurations of resistivity log:
short-normal: has the smallest distance between 2 adjacent electrodes (40 cm (16 in or less)). It is
the most sensitive to thin layers but is also influenced by the drilling mud, short normal devices are
considered to investigate only the invaded zone
long-normal: long normal (162 cm (64 in)) devices are considered to investigate both the invaded
zone and the zone where native formation water is found
lateral: Lateral log has the longest distance between two adjacent electrodes (18 feet 8 inches). It
samples resistivity over a large section of sediment/rock away from the borehole. Lateral log may
miss thin beds.
6.5 Types of resistivity logs:
There are many different types of resisitivity logs, which differ primarily in how far into the rocks
they measure the resistivity. Because drilling fluids tend to force their way into the surrounding rock,
resistivity logs with shallow depths of investigation are unable to see beyond an "invasion zone" to
determine the true formation water resistivity of permeable rocks. Instead, these logs measure thelower resistivity of the contaminated zone. Thus, by pairing logs with deep and shallow depths of
investigation, it is possible to measure permeability by looking at the resistivity diffences between
the logs. The acronyms of some of the more popular resistivity logs are listed below:
AIT (Array Induction Tool) - the resistivity log of the future. It measures five depths
of investigation.
DIL (Dual Indiction Log) - a frequently used log with deep and medium depths of
investigation.
DLL (Dual Laterolog) - a frequently used log with deep and medium depths of
investigation.
LAT (Lateral Log)- an obsolete log with a deep depth of investigation.
LN (Long Normal) - an obsolete log with a deep depth of investigation.
SFL (Spherically Focused Log) - a frequently used log with a shallow depth of
investigation.
SGR (Shallow Guard Log) - a frequently used log with a shallow depth ofinvestigation.
SN (Short Normal) - an obsolete log with a shallow depth of investigation.
resistivity logging systems may be calibrated at the surface by placing fixed resistors between theelectrodes. The formula used to calculate the resistor values to be substituted in the calibration
network shown in fig(6.3) .
Figure( 6.3). System for calibrating resistivity equipment