a new capture and inelastic spectroscopy tool takes...

SPWLA 53rd Annual Logging Symposium, June 16–20, 2012

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A NEW CAPTURE AND INELASTIC SPECTROSCOPY TOOL TAKES

GEOCHEMICAL LOGGING TO THE NEXT LEVEL

R. J. Radtke, Maria Lorente, Bob Adolph, Markus Berheide, Scott Fricke, Jim Grau, Susan

Herron, Jack Horkowitz, Bruno Jorion, David Madio, Dale May, Jeffrey Miles, Luke Perkins,

Olivier Philip, Brad Roscoe, David Rose, and Chris Stoller, Schlumberger

Copyright 2012, held jointly by the Society of Petrophysicists and Well Log

Analysts (SPWLA) and the submitting authors.

This paper was prepared for presentation at the SPWLA 53rd Annual Logging

Symposium held in Cartagena, Colombia, June 16-20, 2012

ABSTRACT

The increasing complexity of today’s reservoirs

demands an accurate understanding of formation

composition and mineralogy. This is particularly true

for unconventional reservoirs, in which quantification

of both mineralogy and organic carbon is critical for

resource evaluation. The new geochemical

spectroscopy tool described here combines the

advantages of inelastic and capture gamma ray

spectroscopy, opening new avenues for detailed

description of complex reservoirs. Capitalizing on

advances in technology, the new service provides

higher precision and improved accuracy for the analysis

of key elements in rock formations and simultaneously

offers a standalone quantitative determination of total

organic carbon (TOC). The measurements are offered at

faster logging speeds. Eliminating the americium-

beryllium (241

AmBe) radioisotopic source makes

combination with traditional measurements a much

more attractive and viable logging option for both

conventional and unconventional markets.

INTRODUCTION

Geochemical logging was introduced over 30 years ago,

starting with a wireline tool based on a pulsed-neutron

generator (PNG) and a thallium-doped sodium iodide

(NaI(Tl)) scintillation detector (Hertzog, 1980). The

emergence of new scintillation detectors led to tools

based on gadolinium oxyorthosilicate (GSO) (Scott et

al., 1991) and bismuth germanate (BGO) (Herron and

Herron, 1996a), both of which are still in active use.

Ruggedization of PNG technology enabled the

introduction of nuclear spectroscopy to logging-while-

drilling (LWD) environments (Weller et al., 2005).

Recent developments include combining capture and

inelastic spectroscopy (Pemper et al., 2006; Herron et

al., 2011), introducing new scintillation detectors

(Odom et al., 2008), and innovating on a traditional

design (Galford et al., 2009).

Existing spectroscopy tools suffer from several

technical shortcomings. The first of these is

measurement precision, which is inversely proportional

to the square root of logging speed. The precision of

measured elements is a function of count rate, and

currently the maximum count rate is limited by the

available scintillator, photomultiplier tube (PMT), and

electronics technologies. Precision and accuracy of

elemental concentrations are also compromised at

borehole temperatures because, for the scintillation

detectors currently in use, the spectral resolution and

performance degrade rapidly with increases in

temperature. Tools using a BGO detector, for example,

require some form of downhole temperature control and

have a limitation of time at temperature. Lastly, systems

using PNGs must cleanly separate the capture and

inelastic spectra, which can be very challenging.

These shortcomings are largely overcome by the tool

described in this paper. The new tool improves the

precision and accuracy of all elements traditionally

measured by capture spectroscopy. In addition, it

successfully integrates capture and inelastic gamma ray

spectroscopy, making it possible to measure carbon

accurately, account for inorganic carbon from carbonate

minerals, and determine total organic carbon (TOC),

which is essential for the evaluation of many

unconventional plays such as shale gas and shale oil.

Merging capture and inelastic data also significantly

improves precision, accuracy, and interpretation

consistency. This is particularly true for magnesium, a

key element for differentiating calcite from dolomite.

The tool’s measurements, including TOC, do not

require a calibration to core or the use of complex,

local, empirical, single- or multiple-tool interpretation

models.

This paper provides details on the technology that

makes these measurement breakthroughs possible and

shows several field examples highlighting the

performance.

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TOOL DESCRIPTION

The basic layout of the tool is shown in Figure 1. The

outer diameter is 114 mm (4.5 in.), which is smaller

than that of previous generation openhole spectroscopy

tools. It is rated to 138 MPa (20,000 psi) and 175°C.

The measurement section contains a deuterium-tritium

(d-T) PNG, which emits 14-MeV neutrons. The

neutrons interact with the formation to produce gamma

rays that are detected by a scintillation detector.

Tungsten shielding reduces the direct passage of

neutrons and gamma rays from the generator to the

detector. The housing near the detector is surrounded

with a thermal neutron shield containing boron to

reduce the number of capture gamma rays produced

from the tool.

The combination of a state-of-the-art scintillation

detector, a high-output pulsed neutron generator, and a

very fast pulse processing system opens up new

possibilities in downhole spectroscopy logging. The

following sections provide details on the technological

advances that underlie the unprecedented tool

performance.

Fig. 1 Measurement section of the spectroscopy tool.

New Spectroscopy Detector - The tool uses a large

cerium-doped lanthanum bromide (LaBr3:Ce) gamma

ray detector (Van Loef et al., 2001; Saint Gobain

Crystals, 2006; Stoller et al., 2011), which is coupled to

a state-of-the-art high-temperature photomultiplier.

LaBr3:Ce is a very fast scintillator with high light

output and excellent spectral resolution. In addition,

LaBr3:Ce has outstanding high-temperature

performance with only a minimal loss in light output

and resolution at temperatures up to 200°C.

A comparison of the properties of LaBr3:Ce with other

scintillators commonly used in downhole spectroscopy

applications is shown in Table 1. The decay time of the

light from the scintillator is an order of magnitude

faster than NaI(Tl) and BGO. The fast decay makes

very high counting rates possible and consequently

improves measurement precision.

The scintillator also produces much more light per

incident photon than other materials. The light yield is

approximately 50% larger than NaI(Tl), which has

historically been considered the benchmark for high

light output in a scintillator. High light output translates

directly into improved spectral resolution, because the

counting statistics associated with the light collected

from the scintillator is one of the main factors that

determine resolution.

Table 1 Summary of some essential room temperature

performance parameters of scintillators used for

downhole spectroscopy applications

Property NaI(Tl) BGO LaBr3:Ce

Density

(g/cm3) 3.67 7.13 5.29

Effective Atomic

Number 50.8 75.2 46.9

Primary Decay

Time (ns) 230 300 25

Light Yield

(photons/keV) 43 8.2 61

Source: Lecoq et al., 2006.

Fig. 2 Temperature dependence of scintillator light

yield for LaBr3:Ce, NaI(Tl), and BGO. LaBr3:Ce has

higher light output over the entire range of logging

temperatures, unlike NaI(Tl) and BGO.

Of great importance in a logging tool, the performance

of LaBr3:Ce is maintained at high temperatures. This

can be seen by comparing the light output of LaBr3:Ce,

NaI(Tl), and BGO as a function of temperature (Figure

2). While the light output of BGO is already low at

25°C, it drops significantly with small temperature

increases. Above 60°C, the light output is too low to

make acceptable logging measurements. This makes it

necessary to use thermal protection in BGO-based

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 50 100 150 200

Rel

ati

ve

lig

ht

yie

ld

Temperature (°C)

LaBr3:Ce

NaI(Tl)

BGO


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spectroscopy tools and limits the available time in most

boreholes for useful measurements. This limitation

excludes long-duration operations such as drillpipe

conveyance or tractoring. In contrast, LaBr3:Ce

maintains its high light output over the entire

temperature range.

Fig. 3 Elemental standard spectra measured with

LaBr3:Ce and BGO detectors at room temperature

(upper panel) and at higher temperatures (lower

panel).

LaBr3:Ce has spectral resolution unmatched in a

downhole tool. An example is shown in the top panel of

Figure 3, where elemental standard spectra for

LaBr3:Ce are compared with those of BGO at room

temperature. The features in the LaBr3:Ce standards are

clearly sharper and better defined. This allows any

measured spectrum to be more easily decomposed into

its component parts, resulting in improved precision of

spectroscopic answers.

As described previously, a decrease in light output

translates into undesirable spectral degradation. The

elemental standard spectra for BGO (Figure 3, orange

curves) show significant broadening and loss of

definition as temperature increases from room

temperature to the BGO maximum operating

temperature of 60°C. On the other hand, the standard

spectra from LaBr3:Ce (Figure 3, blue curves) are

almost identical between room temperature and the

much higher temperature of 150°C.

New-Generation Pulsed Neutron Generator - Neutrons

are produced by a new, high-performance d-T PNG

developed to meet the needs of this advanced

spectroscopy tool. The PNG eliminates the need for a

radioisotopic source such as 241

AmBe, thus reducing

operational, transportation, and safety risks.

The high neutron output of the PNG (capable of 3 × 108

neutrons/s nominal and higher) enables the tool to make

the best use of the fast LaBr3:Ce scintillator by

providing a very high counting rate and consequently

better measurement precision and faster logging speeds.

This output is about a factor of 8 higher than that of

present radioisotope sources.

The PNG is designed to allow clean separation of

inelastic and capture gamma rays, improving the quality

of both measurements. A necessary condition for this

separation is a well-defined, repeatable neutron burst

shape, which enables optimum timing of the inelastic

and capture measurements and a capture measurement

uncontaminated by inelastic gamma rays. To ensure

that this condition is met, the PNG uses a hot cathode-

based Minitron* neutron tube. The hot cathode

technology produces a crisp 8.0-µs burst with rise and

fall times faster than 400 ns (Figure 4). The timing of

the burst is stable and predictable, making it possible to

begin clean capture spectra acquisition very close to the

burst for maximum count rates.

The PNG has been designed for reliable operation over

extended periods at temperatures up to 175°C. The

technology builds on what was developed for the

successful LWD PNG (Weller et al., 2005). Extensive

test facilities were constructed and used to evaluate

multiple PNG design generations. To date, over 3,600

* Mark of Schlumberger.

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job-equivalent runs at maximum environmental

conditions have been completed with several dozen

PNG units. The extensive test results, accumulated over

several years of development, were used for continuous

design improvements to provide high reliability and

performance at tool introduction.

Fig. 4 Neutron burst shape used for spectroscopy.

Electronics - The combination of the fast LaBr3:Ce

scintillator and the high-output PNG produces a very

high counting rate, often in excess of 2,500,000

counts/s. Processing such fast signals requires the use

of specialized electronics. The electronics must provide

excellent spectroscopy performance and outstanding

pile-up rejection to limit spectral distortion due to

nearly coincident gamma ray arrivals.

The pulse height analyzer system that accomplishes

these tasks is shown schematically in Figure 5. Gamma

rays are detected in the large LaBr3:Ce scintillator

described previously, which is coupled to a high-

temperature spectroscopy photomultiplier with an

integrated high-voltage supply and preamplifier. The

signals from the preamplifier enter an integrator. The

integrated signal is digitized by an analog-to-digital

converter (ADC) and processed to obtain the pulse

height, which is accumulated in a pulse height

histogram that provides a spectrum of counts versus

pulse height.

Tool Characterization - To translate the spectra

acquired by the tool into petrophysically relevant

elemental concentrations, standards and sensitivities are

needed. Elemental standards represent the distinct

spectral signature of each element detected with the

tool. As described in the next section, measured spectra

can then be represented by a linear combination of the

standards after correction for environmental and

electronic factors. The coefficients from the

decomposition are called yields. The elemental

sensitivities relate the yields to the elemental weight

fractions which are of interest; these sensitivities are

largely a function of neutron capture cross section.

Fig. 5 Schematic of the spectroscopy acquisition

system. Signals from the detector pass through an

integrator and analog-to-digital converter (ADC) and

are then processed to form the pulse-height spectrum.

Characterization for this tool relies primarily on

extensive measurements in real and laboratory

formations. Many of the formations used were designed

specifically for geochemical applications. In addition to

the elements conventionally measured by capture (H,

Na, Cl, K, Ti, Cr, Ni, Ba, and Gd), inelastic (C, O), or

both types of spectroscopy (Mg, Al, Si, S, Ca, and Fe),

the characterization includes other elements, such as

Mn and several other metals (see Table 2). Figure 6

shows some of the formations used to measure

elemental standards and sensitivities.

Mathematical modeling has been employed to optimize

the design of the tool for both capture and inelastic

spectroscopy and to construct representative formations

for characterization. The modeling is based on the

industry-standard Monte Carlo N-Particle (MCNP)

transport code (Pelowitz, 2008). The MCNP code was

adapted to simulate spectroscopy measurements by

adding the capability to record detected gamma rays by

the isotope and nuclear reaction that created them and


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by the region where they were produced. Converting

the gamma ray response to a pulse height spectrum is

accomplished through the GAMRES code (Evans,

1981), which was tailored with the measured nonlinear

light output of the LaBr3:Ce scintillator. The modeling

results are in very good agreement with measured

spectra. However, any simulation of this kind will

suffer from inaccuracies in even the most up-to-date

neutron-gamma cross sections and in realistically

simulating light collection in the scintillator, PMT

response, and pulse processing. Experimental

measurements for derivation of elemental standards and

sensitivities thus remain the procedure of choice for

obtaining unbiased spectroscopy answers.

Table 2 List of some elements that can be determined

through capture or inelastic gamma ray spectroscopy.

Element Description Capture Inelastic

Al Aluminum √ √

Ba Barium √ √

C Carbon

√

Ca Calcium √ √

Cl Chlorine √

Cu Copper √

Fe Iron √ √

Gd Gadolinium √

H Hydrogen √

K Potassium √

Mg Magnesium √ √

Mn Manganese √

Na Sodium √

Ni Nickel √

O Oxygen

√

S Sulfur √ √

Si Silicon √ √

Ti Titanium √

PHYSICS AND INTERPRETATION

Transforming the spectra acquired by the tool into

petrophysical quantities is an involved process. In brief,

neutrons are emitted from the PNG and produce gamma

rays. The pulse height spectra of the detected gamma

rays are recorded as a function of time relative to the

pulsing sequence of the PNG. Each spectrum is

decomposed into a linear combination of standard

spectra from individual elements. The coefficients of

the linear combination are converted to elemental

weight fractions, which may be further analyzed to

produce mineralogy. These steps are described in more

detail in the following sections.

Fig. 6 Some of the geochemical formations used in the

tool characterization.

Nuclear Interactions, Gamma Ray Measurements, and

Data Acquisition - The well-defined burst of the PNG

creates a population of high-energy (14-MeV) neutrons

around the tool which interact and moderate to thermal

energies on the scale of microseconds. These neutrons

induce the emission of gamma rays from nuclei in the

formation, borehole, and tool via two primary

interactions: inelastic scattering and thermal neutron

capture. In the case of inelastic scattering, which can

occur only above specific neutron energy thresholds, a

portion of the incident neutron energy is transferred to

the target nucleus, which de-excites by emitting gamma

rays at one or more characteristic energies. In thermal

neutron capture, neutrons near equilibrium temperature

are absorbed by a target nucleus, which de-excites by

emitting a different set of characteristic gamma ray

energies. These “fast” and “slow” interactions are

illustrated in Figure 7. The gamma rays themselves

undergo scattering as they travel toward the detector,

where they deposit some or all of their remaining

energy. The principle of the geochemical logging

measurement is that the total detected gamma ray

spectrum can be deconstructed based on each element’s

characteristic standard spectrum, as discussed below.

Elements that are typically found in the downhole

environment exhibit a wide range of cross sections for

thermal neutron capture. One advantage of inelastic

interactions is that they are less affected by

environmental factors including the presence of

elements such as chlorine, which have high thermal-

capture cross sections and can significantly reduce the

population of thermal neutrons available for capture by

elements of interest in the formation. Table 2 presents a

list of some of the elements that can be measured

through capture or inelastic gamma ray spectroscopy.

For some elements, capture and inelastic gamma rays

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are available, but both are not necessarily used or useful

for the determination of elemental concentrations.

Fig. 7 Conceptual sketches of the two primary neutron-

gamma interactions in geochemical logging, showing a

fast neutron undergoing inelastic scattering (top) and

thermal neutron capture (bottom)

The timing sequence of the PNG and its connection

with the spectral data is shown in Figure 8. The

inelastic spectrum is acquired during an 8.0-s-long

neutron burst in a time gate denoted SBUR. The capture

spectra come from early (SEAR) and late (SLAT)

capture gates immediately following the burst plus a

capture gate (STAU) that follows a series of 50 bursts.

The STAU gate is also used for a formation neutron

capture cross-section measurement (). This sequence

is repeated 62 times, after which a quiescent period of

approximately 8 ms is used to acquire a background

spectrum (SBKG) to measure radiation from the tool,

formation activation, and background radiation. The

roles of these gates in the processing are described in

the next section. The cycle takes 125 ms to complete

and is repeated 3 times to form each spectroscopy data

frame. The spectral data channels are recorded and used

by the acquisition system for gain regulation of the

PMT, then depth-gated and written out for further

processing.

Major benefits of the well-defined neutron burst are that

there is no contamination by inelastic reactions during

the “capture” gates and that capture data can be

collected very soon after the end of the burst.

Spectra-to-Yields Processing - The purpose of spectra-

to-yields processing is to obtain an accurate suite of

relative elemental yields based on decomposition of the

recorded neutron-induced gamma ray spectra. An

inherent assumption is that the measured spectra can be

correctly represented by a linear combination of known

elemental standard spectra. Given this assumption, the

desired yields can be produced by a linear weighted

least-squares (WLS) fit of the measured spectrum with

a set of standard spectra within a specified energy

range. However, temperature variations and electronics

effects cause spectral distortions, which must be

compensated for by transforming the measured

spectrum’s energy calibration to match that of the

standards. The energy resolution of the standards also

must be matched to that of the measured spectrum. This

methodology is formulated as a nonlinear least-squares

problem and solved utilizing the Levenberg-Marquardt

method (Gill et al., 1981; Grau and Schweitzer, 1987,

1989). With the measured spectrum at the proper

energy calibration and the standards adjusted to match

the resolution of the measured spectrum, the elemental

relative yields can be derived using a linear WLS

method.

Fig. 8 Timing sequence used for data acquisition.


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The spectra-to-yields processing is performed

independently on appropriate time gate spectra to

produce capture and inelastic relative yields. For

capture processing, spectra from the three capture gates

(SEAR, SLAT, and STAU) are summed to create a

total capture pulse height spectrum. The net capture

spectrum to be decomposed is obtained by subtracting

the background spectrum (SBKG) while properly

accounting for duty factors and counting losses. For

inelastic processing, the contributions from capture

must first be removed. This is accomplished by

subtracting the early capture gate (SEAR) from the

burst gate (SBUR) using a subtraction factor

determined from neutron-capture physics, duty factors,

and counting losses. The result is a net inelastic

spectrum and associated variance (Figure 8).

By definition, the elemental yields are “relative” in that

the sum of either the capture or the inelastic yields for

each spectrum is separately equal to unity. Elemental

relative yields are a function of the volumetric

proportion of an element in the measurement region, as

well as the sensitivity of the tool to each element. These

yields are the starting point for determining quantitative

elemental concentrations and mineralogical volumes.

Yields to Dry-Weight Elements - The conversion of

relative spectral yields from neutron capture into

absolute elemental concentrations is accomplished via a

modified geochemical oxides closure model (Grau and

Schweitzer, 1989; Grau et al., 1989) or through an

iterative inversion technique such as ELAN* elemental

log analysis (Quirein et al., 1986). Oxides closure

models take advantage of common mineralogical

associations to relate the concentration of unmeasured

elements to the concentration of measured elements.

For capture elements, the weight fraction (Wi) of

element i at a given depth is a function of a closure

normalization factor (F) determined at that depth, the

measured relative yield (Yi ) of element i at that depth,

and the tool’s sensitivity (Si) to element i for capture

reactions:

Wi = F(Yi /Si). ............................................... (1)

The normalization factor F is determined at each depth

by solving the simple closure relation:

F (AiYi/Si) = 1, ........................................... (2)

where Ai is a factor accounting for all the unmeasured

elements that are associated with element i.

The inelastic measurement offers a complementary,

independent set of yields from which to extract

elemental concentrations. The inelastic sensitivities to

C and Mg are of particular value. An important

limitation of the inelastic physics is that its set of yields

is not complete enough to form a closure model.

Instead, the normalization factor that converts these

inelastic yields into elemental concentrations is derived

by requiring that the derived dry weights of some of the

elements, which occur in both inelastic and capture

measurements, are consistent. The method ensures that

all the resulting dry weights are mutually consistent for

elements with both capture and inelastic yields. As a

consequence the analysis gains the enhanced precision

of the inelastic Mg measurement and achieves better

accuracy overall.

A second benefit of the inelastic analysis is the

determination of a total carbon concentration. By using

common association factors for carbonate minerals, the

amount of inorganic carbon present can be quantified

and subtracted from the total inelastic carbon to

compute TOC (Herron and LeTendre, 1990). The

combined analysis of inelastic and capture

measurements is part of the generational breakthrough

represented by this tool.

Dry-Weight Elements to Minerals - Mineralogy and/or

lithologic fractions can be derived from elemental

concentration logs through either a sequential

processing method such as SpectroLith* (Herron and

Herron, 1996a, 1996b) or through an inversion

approach (Mayer and Sibbit, 1980; Quirein et al., 1986;

Cannon and Coates, 1990; Peeters and Visser, 1991).

The SpectroLith technique is based on the derivation of

empirical relationships between accurate elemental

concentrations and mineral concentrations. A high-

quality core database of sedimentary rock samples

characterized by dual-range Fourier transform infrared

(DRFT-IR) mineralogy (Herron et al., 1997) and

chemical analyses has been used to develop and evolve

the processing algorithms. Chemical analysis

measurement techniques include X-ray fluorescence,

sulfur by combustion infrared detection technique

(LECO), TOC by elemental analyzer, and trace

elements primarily by induction-coupled plasma mass

spectrometry. In combination, these techniques measure

virtually all of the elements that influence nuclear well

logs or are of interest for formation evaluation. The

DRFT-IR library includes more than 50 mineral

standards representing 28 minerals.

The quantitative elemental concentrations and

processed lithology can be used directly with other log

data in petrophysical interpretation programs to provide

more accurate petrophysical answers (Mayer and

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Sibbit, 1980; Quirein et al., 1986; Cannon and Coates,

1990; Peeters and Visser, 1991).

TOOL APPLICATIONS

The tool performance described in this paper has been

validated by extensive field testing performed in several

locations, mostly in North America, and covering the

main unconventional plays in the United States and

Canada.

Over 50 successful jobs were performed accumulating

more than 500 hours of tool operation with no PNG or

other hardware failures.

Repeatability and Precision – An example of tool

repeatability is shown in Figure 9. The left eight tracks

show two logging passes made at 900 ft/h. The curves

overlay very well in all cases, even for the historically

challenging measurement of Mg and the new

quantification of C. The right eight tracks compare two

passes acquired at 3,600 ft/h. Here, the repeatability is

slightly worse, as expected, but the major rock-forming

elements Si, Ca, Fe, and even Mg would still produce a

good quantitative lithological description of the

formation at much faster logging speeds than existing

tools.

Measurement precision is a function of the tool design,

logging speed, depth averaging, and logging

environment. Borehole diameter and salinity, formation

porosity and salinity, mud and formation composition,

and neutron output are all contributing factors. A tool

planner is available to estimate the precision of

elemental concentrations and interpreted properties

such as matrix density or lithology for a given

environment. The algorithms used in the tool planner

are based on MCNP modeling that was benchmarked to

experimental data. The recommended logging speed

will depend on the required precision for a given

measurement of interest. As suggested by Figure 9, if

only basic mineralogy is needed and wellsite efficiency

is critical, logging speeds of up to 3,600 ft/h may be

achieved. A slower logging speed may be required for

more demanding applications such as carbon

quantification, precise dolomite determination, or

logging in high-salinity borehole fluids.

Accuracy – The elemental weight fractions measured

by the tool can be used to evaluate the mineral

composition of a formation. Interpretation of log data to

determine porosity, permeability, saturation and

geomechanical properties generally depends on

mineralogy. The accuracy of the elemental weight

fractions must therefore be validated to provide

confidence in any mineralogy or other formation

properties derived from them.

Figures 10-13 provide four examples that compare the

elemental weight fractions measured by the tool with

those analyzed from core. Each figure displays a

mineralogy column and elemental concentrations of

silicon, calcium, iron, magnesium, sulfur, potassium,

aluminum, titanium, manganese, and total organic

carbon. The log data are displayed as solid black lines

with shaded areas representing the uncertainties; core

data are displayed as red dots; all values are presented

in weight fraction. The core data are used for

validation only; they are not used for calibration of

elemental concentration logs in any example.

Each of the examples comes from a different basin in

North America. The agreement between core and log

elemental concentrations is very good across a wide

variety of lithologies. Of particular interest is the new

log of TOC, which closely matches the organic carbon

weight fraction measured on core samples. This

information can be extremely valuable when evaluating

organic-rich shale formations as well as conventional

reservoirs.

Examples of the accuracy of the TOC measurement are

given in Figure 14, which presents logs from four wells

in unconventional resource plays from North America.

TOC ranges from 0 to 12 weight percent (displayed as

weight fraction in the figures). Log and core data agree

well over the entire dynamic range in both water-based

and oil-based mud systems. In oil-based muds, TOC

can be biased unless the contribution to the C signal

from the oil in the borehole is properly removed. The

logs were acquired at speeds between 600 and

1,200 ft/h, considerably faster than traditional carbon

logging.

DISCUSSION

Enabled by recent technological advancements, the

measurement capabilities of the new elemental

spectroscopy tool represent an important step beyond

the previous generation of tools. Early field results

highlight the considerable potential of this new service

that enables a range of answer products and

applications not possible with previous tools or tool

combinations.

For the first time, the precision and accuracy of

measured Mg and S enable a solution for mineralogy in

carbonates at standard wireline logging speeds.


9

Logging for TOC becomes a reality for unconventional

resource plays such as shale gas, shale oil, and in situ

thermal processing of oil shales. Improvements in the

precision and accuracy of other measured elements

such as Na and K and quantification of new elements

such as Mn permit a more thorough and accurate

mineralogy interpretation.

Operational aspects also distinguish the new tool. The

improved precision of the measurement translates

directly into faster logging. The electronic source

means that no additional radioisotope source or logging

runs are required to obtain geochemical information.

The higher temperature rating of the detector allows

very long jobs at temperature as with drillpipe or tractor

conveyance. The reduced tool diameter enables logging

in slimmer boreholes. The tool is also combinable with

most wireline openhole services, including triple

combo, nuclear magnetic resonance (NMR), dielectric,

sonic, and imaging tools.

The combination of all these features allows the

simultaneous acquisition of neutron, density, resistivity,

NMR, and capture and inelastic spectroscopy in

demanding environments with a single toolstring. The

availability of real-time analysis of the measurements

supports enhanced real-time log quality control and in-

time decision-making.

Last but not least, the tool is a key step in eliminating

radioisotope sources. The benefits in connection with

health, safety, and the environment and with the costs

of transportation, security, and liability are clear.

ACKNOWLEDGMENTS

The authors thank Energen Resources Corporation and

Whiting Oil and Gas Corporation for supporting field

testing of this tool in their wells and for allowing the

use of their well log data. The authors also gratefully

acknowledge Saint-Gobain Crystals, particularly Peter

Menge and the engineering team, for their contributions

in developing the detector technology described in this

paper. Lastly, we thank the many people on the

Schlumberger development team in Sugar Land,

Princeton, and Cambridge for bringing this tool from

concept to reality. None of the results reported here

would have been possible without their expertise,

dedication, and hard work.

REFERENCES

Cannon, D. E. and Coates, G. R., 1990, Applying

mineral knowledge to standard log interpretation,

Transactions of the 31st Annual SPWLA Logging

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ABOUT THE AUTHORS

R. J. Radtke is a Principal Tool Physicist at

Schlumberger’s Houston Formation Evaluation

Integration Center (HFE) in Sugar Land, Texas. He has

been involved in design, characterization, and algorithm

development for nuclear LWD and wireline tools since

joining Schlumberger in 1999. He graduated from The

University of Chicago in 1994 with a PhD in physics.

Maria Lorente is Wireline Product Champion for new-

generation nuclear tools, based in Sugar Land, Texas.

She started her career as part of the engineering team in

France and then moved to wireline field operations

working in various assignments in the Middle East and

Latin America. She holds a BS degree in electrical

engineering from Universitat Politècnica de València,

Spain, and an MS degree in electrical engineering from

Supélec, France.


11

Bob Adolph is an Engineering Advisor at

Schlumberger’s Princeton Technology Center (PTC) in

Princeton, New Jersey. Interests include wireline and

LWD nuclear measurements, especially gamma ray

spectroscopy, pulsed neutron technology, electronics,

and signal processing. He joined Schlumberger in 1979

and holds a BS in electrical engineering from Rice

University.

Markus Berheide is a Principal Research Scientist in

the Sensor Physics department at Schlumberger-Doll

Research (SDR) in Cambridge, Massachusetts. His

background is in nuclear measurements and enabling

technologies. He holds a Doctorate in physics from

Ruhr-Universität Bochum (Germany).

Scott Fricke is a Principal Tool Physicist at HFE. He

has been involved in software development,

interpretation, and design of nuclear logging tools. He

received a PhD in theoretical nuclear physics from the

University of Minnesota in 1985.

Jim Grau is a Scientific Advisor at SDR. He has been

involved for over 30 years in all aspects of borehole

elemental analysis using nuclear spectroscopy

techniques, including tool design, data acquisition

software, and spectral analysis techniques. Jim received

a PhD in experimental nuclear physics from Purdue

University in West Lafayette, Indiana.

Susan Herron is a Scientific Advisor in the Sensor

Physics department at SDR. Her research interests

include nuclear spectroscopy applications, mineralogy,

and petrophysics. She holds a PhD in Geological

Sciences from State University of New York at Buffalo.

Jack Horkowitz is a Petrophysics Advisor with

Schlumberger working at HFE on a number of projects

including spectroscopy processing, integrated

interpretation products and LWD calipers. Jack joined

Schlumberger in 1995 and holds a PhD in geology from

the University of South Carolina. He is a past President

of SPWLA (2006–07).

Bruno Jorion is a Principal Engineer (Electrical) in the

Engineering department HFE. He received a Diplôme

d'Ingénieur from l'Institut Supérieur d'Electronique du

Nord (ISEN, France) and has been working in several

nuclear and NMR tool designs.

David Madio is a Senior Petrophysicist for

Schlumberger at HFE. His interests include mineralogy,

neutron porosity, and magnetic resonance. He received

a PhD in physics from the University of Pittsburgh in

1996.

Dale May graduated in 1979 with a BS in physics from

Texas Tech University, Lubbock, Texas. He joined

Schlumberger in Midland, Texas, in 1980, as a field

engineer. He has held positions as a field engineer,

specialist engineer, sales engineer, log analyst, and

center manager and is currently an Advisor

Petrophysicist for Schlumberger Oilfield Services.

Jeffrey Miles is a Senior Research Scientist at SDR. He

received a PhD in physics from the Massachusetts

Institute of Technology in 2007. His interests include

the modeling of all aspects of nuclear physics in the

oilfield, with emphasis on neutron-gamma spectroscopy

and algorithms for fast modeling and inversion.

Luke Perkins is a Principal Engineer in the generators

development group at PTC. He received his PhD in

nuclear engineering from U.C. Berkeley and has

worked since 1997 in Manufacturing, Sustaining and

Research and Engineering positions within the oilfield

radiation generators product line, including leading the

development and commercialization of the EcoScope*

pulsed neutron generator.

Olivier Philip is a Principal Engineer in the detector

development group at PTC. He received a PhD in

nuclear engineering from Texas A&M University. He

worked on nuclear tool design in the Schlumberger

Sugar Land Product Center from 1996 to 2000. Since

2000, he has worked on photomultiplier and detector

design and development for several downhole

applications.

Brad Roscoe is a Scientific Advisor and the Nuclear

Program Manager at SDR. His areas of interest include

nuclear detectors, nuclear sources, gamma-ray

spectroscopy, and measurement integration. He earned

a PhD in nuclear engineering in 1981 from the

University of Illinois at Urbana-Champaign.

David Rose is Principal Petrophysicist and Manager of

Interpretation Engineering for Nuclear Answer

Products at HFE. He holds a Bachelors in geophysics

from the Colorado School of Mines.

Chris Stoller is a Scientific Advisor at PTC. He joined

Schlumberger in 1986 and has worked on a variety of

nuclear tool projects in wireline and LWD. He received

a PhD in physics from the Swiss Federal Institute of

Technology in Zurich in 1976.

12


ADDITIONAL FIGURES

Fig. 9 Example of elemental weight fractions repeatability at two different logging speeds, 900 ft/h in the left eight

tracks and 3,600 ft/h in the right eight tracks. The well is located in Central Texas and was drilled with an 8-in. bit

size and filled with fresh water. This interval consists of alternating zones of very pure limestone where Ca

approaches 0.4, the value of calcite; dolomite where Mg approaches 0.13, the value in dolomite; and a small

amount of sandstone (Si = 0.47 in quartz and about 0.3 in feldspars). Limestone and dolomite are easily

distinguishable from the Ca and Mg concentrations even at 3,600 ft/h. The very low concentrations of Fe, S, K, and

Al are also properly reproduced in the log.

900 ft/h 3,600 ft/h


13

Fig. 10 Field example from a well in North Dakota. Zones A, B and C are part of the Bakken formation, and zone D

corresponds to the Three Forks formation. The figure compares elemental weight fractions measured by the tool

(black lines) to those derived from core analysis (red points). The yellow shaded areas along the black lines

represent the uncertainties (which are very small in this example). The track to the left of the depth track shows the

mineralogy, and the track to the right of the depth track shows an enhanced core photograph. Overall there is a

very good match between the measurements and the core, especially the total organic carbon (TOC) measurement

(far right track) ranging from nearly zero (zones B and D) to more than 12 weight percent (zones A and C). The

core photograph shows the highly laminated nature of this formation, which can cause occasional scatter when

comparing log data to standard core plug data. The logging speed was 600 ft/h.

Zon

e A

Zon

e B

Zon

e C

Zon

e D

x125

x100

x150

x175

14


Fig. 11 Field example from Western Canada showing two intervals from the same well. The figure compares

elemental weight fractions measured by the tool (black lines) to those derived from core analysis (red points). Due

to the low salinity mud system, the bit size, and a logging speed of 1,080 ft/h, the measurement statistical

uncertainties are less than the thickness of the lines. The track to the left of the depth track shows the mineralogy.

There is very good agreement between core and log for all elemental concentrations and total organic carbon

(TOC).

x800

x900

x100

x150

x850


15

Fig. 12 Field example from West Texas showing three intervals from the same well. The figure compares elemental

weight fractions measured by the tool (black lines) to those derived from core analysis (red points). The yellow

shaded areas along the black lines represent the uncertainties. The track to the left of the depth track shows the

mineralogy. Overall there is a very good match between the measurements and the core. The logging speed was 900

ft/h.

Fig. 13 Field example from the Southeastern United States showing two intervals from the same well. The figure

compares elemental weight fractions measured by the tool (black lines) to those derived from core analysis (red

points). The yellow shaded areas along the black lines represent the uncertainties. The track to the left of the depth

track shows the mineralogy. Note the change in elemental concentrations and the good agreement between log and

core data for all elements in both the clay-bearing and quartz-rich formations. The logging speed was 1,800 ft/h.

x100

x300

x500

x050

x450

16


Fig. 14 Comparison of total organic carbon (TOC) measured by the new tool and derived from core from four wells

in unconventional formations throughout North America. For each well, the first track shows the total carbon (TC,

in red) derived from the inelastic measurement and the total inorganic carbon (TIC, in black) computed from

carbonate minerals using capture spectroscopy elements. The separation between the two curves is TOC, which is

displayed in the second track with core data (red dots) for validation.

x000

x050

x100

x150

x200

x250

x300

x350

Water-Based Mud Oil-Based Mud

x000

x100

x200

x300

x400

x500

x600

x075

x100

x125

x150

x175

x200

x225

x250

x000

x100

x200

x300

x400

x500

a new capture and inelastic spectroscopy tool takes...

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