4 Oilfield Review

Formation Density from a Cloud, While Drilling

Environmental, health and security concerns have encouraged service companies

to search for alternatives to the traditional logging sources relied on for formation

density measurements. Scientists recently developed a reliable LWD measurement

that uses a pulsed neutron generator similar to those that have been deployed in

wireline logging tools for decades.

Françoise Allioli Valentin Cretoiu Marie-Laure MauborgneClamart, France

Mike EvansSugar Land, Texas, USA

Roger GriffithsPetaling Jaya, Malaysia

Fabien HarangerChristian StollerPrinceton, New Jersey, USA

Doug MurrayAbu Dhabi, UAE

Nicole ReichelStavanger, Norway

Oilfield Review Summer 2013: 25, no. 2. Copyright © 2013 Schlumberger.For help in preparation of this article, thanks to Doug Aitken, Sugar Land, Texas.EcoScope and NeoScope are marks of Schlumberger.

Formation density logs first appeared in the mid-1950s. Henri Doll, a Schlumberger research sci-entist who is credited with the development of the density measurement and many other petro-physical measurements in use today, received a patent for the concept in 1951. The formation density tool he helped design uses a radioisotopic source that emits gamma rays and then counts the gamma rays that return to the tool after pass-ing through the formation. Recently, a new tech-nique has been introduced that eliminates the traditional gamma ray source in logging-while-drilling (LWD) applications.

Density tools were originally referred to as gamma-gamma density (GGD) devices because gamma rays were emitted from a logging source and then returning gamma rays that passed through the formation were counted by the tool.1

The hardware and the electronics used in count-ing those returning gamma rays have undergone evolutionary changes over the past half century, yet the source has remained a fundamental requirement for formation density logging.

Traditional wireline and LWD formation den-sity tools use a cesium [137Cs] gamma ray source.2

To gain a statistically precise measurement, a 63-gigabequerel (GBq) or higher source strength is normally used.3 Density tools are not the only tools that use sources for petrophysical measure-ments. Traditional thermal neutron porosity mea-surements rely on americium beryllium [241AmBe] sources to generate the neutrons used in the measurement.

Service companies go to great lengths to mini-mize the risks associated with the use of sources; these devices must be handled carefully to avoid health, security and environmental concerns.4 In a number of locations throughout the world, the use of traditional source material is being dis-couraged or even banned. In response, service companies have sought to develop alternatives to tools that require sources.5 Increasingly, pulsed neutron generators (PNGs) are replacing 241AmBe neutron sources in both LWD and wire-line applications.6

PNGs produce high-energy, fast neutrons using a charged particle accelerator. Inelastic collisions between these fast neutrons and the nuclei of a variety of atoms found in formation fluids and minerals can put those nuclei in an excited state. Typically, the nuclei return to ground state by emitting one or more gamma rays. These gamma rays form a cloud that can act as a distributed source in the formation. The gamma rays undergo attenuation as they travel through the formation. As in the case of a radio-isotopic source, the attenuation of these gamma rays depends mainly on the electron density of the materials making up the formation.

Scientists have developed a technique that takes advantage of the distributed gamma ray cloud to compute formation density, although they first had to develop a method that accurately modeled gamma ray transport from the forma-tion to one or more detectors on a tool. The resul-tant bulk density measurement is similar to that

Summer 2013 55

from a GGD tool, but it comes from the neutron-induced gamma rays. The density derived from this technique is referred to as a sourceless neu-tron gamma density (SNGD) measurement.7

This article presents the SNGD measurement theory and discusses some of the advantages of a sourceless LWD density tool. Field results vali-date this new technique.

As Low as Reasonably AchievableTraditional sources used for petrophysical analy-sis are protected and isolated while being trans-ported to and from drilling rigs and are stored in shields that protect personnel from exposure. Pressure vessels that house the radioactive ele-ments are made from materials designed to pro-tect sources from mechanical damage and

corrosion in the harsh wellbore environment. While inserting a source into a logging tool, work-ers follow strict safety practices to eliminate potential for exposure. When the tool is lowered below the rig floor, the potential for human expo-sure goes with it. Sources must be handled care-fully, but when established safety precautions are followed, there is little risk of exposure.

1. In this article, a source refers to a radioisotopic device used in petrophysical logging tools that emits ionizing radiation.

2. The radioisotope 137Cs has a half-life of 31.17 years and emits gamma rays with an average energy level of 662 keV.

3. A becquerel (Bq) is the activity of a quantity of radioactive material in which one nucleus decays per second. Prior to the adoption of Bq as a standard SI unit of measurement, radioactivity was expressed in curies (Ci), which was the radioactivity of 1 g of the radium isotope 226Ra. 1 GBq = 0.027027 Ci.

4. Evans M, Allioli F, Cretoiu V, Haranger F, Laporte N, Mauborgne M-L, Nicoletti L, Reichel N, Stoller C, Tarrius M and Griffiths R: “Sourceless Neutron-Gamma Density (SNGD): A Radioisotope-Free Bulk Density Measurement: Physics, Principles, Environmental Effects, and Applications,” paper SPE 159334, presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, October 8–10, 2012.

5. Reichel N, Evans M, Allioli F, Mauborgne M-L, Nicoletti L, Haranger F, Laporte N, Stoller C, Cretoiu V, El Hehiawy E and Rabrei R: “Neutron-Gamma Density (NGD): Principles, Field Test Results and Log Quality Control

of a Radioisotope-Free Bulk Density Measurement,” Transactions of the SPWLA 53rd Annual Logging Symposium, Cartagena, Colombia, June 16–20, 2012, paper GGG.

6. For more on pulsed neutron generators: Adolph B, Stoller C, Archer M, Codazzi D, el-Halawani T, Perciot P, Weller G, Evans M, Grant J, Griffiths R, Hartman D, Sirkin G, Ichikawa M, Scott G, Tribe I and White D: “No More Waiting: Formation Evaluation While Drilling,” Oilfield Review 17, no. 3 (Autumn 2005): 4–21.

7. The term sourceless indicates that this measurement does not use radioisotopic sources.

6 Oilfield Review

In the early days of the nuclear age, whichcoincided with the development of many of thetools used in petrophysical analysis, radiationsafety practices focused on time, distance andshielding: Minimize exposure time, keep maxi-mum reasonable distance from radiation sourcesand maintain barriers (shielding) between peopleand material. These principles are still appliedtoday for working with traditional sources, andexposure limits have been established to ensurethe safety and health of workers who routinelyhandle these materials. Workers are also closelymonitored to determine exposure levels.

Observations of the long-term effects of radia-tion on humans resulting from surface detona-tion of nuclear devices, however, led scientists todevelop a new methodology for dealing withhuman exposure. As low as reasonably achievable(ALARA) has emerged as the standard for regula-tors. The goal of ALARA is to eliminate exposurewhenever and wherever possible, which hasdriven service companies to investigate alterna-tives to traditional sources such as 137Cs and241AmBe. A PNG is one example of an alternativeto traditional sources.8

A PNG is a miniature particle generator.Deuterium [2H] and tritium [3H] are acceleratedinto a tritium-doped target, and high-energy(approximately 14 MeV) neutrons are released(above). When not electrically energized, PNGsdo not emit external radiation. Scientists andengineers developed the first PNGs in the 1950s.These devices have since been adopted for many

downhole applications, including neutron poros-ity tools, cased hole formation evaluation toolsand capture and inelastic spectroscopy services.

PNGs have emerged as a viable alternative to241AmBe sources. For LWD operations, turbinegenerators have been developed to supply thedownhole electrical power needed to operatePNGs. This advance has allowed design engineersto incorporate PNGs in services such as theEcoScope multifunction logging-while-drilling ser-vice and the NeoScope tool.9 Attempts to replace137Cs sources used in GGD tools used for formationdensity, considered by many geoscientists to beone of the most critical parameters for the quanti-tative determination of formation porosity, havenot met with similar success until recently.

Scientists have been unable to replace137Cs-dependent measurements for a number ofreasons. For example, there is no comparableelectronic gamma ray generator, and replacingother sources was deemed a higher priority. Thehalf-life of 241AmBe is 432 years, much longerthan the approximately 30-year half-life of 137Cs.The activity of an 241AmBe source is higher andalso more difficult to shield.10 If an LWD loggingtool becomes stuck in a well, operators mustensure that the source will remain in place,intact and isolated for hundreds or even thou-sands of years. The shorter half-life of 137Cs andits lower radiotoxicity do not remove the risk,but, compared to 241AmBe, there is a reducedpotential for long-term consequences.11

As a way to mitigate risk associated with241AmBe sources, some operators have opted touse PNG-based wireline and LWD neutron poros-ity tools exclusively rather than tools with a tradi-tional source. Additionally, the prospect thatsome countries may mandate the elimination oftraditional sources entirely is a concern to bothoperators and service companies.

Another reason for the delay in replacingdensity sources is that bulk density resultingfrom the GGD measurement is a fairly straight-forward petrophysical parameter that has beenaccepted by the interpretation community fordecades. Replacing GGD tools with SNGD toolsadds a greater level of complexity and intro-duces some differences in measurement phys-ics.12 As a consequence, scientists have investedconsiderable time and resources in understand-ing the physics involved in using induced gammarays for density measurements. In 2005, scien-tists and engineers at Schlumberger introducedthe algorithms needed to compute an SNGDmeasurement. They were able to demonstratethat a sourceless density measurement that rep-licated traditional formation density measure-ments could be produced. Seven years later,they launched the first commercial PNG-basedLWD gamma density tool in the oil and gasindustry. This tool delivers a high-quality bulkdensity measurement comparable to that of tra-ditional GGD tools. Because the technique usesa PNG in place of a traditional source, the toolcomplies with ALARA objectives.13

8. For more on radioactive sources used in logging tools:Aitken JD, Adolph R, Evans M, Wijeyesekera N,McGowan R and Mackay D: “Radiation Sources inDrilling Tools: Comprehensive Risk Analysis in theDesign, Development and Operation of LWD Tools,”paper SPE 73896, presented at the SPE InternationalConference on Health, Safety and Environment in Oiland Gas Exploration and Production, Kuala Lumpur,March 20–22, 2002.

9. Japan Oil, Gas and Metals National Corporation(JOGMEC), formerly Japan National Oil Corporation(JNOC), and Schlumberger collaborated on a researchproject to develop LWD technology that reduces theneed for traditional chemical sources. Designed aroundthe pulsed neutron generator (PNG), NeoScope andEcoScope services use technology that resulted fromthis collaboration. The PNG and the comprehensivesuite of measurements in a single collar are keycomponents of the NeoScope and EcoScope servicesthat deliver game-changing LWD technology.

10. Sources that emit gamma rays can be shielded usinglead, although lead is not an effective shield forneutrons. Shields for neutron sources generallycontain polyethylene.

11. Aitken et al, reference 8.12. In some regions, operators consider the anhydrite

measurement a validation of proper tool calibration. Thisvalue—a density of 2.98 g/cm3—is outside the quotedformation density range of the SNGD measurement.

13. The PNG used in the NeoScope tool contains a smallamount—1.6 Ci—of tritium, a radioisotope of hydrogen.The half-life of tritium is 12.3 years. Tritium is also usedin conjunction with phosphorous in luminous watch dialsand exit signs in buildings.

> Pulsed neutron generator (PNG). PNGs are self-contained particleaccelerators that produce neutrons using a fusion reaction. A highvoltage potential accelerates ionized deuterium and tritium isotopes ofhydrogen toward a target doped with tritium (top). The fusion reaction(bottom) results in the production of a 4He nucleus and a neutron. Thereaction energy is transferred into the kinetic energy of the two particlesand is converted into heat when the particles are stopped in matter. Theneutrons leave the reaction with very high speed, having kinetic energy ofapproximately 14 MeV of the total 17.6 MeV released. When the main poweris disconnected, the PNG produces no neutrons.

Target

High-voltagesupplyControls

Mainpower

On-offswitch

Ion source

Pulsed Neutron Generatorn

Deuterium2H

Tritium3H 4He

Heliumn

Neutron E (17.6 MeV)

nnp+

n np+

n np+ p++ + + Kinetic

energy

Summer 2013 7

More Than Just DensityThe scientists who developed the SNGD modelworked with engineers to include this new designconcept in the NeoScope sourceless formationevaluation while drilling service. Six petrophysi-cal measurements are incorporated in theNeoScope platform—SNGD, neutron porosity,elemental capture spectroscopy, sigma, resistiv-ity and azimuthal natural gamma ray—and theyare collocated on a single, relatively short collar(above). The NeoScope LWD tool is generallylocated close to the bit, giving well placementengineers early and precise geosteering data.Near-bit positioning allows the tool to make mea-surements when drilling fluid invasion is stillminimal, which further simplifies data interpre-tation and modeling. This is especially importantfor sigma measurements. The NeoScope tool alsocontains sensors to measure hole size, annularpressure and temperature, near-bit boreholeinclination and triaxial shock and vibration.

In addition to collocated measurements closeto the bit, the NeoScope tool design has otherbenefits; the SNGD measurement has a greaterdepth of investigation (DOI) than traditionalGGD tools have and is less dependent on wellborewall contact for accurate measurements. Even asmall standoff for the GGD tools may result incompromised measurements, and hole rugosityhas always been problematic for traditional den-sity tools (right).

The SNGD measurement is collocated withthe other neutron-based measurements andresistivity measurements. Conventional loggingstrings often have separate tools for each mea-surement. Collocating the sensors reduces theeffects of irregular tool movement that cancause misalignment of depth reference points.Collocation also simplifies interpretation because

the sensors are simultaneously measuring thesame formation volume under identical staticand dynamic conditions.

The NeoScope service measures neutron-based petrophysical properties, along with bulkdensity. Most wireline and historical neutronporosity data come from tools that use 241AmBesources; the NeoScope service provides a compa-rable thermal neutron measurement. Formationhydrogen index (HI), the basis of neutron poros-ity computation, is also an output of the tool. Theneutron count rates in near and far helium-3detectors are used to determine HI and thermalneutron porosity. Compared with traditionalthermal neutron porosity, this PNG-based HI isless sensitive to environmental conditions.

Sigma—another output available from theNeoScope tool—is the macroscopic thermalneutron capture cross section of the formation.Sigma is a measurement of the formation’s abil-ity to capture, or absorb, thermal neutrons, andthe measurement can provide resistivity-inde-pendent fluid saturation in the presence ofsaline formation water. High-energy, fast neu-trons are emitted by the tool, slowed by colli-sions with the nuclei of elements in theformation—primarily hydrogen—and thenabsorbed by receptive atoms and molecules.After these neutrons are absorbed, capturegamma rays are generated, which are countedby the detectors. The rate at which thermal neu-

>NeoScope LWD logging tool and its capabilities. Engineers designed the NeoScope tool (bottom) with several collocated petrophysical measurementson a single 7.6-m [25-ft] collar. The table (top) summarizes the tool’s capabilities.

Pulsed neutron generatorNeutron flux

detector

Near epithermal detector

Neutron-gamma density Array resistivity

Dual ultrasonic caliper

Three-axis shock andvibration

Azimuthal gamma ray

Near-bit inclination

Annular pressure whiledrilling

Neutron porosity

Spectroscopy

Sigma Temperature

PNG-Based Measurements Other Measurements

Near thermalneutron detectors

Far thermal neutron detectors

Short-spacing gamma ray detector

Long-spacinggamma ray detector

> Greater DOI of the SNGD measurement. Traditional GGD measurements, such as from LWD azimuthaldensity tools, read only a few inches into the formation (left, red) and have a narrow measurementaperture (right). Hole rugosity may negatively impact the quality of the measurement. Although theSNGD (green) has a greater DOI, which results in a measurement that is less sensitive to rugosity andstandoff, it does not have an azimuthal component.

0 2 4 6 8 10 12

0.2

0.4

0

0.6

0.8

1.0

Frac

tion

of re

spon

se

Depth into formation, in.

Azimuthaldensity

Plan View

Depth ofinvestigation

Borehole

SNGDmeasurement volume

GGD dataSNGD data

8 Oilfield Review

trons are captured depends on the capture crosssection—sigma—of the element absorbing them.

The capture cross section of chlorine, whichis the strongest neutron absorber of common ele-ments encountered in well logging, is higher thanthat of oil or gas. If the porosity and formationwater salinity are known, the water saturationcan be determined from sigma. Because the mea-surement is acquired near the bit, it is possible todetermine sigma in the absence of mud filtrateinvasion. This establishes a reliable baseline forcomparison with future cased hole sigma logs.

An added benefit of water saturation com-puted from sigma data occurs when logging inhigh-angle wells. When high-angle and horizon-tal wells cross or approach bedding planes withresistivity contrasts, the resistivity measure-ments often exhibit anomalous readings.Because sigma data are not similarly affected atbed boundaries, saturation measurements com-puted from sigma may be more accurate thantraditional computations that are based onArchie’s equation.

Missing from the SNGD measurement isthe photoelectric factor (PEF) measurement.Conventional density tools include this lithologyindicator for inferring the rock matrix—a crucialinput for computing density porosity. Although thePEF measurement is not available with the newtechnique, the NeoScope tool provides neutron cap-ture spectroscopy, which delivers formation ele-mental composition information. These data offerpetrophysicists a more reliable and accurate lithol-ogy determination than do PEF measurements.

The primary drivers for development of asourceless density tool have been environmentaland security concerns. In some areas of the world,regulations prevent drillers from reentering a res-ervoir in which a traditional source has been leftbehind in a stuck drilling assembly. Because PNGsare inactive and cannot produce neutrons whencirculation ceases, operators are often permittedto drill sidetrack wells very near a wellbore inwhich a sourceless tool has been lost.14

The radioisotope-free nature of the NeoScopeservice is also attractive in unconventional playsbecause many of these are located near populationcenters, where the public may be wary of tradi-tional sources. There are no traditional sourceswith the NeoScope service, completely eliminatingtheir transportation and handling at the wellsite.The NeoScope service provides real-time naturalgamma ray images to steer the well, triple combodata for petrophysical analysis and spectroscopiclithology information to accurately evaluate reser-voir quality, but avoids raising public concernregarding the presence of radioactive sources.

> Compton scattering of gamma rays. For traditional density tools (left), gamma rays are emitted bya source and then interact with the formation in three main ways. Compton scattering (right) is theprimary interaction related to bulk density measurements. Pair production and photoelectric effect(not shown) are the other two interactions. For most well logging situations, the amount of Comptonscattering is related to the electron density of the atoms that make up the minerals and fluids in theformation. Electron density is directly related to bulk density. The formation bulk density is computedfrom the number of gamma rays that make their way from the source, through the formation and backto the detectors. Higher density results in fewer returning gamma rays compared with measurementsin lower density formations.

Nuclear source

Detectors

FormationGamma ray

Scatteredgamma rayIncident gamma ray

Compton Scattering

e–

> Life of a neutron. Both electronic and traditional sources emit high-energy,fast neutrons. Neutrons from the PNG electronic source used in theNeoScope tool have an initial kinetic energy of about 14 MeV but in a fewmicroseconds reach thermal energy level (approximately 0.025 eV). Duringthose first few microseconds, before neutron kinetic energy falls belowabout 1 MeV, the neutrons experience inelastic collisions that producegamma rays. These are the gamma rays used for SNGD processing. Afterseveral microseconds, the neutrons reach thermal energy level and areeventually captured. The capturing atoms generate gamma rays to return toground state.

102

104

106

100

200 40010–2

Neu

tron

ener

gy, e

V

Electronic source

High energy

Epithermal energy

Intermediate energy

Traditional source

Neutron energyleaving source

Inelasticregion

Neutrons with thermal energy

Capturegamma ray

emitted

Averagethermalenergy

0.025 eV

Time, μs

Summer 2013 9

14. In 1999, the US Nuclear Regulatory Commission (NRC)modified existing regulations to exempt PNGs from wellabandonment procedures applied to radioisotopicsources. For more: NRC: “Regulatory Analysis of EnergyCompensation Sources for Well Logging and OtherRegulatory Clarifications—Changes to 10 CRF Part 39,”Office of Nuclear Materials Safety and Safeguards(December 1999), http://pbadupws.nrc.gov/docs/ML0036/ML003690515.pdf (accessed April 29, 2013).

15. Compton scattering occurs when a gamma ray collideswith an electron, transferring part of its energy to theelectron, while itself being scattered. The gamma raycontinues at a reduced energy. The degree of Compton

It’s Not SimpleThe physics of formation density measurementswith GGD tools is relatively straightforward. As the137Cs in a typical logging source decays, it emitsabout 5.0 × 1010 gamma rays/s (GR/s). These GRsinteract with the electrons of atoms in the forma-tion in a variety of ways but primarily by Comptonscattering (previous page, top).15 These interac-tions result in most of the GRs being absorbed bythe formation, but a few travel back to detectors inthe tool located a fixed distance from the source.Formation density measurements are computedfrom the number of gamma rays traveling from thesource to the detectors.

From the original pool of GRs emitted by thesource, a small fraction of the scattered gammarays—a few hundred to more than 10,000 GR/s—will make it to the detectors. High-density rockswith little porosity result in fewer GRs returningto the tool than occurs in porous rocks filled withwater, oil or gas.

Gamma ray output can also vary from sourceto source. To compensate for differences in indi-vidual sources and detector efficiencies, eachtool is calibrated to a fixed reference so the tooldelivers the correct downhole density value.

As previously noted, engineers have success-fully developed tools that replace the 241AmBesource with PNG-based tools for both neutronporosity and capture spectroscopy. The pursuit ofa high-quality, radioisotope-free density mea-surement has been more elusive because of thelack of electronic gamma ray emitters analogousto PNGs to replace 137Cs. To overcome this hur-dle, Schlumberger scientists adapted some of theprinciples used for neutron-based measure-ments, such as spectroscopy and sigma, todevelop the SNGD measurement.

PNGs generate high-energy neutrons in shortbursts. Neutrons leave the tool and interact withthe various elements of the formation rocks andfluids. The interactions that have the greatesteffect are predominantly elastic collisions withhydrogen nuclei (previous page, bottom). Withsuccessive collisions, the initial high-energy neu-trons slow down and reach thermal energy level.16

Thermal neutron porosity tools count the numberof thermal neutrons that arrive back at the tool;from this count rate, the traditional thermal neu-tron porosity is computed.17

Not all the collisions are elastic. Immediatelyafter the initial burst of neutrons from the PNG,but before the neutrons reach thermal level,inelastic collisions occur between the fast neu-trons and atomic nuclei in the formation (aboveright). Inelastic collisions cause some atomicnuclei to become excited and emit one or more

>Neutron interactions. The neutron interactions relevant to petrophysicallogging can be separated into three categories: Inelastic scatter (top),elastic scatter (not shown) and capture (bottom). Inelastic gamma rays aregenerated by the interaction of a fast neutron—typically with energy greaterthan 1 MeV—with a nucleus. The interaction lifts the nucleus into an excitedstate, the neutron emerges with less energy and one or more gamma rays areemitted. Also counted among the inelastic gamma rays are those followinga high-energy nuclear reaction, such as a reaction in which the neutronknocks out a particle—such as an alpha particle, a proton or a secondneutron—from the nucleus. In elastic scattering, the neutron bounces off thenucleus without pushing it into an excited state. The only energy loss is fromthe kinetic energy imparted to the nucleus on which the scattering occurs.Elastic scattering from hydrogen, the essential mechanism underlying theneutron porosity measurement, is a result of the collision between particlesof equal mass—neutron and proton—which causes maximum energy loss.The neutron capture reaction, in which a neutron can be absorbed by anucleus, dominates at low neutron energy. This leaves the absorbing nucleusin an excited state and the resulting deexcitation is accompanied by theemission of gamma rays.

Capturegamma ray

Inelasticgamma rays

Slowneutron

Excited nucleus

Excitednucleus

Inelastic Neutron Scattering

Neutron Capture

n

n

n

scattering depends on the electron density of the targetmaterial. As the electron density increases, there ismore attenuation of gamma ray energy.

16. PNGs emit fast neutrons with a kinetic energy level ofabout 14 MeV. Thermal neutrons have a kinetic energyof about 0.025 eV at room temperature.

17. Weller G, Griffiths R, Stoller C, Allioli F, Berheide M,Evans M, Labous L, Dion D and Perciot P: “A NewIntegrated LWD Platform Brings Next-GenerationFormation Evaluation Services,” Transactions of theSPWLA 46th Annual Logging Symposium, New Orleans,June 26–29, 2005, paper H.

10 Oilfield Review

GRs as they return to ground state. Scientists areable to use the energy spectrum of inelastic GRs toidentify elements such as carbon, oxygen, silicon,calcium, iron and sulfur. Engineers use the volu-metric yields of these elements to compute lithol-ogy, and this is the basis of neutron spectroscopymeasurements. The energy spectrum of inelasticgamma rays is also the basis of carbon/oxygen ratiotools, which are used to identify hydrocarbon-bearing zones in cased holes.

During the short period of inelastic collisions,a GR cloud forms (below). This artificially gener-ated cloud emits around 108 GR/s, about twoorders of magnitude lower than the number emit-ted by a typical 137Cs source. Scientists havedetermined, however, that there are sufficientGRs produced to function in a manner similar tothat of a traditional source. The GR cloud isshort-lived because the neutrons that create itcollide with other nuclei, rapidly slow to thermallevel and are subsequently captured.

The number of gamma rays that result frominelastic collisions and reach the detectors fromthe GR cloud is influenced by three factors: the

fast neutron transport from the PNG to thepoint where inelastic GRs are produced withinthe formation, the subsequent transport of GRsfrom their origin back to the detectors in thetool and the electron density of the formation.The GRs generated in the formation by inelasticinteractions move rapidly through the forma-tion, interacting in a manner similar to GRs gen-erated by a radioisotopic source, and they areattenuated by collisions with electrons withinthe formation primarily through Compton scat-tering (above). Properly characterized, thecounts at the detector are used to compute elec-tron density, which in turn is used to computethe formation bulk density.18

If only inelastic GRs were present, the charac-terization would be more easily performed; how-ever, another major source of GRs complicates themeasurement. Fast neutrons eventually becomethermal neutrons and are captured by atoms inthe formation. Nuclei that capture thermal neu-trons emit GRs to return to a stable energy statein a manner similar to the emission of GRs result-ing from inelastic collisions. The population den-sity of thermal neutrons available for capture isdirectly related to the number of hydrogen atoms

in the formation. In a typical downhole environ-ment, the element with the highest probability ofabsorbing thermal neutrons is chlorine [Cl],whose number density is related to the salinity ofthe formation fluids. The SNGD measurement isbased only on GRs generated by the inelastic col-lisions. To correctly compute the bulk densityvalue, the contributions from capture GRs result-ing from neutron capture must be quantified andremoved from the measurement.19

Engineers must also account for the variabil-ity of the initial source strength. The output of atraditional source may vary, depending on ageand activity level of the radionuclide, but the out-put is fairly constant and its change over time ispredictable. Calibration of GGD tools accountsfor variability between sources and detector effi-ciencies by correcting to a known reference. Theoutput of a PNG is not as predictable and mayvary over short periods of time and even betweenbursts. A control loop in the NeoScope tooladjusts the PNG to maintain a constant averageoutput, and the tool includes a detector at the

> Inelastic gamma ray cloud. The PNG generatesneutrons that move away from the source andcollide inelastically with atoms in the formation(blue shading). These collisions cause a cloudof inelastic gamma rays to form (green shading).Some of these gamma rays will travel back to thetool and be counted by the detectors.

PNG

Inelasticgamma ray

source volume

Inelasticgamma rayscattering

volume

Neutrondetector

Inelasticscattering

Gamma raydetector

>Nuclear transport and long-spacing detector response. The responseof the long-spacing gamma ray detector (black) is largely determinedby neutron (blue) and gamma ray transport (red). Neutron transport isrelated to the interactions of neutrons with atomic nuclei in the formation.Inelastic gamma rays are produced during inelastic scattering of fastneutrons. Elastic scattering, which occurs primarily when neutronscollide with hydrogen nuclei, reduces the energy of the fast neutronsbelow the threshold for producing inelastic gamma rays. Thus, withincreased formation density (lower porosity), there are fewer hydrogennuclei available for elastic scattering and, as a result, there are more fastneutrons available for the production of inelastic gamma rays. Gamma raytransport and the number of inelastic gamma ray counts decrease withincreased formation density because the higher electron density providesmore opportunity for gamma ray interactions and energy reduction.

Formation density, g/cm3

Long-spacingdetector response

Gamma raytransport

Neutrontransport

Inel

astic

cou

nt ra

te, c

ount

s/s

18. Reichel et al, reference 5.19. Epithermal neutrons have an energy range between

about 0.02 eV and 10 keV at room temperature.

Summer 2013 11

PNG to determine the neutron output and com-pensate for variations.

To provide the specified 0.025-g/cm3 accuracyfor the density measurement, the SNGD model usesa combination of responses from multiple detectorsand requires a complex and demanding calibration.This calibration consists of correlating the countrates measured by each of the tool’s detectors to

those measured in the same environment with thereference tool. For this purpose, engineers havedesigned a new calibration tank that allows mea-surements over a wide range of count rates (above).

The uncertainties found in downhole log mea-surements arise from the primary measurement,applied corrections and conversion of measuredparameters to formation properties. To mitigate

these uncertainties, the NeoScope service includes aquality control system that begins with general toolsystem hardware and moves to specific sensor func-tions, individual sensor measurements and inte-grated measurements that may involve multipleindividual sensor responses (below). The last step ofthe process is quality control of the final integratedanswers that may use multiple measurements.

>NeoScope calibration device. A special calibration facility was developed specifically for the NeoScope tool. Fourmeasurements are performed in a water-filled tank using a calibration sleeve and a simulated mud channel. With the PNG turnedon, responses are measured in four configurations: sleeve raised, mud channel filled with air (1); sleeve raised, mud channelfilled with water (2); sleeve lowered, mud channel filled with water (3); and sleeve lowered, mud channel filled with air (4). Thesefour measurements allow calibration gains and offsets to be computed and provide quality checks for tool verification.

NeoScope Calibration Facility

1 2 3 4

Aluminumcalibration sleeve

Water

NeoScopetool

Mudchannel

Detectors

Calibrationsleeve

>Multi-input, multioutput measurements. The nuclear portion of the NeoScope tool (left) uses a single PNG to generateneutrons, but the responses from multiple detectors are integrated to produce specific measurements. For example, sigmadata are derived from near thermal, short-spacing gamma ray and long-spacing gamma ray detectors. SNGD data, the mostcomplex measurement from the NeoScope tool, are primarily computed using counts from the long-spacing gamma ray detector,but inputs from the neutron monitor, near epithermal detector, short- and long-spacing gamma ray detectors and far thermaldetectors are required to provide an accurate final answer. The flowchart (right) traces the corrections applied to arrive at thefinal density output.

Long-spacing gamma ray detector

Source output correction(neutron monitor)

Neutron transport correction(near epithermal and far thermal detectors)

Fast neutron correction(short- and long-spacing gamma ray detectors)

Sigma correction

SNGD output

PNG

Neutron monitor

Near epithermal detectorNear thermal detector

Far thermal detector

Short-spacinggamma ray detector

Long-spacinggamma ray detector

Sigma input

Spectroscopy input

Neutron porosity input

Neutron-gamma density input

12 Oilfield Review

Individual quality control considerationsthat may impact accuracy include sensor andhardware functionality, density values withinthe 1.7- to 2.9-g/cm3 range of SNGD and toolstandoff. In addition, environmental qualitycontrols include borehole size, deviation, ROPand formation shaliness, all of which mayimpact measurement accuracy (above). Theindicators are combined into a measurementquality control flag. A green flag suggests thatthe measurement is accurate and within speci-fied limits. A yellow flag indicates that the mea-surement is likely to be within its specified

20. Reichel et al, reference 5.21. Theys P: Log Data Acquisition and Quality Control.

Paris: Editions Technip, 2nd edition, 1999.

range but may require further interpretation,and a red flag means that the measurement isoutside specified accuracy parameters. Thesequality flag values are crucial for comparing theaccuracy of GGD and SNGD measurements.

Field Testing and BeyondField tests for the SNGD measurements con-sisted of comparing them with GGD measure-ments using a modified tool that allowedengineers to acquire both measurements simul-taneously from the same well using the same

bottomhole assembly. Objectives for field test-ing included logging in the following:• clean sandstone, limestone and dolomite

formations• anhydrite• shale• gas and light hydrocarbon reservoirs• large boreholes• deviated and vertical wells.

Scientists compared the GGD measurement,considered the benchmark, with SNGD resultsand accounted for the differences and limitationsof both measurements. Test acceptance criteriawere based on a systematic evaluation of bothmeasurements, and final analysis was based on aset of numerical interpretation criteria.20

The maximum acceptable error when twoindependent measurements are compared is thesum of their individual accuracies. In this case,the acceptable error for the two measurements is0.040 g/cm3 in clean formations and 0.060 g/cm3

in shales.21 The data from the combined toolswere plotted, which allowed engineers to quan-tify any deviation from perfect agreement.

Additionally, scientists had to account forconditions in each well that might impact GGD-to-SNGD comparisons. These conditions includedfiltrate invasion, the presence of gas or lighthydrocarbons that may change with time andvarious drilling conditions, such as mud weight,fluid variations and changes in ROP. If a large dis-crepancy between the two measurements couldbe explained by environmental effects, the testwas considered acceptable. All tests were per-formed in 81/2-in. boreholes.

In a field test of the NeoScope service, theoperator drilled a well with an average inclina-tion of 60° through a sandstone reservoir using1.26-g/cm3 [10.5-lbm/galUS] water-base mud(WBM). The caliper log indicated the boreholewas in gauge, and no GGD data corrections wererequired. Additionally, the GGD data indicatedno major azimuthal effects. Sigma was within arange that indicated minimal correction to theSNGD. In the hydrocarbon-bearing section of theformation, the resistivity log indicated some inva-sion (next page). Because of the difference intheir DOIs, the SNGD and GGD outputs wereslightly different in this zone. By contrast, thesemeasurements were almost identical in a nonin-vaded water-bearing section of the formation.The SNGD data were within accuracy limitsthroughout the well (left).

> Specifications for SNGD and GGD tools.

SNGD GGD

Accuracy

Image capability No Yes

• Clean sandstone, limestone and dolomite

0.025 g/cm3 0.015 g/cm3

• Shale 0.045 g/cm3 0.015 g/cm3

• Salt Not applicable 0.015 g/cm3

Depth of investigation 25 cm [10 in.] 10.2 cm [4 in.]

Axial resolution 89 cm [35 in.] 36 cm [14 in.]

• Anhydrite Not applicable 0.015 g/cm3

Precision at ROP 61 m/h [200 ft/h] 0.018 g/cm3 0.006 g/cm3

Density range 1.7 to 2.9 g/cm3 1.7 to 3.05 g/cm3

> Crossplot comparison. Density data from a GGD tool were compared withdata from an SNGD tool; the data are color-coded by their quality flag value.There is good agreement between the two when SNGD data are withintolerance. The data align well along the ideal axis and are flagged as green.Invasion effects start to occur in the lower density range at approximately2.3 g/cm3. The spread of the data points around the ideal line is attributed todifferences in the axial resolution of the two measurements while crossingvarious layers at high deviations.

2.2 2.4 2.6 2.8 3.02.0

2.2

2.4

2.0

2.6

2.8

3.0

GGD

data

, g/c

m3

SNGD data, g/cm3

Data within toleranceData at limit of tolerance

Summer 2013 13

> Density comparison in an invaded oil zone. The interval from X10 to X40 ft is an oil-bearing sandstone with mud filtrate invasion. The invasion is indicatedby separation in the resistivity curves (Track 2, blue shading). The sandstone below X60 ft (red shading) is water filled, and the lack of separation indicateslittle to no invasion. The NeoScope tool—along with a conventional GGD LWD tool—was run in this well. The density image (Track 3) indicates a fairlyhomogeneous reservoir, as does the lithology computed from spectroscopy data (Track 6). Quadrant density data (Track 5) overlie each other through thetwo sections, as would be expected with the high-quality wellbore conditions. There is excellent agreement between the traditional density (Track 4, red)and the NeoScope density (black), although there is a slight difference between the two datasets in the oil-bearing interval because of the invasion. Thesedata overlie the thermal neutron porosity data (blue) in clean, water- or oil-filled rocks. (Adapted from Reichel et al, reference 5.)

in.8 10

0 150

X10

X20

X30

X40

X50

X60

X70

0

0.2 2,000 1.9 2.9g/cm3

g/cm3

1.9 2.9g/cm3

0 cu 50

1.9 2.9g/cm3

1.9 2.9g/cm3

1.9 2.9g/cm3

ohm.m

0.02 200ohm.m

500RPM

40 –15%gAPI

Gamma Ray

CollarRotation

Depth, ft

Deviation

16-in. Phase Shift Bulk Density UpperNeutron Porosity

(Thermal) Up Density

Bulk Density

Neutron Density

Image-Derived Density

Sigma

22-in. Phase Shift

28-in. Phase Shift

34-in. Phase Shift

40-in. Phase Shift

Ultrasonic Caliper

QualityFlagsClay

Sandstone

Water

Pyrite

–0.8 0.2g/cm3

Density Correction

1.7 2.7

Density Image

0 90degreein.8 10

Density Caliper

Washout

Mudcake16-in. Attenuation

22-in. Attenuation

28-in. Attenuation

34-in. Attenuation

40-in. Attenuation

Resistivity

1.9 2.9g/cm3

Right Density

1.9 2.9g/cm3

Left Density

1.9 2.9g/cm3

Bottom Density

1.9 2.9g/cm3

Average Density

Quadrant BulkDensity Data

14 Oilfield Review

In another field test conducted in a lime-stone formation at the Schlumberger test facil-ity in Cameron, Texas, USA, engineers drilled awell with an average inclination of 25° using1.13-g/cm3 [9.4-lbm/galUS] WBM (above). Thecaliper log indicated hole enlargement in thetop section of the log. In zones where the SNGDquality control flag was yellow, there were sig-nificant differences between the SNGD andGGD data. The density correction on GGD data

was generally between 0.1 and 0.15 g/cm3, whichis not usually indicative of compromised dataquality resulting from hole rugosity, althoughthe quadrant density data clearly showed effectsof the enlarged borehole.

Analysis of these two logs highlighted the valueof the greater DOI of the SNGD measurement. TheSNGD data were borehole corrected and, becauseof the NeoScope tool’s greater DOI, were less influ-enced by variations in the near-borehole environ-

ment. The SNGD curve tracks the thermal neutronporosity curve in clean formations as expected.The SNGD data appear more reliable than the tra-ditional GGD measurement.

A Middle East operator tested the new SNGDdesign in four environments.22 The NeoScope toolwas run in a high-angle, high gas saturation res-ervoir drilled with nonaqueous mud, a high gassaturation reservoir drilled with WBM, an oil-saturated carbonate reservoir drilled with high-

> Comparison of washout effects on density. Density data were acquired using a NeoScope tool and a conventional GGD LWD tool across a predominantlywater-filled carbonate section (Track 6, lithology) of a test well. Caliper data (Track 1) from the NeoScope tool (black) and the traditional density tool (red)indicate an enlarged borehole (blue shading) above and below X12 ft. Resistivity data are presented in Track 2. Track 3 contains density image data fromthe traditional tool, along with azimuthal density from the bottom (red dashed) and upper (green) quadrants, an image-derived density (black) and sigma data(purple). The bulk density data from the conventional tool (Track 4, red) are affected by hole conditions from X10 to X18 ft, but the NeoScope tool providesgood density data (black). The differences in the quadrant data from the traditional GGD tool (Track 5) demonstrate the effects of the enlarged borehole.The left quadrant (blue) and the upper quadrant (green) data are invalid, as is the average computed density (red). The bottom quadrant (pink) and the rightquadrant (dark red) data are closer to the NeoScope density in Track 4. While the NeoScope density has a greater DOI and is less affected by washouts orhole rugosity, the yellow quality flag (Track 7) indicates the measurements are approaching the limits. (Adapted from Reichel et al, reference 5.)

X10

X20

X30

QualityFlagsClay

Sandstone

Carbonatein.8 10

0 150gAPI

Gamma Ray

Ultrasonic Caliper

in.8 10

Density Caliper

Washout

Mudcake

0 500RPM

CollarRotation

Depth, ft

Deviation

0 90degree

0.2 2,000ohm.m

0.02 200ohm.m

16-in. Phase Shift

22-in. Phase Shift

28-in. Phase Shift

34-in. Phase Shift

40-in. Phase Shift

16-in. Attenuation

22-in. Attenuation

28-in. Attenuation

34-in. Attenuation

40-in. Attenuation

Resistivity

1.9 2.9g/cm3

1.9 2.9g/cm3

40 –15%

Neutron Porosity(Thermal)

Bulk Density

Neutron Density

–0.8 0.2g/cm3

Density Correction

1.9 2.9g/cm3

1.9 2.9g/cm3

1.9 2.9g/cm3

g/cm3

0 cu 50

Bulk Density Upper

Bulk Density Bottom

Image-Derived Density

Sigma

1.7 2.7

Density Image

1.9 2.9g/cm3

Up Density

1.9 2.9g/cm3

Right Density

1.9 2.9g/cm3

Left Density

1.9 2.9g/cm3

Bottom Density

1.9 2.9g/cm3

Average Density

Quadrant BulkDensity Data

Summer 2013 15

22. Atfeh M, Al Daghar KA, Al Marzouqi K, Akinsanmi MO,Murray D and Dua R: “Neutron Porosity and FormationDensity Acquisition Without Chemical Sources in LargeCarbonate Reservoirs in the Middle East—A CaseStudy,” Transactions of the SPWLA 54th Annual LoggingSymposium, New Orleans, June 22–26, 2013, paper KKK.

> Density comparison in a barite-weighted mud system. Barite in drilling mud can render PEFmeasurements invalid. PEF is important for inferring lithology, which is used for porosity calculations.In this high-angle Middle East carbonate reservoir, the spectroscopy data from the NeoScope toolprovide mineralogy information (Track 6) that would not have been available from traditional densitytools. For example, the data show dolomite mixed with calcite from X,350 to X,420 ft. In the high-densitycarbonate intervals, such as from X,400 to X,520, the NeoScope density data (Track 4, black) comparefavorably with traditional bulk density (red). Traditional thermal neutron porosity (blue) is presentedalong with a density-corrected thermal neutron porosity (green). The NeoScope tool does not provideazimuthal density or density images as are available from the traditional LWD GGD tool (Track 5).Sigma data (Track 2) may be used to determine changes in hydrocarbon saturation or fluid contactsover time. Track 3 presents resistivity data. (Adapted from Atfeh et al, reference 22.)

Clay

Sandstone

Calcite

Dolomite

in.8 10

in.8 10

Ultrasonic Caliper

0 100gAPI

Gamma Ray

Bit Size

Depth, ft

Sigma

0 50cu 0.2 2,000ohm.m

16-in. Phase Shift

22-in. Phase Shift

28-in. Phase Shift

34-in. Phase Shift

40-in. Phase Shift

Resistivity

Lithology

1.9 2.9g/cm3 1.9 2.75g/cm3

g/cm3

1.9 2.9g/cm3

40 1.95 2.95–15%

40 –15%

Neutron Porosity (Thermal) Bulk Density

Bulk Density

Density ImageNeutron Density

–0.8 0.2g/cm3

Neutron Porosity (Corrected)

Density Correction

X,300

X,400

X,500

X,600

salinity WBM and an oil-saturated carbonatereservoir drilled with low-salinity WBM. To vali-date the measurements, traditional GGD toolswere run for comparison.

The first test was in an 81/2-in. wellbore, inwhich the high-angle well approached 90° devia-tion at TD. The nonaqueous mud system wasbarite-saturated, which invalidated PEF mea-surements from the GGD tool. The reservoir sec-tion was predominantly limestone and theformation density ranged from around 1.95 to2.7 g/cm3. A comparison of the data from the GGDtool with those from the NeoScope SNGD mea-surement shows excellent agreement (right).

One benefit of the NeoScope tool is the avail-ability of neutron capture spectroscopy data.Although the PEF measurement from the tradi-tional tool was affected by barite in the mud sys-tem, lithology could still be determined usingspectroscopy data from the NeoScope tool. Themajority of the interval was limestone, althoughsome dolomite was observed.

The second example was a vertical well drilledwith WBM through a gas-filled carbonate reservoirin the same field as the previous well. Comparisonof GGD with SNGD data again showed good agree-ment across a wide range of values.

A third example was drilled with high-salinityWBM through an oil-saturated carbonate reser-voir. In this highly deviated well, the porositydata from the GGD and SNGD measurementscompared favorably, well within statistical preci-sion limits of the measurements. As is typical ofliquid-filled reservoirs, the neutron porosity datavalues were similar to porosities computed fromformation density data.

A fourth well was a high-angle well drilledwith low-salinity polymer-base WBM. As withthe other three wells, there was excellent agree-ment between the SNGD data and conventionalGGD measurements.

Petrophysical analysis of data from these fourwells demonstrated that in a variety of wells witha wide range of density values, SNGD data fromthe NeoScope tool compare favorably with datafrom conventional density tools. In addition tothe SNGD data, the neutron porosity and resistiv-ity measurements provide a sourceless triple-combo logging option for LWD applications.Sigma and spectroscopy data are added benefitsthat petrophysicists can use to better character-ize and understand reservoirs.

The Pulse of Things to Come?It has been a long time coming, but the intro-duction of SNGD technology may revolutionizeLWD porosity logging. Replacing sources withPNGs has the potential to eliminate exposurerisks and reduce costs associated with sourcestorage, transportation and record keeping.

Introducing similar measurements for wire-line applications is the next obvious step.Unfortunately, modeling borehole effects on themeasurement for wireline tools has been beyondthe reach of current research and software. Itmay take some time, but if traditional sourcescan be replaced in wireline tools, the ALARAstandard—as low as reasonably achievable—willbe reached in the oil and gas industry. —TS