Oilfield Review. Sourceless Density. Routine Core Analysis. Multistage Stimulation. Hydraulic Fracture Design Software.

Transcription

1 Oilfield Review Summer 2013 Sourceless Density Routine Core Analysis Multistage Stimulation Hydraulic Fracture Design Software

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3 Core Analysis: Combining Expertise for Insight into the Reservoir Managing an asset to optimize oil or gas production requires knowing reservoir rock and fluid properties. For example, planning well locations relies on predictions of porosity and other rock properties from seismic surveys. Stimulation and completion designs depend on knowledge of geomechanical strength and permeability from logging and core measurements. Reservoir simulation demands data on a wide range of properties of the formation rocks and fluids on many scales to engineer field production. Evaluation of the rocks and the fluids within them is vital for reservoir development and management throughout the life of a field. While many reservoir properties can be evaluated remotely with seismic or logging studies, the most detailed and accurate measurements of rocks and fluids come from laboratory evaluations of core samples. The new Schlumberger Reservoir Laboratories organization focuses on the interrelationships of rock and fluid properties to help operators understand oil and gas assets. This organization comprises more than 25 laboratories globally and employs standardized procedures and equipment to support analysis of core data. This comprehensive integration of rock and fluid analysis services helps customers reduce risk in making reservoir development decisions. Combining both services in one organization expands the expertise at hand for execution of the experiments and interpretation of the results. One of the most obvious areas of rock-fluid interaction is in enhanced oil recovery (EOR) studies. To forecast improvements from miscible gas and chemical displacements, engineers require rock and fluid properties at reservoir conditions. EOR laboratory core floods use live reservoir fluids under reservoir conditions. Schlumberger has offered advanced fluids expertise, geomechanics and unconventional resource evaluation for years. In 2012, the company added several commercial conventional core analysis laboratories and established a laboratory hub in Houston. These laboratories provide routine and special core analysis capabilities, with a special emphasis on EOR evaluations for miscible gas and chemical flooding. Throughout its history, Schlumberger has used its breadth and depth of knowledge of the subsurface to develop logging tools, logging-while-drilling techniques, fracturing techniques and other services that require understanding of rock-fluid interactions. The new commercial core analysis capabilities build on this long history of rock studies. As a company, we have always answered questions about reservoirs through petrophysical analysis. Now, we extend this tradition to routine core analysis, answering basic questions about a formation: Does the formation contain fluids? Are hydrocarbons present? Can they flow through the formation? (See Core Truth in Formation Evaluation, page 16). The answers come from measurements of porosity, saturation and permeability. These properties are a part of any petrophysical study of a reservoir. In addition, core measurements provide a means to calibrate log interpretations of electrical properties and nuclear magnetic resonance responses to obtain downhole estimates of porosity and saturations. The petrography and sedimentology of the reservoir core can also be evaluated in detail. Reservoir engineering relies on dynamic flow simulations, and core analysis is fundamental to this activity. Multiphase properties relative permeability and capillary pressure come from measurements made in a special core analysis laboratory. Other flow studies conducted in these laboratories are designed to evaluate EOR processes and assess formation damage caused by a variety of sources. Core analysis is often referred to as the ground truth of rock properties. In the laboratory, we can measure properties more precisely and accurately than through remote sensing. But it is also necessary to understand that laboratory measurements may not be reflective of field conditions. Field conditions can be simulated to a greater or lesser degree, but some alterations to the rock caused by drilling and retrieval are difficult to reverse. To get the full picture, it is necessary to integrate the information from all sources laboratory and field. With the wide range of expertise found in Schlumberger, we can provide the necessary perspective. The future of core analysis is bright. We are commercializing our digital core analysis effort with services ranging from whole core to nanoscale imaging and flow simulation. Additional innovative services will be introduced in coming years. Mark A. Andersen Core Physics Domain Head Schlumberger Reservoir Laboratories Houston, Texas, USA Mark A. Andersen, Schlumberger Domain Head for Core Physics in Houston, joined the company in He spent 11 years as an Oilfield Review editor and executive editor before returning to his roots in core analysis to help build a new business for Schlumberger. He began his career in 1981 as a researcher in rock properties at Amoco Research Center in Tulsa. He subsequently spent several years in Stavanger, where he managed the Amoco Norway external research program and wrote Petroleum Research in North Sea Chalk. Mark is the author of many technical papers, including 23 articles for Oilfield Review. He earned a BS degree in engineering physics from the University of Oklahoma at Norman, USA, and MS and PhD degrees in physics from The Johns Hopkins University in Baltimore, Maryland, USA. 1

4 Schlumberger Oilfield Review Executive Editor Lisa Stewart Senior Editors Tony Smithson Matt Varhaug Rick von Flatern 1 Core Analysis: Combined Expertise for Insight into the Reservoir Editorial contributed by Mark A. Andersen, Core Physics Domain Head, Schlumberger Reservoir Laboratories Editor Richard Nolen-Hoeksema Contributing Editors Ginger Oppenheimer Rana Rottenberg Design/Production Herring Design Mike Messinger Illustration Chris Lockwood Tom McNeff Mike Messinger George Stewart 4 Formation Density from a Cloud, While Drilling A recently introduced formation density tool uses a pulsed neutron generator to induce gamma rays in a formation and compute bulk density. The LWD tool that houses the new measurement system is the first to offer a compact logging suite comparable to a triple-combo service, but without the use of radioisotopic sources. Printing RR Donnelley Wetmore Plant Curtis Weeks 16 Core Truth in Formation Evaluation Oil and gas companies obtain physical samples of subsurface formations through coring. Careful testing of these samples allows operators to determine if the rock contains fluid-filled pores, if those pores contain hydrocarbons and if those hydrocarbons are producible. Routine core analysis helps operators answer these questions and more. On the cover: Core analysis is an essential building block of formation evaluation. Most E&P companies rely on the specialized equipment and expertise of a core analysis laboratory to evaluate their core samples. Here, a core specialist removes a core plug from a solvent distillation and extraction device used to clean the core and measure the volume of any fluids contained therein. A computed tomography scan of a core (inset) shows changes in density indicative of variations in mineralogy or porosity. About Oilfield Review Oilfield Review, a Schlumberger journal, communicates technical advances in finding and producing hydrocarbons to customers, employees and other oilfield professionals. Contributors to articles include industry professionals and experts from around the world; those listed with only geographic location are employees of Schlumberger or its affiliates. Oilfield Review is published quarterly and printed in the USA. Visit for electronic copies of articles in English, Spanish, Chinese and Russian Schlumberger. All rights reserved. Reproductions without permission are strictly prohibited. For a comprehensive dictionary of oilfield terms, see the Schlumberger Oilfield Glossary at 2

5 Summer 2013 Volume 25 Number 2 ISSN Multistage Stimulation in Liquid-Rich Unconventional Formations To optimize the economics of producing oil from liquid-rich shales, service companies are refining the completion technology that has made possible the profitable exploitation of these tight formations. Operators are now able to take advantage of new completion tools and systems, which are designed to significantly improve the efficiency and effectiveness of stimulating low-permeability formations. Advisory Panel Hani Elshahawi Shell Exploration and Production Houston, Texas, USA Gretchen M. Gillis Aramco Services Company Houston, Texas Roland Hamp Woodside Energy Ltd. Perth, Australia Dilip M. Kale ONGC Energy Centre Delhi, India 34 Stimulation Design for Unconventional Resources Taking advantage of the combination of horizontal drilling and hydraulic fracturing technologies, operators are able to access ultralow-permeability reservoirs that contain oil and gas. A systematic, engineered completion design approach using a comprehensive workflow management software system is helping make hydrocarbon extraction from unconventional reservoirs more effective. GG GG GG GG GG GG od Stage 15 Stage 14 Stage 13 Stage 12 Stage 11 Stage 10 Stage 9 Stage 8 George King Apache Corporation Houston, Texas Andrew Lodge Premier Oil plc London, England GG Stage 7 G Stage 6 47 Contributors 49 New Books and Coming in Oilfield Review 51 Defining Hydraulic Fracturing: Elements of Hydraulic Fracturing This is the tenth in a series of introductory articles describing basic concepts of the E&P industry. Editorial correspondence Oilfield Review 5599 San Felipe Houston, TX United States (1) Fax: (1) editoroilfieldreview@slb.com Subscriptions Customer subscriptions can be obtained through any Schlumberger sales office. Paid subscriptions are available from Oilfield Review Services Pear Tree Cottage, Kelsall Road Ashton Hayes, Chester CH3 8BH United Kingdom subscriptions@oilfieldreview.com Distribution inquiries Matt Varhaug Oilfield Review 5599 San Felipe Houston, TX United States (1) DistributionOR@slb.com 3

6 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 Mauborgne Clamart, France Mike Evans Sugar Land, Texas, USA Roger Griffiths Petaling Jaya, Malaysia Fabien Haranger Christian Stoller Princeton, New Jersey, USA Doug Murray Abu Dhabi, UAE Nicole Reichel Stavanger, 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 scientist who is credited with the development of the density measurement and many other petrophysical measurements in use today, received a patent for the concept in 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 passing through the formation. Recently, a new technique has been introduced that eliminates the traditional gamma ray source in logging-whiledrilling (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 counting 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 density tools use a cesium [ 137 Cs] 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 measurements. Traditional thermal neutron porosity measurements rely on americium beryllium [ 241 AmBe] sources to generate the neutrons used in the measurement. Service companies go to great lengths to minimize 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 discouraged 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 241 AmBe neutron sources in both LWD and wireline 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 radioisotopic 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 formation to one or more detectors on a tool. The resultant bulk density measurement is similar to that 4 Oilfield Review

7 from a GGD tool, but it comes from the neutroninduced gamma rays. The density derived from this technique is referred to as a sourceless neutron 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 validate this new technique. 1. In this article, a source refers to a radioisotopic device used in petrophysical logging tools that emits ionizing radiation. 2. The radioisotope 137 Cs has a half-life of 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 226 Ra. 1 GBq = Ci. As Low as Reasonably Achievable Traditional sources used for petrophysical analysis are protected and isolated while being transported to and from drilling rigs and are stored in shields that protect personnel from exposure. Pressure vessels that house the radioactive elements are made from materials designed to protect sources from mechanical damage and 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 , presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, October 8 10, 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 corrosion in the harsh wellbore environment. While inserting a source into a logging tool, workers follow strict safety practices to eliminate potential for exposure. When the tool is lowered below the rig floor, the potential for human exposure goes with it. Sources must be handled carefully, but when established safety precautions are followed, there is little risk of exposure. 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): The term sourceless indicates that this measurement does not use radioisotopic sources. Summer

8 Pulsed Neutron Generator Main power n p+ On-off switch Deuterium 2H Controls Tritium 3H > Pulsed neutron generator (PNG). PNGs are self-contained particle accelerators that produce neutrons using a fusion reaction. A high voltage potential accelerates ionized deuterium and tritium isotopes of hydrogen toward a target doped with tritium (top). The fusion reaction (bottom) results in the production of a 4 He nucleus and a neutron. The reaction energy is transferred into the kinetic energy of the two particles and is converted into heat when the particles are stopped in matter. The neutrons leave the reaction with very high speed, having kinetic energy of approximately 14 MeV of the total 17.6 MeV released. When the main power is disconnected, the PNG produces no neutrons. In the early days of the nuclear age, which coincided with the development of many of the tools used in petrophysical analysis, radiation safety practices focused on time, distance and shielding: Minimize exposure time, keep maximum reasonable distance from radiation sources and maintain barriers (shielding) between people and material. These principles are still applied today for working with traditional sources, and exposure limits have been established to ensure the safety and health of workers who routinely handle these materials. Workers are also closely monitored to determine exposure levels. Observations of the long-term effects of radiation on humans resulting from surface detonation of nuclear devices, however, led scientists to develop a new methodology for dealing with human exposure. As low as reasonably achievable (ALARA) has emerged as the standard for regulators. The goal of ALARA is to eliminate exposure whenever and wherever possible, which has driven service companies to investigate alternatives to traditional sources such as 137 Cs and 241 AmBe. A PNG is one example of an alternative to traditional sources. 8 A PNG is a miniature particle generator. Deuterium [ 2 H] and tritium [ 3 H] are accelerated into a tritium-doped target, and high-energy (approximately 14 MeV) neutrons are released (above). When not electrically energized, PNGs do not emit external radiation. Scientists and engineers developed the first PNGs in the 1950s. These devices have since been adopted for many Ion source Helium 4He Neutron n n Target + n n n n p+ p+ n p+ + + High-voltage supply Kinetic energy E (17.6 MeV) downhole applications, including neutron porosity tools, cased hole formation evaluation tools and capture and inelastic spectroscopy services. PNGs have emerged as a viable alternative to 241 AmBe sources. For LWD operations, turbine generators have been developed to supply the downhole electrical power needed to operate PNGs. This advance has allowed design engineers to incorporate PNGs in services such as the EcoScope multifunction logging-while-drilling service and the NeoScope tool. 9 Attempts to replace 137 Cs sources used in GGD tools used for formation density, considered by many geoscientists to be one of the most critical parameters for the quantitative determination of formation porosity, have not met with similar success until recently. Scientists have been unable to replace 137 Cs-dependent measurements for a number of reasons. For example, there is no comparable electronic gamma ray generator, and replacing other sources was deemed a higher priority. The half-life of 241 AmBe is 432 years, much longer than the approximately 30-year half-life of 137 Cs. The activity of an 241 AmBe source is higher and also more difficult to shield. 10 If an LWD logging tool becomes stuck in a well, operators must ensure that the source will remain in place, intact and isolated for hundreds or even thousands of years. The shorter half-life of 137 Cs and its lower radiotoxicity do not remove the risk, but, compared to 241 AmBe, there is a reduced potential for long-term consequences. 11 As a way to mitigate risk associated with 241 AmBe sources, some operators have opted to use PNG-based wireline and LWD neutron porosity tools exclusively rather than tools with a traditional source. Additionally, the prospect that some countries may mandate the elimination of traditional sources entirely is a concern to both operators and service companies. Another reason for the delay in replacing density sources is that bulk density resulting from the GGD measurement is a fairly straightforward petrophysical parameter that has been accepted by the interpretation community for decades. Replacing GGD tools with SNGD tools adds a greater level of complexity and introduces some differences in measurement physics. 12 As a consequence, scientists have invested considerable time and resources in understanding the physics involved in using induced gamma rays for density measurements. In 2005, scientists and engineers at Schlumberger introduced the algorithms needed to compute an SNGD measurement. They were able to demonstrate that a sourceless density measurement that replicated traditional formation density measurements could be produced. Seven years later, they launched the first commercial PNG-based LWD gamma density tool in the oil and gas industry. This tool delivers a high-quality bulk density measurement comparable to that of traditional GGD tools. Because the technique uses a PNG in place of a traditional source, the tool complies with ALARA objectives For more on radioactive sources used in logging tools: Aitken JD, Adolph R, Evans M, Wijeyesekera N, McGowan R and Mackay D: Radiation Sources in Drilling Tools: Comprehensive Risk Analysis in the Design, Development and Operation of LWD Tools, paper SPE 73896, presented at the SPE International Conference on Health, Safety and Environment in Oil and Gas Exploration and Production, Kuala Lumpur, March 20 22, Japan Oil, Gas and Metals National Corporation (JOGMEC), formerly Japan National Oil Corporation (JNOC), and Schlumberger collaborated on a research project to develop LWD technology that reduces the need for traditional chemical sources. Designed around the pulsed neutron generator (PNG), NeoScope and EcoScope services use technology that resulted from this collaboration. The PNG and the comprehensive suite of measurements in a single collar are key components of the NeoScope and EcoScope services that deliver game-changing LWD technology. 10. Sources that emit gamma rays can be shielded using lead, although lead is not an effective shield for neutrons. Shields for neutron sources generally contain polyethylene. 11. Aitken et al, reference In some regions, operators consider the anhydrite measurement a validation of proper tool calibration. This value a density of 2.98 g/cm 3 is outside the quoted formation density range of the SNGD measurement. 13. The PNG used in the NeoScope tool contains a small amount 1.6 Ci of tritium, a radioisotope of hydrogen. The half-life of tritium is 12.3 years. Tritium is also used in conjunction with phosphorous in luminous watch dials and exit signs in buildings. 6 Oilfield Review

9 PNG-Based Measurements Other Measurements Neutron-gamma density Neutron porosity Spectroscopy Sigma Array resistivity Dual ultrasonic caliper Annular pressure while drilling Temperature Azimuthal gamma ray Near-bit inclination Three-axis shock and vibration Near epithermal detector Short-spacing gamma ray detector Far thermal neutron detectors Pulsed neutron generator Neutron flux detector Long-spacing gamma ray detector Near thermal neutron detectors > NeoScope LWD logging tool and its capabilities. Engineers designed the NeoScope tool (bottom) with several collocated petrophysical measurements on a single 7.6-m [25-ft] collar. The table (top) summarizes the tool s capabilities. More Than Just Density The scientists who developed the SNGD model worked with engineers to include this new design concept in the NeoScope sourceless formation evaluation while drilling service. Six petrophysical measurements are incorporated in the NeoScope platform SNGD, neutron porosity, elemental capture spectroscopy, sigma, resistivity and azimuthal natural gamma ray and they are collocated on a single, relatively short collar (above). The NeoScope LWD tool is generally located close to the bit, giving well placement engineers early and precise geosteering data. Near-bit positioning allows the tool to make measurements when drilling fluid invasion is still minimal, which further simplifies data interpretation and modeling. This is especially important for sigma measurements. The NeoScope tool also contains sensors to measure hole size, annular pressure and temperature, near-bit borehole inclination and triaxial shock and vibration. In addition to collocated measurements close to the bit, the NeoScope tool design has other benefits; the SNGD measurement has a greater depth of investigation (DOI) than traditional GGD tools have and is less dependent on wellbore wall contact for accurate measurements. Even a small standoff for the GGD tools may result in compromised measurements, and hole rugosity has always been problematic for traditional density tools (right). The SNGD measurement is collocated with the other neutron-based measurements and resistivity measurements. Conventional logging strings often have separate tools for each measurement. Collocating the sensors reduces the effects of irregular tool movement that can cause misalignment of depth reference points. Collocation also simplifies interpretation because the sensors are simultaneously measuring the same formation volume under identical static and dynamic conditions. The NeoScope service measures neutronbased petrophysical properties, along with bulk density. Most wireline and historical neutron porosity data come from tools that use 241 AmBe sources; the NeoScope service provides a comparable thermal neutron measurement. Formation hydrogen index (HI), the basis of neutron porosity computation, is also an output of the tool. The neutron count rates in near and far helium-3 detectors are used to determine HI and thermal neutron porosity. Compared with traditional thermal neutron porosity, this PNG-based HI is less sensitive to environmental conditions. Fraction of response GGD data SNGD data Sigma another output available from the NeoScope tool is the macroscopic thermal neutron capture cross section of the formation. Sigma is a measurement of the formation s ability to capture, or absorb, thermal neutrons, and the measurement can provide resistivity-independent fluid saturation in the presence of saline formation water. High-energy, fast neutrons are emitted by the tool, slowed by collisions with the nuclei of elements in the formation primarily hydrogen and then absorbed by receptive atoms and molecules. After these neutrons are absorbed, capture gamma rays are generated, which are counted by the detectors. The rate at which thermal neu- Borehole Plan View SNGD measurement volume Azimuthal density Depth of investigation Depth into formation, in. > Greater DOI of the SNGD measurement. Traditional GGD measurements, such as from LWD azimuthal density tools, read only a few inches into the formation (left, red) and have a narrow measurement aperture (right). Hole rugosity may negatively impact the quality of the measurement. Although the SNGD (green) has a greater DOI, which results in a measurement that is less sensitive to rugosity and standoff, it does not have an azimuthal component. Summer

10 Formation Detectors Nuclear source Gamma ray Incident gamma ray Compton Scattering Scattered gamma ray e > Compton scattering of gamma rays. For traditional density tools (left), gamma rays are emitted by a source and then interact with the formation in three main ways. Compton scattering (right) is the primary 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 Compton scattering is related to the electron density of the atoms that make up the minerals and fluids in the formation. Electron density is directly related to bulk density. The formation bulk density is computed from the number of gamma rays that make their way from the source, through the formation and back to the detectors. Higher density results in fewer returning gamma rays compared with measurements in lower density formations. Neutron energy, ev Electronic source Traditional source Neutron energy leaving source Average thermal energy ev Inelastic region High energy Intermediate energy Epithermal energy Neutrons with thermal energy Time, μs Capture gamma ray emitted > Life of a neutron. Both electronic and traditional sources emit high-energy, fast neutrons. Neutrons from the PNG electronic source used in the NeoScope tool have an initial kinetic energy of about 14 MeV but in a few microseconds reach thermal energy level (approximately ev). During those first few microseconds, before neutron kinetic energy falls below about 1 MeV, the neutrons experience inelastic collisions that produce gamma rays. These are the gamma rays used for SNGD processing. After several microseconds, the neutrons reach thermal energy level and are eventually captured. The capturing atoms generate gamma rays to return to ground state. trons are captured depends on the capture cross section sigma of the element absorbing them. The capture cross section of chlorine, which is the strongest neutron absorber of common elements encountered in well logging, is higher than that of oil or gas. If the porosity and formation water salinity are known, the water saturation can be determined from sigma. Because the measurement is acquired near the bit, it is possible to determine sigma in the absence of mud filtrate invasion. This establishes a reliable baseline for comparison with future cased hole sigma logs. An added benefit of water saturation computed from sigma data occurs when logging in high-angle wells. When high-angle and horizontal wells cross or approach bedding planes with resistivity contrasts, the resistivity measurements often exhibit anomalous readings. Because sigma data are not similarly affected at bed boundaries, saturation measurements computed from sigma may be more accurate than traditional computations that are based on Archie s equation. Missing from the SNGD measurement is the photoelectric factor (PEF) measurement. Conventional density tools include this lithology indicator for inferring the rock matrix a crucial input for computing density porosity. Although the PEF measurement is not available with the new technique, the NeoScope tool provides neutron capture spectroscopy, which delivers formation elemental composition information. These data offer petrophysicists a more reliable and accurate lithology determination than do PEF measurements. The primary drivers for development of a sourceless density tool have been environmental and security concerns. In some areas of the world, regulations prevent drillers from reentering a reservoir in which a traditional source has been left behind in a stuck drilling assembly. Because PNGs are inactive and cannot produce neutrons when circulation ceases, operators are often permitted to drill sidetrack wells very near a wellbore in which a sourceless tool has been lost. 14 The radioisotope-free nature of the NeoScope service is also attractive in unconventional plays because many of these are located near population centers, where the public may be wary of traditional sources. There are no traditional sources with the NeoScope service, completely eliminating their transportation and handling at the wellsite. The NeoScope service provides real-time natural gamma ray images to steer the well, triple combo data for petrophysical analysis and spectroscopic lithology information to accurately evaluate reservoir quality, but avoids raising public concern regarding the presence of radioactive sources. 8 Oilfield Review

11 It s Not Simple The physics of formation density measurements with GGD tools is relatively straightforward. As the 137 Cs in a typical logging source decays, it emits about gamma rays/s (GR/s). These GRs interact with the electrons of atoms in the formation in a variety of ways but primarily by Compton scattering (previous page, top). 15 These interactions result in most of the GRs being absorbed by the formation, but a few travel back to detectors in the tool located a fixed distance from the source. Formation density measurements are computed from the number of gamma rays traveling from the source to the detectors. From the original pool of GRs emitted by the source, a small fraction of the scattered gamma rays a few hundred to more than 10,000 GR/s will make it to the detectors. High-density rocks with little porosity result in fewer GRs returning to the tool than occurs in porous rocks filled with water, oil or gas. Gamma ray output can also vary from source to source. To compensate for differences in individual sources and detector efficiencies, each tool is calibrated to a fixed reference so the tool delivers the correct downhole density value. As previously noted, engineers have successfully developed tools that replace the 241 AmBe source with PNG-based tools for both neutron porosity and capture spectroscopy. The pursuit of a high-quality, radioisotope-free density measurement has been more elusive because of the lack of electronic gamma ray emitters analogous to PNGs to replace 137 Cs. To overcome this hurdle, Schlumberger scientists adapted some of the principles used for neutron-based measurements, such as spectroscopy and sigma, to develop the SNGD measurement. PNGs generate high-energy neutrons in short bursts. Neutrons leave the tool and interact with the various elements of the formation rocks and fluids. The interactions that have the greatest effect are predominantly elastic collisions with hydrogen nuclei (previous page, bottom). With successive collisions, the initial high-energy neutrons slow down and reach thermal energy level. 16 Thermal neutron porosity tools count the number of thermal neutrons that arrive back at the tool; from this count rate, the traditional thermal neutron porosity is computed. 17 Not all the collisions are elastic. Immediately after the initial burst of neutrons from the PNG, but before the neutrons reach thermal level, inelastic collisions occur between the fast neutrons and atomic nuclei in the formation (above right). Inelastic collisions cause some atomic nuclei to become excited and emit one or more Inelastic Neutron Scattering Neutron Capture Slow neutron n Inelastic gamma rays Excited nucleus > Neutron interactions. The neutron interactions relevant to petrophysical logging can be separated into three categories: Inelastic scatter (top), elastic scatter (not shown) and capture (bottom). Inelastic gamma rays are generated by the interaction of a fast neutron typically with energy greater than 1 MeV with a nucleus. The interaction lifts the nucleus into an excited state, the neutron emerges with less energy and one or more gamma rays are emitted. Also counted among the inelastic gamma rays are those following a high-energy nuclear reaction, such as a reaction in which the neutron knocks out a particle such as an alpha particle, a proton or a second neutron from the nucleus. In elastic scattering, the neutron bounces off the nucleus without pushing it into an excited state. The only energy loss is from the kinetic energy imparted to the nucleus on which the scattering occurs. Elastic scattering from hydrogen, the essential mechanism underlying the neutron porosity measurement, is a result of the collision between particles of equal mass neutron and proton which causes maximum energy loss. The neutron capture reaction, in which a neutron can be absorbed by a nucleus, dominates at low neutron energy. This leaves the absorbing nucleus in an excited state and the resulting deexcitation is accompanied by the emission of gamma rays. 14. In 1999, the US Nuclear Regulatory Commission (NRC) modified existing regulations to exempt PNGs from well abandonment procedures applied to radioisotopic sources. For more: NRC: Regulatory Analysis of Energy Compensation Sources for Well Logging and Other Regulatory Clarifications Changes to 10 CRF Part 39, Office of Nuclear Materials Safety and Safeguards (December 1999), ML0036/ML pdf (accessed April 29, 2013). 15. Compton scattering occurs when a gamma ray collides with an electron, transferring part of its energy to the electron, while itself being scattered. The gamma ray continues at a reduced energy. The degree of Compton n n Excited nucleus Capture gamma ray scattering depends on the electron density of the target material. As the electron density increases, there is more attenuation of gamma ray energy. 16. PNGs emit fast neutrons with a kinetic energy level of about 14 MeV. Thermal neutrons have a kinetic energy of about 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 New Integrated LWD Platform Brings Next-Generation Formation Evaluation Services, Transactions of the SPWLA 46th Annual Logging Symposium, New Orleans, June 26 29, 2005, paper H. Summer

12 GRs as they return to ground state. Scientists are able to use the energy spectrum of inelastic GRs to identify elements such as carbon, oxygen, silicon, calcium, iron and sulfur. Engineers use the volumetric yields of these elements to compute lithology, and this is the basis of neutron spectroscopy measurements. The energy spectrum of inelastic gamma rays is also the basis of carbon/oxygen ratio tools, which are used to identify hydrocarbonbearing zones in cased holes. During the short period of inelastic collisions, a GR cloud forms (below). This artificially generated cloud emits around 10 8 GR/s, about two orders of magnitude lower than the number emitted by a typical 137 Cs source. Scientists have determined, however, that there are sufficient GRs produced to function in a manner similar to that of a traditional source. The GR cloud is short-lived because the neutrons that create it collide with other nuclei, rapidly slow to thermal level and are subsequently captured. The number of gamma rays that result from inelastic collisions and reach the detectors from the GR cloud is influenced by three factors: the Inelastic gamma ray source volume Inelastic gamma ray scattering volume PNG Neutron detector Inelastic scattering Gamma ray detector > Inelastic gamma ray cloud. The PNG generates neutrons that move away from the source and collide inelastically with atoms in the formation (blue shading). These collisions cause a cloud of inelastic gamma rays to form (green shading). Some of these gamma rays will travel back to the tool and be counted by the detectors. Inelastic count rate, counts/s Long-spacing detector response Gamma ray transport Neutron transport Formation density, g/cm 3 > Nuclear transport and long-spacing detector response. The response of the long-spacing gamma ray detector (black) is largely determined by neutron (blue) and gamma ray transport (red). Neutron transport is related to the interactions of neutrons with atomic nuclei in the formation. Inelastic gamma rays are produced during inelastic scattering of fast neutrons. Elastic scattering, which occurs primarily when neutrons collide with hydrogen nuclei, reduces the energy of the fast neutrons below the threshold for producing inelastic gamma rays. Thus, with increased formation density (lower porosity), there are fewer hydrogen nuclei available for elastic scattering and, as a result, there are more fast neutrons available for the production of inelastic gamma rays. Gamma ray transport and the number of inelastic gamma ray counts decrease with increased formation density because the higher electron density provides more opportunity for gamma ray interactions and energy reduction. fast neutron transport from the PNG to the point where inelastic GRs are produced within the formation, the subsequent transport of GRs from their origin back to the detectors in the tool and the electron density of the formation. The GRs generated in the formation by inelastic interactions move rapidly through the formation, interacting in a manner similar to GRs generated by a radioisotopic source, and they are attenuated by collisions with electrons within the formation primarily through Compton scattering (above). Properly characterized, the counts at the detector are used to compute electron density, which in turn is used to compute the formation bulk density. 18 If only inelastic GRs were present, the characterization would be more easily performed; however, another major source of GRs complicates the measurement. Fast neutrons eventually become thermal neutrons and are captured by atoms in the formation. Nuclei that capture thermal neutrons emit GRs to return to a stable energy state in a manner similar to the emission of GRs resulting from inelastic collisions. The population density of thermal neutrons available for capture is directly related to the number of hydrogen atoms in the formation. In a typical downhole environment, the element with the highest probability of absorbing thermal neutrons is chlorine [Cl], whose number density is related to the salinity of the formation fluids. The SNGD measurement is based only on GRs generated by the inelastic collisions. To correctly compute the bulk density value, the contributions from capture GRs resulting from neutron capture must be quantified and removed from the measurement. 19 Engineers must also account for the variability of the initial source strength. The output of a traditional source may vary, depending on age and activity level of the radionuclide, but the output is fairly constant and its change over time is predictable. Calibration of GGD tools accounts for variability between sources and detector efficiencies by correcting to a known reference. The output of a PNG is not as predictable and may vary over short periods of time and even between bursts. A control loop in the NeoScope tool adjusts the PNG to maintain a constant average output, and the tool includes a detector at the 18. Reichel et al, reference Epithermal neutrons have an energy range between about 0.02 ev and 10 kev at room temperature. 10 Oilfield Review

13 NeoScope Calibration Facility NeoScope tool Mud channel Calibration sleeve Aluminum calibration sleeve Water Detectors > NeoScope calibration device. A special calibration facility was developed specifically for the NeoScope tool. Four measurements are performed in a water-filled tank using a calibration sleeve and a simulated mud channel. With the PNG turned on, responses are measured in four configurations: sleeve raised, mud channel filled with air (1); sleeve raised, mud channel filled with water (2); sleeve lowered, mud channel filled with water (3); and sleeve lowered, mud channel filled with air (4). These four measurements allow calibration gains and offsets to be computed and provide quality checks for tool verification. PNG to determine the neutron output and compensate for variations. To provide the specified g/cm 3 accuracy for the density measurement, the SNGD model uses a combination of responses from multiple detectors and requires a complex and demanding calibration. This calibration consists of correlating the count rates measured by each of the tool s detectors to those measured in the same environment with the reference tool. For this purpose, engineers have designed a new calibration tank that allows measurements over a wide range of count rates (above). The uncertainties found in downhole log measurements arise from the primary measurement, applied corrections and conversion of measured parameters to formation properties. To mitigate these uncertainties, the NeoScope service includes a quality control system that begins with general tool system hardware and moves to specific sensor functions, individual sensor measurements and integrated measurements that may involve multiple individual sensor responses (below). The last step of the process is quality control of the final integrated answers that may use multiple measurements. Neutron monitor PNG Long-spacing gamma ray detector Source output correction (neutron monitor) Near epithermal detector Near thermal detector Short-spacing gamma ray detector Far thermal detector Long-spacing gamma ray detector Sigma input Spectroscopy input Neutron porosity input Neutron-gamma density input Neutron transport correction (near epithermal and far thermal detectors) Fast neutron correction (short- and long-spacing gamma ray detectors) Sigma correction SNGD output > Multi-input, multioutput measurements. The nuclear portion of the NeoScope tool (left) uses a single PNG to generate neutrons, but the responses from multiple detectors are integrated to produce specific measurements. For example, sigma data are derived from near thermal, short-spacing gamma ray and long-spacing gamma ray detectors. SNGD data, the most complex 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 thermal detectors are required to provide an accurate final answer. The flowchart (right) traces the corrections applied to arrive at the final density output. Summer

14 > Specifications for SNGD and GGD tools. Individual quality control considerations that may impact accuracy include sensor and hardware functionality, density values within the 1.7- to 2.9-g/cm 3 range of SNGD and tool standoff. In addition, environmental quality controls include borehole size, deviation, ROP and formation shaliness, all of which may impact measurement accuracy (above). The indicators are combined into a measurement quality control flag. A green flag suggests that the measurement is accurate and within specified limits. A yellow flag indicates that the measurement is likely to be within its specified GGD data, g/cm 3 SNGD GGD Density range 1.7 to 2.9 g/cm to 3.05 g/cm 3 Precision at ROP 61 m/h [200 ft/h] g/cm g/cm 3 Accuracy Clean sandstone, limestone and dolomite g/cm g/cm 3 Shale g/cm g/cm 3 Salt Not applicable g/cm 3 Anhydrite Not applicable g/cm 3 Axial resolution 89 cm [35 in.] 36 cm [14 in.] Depth of investigation 25 cm [10 in.] 10.2 cm [4 in.] Image capability No Yes Data within tolerance Data at limit of tolerance range but may require further interpretation, and a red flag means that the measurement is outside specified accuracy parameters. These quality flag values are crucial for comparing the accuracy of GGD and SNGD measurements. Field Testing and Beyond Field tests for the SNGD measurements consisted of comparing them with GGD measurements using a modified tool that allowed engineers to acquire both measurements simultaneously from the same well using the same SNGD data, g/cm 3 > Crossplot comparison. Density data from a GGD tool were compared with data 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 within tolerance. The data align well along the ideal axis and are flagged as green. Invasion effects start to occur in the lower density range at approximately 2.3 g/cm 3. The spread of the data points around the ideal line is attributed to differences in the axial resolution of the two measurements while crossing various layers at high deviations. bottomhole assembly. Objectives for field testing 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 results and accounted for the differences and limitations of both measurements. Test acceptance criteria were based on a systematic evaluation of both measurements, and final analysis was based on a set of numerical interpretation criteria. 20 The maximum acceptable error when two independent measurements are compared is the sum of their individual accuracies. In this case, the acceptable error for the two measurements is g/cm 3 in clean formations and g/cm 3 in shales. 21 The data from the combined tools were plotted, which allowed engineers to quantify any deviation from perfect agreement. Additionally, scientists had to account for conditions in each well that might impact GGDto-SNGD comparisons. These conditions included filtrate invasion, the presence of gas or light hydrocarbons that may change with time and various drilling conditions, such as mud weight, fluid variations and changes in ROP. If a large discrepancy between the two measurements could be explained by environmental effects, the test was considered acceptable. All tests were performed in 8 1 /2-in. boreholes. In a field test of the NeoScope service, the operator drilled a well with an average inclination of 60 through a sandstone reservoir using 1.26-g/cm 3 [10.5-lbm/galUS] water-base mud (WBM). The caliper log indicated the borehole was in gauge, and no GGD data corrections were required. Additionally, the GGD data indicated no major azimuthal effects. Sigma was within a range that indicated minimal correction to the SNGD. In the hydrocarbon-bearing section of the formation, the resistivity log indicated some invasion (next page). Because of the difference in their DOIs, the SNGD and GGD outputs were slightly different in this zone. By contrast, these measurements were almost identical in a noninvaded water-bearing section of the formation. The SNGD data were within accuracy limits throughout the well (left). 20. Reichel et al, reference Theys P: Log Data Acquisition and Quality Control. Paris: Editions Technip, 2nd edition, Oilfield Review

15 Resistivity Mudcake Washout Density Caliper 8 in. 10 Ultrasonic Caliper 8 in. 10 Gamma Ray 0 gapi in. Attenuation 34-in. Attenuation 28-in. Attenuation 22-in. Attenuation Density Image 16-in. Attenuation 0.02 ohm.m g/cm Deviation 40-in. Phase Shift Sigma 0 degree in. Phase Shift 0 cu 50 Collar 28-in. Phase Shift Image-Derived Density Rotation 22-in. Phase Shift 1.9 g/cm RPM in. Phase Shift Bulk Density Upper Depth, ft 0.2 ohm.m 2, g/cm Density Correction 0.8 g/cm Neutron Density 1.9 g/cm Bulk Density 1.9 g/cm Neutron Porosity (Thermal) 40 % 15 Quadrant Bulk Density Data Average Density 1.9 g/cm Bottom Density 1.9 g/cm Left Density 1.9 g/cm Right Density 1.9 g/cm Up Density 1.9 g/cm Pyrite Water Sandstone Clay Quality Flags X10 X20 X30 X40 X50 X60 X70 > 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 indicated by separation in the resistivity curves (Track 2, blue shading). The sandstone below X60 ft (red shading) is water filled, and the lack of separation indicates little 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 fairly homogeneous reservoir, as does the lithology computed from spectroscopy data (Track 6). Quadrant density data (Track 5) overlie each other through the two 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. These data overlie the thermal neutron porosity data (blue) in clean, water- or oil-filled rocks. (Adapted from Reichel et al, reference 5.) Summer

16 Mudcake Washout Density Caliper 8 in. 10 Ultrasonic Caliper 8 in. 10 Gamma Ray 0 gapi 150 Deviation 0 degree 90 Collar Rotation 0 RPM 500 Depth, ft Resistivity 40-in. Attenuation 34-in. Attenuation 28-in. Attenuation 22-in. Attenuation 16-in. Attenuation 0.02 ohm.m in. Phase Shift 34-in. Phase Shift 28-in. Phase Shift 22-in. Phase Shift 16-in. Phase Shift 0.2 ohm.m 2,000 Density Image 1.7 g/cm Sigma 0 cu 50 Image-Derived Density 1.9 g/cm Bulk Density Bottom 1.9 g/cm Bulk Density Upper 1.9 g/cm Density Correction 0.8 g/cm Neutron Density 1.9 g/cm Bulk Density 1.9 g/cm Neutron Porosity (Thermal) 40 % 15 Quadrant Bulk Density Data Average Density 1.9 g/cm Bottom Density 1.9 g/cm Left Density 1.9 g/cm Right Density 1.9 g/cm Up Density 1.9 g/cm Carbonate Sandstone Clay Quality Flags X10 X20 X30 > Comparison of washout effects on density. Density data were acquired using a NeoScope tool and a conventional GGD LWD tool across a predominantly water-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 from the 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 provides good 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 right quadrant (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 or hole rugosity, the yellow quality flag (Track 7) indicates the measurements are approaching the limits. (Adapted from Reichel et al, reference 5.) In another field test conducted in a limestone formation at the Schlumberger test facility in Cameron, Texas, USA, engineers drilled a well with an average inclination of 25 using 1.13-g/cm 3 [9.4-lbm/galUS] WBM (above). The caliper log indicated hole enlargement in the top section of the log. In zones where the SNGD quality control flag was yellow, there were significant differences between the SNGD and GGD data. The density correction on GGD data was generally between 0.1 and 0.15 g/cm 3, which is not usually indicative of compromised data quality resulting from hole rugosity, although the quadrant density data clearly showed effects of the enlarged borehole. Analysis of these two logs highlighted the value of the greater DOI of the SNGD measurement. The SNGD data were borehole corrected and, because of the NeoScope tool s greater DOI, were less influenced by variations in the near-borehole environment. The SNGD curve tracks the thermal neutron porosity curve in clean formations as expected. The SNGD data appear more reliable than the traditional GGD measurement. A Middle East operator tested the new SNGD design in four environments. 22 The NeoScope tool was run in a high-angle, high gas saturation reservoir drilled with nonaqueous mud, a high gas saturation reservoir drilled with WBM, an oilsaturated carbonate reservoir drilled with high- 14 Oilfield Review

17 salinity WBM and an oil-saturated carbonate reservoir drilled with low-salinity WBM. To validate the measurements, traditional GGD tools were run for comparison. The first test was in an 8 1 /2-in. wellbore, in which the high-angle well approached 90 deviation at TD. The nonaqueous mud system was barite-saturated, which invalidated PEF measurements from the GGD tool. The reservoir section was predominantly limestone and the formation density ranged from around 1.95 to 2.7 g/cm 3. A comparison of the data from the GGD tool with those from the NeoScope SNGD measurement shows excellent agreement (right). One benefit of the NeoScope tool is the availability of neutron capture spectroscopy data. Although the PEF measurement from the traditional tool was affected by barite in the mud system, lithology could still be determined using spectroscopy data from the NeoScope tool. The majority of the interval was limestone, although some dolomite was observed. The second example was a vertical well drilled with WBM through a gas-filled carbonate reservoir in the same field as the previous well. Comparison of GGD with SNGD data again showed good agreement across a wide range of values. A third example was drilled with high-salinity WBM through an oil-saturated carbonate reservoir. In this highly deviated well, the porosity data from the GGD and SNGD measurements compared favorably, well within statistical precision limits of the measurements. As is typical of liquid-filled reservoirs, the neutron porosity data values were similar to porosities computed from formation density data. A fourth well was a high-angle well drilled with low-salinity polymer-base WBM. As with the other three wells, there was excellent agreement between the SNGD data and conventional GGD measurements. Petrophysical analysis of data from these four wells demonstrated that in a variety of wells with a wide range of density values, SNGD data from the NeoScope tool compare favorably with data from conventional density tools. In addition to the SNGD data, the neutron porosity and resistivity measurements provide a sourceless triplecombo logging option for LWD applications. Sigma and spectroscopy data are added benefits that petrophysicists can use to better characterize and understand reservoirs. 22. Atfeh M, Al Daghar KA, Al Marzouqi K, Akinsanmi MO, Murray D and Dua R: Neutron Porosity and Formation Density Acquisition Without Chemical Sources in Large Carbonate Reservoirs in the Middle East A Case Study, Transactions of the SPWLA 54th Annual Logging Symposium, New Orleans, June 22 26, 2013, paper KKK. Bit Size 8 in. 10 Ultrasonic Caliper 8 in. 10 Gamma Ray 0 gapi 100 Depth, ft X,300 X,400 X,500 X,600 Sigma Resistivity 40-in. Phase Shift 34-in. Phase Shift 28-in. Phase Shift 22-in. Phase Shift 16-in. Phase Shift 0 cu ohm.m 2,000 Density Correction 0.8 g/cm Neutron Porosity (Corrected) 40 % 15 Bulk Density 1.9 g/cm Density Image Neutron Density 1.9 g/cm g/cm Neutron Porosity (Thermal) Bulk Density 40 % g/cm Lithology Dolomite Sandstone > Density comparison in a barite-weighted mud system. Barite in drilling mud can render PEF measurements 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 tool provide mineralogy information (Track 6) that would not have been available from traditional density tools. For example, the data show dolomite mixed with calcite from X,350 to X,420 ft. In the high-density carbonate intervals, such as from X,400 to X,520, the NeoScope density data (Track 4, black) compare favorably with traditional bulk density (red). Traditional thermal neutron porosity (blue) is presented along with a density-corrected thermal neutron porosity (green). The NeoScope tool does not provide azimuthal 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 contacts over time. Track 3 presents resistivity data. (Adapted from Atfeh et al, reference 22.) The Pulse of Things to Come? It has been a long time coming, but the introduction of SNGD technology may revolutionize LWD porosity logging. Replacing sources with PNGs has the potential to eliminate exposure risks and reduce costs associated with source storage, transportation and record keeping. Calcite Introducing similar measurements for wireline applications is the next obvious step. Unfortunately, modeling borehole effects on the measurement for wireline tools has been beyond the reach of current research and software. It may take some time, but if traditional sources can be replaced in wireline tools, the ALARA standard as low as reasonably achievable will be reached in the oil and gas industry. TS Clay Summer

18 Core Truth in Formation Evaluation The nature of subsurface exploration forces oil and gas companies to investigate each reservoir remotely, primarily through well logs, seismic surveys and well tests. Through analysis of rock samples obtained downhole, core laboratories provide a wealth of information about lithology, porosity, permeability, fluid saturation and other properties to help operators better characterize the complex nature of the reservoir. Mark A. Andersen Brent Duncan Ryan McLin Houston, Texas, USA Oilfield Review Summer 2013: 25, no. 2. Copyright 2013 Schlumberger. For help in preparation of this article, thanks to Angela Dippold Beeson, David Harrison, Mario Roberto Rojas and Leslie Zhang, Houston; Carlos Chaparro and Adriano Lobo, Ecopetrol, Bogotá, Colombia; Alyssa Charsky, Michael Herron and Josephine Mawutor Ndinyah, Cambridge, Massachusetts, USA; William W. Clopine, ConocoPhillips Company, Houston; Rudolf Hartmann, BÜCHI Labortechnik AG, Flawil, Switzerland; Thaer Gheneim Herrera, Bogotá, Colombia; Wendy Hinton, Himanshu Kumar and David R. Spain, BP, Houston; Upul Samarasingha, Salt Lake City, Utah, USA; Tony Smithson, Northport, Alabama, USA; and Elias Yabrudy, Coretest Systems, Morgan Hill, California, USA. Techlog, TerraTek and XL-Rock are marks of Schlumberger. PHI-220 Helium Porosimeter is a mark of Coretest Systems, Inc. LECO is a mark of the LECO Corporation. Cores provide essential data for the exploration, evaluation and production of oil and gas reservoirs. These rock samples allow geoscientists to examine firsthand the depositional sequences penetrated by a drill bit. They offer direct evidence of the presence, distribution and deliverability of hydrocarbons and can reveal variations in reservoir traits that might not be detected through downhole logging measurements alone. Through measurement and analysis of porosity, permeability and fluid saturation from core samples, operators are better able to characterize Whole Core Segment 1 ft Full Diameter Core Analysis pore systems in the rock and accurately model reservoir behavior to optimize production. Core analysis is vital for determining rock matrix properties and is an important resource for formation characterization. The process known as routine core analysis helps geoscientists evaluate porosity, permeability, fluid saturation, grain density, lithology and texture. Routine core analysis laboratories (RCALs) frequently provide a variety of additional services such as core gamma logging for correlating core depth with wellbore logging depth, core computed tomography (CT) scans for 1 ft Core Plug Analysis 2.5 to 3 in. 3 ft 1 ft 1 ft 1 ft 1 ft > Divided cores. At the wellsite, whole cores are typically cut into smaller segments for ease of shipping. At the laboratory, the whole core segments may be cut and subsampled. 16 Oilfield Review

19 characterizing rock heterogeneity and core photographs for documenting and describing the core. When operators need to understand complex reservoir behaviors, they turn to special core analysis for detailed measurements of specific properties. Special core analysis laboratories (SCALs) are typically equipped to measure capillary pressure, relative permeability, electrical properties, formation damage, nuclear magnetic resonance (NMR) relaxation time, recovery factor, wettability and other parameters used for calibrating logs. SCAL services are also used to characterize reservoirs for enhanced oil recovery (EOR) and for studying multiphase flow and rock-fluid interactions. Only a few samples are selected for these extensive tests, some of which require weeks to complete. For years, Schlumberger has maintained a number of core analysis laboratories to support research into wireline tool response, drilling fluid chemistry, formation damage, EOR or completion technology. However, these facilities did not provide core analysis on a commercial scale. Until recently, the company s commercial core analysis services were centered in Salt Lake City, Utah, USA, where the TerraTek rock mechanics and core analysis facility is known for its focus on geomechanics and unconventional reservoirs. The 2012 inauguration of Schlumberger Reservoir Laboratories has opened the way for integrating rock measurement technologies with fluids expertise to help customers better understand reservoir behavior. Schlumberger now offers rock and fluid analysis through 27 laboratories around the globe. Several companies offer similar analyses of conventional cores. This article focuses on routine analysis of conventional sandstone and carbonate cores carried out by specialists at the Schlumberger Reservoir Laboratory in Houston. Sample Sizes Cores come in a variety of lengths and diameters (previous page). The information extracted from a core depends in part on the size and quantity of the core, which control the types of analyses that may be performed. To meet customer needs, the core analysis laboratories must be flexible enough to process the various types of core sent from the wellsite, be they bottomhole cores or sidewall cores. Summer

20 Bottomhole cores, also referred to as whole cores, or conventional cores, are obtained during the drilling process using a special coring bit (below). The cores typically range in diameter from 4.45 to 13.3 cm [1.75 to 5.25 in.] and are generally drilled in 10-m [30-ft] increments, which correspond to the length of the core barrel or its liner. Whereas a conventional bit is designed to grind away the rock at the bit face, the doughnut-shaped coring bit creates a cylinder of rock that passes through the center of the bit and is retained in a protective core barrel. When the core barrel is full, the driller pulls the assembly out of the hole, and a wellsite coring specialist lays the barrel liner on the pipe rack. The liner, with core inside, is then scribed with depth markings and orientation lines. For ease of shipping, the metal liner is usually cut into 1-m [3-ft] segments and sealed at each end. To prevent shifting during transit, the wellsite corehandling team may inject epoxy or foam into the liner to stabilize the core. For sidewall cores (SWCs), the process is far less involved. SWCs are obtained by a wireline sampling device, which is typically run in the hole near the conclusion of an openhole wireline logging job after the operator consults the logs to identify zones that merit sampling. The SWC device can extract up to 90 samples from the side of the wellbore at selected depths. Once on the surface, sidewall cores are retrieved from the tool, sealed in individual bottles and shipped to the laboratory for analysis. Percussion-type sampling devices obtain SWCs measuring from about 2.86 to 4.45 cm [1.125 to 1.75 in.] in length by 1.75 to 2.54 cm [0.688 to 1 in.] in diameter. Percussion sampling devices are known as core guns because they use small explosive charges to propel individual core barrels, called bullets, into the formation. The core barrels are attached to the gun with strong cables that are used to pull the core bullet from the borehole wall as the gun is reeled uphole. By contrast, rotary cores are cut from the formation using a miniature, horizontally oriented coring bit. The XL-Rock large-volume rotary sidewall coring tool can drill cores 6.4 cm [2.5 in.] long by 3.8 cm [1.5 in.] OD from the side of the borehole. This device produces core samples that have more than three times the volume of percussion SWCs. A third type of rock sample is the core plug. The core plug is extracted from segments of whole core. These plugs are taken as a representative subsample of the whole core and are useful in analyzing intervals of relatively homogeneous core. Core plugs in conventional reservoirs are routinely taken at 0.3-m [1-ft] intervals along the length of the core and measure about 6.4 cm long by 2.54 or 3.8 cm in diameter. Variations in lithology may require smaller sampling intervals, but if the core is highly heterogeneous, as seen in vugular or fractured carbonates or thinly laminated sand-shale intervals, the operator may elect to analyze the whole core rather than plugs. Initial Processing The basic workflow for conventional core analysis moves from receiving to preliminary imaging, then to preparation and analysis. Each process involves several steps. Whole cores typically require more initial processing than sidewall cores. Although routine core analysis provides a standard set of measurements, not all cores go through the entire workflow described here. At the laboratory, cores are received and inventoried. Whole cores are run through a core gamma ray logger, which measures gamma rays that are naturally emitted from the cores. By comparing core gamma ray measurements to LWD or wireline gamma ray logs, geoscientists can correlate core depth to log depth and identify intervals from which core may have been lost or damaged. The core gamma ray logging device uses a conveyor to move the core either exposed or sealed within the liner past a gamma ray detector. The detector scans the core along its length from bottom to top, which replicates the logging sequence used in obtaining wireline logs. Next, the core is run through a computed tomography scanner to obtain a CT image. The CT device obtains a 3D image of the whole core, taking a series of closely spaced scans that can be sliced at any point or orientation to create a virtual slab of the core. The CT scanner permits a quick reconnaissance across the core. When zones of interest are identified, they may be scanned again for detailed examination (next page). CT scanning is especially useful for detection and evaluation of internal features such as bedding planes, vugs, nodules, fossils and frac- > Coring bit. This polycrystalline diamond compact (PDC) bit employs a fixed cutter design that leaves the center of the borehole untouched. The bit creates a cylindrical core of the formation that passes through the middle of the bit, to be retained within the bottomhole assembly. 1. For more on CT scans in oilfield applications: Kayser A, Knackstedt M and Ziauddin M: A Closer Look at Pore Geometry, Oilfield Review 18, no. 1 (Spring 2006): Passey QR, Dahlberg KE, Sullivan KB, Yin H, Brackett RA, Xiao YH and Guzmán-Garcia AG: Digital Core Imaging In Thinly Bedded Reservoirs, in Dahlberg KE (ed): Petrophysical Evaluation of Hydrocarbon Pore-Thickness in Thinly Bedded Clastic Reservoirs. Tulsa: The American Association of Petroleum Geologists, AAPG Archie Series, no. 1 (June 30, 2006): Perarnau A: Use of Core Photo Data in Petrophysical Analysis, Transactions of the SPWLA 52nd Annual Logging Symposium, Colorado Springs, Colorado, USA, May 14 18, 2011, paper Z. 18 Oilfield Review