gamma ray

LWD-20-70-0000-02-03 COMPANY CONFIDENTIAL

Section 03: Ver 01-2006 INTEQ, A BAKER DIVISION 3 - 1

Gamma Ray

Table of Contents

Section Description Page Rev Date

Service Quality

Gamma Ray ...................................................................................................................... 2 Overview ............................................................................................................... 2

Depth of Investigation ............................................................................... 2 Vertical Resolution.................................................................................... 2 Statistical Precision................................................................................... 3 Dual Azimuthal Gamma Service............................................................... 3 Average Count Rate ................................................................................. 5 Well Path Constraints ............................................................................... 7

Presentation .......................................................................................................... 9 Calibration ........................................................................................................... 10

Primary Calibration ................................................................................. 10 Spectral Biasing...................................................................................... 11 Master Calibration................................................................................... 11 Wellsite Verification ................................................................................ 13

Environmental Corrections .................................................................................. 14 Borehole size and mud weight................................................................ 14 Potassium content .................................................................................. 14 Corrections at the wellsite....................................................................... 14

Typical Log Response......................................................................................... 20 Other factors affecting gamma response................................................ 21 Correlation .............................................................................................. 22 Comparison to Wireline .......................................................................... 22

Summary ............................................................................................................. 24

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COMPANY CONFIDENTIAL LWD-20-70-0000-02-03

3 - 2 INTEQ, A BAKER DIVISION Section 03: Ver 01-2006

Gamma Ray

Gamma Ray

Gamma ray scintillation detectors are incorporated into several instruments, including collar based and legacy probe based tools, including:

Collar Tools Probe Tools OnTrak 4 ¾", 6 ¾", 9 ½" RNT 6 ¾", 9 ½"

MPR 3 ⅛", 6 ¾", 9 ½"

Collar tools - 2 x detectors (180o apart) under hatch covers RNT OnTrak

Two Gamma sensor packages are housed under two hatch covers located 180 degrees from each other to permit dual gamma measurements. With the scribe line on the high side, Gamma #1 is 90 degrees to the left.

Probe tools - probe centered inside the collar SRIG (1 ¾” probe) used with various MPR collars NaviGamma (2” probe) used with NaviTrak Navi-185 (1 ¾” probe) Legacy TELECO probe based tools (3” probe)

Overview In sedimentary rocks the major contribution to natural gamma ray emission primarily comes from the potassium isotope 40K, and uranium, and thorium which exists in clay minerals such as illite and montmorillonite. Each element emits gamma rays which are distinctive in both number and energy:

40K decays to 40Ar - peak 1.46 MeV 238U decays through a series of isotopes - peak 1.76 MeV 232Th decays through a series of isotopes - peak 2.62 MeV

The total gamma ray measurement is a combination of the potassium, uranium and thorium elements present in the rock and fluid of any particular geological horizon. Higher gamma ray readings typically, but not always occur in front of shale beds. Applications include depth correlation with other logs, determination of stratigraphic profiles and the estimation of shale content in reservoir rocks.

Depth of Investigation It is estimated that 90% of detected gamma rays come from within 6 inches of the formation. The equivalent depth of investigation (50% level) is therefore somewhat less than 6 inches from the borehole wall.

Vertical Resolution Bed resolution and factors affecting it are important to all well logging methods. Vertical bed resolution of the gamma log is a function of: logging speed (R), detector length (L) and data acquisition time (DT).

All gamma-ray logging instruments make measurements over a characteristic sampling period by use of a time constant (DT), which in LWD is referred to as the acquisition time window. This time window is chosen to be long enough to reduce statistical fluctuations inherent in all nuclear measurements, but short enough to preserve boundary resolution. Generally this is set to 4 x 2.5 sec (10 second sample



Gamma Ray

rate) for both memory and real-time data. Azimuthal gamma ray measurements require an extended sample time of 25 seconds for acquiring data over 8 sectors.

Statistical Precision The discrete nature of nuclear counting takes the form of a Poisson distribution, and the standard deviation is given by the square root of the number of counts.

where, N = total counts

Therefore statistical precision is improved by increasing the count rate. For passive nuclear measurements such as gamma ray, statistical precision can be improved by reducing logging speed (i.e. rate of penetration), increasing detector size, or averaging multiple passes. However, the time-depth conversion and fixed level spacing conversion effectively places a limit beyond which slower ROP’s will not result in an improvement in vertical resolution.

Vertical averaging can also improve the appearance of statistical data, but can result in a loss of vertical resolution.

Dual Azimuthal Gamma Service The azimuthal gamma service (gamma elite) uses two gamma detectors positioned 180 degrees apart which acquire counts as the tool rotates. Combining count rates from two detectors improves statistics and provides redundancy. A magnetometer within the tool senses the Earth’s magnetic field relative to the tool-face, enabling counts to be divided into eight azimuthal sectors (numbered 0 to 7). Sectors are oriented to the tool-face using inclination and azimuthal information from the MWD platform being run with the tool (e.g. MPR, OnTrak). High frequency tool-face updates allow the sectoring process to function at rates of up to 400 rpm.

While rotating, the counts detected in each sector are binned over a defined acquisition time. Counts are divided by the time per sector in order to obtain a value for counts per second.

Adjacent sector data can be combined into quadrants for transmission to surface in real-time (Figure 1).

Real-time data transmission is user selectable – Up/Down, or Up/Down/Left/Right

Quadrant Sector Up 7+0 Down 3+4 Left 5+6 Right 1+2

Bed boundary detection at low contrast (<30 API) is possible with 25 sec default acquisition time.

Acquisition time is user configurable and can be changed by downlink for bed contrasts < 15 API.

Azimuthal gamma data can be acquired at rotational speeds of up to 400 rpm. Directional information comes directly from the OnTrak navigation package.

0

1

2

34

5

6

7

D

U

L R

N=σ

Figure 1 – Azimuthal Gamma Sector Allocation



Gamma Ray

The example below illustrates the relative shift between upper and lower quadrant gamma curves as the tool passes through a formation boundary with significant apparent dip.

Sector allocation is based upon the following criteria:

If Inclination is < 5 degrees – directional package transmits magnetic tool face (MTF). Image logs are presented relative to North - i.e. scaled 0-360 degrees across 10 chart divisions.

If inclination > 5 degrees – directional package transmits gravity tool face (GTF). This information is derived from the magnetometers using the azimuth and inclination as determined by the last “flow on” survey. Image logs are scaled relative to high side - i.e. High-Low-High across 10 chart divisions.

The OnTrak navigation package (directional package) calculates and transmits tool face values at 60 Hz (once every 16 msec).

Real-time status flags Real-time status bits are transmitted to surface to indicate:

Lack of a valid flow-on survey

Inadequate tool face information

Consecutive tool face measurements that are not realistic

Magnetometer vector sum Hx and Hy that is too small (occurs when wellpath coincides with Hz axis)

The Gamma Elite service requires borehole azimuth and inclination data from the flow-on survey to derive gravity tool face data from the rotational magnetic tool-face measurement. A full survey is taken when the tool is powered up in non-rotating mode. The last good power-on survey is stored, allowing data to be written to memory even if no survey is taken after a power-on sequence (assuming an initial valid survey is taken). If a bad or “outdated” survey (i.e. a survey that does not properly reflect the actual borehole azimuth and/or inclination) is recorded, the real-time data observed at surface will be incorrect. In the unlikely event that this should occur, sector allocation can be corrected through post-processing.

Note The accuracy of the sectoring process depends on the accuracy of the survey.

0102030405060708090

100110120130

3195 3196 3197 3198 3199 3200 3201 3202 3203 3204 3205 3206 3207

Gamma Top Quadrant

Gamma Bottom Quadrant

Gamma Top Quadrant

Gamma Bottom Quadrant

Y

X

Y

X

Y

X

Gamma Bottom

Gamma Top

Sand

Shale

XX

Example:Bit OD: 8 1/2”Gamma Top Quad. Change@ 3201 mGamma Top Quad. Change@ 3203.2 mAverage Inclination: 88°Incident Angle: Atan (Y/X)= 5.6°Formation Apparent Dip: 90°- inc + Atan (Y/X) =7.6°

Figure 2 - Example showing UP/DOWN quadrant data for an inclined bed



Gamma Ray

If a survey is inadvertently influenced by tool rotation (not allowing enough time to take the survey) gamma data will not be correctly assigned to appropriate tool-face sectors.

For field operation, it is not necessary to acquire a full survey before rotating the pipe. A full survey is taken within 19 seconds of power-up (9 seconds system configuration and 10 seconds survey).

However, to verify that a survey has been taken, the operator must verify the data transmission sequence received at surface, including the survey FID (format identification parameter). The FID number represents the survey format structure for the service being run (for example, 0 or 2 for AutoTrak, 0 or 4 for OnTrak).

This tool acknowledgement provides a good indication that a full survey has been acquired and the driller can be informed to start rotating. Survey data should immediately be checked for accuracy. If there is any reason to doubt survey quality, the survey must be retaken to ensure proper sectoring. The accuracy of the full survey should be verified before the drill pipe is rotated – unless constrained by the customer.

Average Count Rate For quadrant data, the count rates of both gamma sensors are “normalized” for the API correction factors and calculated for each of the four quadrants by applying:

Where,

CR1 = count rate gamma sensor 1 CR2 = count rate gamma sensor 2 API1 = API correction factor for gamma sensor 1 API2 = API correction factor for gamma sensor 2

The normalized count rate for each of the four quadrants is transmitted to surface. At surface each count rate is multiplied by the averaged API correction factor in order to obtain API values for each quadrant:

Sectored CPS values for real-time telemetry can be derived using:

Both detectors (“normalization method”) for improved statistic reliability, or selecting

Channel A or channel B (if one detector fails)

Note To ensure valid survey data is acquired, the wellsite engineer must adhere to standard survey

procedures after each stand of drill-pipe has been drilled down.

1

221

22211 −

+

∗∗+∗

=APIAPIAPICRAPICRCRQuadrant

221 APIAPICRAPI QuadrantQuadrant

+∗=



Gamma Ray

In the event of a single detector failure, OnTrak Elite auto-detects the failure and automatically switches to “single scintillator mode” for real-time transmission.

Low and high gamma thresholds for mode switching are set by the wellsite engineer, prior to running in hole. If a gamma measurement falls outside the set range a status bit is set and the tool uses only counts from the remaining detector. The unit is “cps” or counts per second (not MWD API units). The recommended low and high threshold values are 1 and 150 respectively.

The downlink to select one specific or both detectors for real-time transmission has no effect on the azimuthal gamma measurement which is controlled by the auto-detect-functionality.

Apparent and corrected API values are derived from the base counts for each of the 4 sectors by the surface system (RTProc):

Base Apparent Corrected GRBUX → GRAUX → GRCUX

GRBDX → GRADX → GRCDX

GRBRX → GRARX → GRCRX

GRBLX → GRALX → GRCLX

The ‘X’ suffix denotes real-time curve data. U, D, L, R refers to Up, Down, Left, Right quadrants respectively.

Selection of which quadrants to transmit in real-time is user configurable.

For special applications such as real-time geosteering and reservoir navigation, data can be displayed as curve and image plots that allow instantaneous apparent and true dip picking.

RigLink (v.2.0 and above) can display quadrant information for real-time imaging. Figure 3 shows a gamma ray image log on a RigLink user screen.

Figure 3 - User configurable RigLink display screen with 4 quadrant real-time gamma data displayed as log curves in track 1 and as an image plot in track 5



Gamma Ray

Well Path Constraints

Magnetic dip is a measure of the angle between the magnetic field lines and a tangent to the surface of the Earth. This angle depends on geographic location but it is always perpendicular at the magnetic poles and becomes almost parallel to the Earth’s surface close to the equator. It is important to note that by convention, the dip angle is considered to be positive in the geomagnetic Northern hemisphere and negative in the geomagnetic Southern hemisphere. Also note that the geomagnetic hemispheres generally approximate to the geographic hemispheres but are not exactly equal – the magnetic North Pole is not at the same location as the geographic North Pole.

The magnetometer will not operate correctly when the tool axis is parallel to the magnetic field lines.

The following examples show where these conditions could occur. Since all measurements have a certain degree of error the tool may be affected at slightly different values. It is therefore necessary to assign a window within which the azimuthal measurement should be regarded as unreliable. The window of error is defined as + 5 degrees of the theoretical well inclination and + 5 degrees of magnetic North (or South). The client should be informed that data quality will be affected at inclinations that fall within the window of error.

Case 1: Well azimuth is between 355o and 5o of magnetic North

When logging due North, the borehole inclination where the tool face is unreliable = 90o – dip angle

Example 1: Location Northern hemisphere, magnetic dip: 60o Inclination where tool-face is unresolved is 90o – magnetic dip = 90o-60o=30o Considering the inclination error window we will consider data from 25o to 35o inclinations

to be unreliable.

Example 2: Location Southern hemisphere, magnetic dip: -10o Inclination where tool-face is unresolved is 90o – magnetic dip = 90o-(-10o) = 100o Considering the inclination error window we will consider data from 95o to 105o inclinations

to be unreliable. Such high inclinations are rarely seen, but shown here for demonstration.

Case 2: Well azimuth is between 175o and 185o of magnetic South

When logging due North, the borehole inclination where the tool-face is unreliable = 90o + dip angle

Example 1: Location Southern hemisphere, magnetic dip: 30o Inclination where tool-face is unresolved is 90o + magnetic dip = 90o+ 30o = 120o Considering the inclination error window we will consider data from 115o to 125o

inclinations to be unreliable. Such high inclinations are rarely seen, but shown here for demonstration.

Note Caution must be exercised when well planning azimuthal imaging jobs. The magnetometer packageis a two axis device used to determine magnetic tool-face of the package itself and subsequently thegravity tool-face of the short and long spaced detectors. Since the magnetometer measures only thexy transverse component of the magnetic field, there are limitations on its application. Correctoperation depends on the geographic location of the well, the borehole inclination, azimuth andmagnetic dip at that specific location.



Gamma Ray

Example 2: Location Southern hemisphere, magnetic dip: -20o Inclination where tool-face is unresolved is 90o + magnetic dip = 90o+ (-20o) = 70o Considering the inclination error window we will consider data from 65o to 75o inclinations

to be unreliable.

It is important to confirm the well plan before drilling commences. The customer should be made aware of the possible implications to image quality due to magnetometer limitations.

Determine the correct well location (ie whether in the geographic Northern or Southern hemisphere) and use the dip value by the BGGM program to determine the geomagnetic hemisphere. Assigning a positive or negative sign to the dip value, based on geographic hemisphere may be incorrect.

Use the latest released version of BGGM within the Advantage system to obtain an accurate value of magnetic dip. The magnetic dip value can change over time as the magnetic poles move. In some locations the change is small, however there are locations that can show large rates of change, especially close to the magnetic poles themselves.

Magnetometer functionality

Sector positions are derived from an out of phase sinusoidal relationship of voltage from X and Y magnetometers as the tool rotates through the Earth’s magnetic field, at an update frequency of 200 times per second.

The following information must be programmed into the tool prior to running in hole:

Local magnetic dip (72 degs nominal value in North Sea)

Total local magnetic field strength ~ 49500nT Inclination and Azimuth is required from the AutoTrak directional module.

The Azimuthal density calculations, being magnetically derived are also subject to certain physical limitations:

Drilling normal at an angle at right angles to the Earths magnetic field, the XY Axis magnetometers do not get enough information to calculate sector positions. (For the North Sea this is approximately equal to 20 degrees inclination due South or 110 degrees inclination due North)

Drilling close to magnetic anomalies such as casing, debris, drilling past lost tools etc. will cause a distortion of the sector positions.

Note Whenever azimuthal data is acquired, the wellsite engineer must initiate an LWD Image Service QC and Processing Summary form. The form provides an image analyst with critical information

regarding tool configuration and a record of events during acquisition.

A copy of the form is located in the Appendix



Gamma Ray

Azimuthal Data Quality Control

The quality control of azimuthal data is discussed separately in the Image Quality Control Guide – See Appendix C.

Factors that have an impact on azimuthal data quality include:

Pre-job planning - primary and critical to the acquisition program Survey accuracy - correct survey procedures must be followed Drilling practices - ROP, BHA configuration, stick-slip and bit selection Borehole geometry Resampling of data Interpolation of data

Presentation Standard curves for single and dual detector gamma ray instruments are listed below. Gamma is normally scaled 0-100 or 0-150 API (location or customer dependent). Mnemonics are updated as new services are introduced. Users should refer to www.bakerhughesdirect.com in order to verify that they have the latest information. Data can be acquired at the surface in real-time, or stored downhole in tool memory. Curve naming convention is in the form xxx$ where $ is specified below: xxxM Memory xxxX Real time

Primary Optical Curves - Single detector GRAM Gamma Ray - Apparent API GRBM Gamma Ray - Base cps GRCM Gamma Ray - Corrected MWD-API GRAFM Gamma Ray - Filtered API GRCFM Gamma Ray - Borehole Corrected - Filtered API

GRIM Gamma Ray - Data Point Indicator point GRSIM Gamma Ray - Sliding Indicator no units GRTM Gamma Ray - Time Since Drilled minutes GRDDM Gamma Ray – Data Density pts/ft

0 150Gamma Ray(API)



Gamma Ray

Primary Optical Curves - Dual detector (additional to above) GR1AM Gamma Ray - Apparent - Detector 1 MWD-API GR2AM Gamma Ray - Apparent - Detector 2 MWD-API GR1BM Gamma Ray - Base - Detector 1 cps GR2BM Gamma Ray - Base - Detector 2 cps GR1CM Gamma Ray - Corrected - Detector 1 MWD-API GR2CM Gamma Ray - Corrected - Detector 2 MWD-API GR1AFM Gamma Ray - Detector 1 - Apparent - Filtered API GR2AFM Gamma Ray - Detector 2 - Apparent - Filtered API GTFM Gamma Tool-face degrees

Primary Optical Curves - Azimuthal Gamma (additional to above)

GRAM Gamma Ray API GRAS0M Azimuthal Gamma Ray - Apparent - Sector 0 API GRAS1M Azimuthal Gamma Ray - Apparent - Sector 1 API GRAS2M Azimuthal Gamma Ray - Apparent - Sector 2 API GRAS3M Azimuthal Gamma Ray - Apparent - Sector 3 API GRAS4M Azimuthal Gamma Ray - Apparent - Sector 4 API GRAS5M Azimuthal Gamma Ray - Apparent - Sector 5 API GRAS6M Azimuthal Gamma Ray - Apparent - Sector 6 API GRAS7M Azimuthal Gamma Ray - Apparent - Sector 7 API

Calibration

Primary Calibration The unit of measure is the API gamma ray unit, based on an artificially radioactive concrete block located at the University of Houston, Texas, USA. The block is defined to have a radioactivity of 200 American Petroleum Institute (API) units, a value considered to be twice the radioactivity of a typical shale and sufficient to provide the required measurement span.

Most LWD tools do not fit into the API pit, therefore secondary calibration standards have been established.

The values of the blocks are defined by wire-line instruments calibrated in API GR pit

Reference condition is a 10" borehole filled with 8.33 lb/gal water (chosen for tool fit)

All LWD tools read the same value in the granite block

6" boreholes are used for the verification of corrections

Limestone blocks are used to evaluate the effect of different spectral composition



Gamma Ray

Spectral Biasing Potassium (K), uranium (U) and thorium (Th) are the three most abundant, naturally occurring radioactive elements. Each one emits gamma rays that have characteristic energy levels. A typical spectrum is shown in Figure 4.

Potassium (40K) - 1.46 MeV

Uranium (238U) - 1.76 MeV

Thorium (232Th) - 2.62 MeV

In contrast to most wireline tools, logging-while-drilling (LWD) tools have thicker housings that cause a different spectral response to the three sources of radioactivity, and therefore a different total gamma ray reading in certain formations. To compensate for this logging-while-drilling tools are calibrated using a routine that corrects for spectral biasing effects.

The gamma calibration block is the primary standard for calibrating gamma ray logs. However, even when properly calibrated, different gamma ray tools will not necessarily have identical readings downhole because their detectors can have different spectral sensitivities. The only time they will read the same is when the downhole formation contains the same proportions of uranium, thorium and potassium as the Houston standard.

Master Calibration Gamma ray tools are calibrated in the shop using a portable standard and a software routine called "GammaCal", which accounts for spectral biasing and tool type.

Background counts are determined with a non-radioactive calibrator “blank” in place. The blank calibrator is then exchanged for one containing a gamma ray source with a spectrum equivalent to the API pit, and a ten minute count rate is performed. The reference standard assumes that water is present between the calibrator and the tool; a correction is therefore applied because water is not used in the shop calibration procedure.

The figure shows a calibrator positioned over an OnTrak gamma ray sensor hatch.

INTEQ gamma ray tools do not differentiate between uranium, thorium or potassium gamma rays, they measure only total gamma ray counts. However, potassium gamma rays having lower energy than those associated with uranium and thorium, are attenuated more rapidly by the steel of the tool body itself. Lower energy gamma rays have a higher cross section of interaction with surrounding atoms, and tend to be filtered out of the formation gamma spectrum. The variation in response of LWD tools where the spectrum reaching the detector is biased toward higher energies is referred to as "spectral biasing".

The GammaCal calibration routine is designed to reduce or eliminate this spectral biasing effect.

1.46 MeV - Potassium

1.76 MeV - Uranium

2.62 MeV - ThoriumCou

nts

Energy (MeV)

Figure 4 – Typical gamma ray spectrum



Gamma Ray

Two types of portable calibrators are used for calibrating INTEQ gamma ray tools. There are slight variations in activity for same size calibators. For example a 10” Amersham may have an assigned API value of 265 in one location, whereas a 10” Amersham in another location may have an API value of 263.

10” Amersham calibrator

8.5” Amersham calibrator

The 10” calibrator originates from TELECO and 8.5” calibrator originates from EXLOG. In order to assign values to both types of calibrators we have measured the count rates for gamma ray sensors, gamma1 and gamma2, when placed in the Amersham Source and Amersham Blank. The latter measurement is needed to exclude ambient background radiation.

The calibrator values have been defined so that they correspond to a reference condition of 10” water filled bore-hole.

The calibration is designed to ensure that log response is identical when corrected back to the reference condition, but the effect of borehole size, mud weight and potassium (KCl) content must be corrected for. [Discussed later under - Environmental Corrections]

Calibrator Values The activity for each calibrator is determined at the time of manufacture. Each, same size calibrator may have a slightly different assigned value. The assigned API value is reported on the shop calibration summary.

A gamma ray calibration summary as presented on a log is shown below.

Calibration Explanation Tool specific information is located in the top section of the calibration summary; including tool size, tool type, date & time, firmware, calibrator asset # and value.

The example shown is for a 6 ¾” OnTrak tool with dual azimuthal gamma detectors. Recorded parameters for sensors #1 and #2 are the same (+ 90o and -90o either side of tool face).

Background (cps) = Background count rate with no calibrator (or source in vicinity of tool)

Calibrator On (cps) = Count rate with calibrator in place

CR Differential = Count rate difference between Calibrator ON and Background

Tool Size: 6.75 Tool Type: Asset #: Build No. 008

Tool Firmware: Location: Calibrator Asset:

Test Software: Unit: Detector: Calibrator Value (API)

Sensor #1 (-90)2.15 2.91 800 1800

Sensor #2 (+90)2.15 2.91 800 1800

200

200

200

Scintillator

Calibrator (gAPI)

HV(V)

Backround (gAPI)

Calibrator On (gAPI)

14.79

13.95

Gamma Ray Calibration Summary

10049368

77677TK101 145-15

OnTrak 15 Aug 2005 10:23Date/Time:

77763TI002

Houston

Bldg C

1450

1450 214.33

Background (cps)

Calibrator On (cps)

CR Differential

API Correction

Factor

87.4

6.09

93.88

93.41 87.32

6.48

213.962.2905

2.283



Gamma Ray

API Correction Factor = A factor applied to the raw count rate to normalize to the calibrator value. Also known as the Gamma Ray API Correction Factor (GRAPIF) which converts instrument response into engineering units.

[Calibrator ON – Background] x GRAPIF should equal Calibrator Value.

For detector #1: [105.5-17.36] * 2.78 = 209.8 (the stated calibrator value).

HV = High voltage applied to photomultiplier tube

Background (gAPI) = Background API value attributed to background count rate

Calibrator On (gAPI) = Calibrator On API value

Calibrator (gAPI) = API value assigned to the calibrator Downhole processing "normalizes" the count rates of both gamma sensors for the API correction factors and calculates them for each of the four quadrants. The normalized count rate for each of the four quadrants is then transmitted to surface. At the surface, each count rate is multiplied by the average API correction factor in order to obtain API values for each quadrant.

Wellsite Verification A verification check is performed before and after each run in hole. This is to verify that the tool is functioning correctly before it is run in hole, that it has not been damaged while in hole, and gives confidence that acquired data is valid throughout the logged interval.

The wellsite verification takes the form of a background count rate check. The rig crew must ensure that other radioactive logging sources (density and neutron) that may be present, do not interfere with the before-log and after-log verifications.

A composite log may require data from several runs to be combined together, each requiring a pre- and a post-run verification to be performed. The post-run verification for one run becomes the pre-run verification for the next, as shown in the verification summary below.

Background (cps) = Background count rate at wellsite with no calibrator (or source in vicinity of tool)

Standard Deviation (σ) = The standard deviation of ten background acquisition cycles. Tolerance values AA, BB, CC and DD are set at –0.1σ & +10.0σ of the shop calibration value.

Tool Size: 6.75 Tool Type: Asset #: Build No. 008

Post-Run

Pre- Run Time

AA BB CC DD

n/a 1 12:34

1 2 08:32

2 3 11:34

3 n/a 21:15

Gamma Ray Pre-Run / Post-Run Verification Summary

Background (cps)

Standard Deviation

Standard Deviation

Sensor #1 (-90) Sensor #2 (+90)

OnTrak 10049368

Refer to heading remarks for any out of tolerance values:

17 July 2005 4.39 0.28

2.11 0.25

4.66 0.27

0.23

14 July 2005

15 July 2005

3.87

5.24

0.27

4.34 0.25

5.32

0.28

3.45 0.23

Background (cps)

12 July 2005

Date



Gamma Ray

Environmental Corrections Calibration ensures that gamma ray logs, regardless of tool type and detector crystal size, have an identical response when in the reference condition. Gamma ray log reference condition is a 10 inch borehole, filled with water (8.33 ppg) However, we require that gamma ray logs be the function of formation only and are independent of the bore-hole size and mud. Environmental corrections are therefore required for:

Borehole size and mud weight Potassium content

Potassium in the mud emits mono-energetic gamma rays of 1.46 MeV that rescatter in the mud, in the drill collar and in the detector assembly before being detected. If these background gamma rays were not taken into account the log measurement would be too high. The purpose of potassium corrections is to remove these unwanted background gamma rays. This background is a function of the borehole size, mud weight and the potassium concentration within the mud. Geometric parameters such as tool size and detector configuration are accounted for in the tool response model.

Borehole size and mud weight For a given borehole, as mud density increases, the attenuation of gamma radiation increases due to the denser medium interacting with the gamma rays. The degree of attenuation depends on the diameter of the borehole and the density of the mud. The effects are greatest in large holes with heavy mud. A correction can be applied to the apparent gamma value to correct for the effect of mud weight.

The magnitude of observed gamma radiation is affected by the size of the hole and the size of the collar surrounding the gamma ray sensor. As borehole size increases in relation to any one tool size, fewer gamma rays will reach the detector. One tool size may be used in different borehole sizes, and more than one tool size is normally employed in a well. To allow quantitative comparisons between different hole sizes and tool sizes, the measured gamma radiation can be corrected to a standard set of conditions. Standard conditions for each tool size are normalized to each other, allowing direct comparison of data from different tool sizes with only minimal corrections. If the assumption of an in-gauge borehole is valid, corrections need only to be made for tool size and bit size.

Gamma rays lose both energy and intensity while passing through the borehole fluids. The amount of gamma ray absorption depends upon the density of the intervening medium between the formation and the detectors. A correction is required for the mud weight and borehole diameter.

Potassium content Since a gamma ray sensor detects gamma radiation regardless of its source, it is necessary to distinguish formation-related from non-formation-related gamma radiation. The primary sources of non-formation gamma radiation are potassium compounds such as KCl, KOH and K-Lig, which are commonly added to the drilling mud. Potassium in the mud may have a much larger effect on the observed response than potassium within the formation. Probe based gamma sensors are surround by drilling fluid within the drill collar bore, while collar based gamma sensors will be surrounded by mud in the annulus.

Corrections at the wellsite The wellsite engineer maintains a table of mud parameters throughout the course of the job. The table is updated when mud properties change, and a note of the time is recorded.



Gamma Ray

Borehole Size and Mud Weight Corrections

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0

Borehole Diameter (inches)

Cor

rect

ion

Fact

or

20 ppg

18 ppg

16 ppg

14 ppg

12 ppg

10 ppg

8.4 ppg

0.9

Real-time gamma data is corrected using the most recent mud weight and potassium content information. A fixed borehole size (bit size) is used to correct real-time data for borehole size. Once the tool is on surface, memory data is processed using the same time tagged mud properties table.

If caliper data is available (e.g. density - acoustic standoff ASO) the borehole size correction can be applied to the memory data using a validated caliper measurement.

Borehole size and mud weight worked examples

Natural gamma rays originating in the formation are attenuated as they pass through the mud in the borehole, resulting in lowered gamma ray counts. The degree of attenuation depends upon the diameter of the borehole and the density of the mud; the effects are greatest in large boreholes with heavy mud. The borehole size and mud density charts in this section are used to determine a correction factor that can be applied. Refer to the INTEQ Log Interpretation Chartbook. To determine the correction factor, select the borehole size on the x-axis and project a vertical line to intersect the curve corresponding to the appropriate mud density. Read the correction factor from the y-axis at the point of intersection. Multiply this factor by the apparent gamma ray value taken from the log to determine the correct API gamma ray value. Gamma detectors are calibrated in API units and corrected to a 10” borehole filled with water (8.33 ppg). Example: Given the following information, determine the corrected API gamma ray value for

borehole size and mud weight. Mud Density: 10 lb/gal Tool Service: OnTrak Tool Size: 4 ¾” Borehole Size: 7.5” Apparent GR: 85 API

Step 1: Determine the borehole size and mud weight correction from the appropriate borehole

size and mud weight correction chart (4 ¾ OnTrak). Entering the borehole size and moving vertically upwards to the 10 lb/gal mud line

indicates a correction factor of 0.9

Corrected GR API value given by 0.9 x 85 ≈ 76.5 API



Gamma Ray

Mud Potassium Corrections ( 1% K by Weight )

0

1

2

3

4

5

6

7

8

6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0

Borehole Diameter (inch)

Cor

rect

ion

per 1

% P

otas

sium

(API

)

20 ppg

18 ppg

16 ppg

14 ppg

12 ppg

10 ppg

8.4 ppg

2.5

Potassium Content Potassium contains approximately 0.0118 % by weight of the isotope K40 which emits gamma rays that must be subtracted from the total count rate measured at the detector. Gamma response is corrected to a 10 inch borehole filled with water (8.33 ppg) that has zero potassium content.

The potassium correction chart indicates the API value that must be subtracted from the gamma ray log value, per 1% of potassium by weight contained in the whole mud system.

Example: Given the following information, determine the corrected API gamma ray value for potassium

content, borehole size and mud weight.

Mud additives: KCl = 70,000 mg/l (whole mud) Mud Density: 10 lb/gal Tool Service: OnTrak Tool Size: 4 ¾” Borehole Size: 7.5” Apparent GR: 85 API

Step 1: Determine percent potassium by weight

Atomic weights: K = 39.1 Cl = 35.45

Relative % K by wt. in KCl given by 525.055.741.39

45.351.391.39

==+

Wt. of K (gm) in 70,000 mg/l solution 75.36000,1

525.0000,70=

×

Mud density = 10 lb/gal Convert density in lb/gal to g/cc [1 g/cc = 8.33 lb/gal] 10 ÷ 8.33 = 1.198 g/cc Weight of 1 liter (1000 grams) mud = 1198 gram

Total %K by weight = 36.75 ÷ 1198 = 0.03 = 3% Repeat the calculation for other potassium additives present and add total %K by weight.

Step 2: Determine potassium correction in API units per 1% of potassium.

Enter potassium mud correction chart at 7.5 on the x-axis and project vertically upwards to intersect with the 10 ppg mud line. Corresponding API correction per 1% potassium = 2.5



Gamma Ray

Step 3: Determine Correction Correction per %K multiplied by total % K 2.5 x 3 = 7.5 API Potassium Corrected API response = 85 – 7.5 = 77.5 API units

Step 4: Perform mud weight and borehole size correction as in previous example

Potassium Content – Mud Report Some clients require Gamma logs to be corrected for mud Potassium content, primarily for KCl or K2CO3 water based mud systems. Oil based mud systems generally do not report mud Potassium content.

Procedure This procedure applies to all gamma ray tools calibrated using the GammaCal routine: Based on tool type selection, real-time and memory processing software computes two parameters for natural Formation Gamma measurement:

Gamma Ray Apparent (GRA$) and Gamma Ray Corrected (GRC$)

If mud potassium content is entered into the surface system, both parameters will be corrected for potassium content. If a mud potassium content of zero (0) is entered, the only corrections applied to Gamma Ray Apparent (GRA$) are for mud weight and borehole size, in order to compute Gamma ray Corrected value.

For Real-time Advantage Systems the Potassium data entry is under:

Real-Time Processing > MWD > Re-log Tare and Environmental Information Advantage Memproc will use this value when memory data is processed. Mud potassium content value is entered into the surface system as percent K+. The percent K+ (K+ %) is defined as the mass of potassium (solute) in grams per 100 grams of solution. The mass units could be any other units (for eg. pounds, kilograms). However it is important that mass of the solute (K+) is compared with similar 100 mass units of the solution. The mass of K+ in grams is generally not directly reported in mud reports but needs to be determined from the mass KCl or K2CO3 in solution. Correction for mud potassium is generally applied to water based muds that contain potassium chloride (KCl) or potassium carbonate (K2CO3). Of the two mud types mentioned here, KCl based muds are more common; although K2CO3 based drilling fluids are increasingly being used for slim hole drilling. Although small amounts of potassium may be present in seawater based drilling fluids, there is no reason to apply this correction for minor amounts of potassium. The decision to apply this correction is based upon client requirements. If asked to apply mud potassium corrections to the gamma ray the client should be made aware that the correction applied will be based on the values shown by the mud report or as reported by the mud engineer. Actual concentrations of the mud potassium content may vary as drilling progresses, hence there will be a need to update the mud K table on a regular basis. Determine the mass of KCl or K2CO3 in applicable mass units per 100 mass units of solution (drilling fluid). The word solution in this case implies the drilling fluid. This is because the drilling fluid, which is made from dissolving the salt (KCl) in water, is often weighted using weighting material such as Barite or Haematite. During the weighting process, while the mass of the solute remains constant , the mass of the solution may undergo a change due to the addition of weighting material. Therefore in 1 litre of drilling fluid, part of the water (solution) is replaced by the weighting material. There is also a small reduction in the mass of the solute per litre of drilling fluid. However the amount of reduction in the mass of the solute will differ depending upon the density of the weighting material.



Gamma Ray

Mud reports generally report concentration of KCl or K2CO3 in mg / litre. The concentration thus referenced refers to mass per volume and not mass per 100 mass units. Additionally this concentration may be referenced to one litre (1 litre) of mud filtrate and may not represent the KCl or K2CO3 concentration in one litre (1 litre) of mud. KCl or K2CO3 must be expressed in grams per liter of whole mud - NOT grams per liter of filtrate The mass of 1 litre of mud can always be found since the density and volume of the mud sample are known. Given below is the process to obtain % K from drilling fluid. The notes below provide a method of converting KCl in mg / litre of mud filtrate to mg / litre of drilling mud.

Shown below is an example showing the actual calculation. The units used are metric.

Case 1: Calculate %K+ by mass when KCl is reported as mg/litre

This example assumes that the reported KCl concentration is for litre of mud.

Step 1: Convert KCl in mg / litre to KCl in grams / litre

KCl (grams/litre)= KCl (mg /litre) / 1000

Step 2: Calculate the mass in grams of 1 litre of mud

Mass= Volume * Mud Density

Where mud density is in g / cc and Volume is in cm3 or milli litres , and the mass of mud is in grams.

Thus we obtained mass of KCl(in grams) per mass (in grams) of 1 litre of drilling mud.

Step 3: Calculate mass percent of KCl per 100 grams of mud

If we obtain the mass of 1 liter of mud as 1300 grams and the mass of KCl as 50 grams

Let the mass of KCl in 100 grams of mud = X

Thus X= (50 * 100)/1300=3.8 grams of KCl per 100 grams of mud = 3.8 %

Compute mass of 1 litre of drilling mud

Mass = Volume(cm3) * Mud Density (g/cc)

*Obtain KCl or K2CO3 in grams per liter of drilling mud

Convert KCl or K2CO3 in grams / litre to grams / 100 grams of mud

Calculate K % from by multiplying with fraction of K+ part of the molecular weight of KCl or K2CO3



Gamma Ray

Step 4: Calculate Percent K per 100 grams of drilling mud

Obtain % K from % KCl

% K+ = KCl (g/100 g) * 0.5244

= 3.8 * 0.5244 (from above example)

= 1.99

Where 0.5244 is the % by weight of potassium from the molecular weight of KCl

Case 2: Calculate %K+ by weight when K2CO3 is reported as mg/litre

Step 1-3: Follow previous KCl mud procedure

Step 4:

% K+ = K2CO3 (g/100g) * 0.5658

Where 0.5658 is the percentage by weight of potassium from the molecular weight of K2CO3

The procedure to convert KCl mg / litre of mud filtrate to KCl grams / litre of drilling mud is detailed below.

The Drilling fluids report states the following:

1. Concentration of KCl in the mud filtrate

2. Percentage of water in the drilling mud (From Retort data)

Example : KCl in mud filtrate = 20200 mg/l

Percentage of water in mud = 80 %. (The remaining 20 % are Solids + Oil Fractions)

Check density of KCl solution from Mud Facts book using the KCl concentration in the mud filtrate.

For example in this case the density is 1.011 g / cm3 or 8.42 lb/gal for a KCl solution having 20200 mg/l of KCl.

Thus KCl (g/cm3) in Drilling mud = 1.011 * 0.8= 0.8088 g / cm3 mud or 808 grams per litre of mud.

Note: The KCl will not exist in solution in the oil phase. Also it is expected that there is no KCl in the solid phase.

Implementation:

Use Mud Facts reference tables for potassium chloride solutions. These can be downloaded from www.BakerHughesDirect.com (Fluids > toolbox > manuals)

The example shown here is in metric units. Regions using other units can use the same process. The final output is a percentage (%K) which has no units.

Small amounts of KOH are sometimes added to KCl based muds. These are generally minor amounts and will not affect the calculation.

Baker Hughes Fluids is a primary supplier of K2CO3 based muds.

Conversions: KCl (g/l) = KCl(lb/bbl) * 2.85714 KCl (mg/l) = Cl– * 2.101



Gamma Ray

Typical Log Response

Shale generally contains much higher quantities of radioactive substances than sandstones and carbonates (limestone and dolomite). Therefore the gamma ray sensor in most cases easily distinguishes between shales and non-shale formations.

Shales are generally identified by high gamma ray readings (>100 API) as they contain radioactive minerals that have been adsorbed by clay particles (e.g. illite and montmorillonite).

Non-shale formations (sandstones and carbonates) are identified by relatively low gamma ray readings (lower than 60 MWD-API units)Clean sandstones and carbonates normally exhibit low levels of natural radioactivity,

Non-radioactive evaporites like salt (except potassium salt), anhydrite or gypsum can be identified by their conspicuously low radiation activity. Coal is also characterized by its very low gamma ray response.

Gamma repeatability should be within ± 3 to 5 API units. Note that if back-plot gamma data is acquired and there is a neutron tool in the BHA, the neutron source may cause the formation to activate and give anomalously high gamma readings or spikes for a considerable period of time.

0 50 100

Shaly sand

Shale

Very shaly sand

Clean limestone

Dolomite

Shale

Clean sand

Coal

Shaly sand

Anhydrite

Salt

Volcanic Ash

Gypsum

25

(API)

150115

Shale Line

Clean sand base line

90 API



Gamma Ray

General Log Responses

Other factors affecting gamma response Any drilling related events or sensor response related events may have an effect on the quality on the log, such as:

Cored intervals

Significant mud weight, mud chemistry changes (pumping of high viscosity pills)

Lost circulation (barite build up in formation)

Mud hydraulics, bottom hole cleaning and cuttings

Excessive shocks / downhole vibrations / sticky hole conditions

Bit wear, under-gauge hole

Directional well path influences

Excessive weight on bit (drill-string bending)

Reaming / re-logging / back-reaming events (Specify purpose of reaming; to recover lost data or for time lapse analysis)

Incorrect calibration (gammacal)

Exposure time (for reaming runs)

High penetration rate may result in data gaps or low data density (real-time and memory)

Surface equipment problems (such as power failures)

Decoding problems (if real time log is presented)

Tool Failures

Incorrect tool power up procedures (connections)

Lithology Coding GR Density Neutron (Limestone) 6 3/4" CCN

Acoustic Resistivity Pe

Sandstone

Low (Unless RA min)

2.65 - 3.25 53 (zero porosity)

High (zero porosity)

1.81

Limestone

Low 2.71 0 47.5 High 5.08

Shale

High 2.2 – 2.7 (water content)

High (water content)

50 – 150 (water content)

Low (water content)

1 – 5

Dolomite

Low (Higher if U)

2.87 - 2.05 43 High 3.14

Anhydrite

V.Low 2.98 -1.57 50 V.High 5.06

Salt

Low (Unless K Salt)

2.03 (1.87)

- 2.75 67 (74)

V.High 4.65

Water

0 1.0 – 1.1 (Salt & Temp)

100 189 – 218 0 – infinite (Salt & Temp)

0.36 (+Salt)

Oil

0 0.6 – 1.0

70 – 100 (H Index)

210 – 240 (API)

V.High Low

Gas

0 0.2 – 0.5

10 – 50 (H Index)

626 – 910 V.High Low



Gamma Ray

In the US Gulf coast and other locations where unconsolidated formations with swelling clays are drilled, balls of viscous suspensions of swelling clay (gumbo) may be present in the mud column. These 'balls' of clay can give spurious peaks on the gamma curve. They can usually be identified by: lack of corresponding response on resistivity or acoustic devices, or reductions in caliper reading.

When starting a new section GRAPICF (Gamma Ray API Calibration Factor) must not be adjusted to match values that were measured in the previous section. The GRA* is not corrected for hole size. The curve GRAPICF should not be changed.

Correlation The gamma ray is the primary depth correlation device for most services. It should correlate closely within 1 ft. (0.3 m) with overlap gamma ray runs in the same well and gamma ray runs from other combination services over the same interval. Depths must agree within 1/2 ft (0.15 m) with other services run in the same combination. The gamma ray values should agree (within ± 5 API) with other gamma ray runs in the same well, after environmental corrections have been applied.

Statistical variations are directly dependent upon rate of penetration. However, low ROP can result in a highly statistical log that requires smoothing.

Comparison to Wireline Some fundamental differences exist between LWD and wireline gamma ray data, and only rarely do the logs overlay exactly. Statistical variations associated with LWD logs are often considerably less than those of wireline because wireline logging speeds are greater (1800 ft/hr) than LWD average rates of penetration (200 ft/hr).

Wireline gamma ray logs are recorded on a true API scale. Historical LWD logs are recorded on "apparent API" scales. Sufficient modeling work had not been carried out to characterize early LWD logs for the effects of spectral biasing. Later LWD logs, where the tools have been calibrated using GammaCal software mean that INTEQ gamma ray logs are now recorded on a "true API scale"

LWD collars and hatch covers can have effective wall thicknesses of ½” to 3", while wireline gamma ray tool housings are typically 1/4” to 3/8”. Thus, the LWD measured gamma ray spectrum is biased to enhance potassium relative to thorium and uranium. For this reason, the LWD gamma ray data can appear to be lower than wireline values in formations that are rich in thorium and/or uranium. After borehole correction, the two types of logs may have identical values, particularly in formations with spectral characteristics similar to the API pit.

LWD bed resolution is improved, compared with wireline, because of the slower logging speeds. LWD formation measurements are acquired before significant hole enlargement occurs (reduced borehole size correction).

LWD logs experience less mud volume attenuation since the gamma sensors are housed in drill collars that typically have larger OD's than the wireline tools.

Differences may be observed in run-by-run comparisons where wireline instruments have been run either centralized or decentralized. Offset log data may or may not be corrected for borehole size – the log heading should specify.



Gamma Ray

The log example below shows the comparison between a 6 3/4" OnTrak and a Baker Atlas 3 5/8" wireline tool, in an 8 1/2" borehole. Time difference between the two logs is approximately 14 hours. The responses are in generally good agreement.



Gamma Ray

Summary When reviewing the gamma ray log the following points should be considered:

Hole Size – enlarged hole may reduce the gamma count Mud weight – increase in mud weight will reduce the gamma count Potassium content – increase in potassium content will increase the gamma count Gamma base counts – For dual detector tools, GR1 and GR2 should read approximately the same while

rotating, check the constants if not. Intermittent or sudden decrease in gamma values - may indicate a detector failure.

Compare real-time and memory data for correlation and agreement

Look for saw tooth response (gamma ray gradually decreasing until it jumps back up) or other cyclic behavior coinciding with stand lengths

Use QC plot to check that there are no differences in the character of the gamma base and the apparent values. If an two detector system has been run (RNT, OnTrak, etc.) check the two gamma sensors read the same (while rotating).

After a re-log is performed it is important to plot the re-log data together with the “original pass” data. Evaluate the depth discrepancies and depth shift re-log data if necessary. Document any shifts in the header remarks section.

Peaks on the log can be caused by severe downhole conditions. Excessive torque can result in invalid data being reported.

Base and apparent counts should exhibit the same trends

Check for data gaps or response jumps

When sliding across a formation-shale boundary at high relative dip, detector1 and detector2 counts may separate

Most Gamma logs are acquired with dual detector tools & rotary drilling systems. If dual detector tools are used with motors:

Expect some curve separation in sliding zones

Expect curves to overlay when rotating (except across contrasting formations with high apparent dip)

gamma ray

Documents

gamma log

gamma response

probe tools probe

gammaray logging instruments

dual gamma measurements

natural gamma ray emission

total gamma ray measurement

higher gamma ray readings