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Special Section: B o r e h o l e g e o p h y s i c s a n d s on i c l o g g i n g

March 2015

Volume 34, No. 3

TTI Migration

Orthorhombic Migration

Is complex geology yielding poor imaging results anduncertainty in your energy investment decisions?Get more accurate velocity models and pre-stack depth migrationswith Orthorhombic imaging.Orthorhombic imaging consists of a set of tools that allow azimuthal anisotropy to be incorporatedinto velocity model building and pre-stack depth migration. In areas of complex geology, especiallywhere fractures are present, commonly used Tilted Transverse Isotropy (TTI) may not be sufficientfor describing moveout variations with azimuth. To answer this, TGS has developed a completesuite of Orthorhombic imaging toolstomography with Kirchhoff, RTM and Beam migrations.

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The Leading Edge

Table of Contents274.. ..... Foundation News: Foundation Board of Directors proudly welcomes its new leaders, P. Allison308.. ..... U ltimate use of prestack seismic data: Integration of rock physics, amplitude-preservedprocessing, and elastic inversion, S. Z. Sun, P. Yang, L. Liu, X. Sun, Z. Liu, and Y. Zhang316.. ..... C onference Review: IPTC KL breaks all records, J. M. Reilly320.. ..... W orkshop Review: Report on the SEG/KOC joint workshop in Kuwait: Seismic multiples Are they signal or noise?, A. El-Emam, C. Kostov, and M. Hadidi326.. ..... M eter Reader: Generating a high-resolution global gravity model for oil exploration: Part 1 Land data compilations, J. D. Fairhead

Departments266........ Editorial Calendar268........ Presidents Page272........ From the Other Side336........ Seismos338........ Memorials340........ Personals342........ Reviews346........ Announcements350........ SEAM

332.. ..... S EG Wiki Interview Series: Hall: Its up to us to build the future of the science, I. Farley

354........ Student Zone

348.. ..... State of the Net: From ORCID iDs to SEG Wiki help, I. Farley

358........ Meetings Calendar362........ Membership

Special section: Borehole geophysics and sonic logging

364........ Ad Index

276.. ..... I ntroduction to this special section: Borehole geophysics and sonic logging, T. Smith, C. Torres-Verdn,and A. C. H. Cheng278.. ..... C an we ever trust the shear-wave log?, A. Cheng286.. ..... I n situ calibrated velocity-to-stress transforms using shear sonic radial profiles for time-lapseproduction analysis, J. A. Donald and R. Prioul296.. ..... A nisotropy estimate for the Horn River Basin from sonic logs in vertical and deviated wells,C. Sayers, L. den Boer, S. Dasgupta, and B. Goodway

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PRESIDENT-ELECTJohn BradfordBoise State UniversityDepartment of Geosciences1910 University DriveBoise, ID 83725, USATel: [email protected]

DIRECTOR AT LARGEChris KrohnExxonMobil Upstream Research Co.Nature 2, 3B.35422777 Springwoods Village ParkwaySpring, TX 77389 USATel: [email protected]

FIRST VICE PRESIDENTRobert R. StewartUniversity of HoustonEarth and Atmospheric SciencesSR1 131CHouston, TX 77204, USATel: [email protected]

DIRECTOR AT LARGEGuillaume CamboisPGSLilleakerveien 4CP.O. Box 251 LilleakerOslo, Norway 0216Tel: +47 4143 [email protected]

SECOND VICE PRESIDENTEve SpruntEve Sprunt and Associates3753 Oakhurst WayDublin, CA 94568, USATel: [email protected]

DIRECTOR AT LARGEGustavo J. CarstensCalle 6 e/526 y 527 #5501900 La Plata, Buenos Aires,ArgentinaTel: +54 911 4439 [email protected]

TREASURERAlison Weir SmallParallel Petroleum LLC1004 North Big Spring StreetSuite 400Midland, TX 79701, USATel: [email protected]

DIRECTOR AT LARGEMaurice NessimSchlumberger10001 Richmond AvenueHouston, TX 77042, USATel: [email protected]

EDITOREvert SlobDelft University of TechnologyStevinweg 12628 CN, Delft, The NetherlandsTel: [email protected]

DIRECTOR AT LARGEXianhuai ZhuConocoPhillips600 North Dairy AshfordHouston, TX 77079, USATel: [email protected]

Julie ShemetaMEQ Geo Inc.Highlands Ranch, CO, USATel: [email protected]

PAST PRESIDENTDon SteeplesUniversity of KansasDepartment of Geology903 Juniper Box 99Palco, KS 67657, USATel: [email protected]

CHAIR OF THE COUNCILMike GraulTexSeis, Inc.10810 Katy Freeway, Suite 201Houston, TX 77043, USATel: [email protected]

Tracy J. StarkStark Reality, Inc.5021 Sparrows Point DrivePlano, TX 75023, [email protected]

THE LEADING EDGE

March 2015

THE LEADING EDGE EDITORIAL BOARD

S E G B OA R D O F D I R E C TO R S

DIRECTOR AT LARGEEdith J. MillerChevron ETC Perth250 St. Georges TerraceQV1 Building Level 13Perth, WA 6000, AustraliaTel: +61 8 9485 [email protected]

The Leading Edge (Print ISSN 1070-485X; Online ISSN 1938-3789) is published monthly by the Society of ExplorationGeophysicists, 8801 S. Yale Ave., Tulsa, Oklahoma 74137 USA; phone 1-918-497-5500. Periodicals postage paid atTulsa and additional mailing offices. Print subscriptions for members of the Society in good standing are includedin membership dues paid at the World Bank III and IV rate. Dues for Active and Associate members for 2015 varydepending on the three-tiered dues structure based on World Bank classification of the members country ofcitizenship or primary work residence. Dues are US$95 (World Bank IV countries), $50 (World Bank III countries),and $13 (World Bank I and II countries). Dues for all Student members regardless of country of citizenship or primaryresidence are $21 and include online access to journals. Students may receive TLE in print by paying an additional$38. Print and online single-site subscriptions for academic institutions, public libraries, and nonmembers are asfollows: $205, Domestic (United States and its possessions); $250 Rest of world. For corporations and governmentagencies, print and online single-site subscriptions are: $1,105, Domestic (United States and its possessions); $1,150Rest of world. Rates are subject to change without notice. See www.seg.org/publications/subscriptions for orderinginformation and details. Single-copy price is $19 for members and $38 for nonmembers. Postage rates are availablefrom the SEG business office. Advertising rates will be furnished upon request. No advertisement will be acceptedfor products or services that cannot be demonstrated to be based on accepted principles of the physical sciences.Statements of fact and opinion are made on the responsibility of the authors and advertisers alone and do notimply an opinion on the part of the officers or members of SEG. Unsolicited manuscripts and materials will not bereturned unless accompanied by a self-addressed, stamped envelope. Copyright 2015 by the Society of ExplorationGeophysicists. Material may not be reproduced without written permission. Printed in USA.

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CHAIRMANCarlos Torres-VerdnUniversity of Texas Department of Petroleum and Geosystems Engineering1 University Station, Mail Stop C0300Austin, TX 78712-0228, USATel: [email protected]

PRESIDENTChristopher LinerUniversity of ArkansasDepartment of Geosciences218 Ozark HillFayetteville, AR 72701, USATel: [email protected]

Doug FosterConocoPhillips600 North Dairy AshfordHouston, TX USA, 77079Tel: [email protected]

Ezequiel F. GonzalezShell Exploration and Production150 North Dairy AshfordHouston, TX 77079, USATel: [email protected]

John LaneOffice of GroundwaterU. S. Geological Survey11 Sherman PlaceStorrs-Mansfield, CT 06269, USATel: +1-860-487-7402 [email protected] NasserHess Corporation1501 McKinney StreetHouston, TX 77010, USATel: [email protected]

STEVEN DAVIS, SEG executive directorTED BAKAMJIAN, director, publicationsJENNY KUCERA, managing editorROWENA MILLS, associate editorTONIA GIST, senior graphic designerJILL PARK, graphic designerAdvertising information and rates:HEATHER WALKE, phone 1-918-497-5524.Editorial information: phone 1-918-497-5521;fax 1-918-497-5565; e-mail [email protected] information: [email protected].

POSTMASTER: Send changes of address toThe Leading EdgeBox 702740, Tulsa, OK 74170-2740 USA

EXPLORET H E M U LT I - P H Y S I C S

FRONTIERMore than regionalreconnaissance.Its an exciting time for multi-measurementmethodologies. Whether youre conductingregional reconnaissance or developing an assetarea with extensive seismic and well control,integrating and interpreting all possible geophysicalmeasurements can uncover basement-to-surfaceinsights that drive prospectivity and wellproductivity. By integrating low-cost, low-touchairborne geophysical data, NEOS can makeyour prospects even more valuablein bothconventional and unconventional plays.With NEOS, expand your horizons.

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The Leading Edge

Editorial CalendarIssue Special Section theme

Due date

Guest editors

2015Apr. . . . Drilling hazards and deep-sea technology/OTC . . . . . . past due . . . . . . . . .Carlos Torres-Verdn, [email protected] Dev, [email protected] Smith 1 , [email protected]. . .Seismic-assisted well geosteering . . . . . . . . . . . . . . past due . . . . . . . . . Carlos Torres-Verdn 2 , c [email protected] . . .Injection-induced seismicity. . . . . . . . . . . . . . . . . . past due . . . . . . . . . Rob Habiger, [email protected] Beroza, [email protected] Shemeta 1 , [email protected] . . . . Multiples from attenuation to imaging . . . . . . . . . . . . 15 Mar 2015. . . . . . .Alejandro Valenciano, [email protected] Chemingui, [email protected] Gonzalez 1 , [email protected]. . .Passive seismic source mechanisms . . . . . . . . . . . . . 15 Apr 2015. . . . . . . David Lumley, [email protected] Kamei, [email protected] Nakata, [email protected] Shemeta 1 , [email protected]. . . . Uncertainty assessment. . . . . . . . . . . . . . . . . . . . . . . . 15 May 2015. . . . . . .Ezequiel Gonzalez 1 , [email protected] Mukerji, [email protected] Djikpesse, [email protected]. . . . Education in the geosciences . . . . . . . . . . . . . . . . . .15 Jun 2015. . . . . . . . Lisa Buckner, [email protected] Webb, [email protected] y Stark 1 , [email protected]. . . . Resource plays I: Rock physics. . . . . . . . . . . . . . . . . .15 Jul 2015. . . . . . . . Lev Vernik, [email protected] Avseth, [email protected] Nasser 1 , [email protected]. . . . Resource plays II: Geophysics . . . . . . . . . . . . . . . . . .15 Aug 2015 . . . . . . . Doug Foster 1 , [email protected]

12

TLE Editorial Board coordinatorBoard coordinator/guest editor

TLE publishes special sections covering all aspects of applied geophysics and related disciplines. Submission of special-section articlesis open to all. Please send articles to the lead guest editor for the special section; submission instructions are listed below. Board coordinators work with guest editors to coordinate and support the review process and may also serve as guest editors.Notice to authorsTLE publishes articles on all areas of applied geophysics and disciplines whichimpact it. To submit a paper for possible publication in a specific issue, pleasee-mail an inquiry to the appropriate guest editor for that issue. Authors areencouraged to submit their papers at any time, regardless of whether they fit theeditorial calendar. To submit an article on an unscheduled topic, contact JennyKucera, TLE managing editor, [email protected] or 1-918-497-5521.Electronic submission of articlesElectronic submissions should include the manuscript file, figures and othergraphics, a PDF of the manuscript and figures, and the authors contactinformation. These files can be uploaded to an FTP site (the preferred method)or burned to a CD and mailed to the appropriate editor. Once accepted for TLE,the files will be opened and edited on a Mac or a PC using various softwareapplications. To simplify conversion, figures should be submitted in TIFF, PDF

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or EPS (.tif, .pdf or .eps) file formats, with a resolution of at least 300 dpi(pixels per inch). For assistance with electronic submission, contact Jill Park,[email protected] or 1-918-497-5570. More details are online at http:// www.seg.org/resources/publications/ tle/submission-guidelines.Notice to lead authorsLead authors of articles published in TLE who are not members of SEG shouldapply for a one-year free membership and subscription to TLE by contactingMember Services, fax 1-918-497-5565 or [email protected]. Lead or corresponding authors also are required to sign a copyright transfer agreement, whichgives TLE permission to publish the work and details the magazines and theauthors rights. TLE staff will send a form to be signed and sent back after thearticle is accepted for publication. The form can be downloaded at http://www.seg.org/documents/10161/74670/SEG_Copyright_form.pdf.

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Impact factors in exploration geophysics

F

inding something valuable in the subsurface is at the coreof exploration geophysics. Individuals and groups adept atthis search have often been handsomely rewarded. Any technical advancement or deeper understanding that helps in thequest is similarly well regarded. Although subsurface resourcesare essential to our prosperity, their value fluctuates: Many SEGmembers probably check (perhaps with trepidation) the dailyprice of WTI or Brent crude oil.When the price of commodities soars, our geophysical services and expertise are widely sought. This is nicely expressedin salaries for exploration geoscientists see, for example, theSEG Membership Compensation Survey (Clark, 2012). University programs in applied geophysics burst with enthusiasticstudents, membership in geoscience societies expands, and innovation is in high gear. This virtuous activity leads to discovery,production and, apparently now, an energy oversupply. Withthese fluctuations, what are geophysicists to do?Clearly, our companies and institutions books must somehow balance. But one cost-effective way to keep the people andscience of geophysics inspired and advancing is to continue tosupport our professional societies. SEG meetings and communications, through which we share best practices, are excellentplaces and means to maintain motivation and to hone costs. OurSEG publications, instruction, and network will provide background and structure for the next upswing. The fees and budgetsof our geoscience societies are quite modest (millions of dollars)compared with the size of the industry that they service (billionsof dollars).Although the value of a commodity is summarized by itsprice, we might inquire as to the valuation of companies, organizations, and even individual contributions. Public oil companies are evaluated by many measures, which include findingcosts, acreage owned, barrels of oil per day (BOPD) produced,reserves, production replacement ratios, BOPD per employee,indebtedness, cash flow, profitability, and dividends. Professional organizations might be assessed by their number of members,funding, activity, outreach, awards, testimonials, and publication impact factors.For example, SEGs flagship journal, Geophysics, can beobjectively ranked (e.g., via its Scientific Journal Rankings index). It is generally at the top of all applied-geophysics journalsand has increased its rating substantially in the last decade. Thisis an important measure of our Societys scientific contributionand impact.What about individuals? Any full evaluation of a personsprofessional contributions would have many components. Certainly, much of an individuals impact might never be measured.And theres wisdom in suggesting that measurement and comparison should be avoided altogether because they lead to eitherarrogance or jealousy. Nonetheless, there are some positive factors which can be counted.Our most popular SEG book is none other than SheriffsEncyclopedic Dictionary, with more than 7500 copies sold in

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SEGs flagship journal, G eophysics , canbe objec tively ranked (e.g., via itsScientific Journal Rankings index).It is generally at the top of allapplied-geophysics journals and hasincreased its rating substantially inthe last decade. This is an importantmeasure of our Societys scientificcontribution and impact.the last 15 years. Books on seismic data processing (Yilmaz),amplitude interpretation (Hilterman), anisotropy (Thomsen),and reservoir geophysics (Abriel) have each sold more than 4000copies in the last 15 years.Determining the impact of publications is subject to debate, but it is estimated by various factors, including the hscore (Liner, 2009). The h-score is a writers number of papersthat have been cited that number or more times in other publications (that is, an author would have an h-score of 20 if heor she had published 20 papers that had been cited at least 20times). Google Scholars Web crawlers assiduously seek outpublications and their citations, making evaluations like thispossible.Although we can count citations, full impact in explorationgeophysics is much richer. At least as meaningful but harderto determine could be who has discovered the most oil. Whoinvented the technology that has found the most ore? Whichperson wrote the best depth migration? Who first invented andimplemented the FFT on a computer? (Answer: Vern Herbert atChevron in about 1962, but he never published it.)Thinking of impact on a larger scale, Time Books (Knauer,2012) has selected the 100 most influential people of all timeand gathered them into four groups: beacons of the spirit, explorers and visionaries, leaders of the people, and architects ofculture. Directly relevant to geophysics, most of the recognizable scientists are in the category of Times explorers (e.g., Copernicus, Columbus, Darwin, Babbage, Einstein, Bell, Jobs) those who sought to chart and understand the world and createthings that never existed before.Skiena and Ward (2014) search large data sets using various algorithms to try to rank the most significant people inhistory. Scientists rank well: Aristotle (eighth), Darwin(12th), Einstein (19th), Newton (21st), and Linnaeus (31st).Most of these would be regarded as geniuses. They werebright, dedicated, and usually well mentored and connected. As German philosopher Schopenhauer said (Grossman,2013), Talent hits a target no one else can hit; genius hits atarget no one else can see.

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March 2015

THE LEADING EDGE

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Inspiring, connecting, mentoring, and assisting are whereSEG may help to develop the next generation of talents, targets,and geniuses. And we need it. With the passing in the last fewmonths of some of the founders of modern geophysics and ourbrightest lights (Mike Batzle, Frank Levin, Bob Sheriff, andTad Ulrych), the torch is being passed on.We have many challenges to meet replacing about 93million BOPD global oil consumption, how to handle CO2,finding fresh water, better understanding induced seismicity,implementing 3D anisotropic full-waveform inversion. Fortunately, there is strong evidence that happiness is a by-product ofconstructive effort (e.g., Brand and Yancey, 1993; Lyubomirsky,2013).So lets get to it. And kudos to all those who have helped discover resources and develop our people and geoscience. Youvemade quite an impact.

References

Brand, P., and P. Yancey, 1993, Pain: The gift nobody wants: HarperCollins.Clark, D., 2012, SEGs 2011 membership compensation survey: TheLeading Edge, 31, no. 5, 522524, http://dx.doi.org/10.1190/tle31050522.1.Grossman, L., 2013, A fire in the flint, in Secrets of genius: Discovering the nature of brilliance: Time Books.Knauer, K., ed., 2012, The 100 most influential people of all time:Time Books.Liner, C. L., 2009, Seismos: A column on the history and cultureof geophysics and science in general: The Leading Edge, 28, no.4, 418419, http://dx.doi.org/10.1190/1.3112755.Lyubomirsky, S., 2013, The myths of happiness: What shouldmake you happy, but doesnt, what shouldnt make you happy,but does: Penguin Press.Skiena, S., and C. B. Ward, 2014, Whos bigger? Where historicalfigures really rank: Cambridge University Press.

Robert R. Stewart

First Vice President

Volume IFundamentals of Signal ProcessingDeconvolutionVelocity Analysis and Statics CorrectionsMigrationDip-Moveout Correction and Prestack MigrationNoise and Multiple Attenuation

Volume II3D Seismic ExplorationEarth Imaging in DepthStructural InversionReservoir Geophysics

March 2015

z YilmazIn addition to a comprehensive update of his original volume on processing,z Yilmaz has expanded the set to include inversion and interpretation.Complete Set (Volumes I and II)Catalog #112APublished 2001, 2,092 pages, HardcoverISBN 978-1-56080-094-1SEG Members $159 $99, List $199 $124E-book eISBN 978-1-56080-158-0 SEG Members $101 $59, List $127 $79

DVDCatalog #112CPublished 2008, 1 DVDISBN 978-1-56080-152-8 SEG Members $99 $59, List $124 $79

To order:Visit www.seg.org/bookmartE-mail [email protected]

THE LEADING EDGE

Seismic Data AnalysisProcessing, Inversion, and Interpretation of Seismic Data

The DVD has been published as PDF files with a robust set of links, includinglinks to cited sources. A single disc contains all contents of the 2,092-pagetwo-volume book set.

DVDComplete contents of two-volumebook setPublished as PDF filesLinks to cited sourcesSingle disc

270

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271

From the Other SideA

column by

L e e L aw ye r

with stories about geophysics and geophysicists

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ocated 70 miles east of Dallas, Texas, is a was a waste of time. We needed common depth point (CDP)!small town called Grand Saline home Sounds unusual, but back then, CDP was not widely accepted.of a salt mine owned by Morton Salt. On The geophysicist was Ken Gilleland. As I recall, he made hisMain Street sits a relatively small building point by showing two seismic lines, one a single-fold line andconstructed in 1977, with walls and door- one a sixfold CDP line (maybe 12-fold) over the same location.ways made from blocks of salt. It houses the On one section, you could see the top and base of the salt formaSalt Palace Museum. Each year in this town tion. On the other, you couldnt.This is where the term turtle showed up. We were lookingis a Salt Festival. The suggested slogan is, Somany salt jokes, so little time. In 1982, that for anticlines, created by early salt movement. The Smackovermine produced 400,000 tons of salt. It is said sits directly on the salt. Unfortunately, there is a way to createthat the salt found here can supply the worlds craving for an anticline where there is no upwelling of the salt. That meansthe next 20,000 years. After that, well need to find a substi- there is no anticline at the level of the objective! It is not easytute. Salt domes make good storage areas because they are to tell whether you have a salt anticline or a turtle because theylook alike. In any case, methods were developed, and an undernice and dry.Grand Saline is an interesting town, but that is not what standing of salt movement grew exponentially.Salt is lighter than the surrounding sediments. When salt isdrew the attention of this column. Salt is the subject. Doctorsrecommend cutting down on the amount of salt you consume, loaded, it takes the direction of least resistance, which is alwaysthereby reducing the flavor of a lot of food items. In the army, upward. Salt does not travel downward because going down hasits just the reverse. They line the troops up on a hot day and more overburden. Clearly, on the TGS seismic data in the Exrequire them to take salt pills. In nature, there are many salt plorer, salt can go sideways and any other direction except downsprings. These often result in a salt flat, attracting deer to these ward. It doesnt need structural movement to set it in motion. Itsalt licks. Cattlemen put out salt blocks for cows to lick. The creates its own structure, and those structures often hold bilcows like the salt blocks, and just like humans, they put on lions of barrels of oil and oil-equivalent gas.If the salt reaches the surface, it might stall out, so to speak.weight. I wonder if the cows blood pressure goes up with anoverconsumption of salt. A hot-sauce manufacturer advertises The salt becomes denser than the lighter sediments surroundingthat it stores its product deep in a salt dome on Avery Island, it. That happens about 2000 ft deep in the Gulf of Mexico. Wesee strong positive-gravity anomalies on top of shallow domes.Louisiana, to ensure flavor.All of this is interesting, but it is the massive salt depos- In the past, we ascribed this to a substantial caprock, mostlyits in the Gulf of Mexico and other places that fascinate me. anhydrite and limestone. Be careful the shallow salt also conEvery month, a seismic section (courtesy of TGS) shows up tributes to the positive-density contrast. There are other interin the AAPG Explorer that boggles the mind. This includes esting questions about salt tectonics, perhaps to be discussedsalt domes, salt welds, allochthonous salt, autochthonous salt, next month.turtles, and many other salt features imbedded in some specular seismic data.To contact the Other Side, write L. C. (Lee) Lawyer, Box 441449,I recall that the industry acquired a sparker survey thatHouston, TX 77244-1449 (e-mail [email protected]).ran parallel to the shoreline off Louisiana and Texas. I believe that was in the early 1950s. The sparkers did not illuminate the subsurface deeply. All one could see was thetops of salt features, the tops only. If one cared to speculate how far down the salt persisted, he or she could calculate a tremendous amount of salt. Of course that wasnttrue. When we acquired adequate data, we realized thatwe were looking only at the tops. We found that the topshad bottoms and that the salt had many shapes. But howPower Plays: Geothermal Energy in Oil and Gas Fieldsdid it get that way?I had a big advantage. We (Chevron) were exploring inConference - May 19-20, 2015 Workshop - May 18 SMU Campus, Dallas, TXeastern Texas. The objective was the Smackover Formation in Desalination Power from flare gas and well watersouthern Arkansas that sits on top of the Louann Salt (Juras Induced seismicity ENAM Community Seismic Experiment Waste heat technology Onshore and offshore thermal maturationsic). The advantage for us was that we could see the top andbottom of the formation. This wasnt always the case. It was aContact Maria RichardsChevron geophysicist who convinced Chevron management214-768-1975www.smu.edu/geothermaland research divisions that shooting single-fold seismic data

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F o u n d at i o n N e w s

Foundation Board of Directorsproudly welcomes its new leaders

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t the Foundations first board meeting of 2015, Mike Forrest, Foundation chair, welcomed aboard five new SEGFoundation Board members. These are Anna Shaughnessy, JulieHardie, Erik Finnstrom, David Bartel, and Peter Cramer. Anna Shaughnessy (Massachusetts Institute of Technology) is the executive director of MITs Department ofEarth, Atmospheric, and Planetary Sciences (EAPS) andbrings many years of experience as an educator and SEGsupporter. Anna adds an academic expertise to the Foundation Board. Julie Hardie (Seismic Exchange, Inc.) is the vice presidentof SEIs legal department. Julies expertise will add depth tothe Foundations governance and will enhance our executionof fiduciary responsibilities to donors. Erik Finnstrom (Statoil) is a seasoned veteran serving asthe senior vice president of exploration excellence at Statoil,Oslo. He was instrumental in Statoils recent support of theFoundation. Statoil is supporting international SEG studentmembership, distinguished lectures, and the InternationalGeosciences Student Conference. Eriks support and uniqueposition internationally will help the Foundation workclosely with more SEG members. David Bartel (Chevron) is an active volunteer with the SEG/Chevron Student Leadership Symposium. He is an avidsupporter of SEG student programs.

Peter Cramer (ConocoPhillips) is exploration managerat ConocoPhillips and brings many years of explorationexperience from his successful career at Shell and ConocoPhillips to enrich our connections to industry. Peter willalso be a great link to other geophysicists in the Houstonarea.Each of these five new Foundation Board members bringsextraordinary leadership skills, experience, and dedication toserve, which strengthens our ability to serve all SEG membersworldwide. As we face current economic challenges, strong,thoughtful leadership is required to manage the Foundationsassets and execute strategies for continued success. Over thenext few years, these new board members will work togetherwith our returning eight board members to build on our pastsuccesses and advance our mission to support the funding ofSEG programs worldwide.Please join me in welcoming these new Foundation leaders to our Foundation team. You can contact us by e-mail [email protected]. Paul AllisonExecutive Director, SEG Foundation

The SEG Foundation announces the newMichael L. Batzle Endowed Scholarship

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he SEG Foundation recently received a gift from LisaBatzle to create a new scholarship honoring MikeBatzle. The Michael L. Batzle Endowed Scholarship fundis available for all donors to support through the Foundations online donation site: seg.org/donate, choosing SEGPrograms.Mike was a devoted mentor and friend to many geosciences students, educators, and professionals. As part of theSEG family, we are proud of all Mikes successes and contributions. The entire SEG family extends its condolences toMikes family.

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Mike Batzle

Mike Batzle teaching at a geophysics fieldcamp for Colorado School of Mines.

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Introduction to this special section:Borehole geophysics and sonic loggingTad Smith 1 , Carlos Torres-Verdn 2 , and Arthur Cheng 3

S

onic logs are most frequently used to tie surface and boreholeseismic amplitude measurements to P- and S-wave velocitiesencountered along wellbore trajectories. Other common uses ofsonic logs include the estimation of dynamic elastic propertiesfor geomechanical analysis, such as wellbore stability studies andthe design and planning of hydrofracturing operations. Modernsonic logs are acquired with multiple transmitters and an array ofclosely spaced receivers (anywhere between eight and 13 receivers) in the form of time waveforms. Sonic transmitters can comein the form of monopole, dipole, and quadrupole actuators. Bydesign, sonic transmitters are immersed in a fluid the borehole mud whereby formation shear waves can be detected andquantified only by the elastic coupling that exists between wavemotion taking place in the borehole fluid and in the surroundingrock formations. Such an elastic coupling gives rise to markedlyfrequency-dispersive behavior of the detected waves that presentssome technical challenges when one interprets their speeds.Depending on the elastic properties of rock formations, shearwave velocity can be estimated from sonic waveforms generatedwith monopole, dipole, and quadrupole sources. In the case ofdipole sources, shear-wave velocity is extracted from the lowfrequency velocity of the so-called flexural wave. Dipole sourceshave explicit directionality in their radiation patterns, thereforelending themselves to detection and analysis of elastic anisotropyin rock formations. Logging-while-drilling acquisition of sonicwaveforms typically uses quadrupole sources to generate a socalled screw wave whose low-frequency velocity asymptotes toward that of the formation shear-wave velocity.Geophysicists are often not aware of the intricacies associated with monopole, dipole, and quadrupole acquisition of sonicwaveforms, especially in the way those intricacies can affectthe reliability and accuracy of estimated shear-wave velocities.Presence of elastic anisotropy offers additional challenges tothe identification and estimation of formation velocities. More12

Houston, Texas.Austin, Texas.

important, there is substantially much more information aboutrock elastic properties in sonic waveforms currently used by geophysicists. One example is the velocity of propagation of Stoneley waves, which can bring about useful information regardingthe anisotropic elastic properties of rocks. The objective of thisspecial section is to highlight some modern concepts and applications of sonic waveforms in formation evaluation, geomechanics, and seismic amplitude ties.We begin this special section with an article from Cheng onsome of the fundamentals of sonic logging. Specifically, this articlereviews the effects of dispersion and anisotropy on shear-wave slowness measurements and methods to take those effects into accountto obtain a more robust shear-wave slowness log. Failure to properlyunderstand and account for these effects can result in errors thatcan have a first-order effect on seismic and geomechanical models.In the second article, Donald and Prioul obtain nonlinear elasticconstants from the inversion of borehole sonic shear radial profiles.The stress-to-velocity relationship determined from these profilesis compared with empirical laboratory data. Results show that thestress sensitivities are significantly stronger with the borehole radialprofiles than the empirical model for all considered stress paths.Finally, we conclude the special section with an article bySayers et al. In this article, kriged estimates of density and vertical P- and S-wave velocities were derived from vertical wells intheir study area. Given that density is scalar and independentof well deviation, comparison of the kriged density provided ablind test of kriging accuracy. However, the measured sonic velocities are systematically higher than the kriged vertical velocities. These differences were then used to estimate the anisotropicparameters at the location of a deviated well. Derived anisotropic parameters were then used to apply a nonhyperbolic moveoutcorrection, , to flatten gathers from a seismic survey 10 km tothe north of the area of interest, within which the anisotropyparameters were estimated by the authors.3Singapore.http://dx.doi.org/10.1190/tle34030276.1.

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Can we ever trust the shear-wave log?Arthur C. H. Cheng 1Abstract

After three decades of shear-wave logging, there are stillquestions on the robustness of the measurement. The perceivedvariability in shear-wave slowness measurements comes fromtwo main sources: (1) the dispersive nature of the flexural andquadrupole waves used in the measurement and (2) anisotropy.Dispersion and anisotropy affect shear-wave slowness measurements, and various methods take those effects into account toobtain a more robust shear-wave slowness log.

Introduction

Accurate and reliable compressional- and shear-wave measurements are the keys to a variety of geophysical and geomechanical applications, including seismicwell ties, input to velocity model building, pore-pressure prediction, wellbore stabilityevaluation, reservoir characterization, and fluid substitution. Inall those, shear-wave measurements are necessary and critical.Traditional acoustic log measurements use an omnidirectionalpressure (monopole) source inside the borehole to detect the refracted compressional and shear head waves along with the guided waves (pseudo-Rayleigh and Stoneley) and use array processingto measure the formation compressional- and shear-wave slownesses (Cheng and Toksz, 1981; Kimball and Marzetta, 1984).However, detecting a refracted shear wave is possible only infast formations where shear slowness (inverse velocity, a more direct measurement for logs) is less than the compressional slownessof the borehole fluid (190 to 300 s/ft, depending on compositionand weight of the mud). If the shear slowness of the formation islarger than the compressional-wave slowness of the borehole fluid,there is no refracted shear head wave, and we cannot measure theformation shear slowness. That situation exists for a large numberof conventional reservoirs as well as for near-surface formations.

Flexural wave dispersion and dispersion correction

formations. The borehole flexes as the wave propagates along,and it is directional. It is dispersive; its slowness changes quiterapidly with frequency. However, it has one key desirable characteristic. It propagates at the formation shear slowness at thelow-frequency asymptote (in a typical formation and boreholesize, about 13 kHz; in a large, e.g., 22-inch-diameter, boreholeand slow formations, it can be as low as 500 Hz or even lower).At high frequencies (about 610 kHz), it propagates at the highfrequency Stoneley or, equivalently, Scholte wave slowness.Figure 1 shows an example of measured flexural waveformsand the associated dispersion curve from a test well. It is clearthat at the low-frequency limit (less than 3 kHz), the dispersioncurve is relatively flat and is measuring the formation shear-waveslowness. It should be pointed out that this dispersion is purelythe result of the geometry of the borehole and is distinct fromdispersion related to viscous flow of pore fluids in rock.The complication of using flexural waves comes in the nonuniform excitation nature of the wave. Figure 2 shows a typicaltheoretical dispersion curve and the associated excitation function for the flexural wave. At the low-frequency limit where theflexural wave is propagating at the shear slowness, there is relatively little excitation energy. The peak of the excitation andthus the most energy contained in the waveform is at a highfrequency corresponding to the inflection point in the dispersioncurve. This creates a problem in processing the data.Traditional time-semblance methods, also known as slowness-time-coherence methods, are influenced by the high-energy, slower high-frequency component of the wave and thuswill result in picking a slowness that is larger than the trueformation slowness. Figure 3 shows a synthetic example. Thisis a well-known artifact of time-semblance processing of theflexural wave.This artifact can be corrected by dispersion-correctiontechniques (Geerits and Tang, 2003). In general terms, theseare model-based estimates of the true formation shear slowness by estimating the frequency at which the measurement is

Zemanek et al. (1984) introduce the use of the dipole acoustic source for shear-wave logging in all formations. Instead of apressure source, the authors used a unidirectional displacement source, generated by a piezoelectric bender element,to generate a flexural wave mode in theborehole, with signals received by another bender element in the borehole.Subsequent evolution of the technologynow uses pressure sensors located azimuthally on the logging tool, and thesignals are combined to generate thedesired flexural wave signal (Tang andCheng, 2004).Unlike the refracted head wave,the flexural wave is a borehole surface- Figure 1. (a) Eight measured flexural waveforms and (b) the dispersion curve associated with themguided wave, and as such, it exists in all from a test well in Texas.

1

National University of Singapore.

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http://dx.doi.org/10.1190/tle34030278.1.


being taken. This can be the frequency with the peak energy inthe spectrum or the centroid frequency of the spectrum (e.g.,Geerits and Tang, 2003).With that and the measured hole size and estimated mudslowness, one can uniquely recover the true formation slowness from the measured one. It should be pointed out that notall contractors do the dispersion correction as part of the normal deliverable. Often, the delivered log, especially one that isdelivered at the well site, is not corrected for dispersion. Thiswill result in a slower shear-wave log and a higher Poissonsratio. One should always make sure that the log is correctedfor dispersion.However, like any other model-based inversion technique,the method works if the model is correct. In the case of acousticlogging, there are many instances in which the model breaksdown. The most common reason is that the formation is nothomogeneous and isotropic, which is a common assumption. Abrief discussion follows on how anisotropy affects the dispersioncurve and thus dispersion correction.

low-frequency end, an increase in results in a faster dispersioncurve at higher frequencies. A dispersion correction based on anisotropic formation model will overcorrect the dispersion, making the resulting shear-wave velocity faster than it should be.Based on the above discussions on dispersion, it is clear thatmodel-based dispersion correction needs to be applied carefullyand properly. The good news is that for modern wireline acoustic tools, the bandwidth of the measurement is usually broadenough for proper frequency analysis (see, e.g., the data in Figure 1), except in some cases in large holes or slow formations.This allows for direct identification of low-frequency shear-waveslowness.

Stress-induced anisotropy

It is well known that stress-induced anisotropy results inazimuthal variations in shear-wave slowness and in changes infar-field versus near-field shear-wave slownesses caused by hoopstress around the borehole (Sinha and Kostek, 1996). Figure 4shows a numerical simulation of the dispersion behavior of thefast and slow shear waves under a uniaxial stress of 10 MPa inBerea Sandstone (Fang et al., 2015). The crossover of the dispersion curves as a function of frequency is well understood and isthe result of stress rotation from far to near field.Because of this behavior, the dispersion curves associated withthe fast and slow directions depart from that of a homogeneousformation with the same slowness. In particular, because the dispersion curve associated with the fast direction becomes slowerat higher frequencies, and vice versa, the dispersion correctionwill result in slower velocities for the fast shear wave and fastervelocities for the slow shear wave, thusreducing the amount of shear-wave anisotropy being measured.

Figure 2. (a) Numerical calculation of flexural dispersion in an open

borehole and (b) associated excitation of the flexural wave as a function of frequency. This illustrates that the maximum excitation is notat the formation shear slowness and thus shows the need for dispersion correction.

VTI anisotropy

A lesser-known effect is that ofVTI, or polar, anisotropy. It is truethat the flexural wave will measurethe slow shear-wave slowness in VTIformation at low frequencies. However, it is less well known that flexuraldispersion responds to the amount ofanisotropy in the formation (Ellefsen et al., 1989). Figure 5 shows anumerical simulation of the flexuraldispersion in three formations withthe same slow shear-wave velocitybut with different shear-wave anisotropy parameters, = 0, 0.15, and 0.3.It is clear that although the threecases have the same slowness at the

Figure 3. Processing example showing (a) time-semblance (or slowness-time-coherence) results inan estimated slowness larger than (b) the actual one.


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Figure 4. Flexural-wave dispersion in a numerically simulated Berea

Sandstone formation with a uniaxial (at 0) applied stress of 10 MPa.The flexural wave in the fast direction becomes slower, and vice versa.After Fang et al., 2015, Figure 6a.

Figure 5. Flexural-wave dispersion in a VTI formation with shearwave anisotropy parameters of = 0, 0.15, and 0.3.

In general terms, modern tools are usually reliable to atleast 16 inches in hole diameter and 600 s/ft shear slownessand sometimes to as much as 22 inches in diameter and 1000s/ft. However, the picking of the low-frequency asymptote isnot trivial, although several approaches are available (Huangand Yin, 2005; Tang et al., 2010; Mukhopadhyay et al., 2013).Furthermore, these approaches are more time consumingand often are not available at the well site. Care must be taken tomake sure a dispersion-corrected log is not just one that is froman isotropic model-based inversion but is based on the actualmeasured dispersion curve. In more challenging environments,it is always advisable to go to the waveforms and check that theprocessing algorithm is picking the actual flexural mode and notthe Stoneley mode, and it is advisable to examine the dispersioncurve.

Dipping-layer/deviated borehole

Shear-wave propagation in an anisotropic formation is complex. In a TI formation, it will split into well-defined fast andslow shear waves. They also can be classified as SV or SH waves,depending on the polarization of the particular motion. The cases of HTI and VTI have been studied well, but the case for intermediate angles has not been studied nearly as much. A flexuralwave propagating in a borehole at an angle to a TI formation,whether it is from a deviated borehole or from a dipping bed, results in some interesting observations, which might cast doubt onthe accuracy of the measurement if the results are not interpreted properly. This is particularly true for logging-while-drilling(LWD) measurements in which borehole deviation is common.Figure 6 shows an example of a borehole intersecting a TIformation at an angle. The formation properties are from an example generated using a crack model (Cheng, 1993). For thisparticular model, the SV and SH waves cross at an incidenceangle of about 50. At less than 50, the SH is faster than the SV.At more than 50, the situation is reversed. This can cause significant misinterpretation of the fast and slow shear directions ina dipping bed or deviated borehole.

Figure 6. (a) Schematic of logging in a deviated borehole. (b) Variationof P, SV, and SH velocities as a function of incidence angle. Dependingon the angle of deviation of the borehole, the observed VP /VS ratiochanges significantly, leading to difficulties in interpretation.

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Moreover, as seen in Figure 6, the P-wave slowness variesdifferently than the two shear waves as a function of incidenceangle. As a result, the measured VP /VS ratio or, equivalently, thePoissons ratio might fall out of the established trend line andcause uncertainty in the interpretation.

Logging-while-drilling shear-waveslowness measurements

The situation with logging while drilling is even morecomplex. Because there is a large steel collar in LWD loggingtools, the flexural wave is influenced heavily by the tool modeat low frequencies and usually does not approach the formationshear-wave slowness (Tang and Cheng, 2004). Instead, thequadrupole wave is used to measure shear-wave slowness ina slow formation (Tang et al., 2002). The quadrupole wave is

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obtained by summing the pressure at the receivers on oppositesides of the tool and subtracting those on the orthogonal sides.Like the flexural wave, the quadrupole wave is dispersive, and itspeak excitation is again at the inflection point of the dispersioncurve (Figure 7; see also Tang et al., 2010).Thus, the quadrupole wave has all the issues associated withthe flexural wave, as discussed above, including those encountered in anisotropic formations and deviated boreholes. However,unlike the flexural wave, the quadrupole wave is omnidirectional. In addition, because the quadrupole sums the signal in onedirection and subtracts it in the orthogonal direction, it does notsplit itself cleanly in HTI formations as the flexural wave does.Thus we have a signal that shifts its phase depending on which

direction the tool is pointing. This will cause significant problemsin LWD measurements because the tool is rotating rapidly.Figure 8 shows a schematic illustrating this effect. Furthermore, the low-frequency asymptote for the quadrupole modeis not as flat as that for the flexural mode (Figure 7), makingpicking the actual slowness more of a challenge, and the lowfrequency asymptote can be arbitrary depending on data qualityand the processor. Validation of the dispersion correction or thelow-frequency pick needs to be done if one wants assurance thatthe picked shear slowness is correct.

Conclusions

Shear-wave logging uses the flexural wave for wireline andthe quadrupole wave for logging while drilling. Both wavemodes are dispersive, and their peak excitations do not correspond to the formation shear slowness. Because of that, conventional time-semblance processing results in a slowness thatis larger than the actual slowness, requiring that the data needto be corrected for dispersion. However, model-based dispersioncorrection will give erroneous results in anisotropic formations,especially in combination with a deviated borehole. Correct results can be obtained by proper analysis of the dispersion curveand identification of the low-frequency asymptote.

Acknowledgments

I would like to thank Bill Langley and Halliburton EnergyServices for the use of flexural-wave data and for many insightful discussions.Figure 7. (a) Numerical calculation of flexural dispersion in a loggingwhile-drilling environment and (b) associated excitation of the quadrupole wave as a function of frequency. Similar to the flexural wave inthe open-hole case shown in Figure 2, the peak excitation is not at theformation shear slowness.

Corresponding author: [email protected]

References

Cheng, C. H., 1993, Crack models for a transversely anisotropicmedium: Journal of GeophysicalResearch: Solid Earth, 98, no. B1,675684, http://dx.doi.org/10.1029/92JB02118.Cheng, C. H., and M. N. Toksoz, 1981,Elastic wave propagation in a fluidfilled borehole and synthetic acoustic logs: Geophysics, 46, no. 7, 10421053, http://dx.doi.org/10.1190/1.1441242.Ellefsen, K. J., C. H. Cheng, and K. M.Tubman, 1989, Estimating phasevelocity and attenuation of guidedwaves in acoustic logging data: Geophysics, 54, no. 8, 10541059, http://dx.doi.org/10.1190/1.1442733.Fang, X., A. Cheng, and M. Fehler, 2015,Investigation of borehole cross-dipoleflexural dispersion crossover throughnumerical modeling: Geophysics,80, no. 1, D75D88, http://dx.doi.Figure 8. Diagram of a quadrupole wave in an azimuthally anisotropic formation (F and S indiorg/10.1190/geo2014-0196.1.cate fast and slow shear slownesses). The quadrupole wave is obtained by summing the pressure atGeerits, T. W., and X. Tang, 2003, Centhe receivers on opposite sides of the tool and subtracting those on the orthogonal sides (see thetroid phase slowness as a tool for disper+ and positions). This illustrates when the positive receivers are (a) in line with the fast direcsion correction of dipole acoustic loggingtion and (b) in line with the slow formation. In the first case, the first-arriving signal is positive,data: Geophysics, 68, no. 1, 101107,whereas in the second case, it is negative, as illustrated in (c) and (d), respectively.http://dx.doi.org/10.1190/1.1543197.

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Huang, X., and H. Yin, 2005, A data-driven approach to extractshear and compressional slowness from dispersive waveformdata: 75th Annual International Meeting, SEG, ExpandedAbstracts, 384387, http://dx.doi.org/10.1190/1.2144349.Kimball, C. V., and T. L. Marzetta, 1984, Semblance processing of borehole acoustic array data: Geophysics, 49, no. 3,274281, http://dx.doi.org/10.1190/1.1441659.Mukhopadhyay, P., A. Cheng, and P. Tracadas, 2013, The differential-phase based time- and frequency-semblance algorithmfor array-acoustic processing and its application to formationslowness measurement: Petrophysics, 54, no. 5, 475481.Sinha, B. K., and S. Kostek, 1996, Stress-induced azimuthalanisotropy in borehole flexural waves: Geophysics, 61, no. 6,18991907, http-//dx.doi.org/10.1190/1.1444105.Tang, X. M., and A. Cheng, 2004, Quantitative borehole acoustic methods: Elsevier Handbook of Geophysical ExplorationSeries No. 24: Seismic Exploration.Tang, X.-M., C. Li, and D. J. Patterson, 2010, A curve-fittingtechnique for determining dispersion characteristics of guided elastic waves: Geophysics, 75, no. 3, E153E160, http://dx.doi.org/10.1190/1.3420736.Tang, X. M., T. Wang, and D. Patterson, 2002, Multipoleacoustic logging-while-drilling: 72nd Annual InternationalMeeting, SEG, Expanded Abstracts, 364367, http://dx.doi.org/10.1190/1.1817254.Zemanek, J., F. A. Angona, D. M. Williams, and R. L. Caldwell,1984, Continuous shear wave logging: Transactions of theSPWLA 25th Annual Logging Symposium, conference paper1984-U.`

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In situ calibrated velocity-to-stress transforms using shearsonic radial profiles for time-lapse production analysisJ. A. Donald 1 and R. Prioul 2Abstract

Borehole acoustic waves are affected by near- and far-fieldstresses within rocks that exhibit stress sensitivity, typically inmedium- to high-porosity formations. Nonlinear, or third-order,elastic constants are obtained from the inversion of borehole sonicshear radial profiles with an elastic wellbore stress model. Thestress-to-velocity relationship determined from these profiles inthe elastic region surrounding the wellbore is used for calibrationto compare with empirical laboratory data traditionally used intime-lapse seismic-feasibility studies to assess simulated production. This analysis enables rock physicists to use the wellbore as alaboratory and to examine the stress dependence of the acousticvelocities from in situ field data in their zone of interest. Laboratory experiments on core samples can yield both empirical andmathematical rock-physics models to describe the relationshipbetween stress and velocity to link rock properties to in situ measurements of acoustic data (seismic and sonic). In an examplefrom offshore Malaysia, full-waveform borehole sonic data areprocessed to produce shear radial profiles in a deepwater environment. The compressional velocities are mainly sensitive to stressin the polarization-propagation direction, and shear velocities aremainly sensitive to stresses in propagation and polarization directions, as expected from nonlinear elasticity. The three compressional and shear velocities vary greatly with vertical stress depending on the stress path because they depend on the three principalstress magnitudes. In contrast, a classical empirical model thatdepends on porosity, clay content, and effective stress cannot capture differences caused by stress path because it relies on only onestress. Results show that stress sensitivities are significantly stronger with borehole radial profiles than the empirical model for allconsidered stress paths (K = -0.5, 0, 0.5, and 1).

Introduction

Time evolution of reservoir-geomechanics properties overthe life of producing reservoirs can be characterized from timelapse seismic data and 3D geomechanics models (Herwanger andKoutsabeloulis, 2011). One key ingredient needed to link seismicdata and geomechanics models is a relationship among the threeprincipal stresses (as well as pore pressure) and the elastic stiffnesses or velocities. Although the variation of the effective elastic moduli of rocks as a function of compressive stress caused bynonlinear elasticity or the closing of cracks has been reported inthe laboratory for more than 40 years (Mavko et al., 1998, section2.4), the most challenging practical task ever since has been tofind a stress-stiffness relationship that captures the representativephysics with few parameters to be calibrated in situ.One such model has been based on the theory of acoustoelasticity (Thurston, 1974), also sometimes called nonlinearelasticity, in which it can be shown that an initially isotropicrock described by two so-called second-order elastic constants12

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that is subjected to three stresses is characterized by only six (instead of nine) effective elastic constants (i.e., it belongs to a special class of orthorhombic media) (Rasolofosaon, 1998) and onlythree so-called third-order nonlinear elastic constants (acting asstress sensitivity parameters). Early applications of acoustoelasticity to rocks in the laboratory showed that nonlinear stresssensitivity constants could be estimated if nonlinearity remainssmall to moderate under the application of stress (Johnson andRasolofosaon, 1996; Winkler and Liu, 1996).One of the important results shown by the acoustoelasticity theory is that compressional velocities are affected mostly bystresses in the direction of propagation-polarization, whereasshear velocities are sensitive to stresses in both propagation andpolarization directions; see evidence from the laboratory in Prioul et al. (2004).In boreholes, the local principal stress directions and magnitudes are known to be perturbed by the presence of the circularcavity, which translates into azimuthal and radial velocity variations (Winkler, 1996). The first manifestation of such velocityvariations is the observation of shear-wave splitting using dipolesonic-logging tools (Esmersoy et al., 1994; Mueller et al., 1994),which can be used to identify stress-related characteristics (Tanget al., 1999; Tang and Cheng, 2004).Furthermore, the analysis of borehole flexural waves fromdipole sonic showed a crossover in flexural dispersions for the radial polarization aligned parallel and normal to the stress direction theoretically (Sinha and Kostek, 1996), experimentallyin the laboratory (Winkler et al., 1998), and in situ with log data(Plona et al., 2000; Sinha et al., 2000), which has now becomethe classical signature of stress-induced anisotropy effects on dipole sonic data.Flexural dispersion curves have been used to estimate radialprofiles of shear moduli (Sinha et al., 2006; Tang and Patterson,2010), which then have been used to estimate in situ nonlinearelastic constants and stress magnitudes (Lei et al., 2012; Donald et al., 2013). Alternative stress-velocity models applied toborehole sonic also have been considered to interpret nonelasticvelocity variations in the one-radius region from the boreholewall (Sayers et al., 2007; Fang et al., 2013).In practice, although all rocks have some degree of stresssensitivity, this phenomenon is more likely to be observed within medium- to high-porosity rocks, given the current accuracyof borehole acoustic-logging technology to resolve changes inslowness with stress (Donald et al., 2013).We present here a case study from offshore Malaysia in whichwe identify clear stress-induced anisotropy signatures and severalzones where assumptions of the acoustoelasticity model are satisfied. We recall several key steps of the method to estimate theminimum and maximum horizontal stresses and the nonlinearand reference parameters that fully describe the velocity-to-stresshttp://dx.doi.org/10.1190/tle34030286.1.


transforms. Then we show that the in situ calibrated transformcan be used to understand stress-path effects on velocities and, asa perspective, could be used for time-lapse seismic and reservoirgeomechanics simulations (Donald et al., 2013).

Identifying stress-induced anisotropy

In a well from offshore Malaysia, full-waveform sonic logswere acquired (Pistre et al., 2005) to obtain compressionalmonopole, cross-dipole, and Stoneley waveforms. The dipolesources (oriented orthogonally to each other) were processed todetermine the fast and slow shear-wave slownesses (slowness= 1/velocity) and the polarization azimuth of the far-field fastshear wave (Esmersoy et al., 1994; Donald et al., 2013), whichare shown in Figure 1.At discrete depths, the fast (red) and slow (blue) flexural andStoneley (cyan) wave-train data were transformed to obtain theslowness-dispersion curves, as shown in Figure 2. The dipolecrossover from the dispersion analysis clearly indicates that thedominant mechanism of anisotropy is differential horizontalstress (Donald et al., 2013). This crossover signature is presentin all the clean zones throughout the logged section.Also note that polarization of the fast shear wave (or fastshear azimuth) is constant through the interval and is independent of tool rotation. In Figure 2, solid lines represent the theoretical homogeneous isotropic dispersion for each wave, takinginto account the borehole fluid bulk modulus and far-field formation moduli (shear and bulk), borehole diameter, and presence of the sonic tool in the wellbore (Donald et al., 2013).The difference between the theoretical model dispersion andthe measured dispersions as a function of frequency then wereused to obtain a dynamic shear modulus as a function of wavelength and as a function of radii from the borehole wall into thefar field as many as seven borehole radii away (so-called shearradial profile) (Sinha et al., 2006; Donald et al., 2013), as shownin Figure 3. We note that the homogenous reference model thatis used in the perturbation model to derive the radial profile alsorequires an estimate of the mud slowness, which often is derivedusing the high-frequency portion of the leaky-P compressionalwave. Alternatively, it is common to calibrate mud slownesswithin a homogenous and isotropic zone.

Figure 1. Cross-dipole anisotropy processing of flexural-wave data

to determine fast and slow shear slownesses, along with polarizationdirection of the fast shear wave. The depth track shows the differences in inline and crossline energies from the fast and slow dipoles.Track 1 shows tool and hole orientation, along with gamma ray andcaliper. Track 2 shows the fast shear azimuth. Track 3 shows theslowness anisotropy (DT-based) and time-based anisotropy (differences in average arrival times) and fast and slow shear slownesses.Track 4 shows the processed waveforms at level 7 of the fast and slowdipole firing.

Stress characterization and parameter estimation

Assuming that one principal stress is vertical, V, we candefine a coordinate system with X 3 pointing to the vertical axis,X1 pointing to the azimuth of maximum horizontal stress H,and X 2 pointing to the azimuth of minimum horizontal stressh. When the rock is stress sensitive, sonic velocities change as afunction of incremental changes in effective stress above and beyond a reference state. In near-vertical wellbores where there areindications of stress-induced anisotropy from dipole dispersions,the slow, fast, and Stoneley shear provide estimates of the threeshear moduli (Donald et al., 2013), c44, c55, and c66, where cij = [1/(shear slowness)]2b. The vertically propagating compressionalwave velocity yields the compressional modulus c 33. Then thethree P-wave moduli and three S-wave moduli can be expressedin terms of the diagonal elements of elastic stiffness tensor asfollows (Donald et al., 2013):


Figure 2. Slowness-dispersion analysis indicating stress-induced

anisotropy with classic crossover behavior between fast and slowdipole firings. Monopole compressional and shear head waves are alsoevident at high frequencies, whereas the dispersive Stoneley waves canbe seen at lower frequencies.

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c11 = bV112 , c22 = bV222, c33 = bV332,c44 = bV322, c55 = bV312, c66 = bV322,

(1)

where b denotes the formation bulk density and Vij (i, j = 1, 2, 3)denotes the velocity of a wave traveling along axis X i and polarized along X j (Donald et al., 2013).Following Lei et al. (2012), stress-sensitivity coefficients forthe compressional moduli rely on Mref , ref, c 111, and c 112, whereasthe stress-sensitivity coefficients for the shear moduli rely onMref , ref , c 144, and c 155. Mref and ref are the two independent second-order elastic constants in a hydrostatically loaded referencestress (the rock is assumed to be isotropic in the unstressed orhydrostatically loaded state). There are three independent thirdorder elastic constants (Donald et al., 2013), c 111, c 112, and c 123,with c 144 = (c 112 c 123)/2 and c 155 = (c 111 c 112)/4.As shown by Pistre et al. (2009) and by Sun and Prioul (2010),the stress regime, or Q factor, can be related to the relative rankingof the shear moduli (Donald et al., 2013), as defined in Table 1.For a given zone that shows stress-induced anisotropy for anormal faulting regime, the ratio of shear moduli to the corresponding formation stresses yields (Donald et al., 2013)c55 c66 c55 c44D= = ,VhHh

(2)

where the acoustoelastic parameter D = 3/2 + (c 155 c 144 )/ 2.With measurements of the three shear moduli, the overburdenstress, and the minimum horizontal stress directly measuredfrom extended leak-off tests (XLOT) or minifracs, equation 2can be rearranged to solve for the maximum horizontal stressdirectly as (Donald et al., 2013)c55 c44H = c c (V h ) + h.5566

[

]

(3)

If both the minimum and maximum horizontal stresses are unknown, then the D parameter must be solved independently.Subsequent work by Lei et al. (2012) shows a method of obtaining D independently from the dipole radial profiles (Donald etal., 2013) combined with a borehole stress model.Inversion of the measured dipole dispersions yields a radialprofile of the shear modulus from the sand face into the far field.In conjunction with the measurements, an equivalent isotropicmodel of the simulated dipole dispersions for each direction(maximum and minimum horizontal stress directions) can begenerated in an anisotropic stress environment. The dipole measurements are affected by a combination of the near-wellborestresses (axial, radial, and tangential) and the far-field stresses (vertical, maximum horizontal, and minimum horizontal)(Donald et al., 2013). By combining the elastic solution fromKirsch (1898) with the effect of the dipole measurements in thenear and far fields by Sinha and Kostek (1996), we obtain thefollowing relationships (Lei et al., 2012):24c55 (r, )|=0 = m1 a2 + 3 (c55 c44 ) a4 + c55,r 2r

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(4)

Figure 3. Shear radial profiles from fast and slow dipoles and from

the Stoneley wave at a single depth. The change in shear slowness isdifferent for each of the three orthogonal shear measurements.

Table 1. Shear moduli, stress regimes, and Q factor.Shear moduli ranking

Stress regime

c55 > c44 > c66

Normal faults

c55 > c66 > c44

Strike-slip faults

c66 > c55 > c44

Thrust faults

Q factor

c55 c440c c 155661

c55 + c66 2c44c55 c44 2

2

3c66 2c44 c55c66 c44 3

24c44 (r, )|= /2 = m2 a2 3 (c55 c44 ) a4 + c44,r2r

(5)

where a is the distance from the wellbore wall; r is the radiusof the wellbore; and m1 and m2 are functions of c 144, c 155, andthe reference moduli. The full derivation is shown in Lei et al.(2012) for the m1 and m2 terms. The model radial profiles fromequations 4 and 5 are compared with the measured radial profiles from the fast (related to c55) and slow (related to c44) dipoles,respectively (Donald et al., 2013).A least-squares regression is performed between the model and measured profiles, as shown in Figure 4. The region inwhich the model and the measurements diverge represents thearea that is not related to elastic behavior, and thus the data fromthat point to the wellbore wall are excluded. This computation isdone at the same sampling rate as the sonic data are processed,and as a result, the nonlinear elastic constants are obtained at 15cm intervals. Once the nonlinear elastic constants are known,the minimum and maximum horizontal stress magnitudes canbe obtained independently.The remaining independent third-order elastic constant c 111can be determined by changes in the compressional modulus between depths z1 and z2 over a reasonably uniform lithology layer.


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Figure 4. Fast and slow radial profiles compared with wellbore elasticmodel. Note the divergence of the model regressions and the field dataat the plastic yielding point.

In stress-sensitive formations, it is common to observe that themoduli increase with depth, and the hydrostatic and overburdenstresses increase accordingly. Equation 6 describes this evaluation using the same inputs as described above (Lei et al., 2012,equation 78):c111 =

Mref|z=z Mref|z=z84c + | +K ref|z=z ,ref|z=z ref|z=z3 155 3 ref z=z1

2

0

0

1

(6)

2

where Kref = Mref 4ref /3.Figure 5 shows the results of the stress-magnitude inversion.The requirements for choosing each zone were a positive indication for dispersion crossover, intrinsic formation isotropy (nolayering) and consistent formation properties over a minimumlength of 3 m, and a minimum of 2% shear slowness anisotropybetween fast and slow dipole sources. The results of the threeshear moduli indicate that this section has a normal stress regime, where c55 > c44 > c66, and the stress Q factor is computed tobe 0.66 (Donald et al., 2013).A result of a leak-off test is plotted for the upper part of theinterval, along with the equivalent mud weight used to drill thewell. Both values are consistent with the results of the minimumhorizontal stress values. The difference between the minimumand maximum horizontal stresses ranges from 2.5 to 3.5 MPa,or 8% to 12%. The sediments are relatively high in porosity, andthe change in the three shear moduli with depth is very evident(Donald et al., 2013).Pore pressure (Pp ) is measured in this section using a wireline formation tester, and Biots alpha () is assumed to be 0.95.As an example, we report the complete stress determinationin Table 2 for a depth of 2504 m. It should be noted that thedual-axis caliper measurements show no ovality over the loggedsection. The difference in the shear moduli is evidence that theprincipal formation stresses are different (Donald et al., 2013).

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Figure 5. Stress magnitude and nonlinear elastic constant analysis

from offshore Malaysia. Gamma ray is displayed in track 1. Bulkdensity and oriented calipers are shown in track 2. Three shear modulifrom fast and slow dipoles and Stoneley are in track 3. Stress magnitudes with pore pressure and overburden stress gradients and calibration points (leak-off test, mud weight, and pore-pressure tests) areshown in track 4. Nonlinear elastic constants from the dipole radialprofiling are shown in track 5.

Table 2. In situ stress magnitudes for a well offshore Malaysia at adepth of 2504 m.

SV

MPa37.4

SH

Sh

MPa

MPa

35.1

Pp

26.8

0.95

MPa

30.9

Table 3. Reference moduli and stress-sensitive constants used forstress determination at 2504 m.Mref

GPa

12.2

ref

GPa3.4

ref

MPa34.5

c144

GPa

1595

c155

GPa

2587

c111

GPa

24,669

Velocity variations under different stress paths

Once all three stress magnitudes and the stress-sensitivity coefficients of the zone of interest have been determined, we have insitu calibrated velocity-to-stress transforms that can be used readily for time-lapse seismic reservoir geomechanics (Donald et al.,2013). Table 3 shows the coefficients for the case study.During primary depletion, vertical effective stress and horizontal effective stress increase within the reservoir because ofa decrease in pore pressure, whereas in the caprock, vertical


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effective stress decreases and horizontal stresses can increase(Herwanger and Horne, 2009). The stress path K is a convenientway to characterize tensor stress changes from an initial stressstate by a single parameter. It is defined as the ratio between thechange in minimum horizontal effective stress and the changein vertical effective stress (Donald et al., 2013):K= h .V

depend on porosity (26%) and clay volume Vclay (5%) as well as aneffective stress Pe (velocities in kilometers per second and stressin kilobars) (Donald et al., 2013):VPEP 89 = 5.77 6.94 1.73Vclay + 0.446(Pe e 16.7P ),

(8)

VS EP 89 = 3.7 4.94 1.57 Vclay + 0.361(Pe e 16.7P ).

(9)

e

e

(7)

We assume here that the maximum and minimum horizontalstresses are changing in the same way (H = h). We considerseveral modes of deformation, i.e., different values of K, to illustrate how seismic velocities change as a function of different stresspaths. For example, under hydrostatic stress changes such as porepressure changes, the horizontal and vertical stresses are increasedsimultaneously by equal amounts, i.e., K = 1 (Donald et al., 2013).If deformation of the reservoir is constrained by a no-lateraldeformation boundary condition (such as in uniaxial strain experiments), elasticity theory tells us that vertical stress changes are associated with horizontal stress changes as 0 < K = (/1 ) < 1,where is Poissons ratio. For laterally unconstrained compressionusing only vertical force (i.e., horizontal stress changes as H = h= 0H = h = 0), the stress path is K = 0. Negative stress paths arepredicted for overburden stretching (K < 0) (Donald et al., 2013).Figure 6 shows the variations of the three compressional velocities as a function of small perturbations of effective vertical stressfor the stress conditions of depth 2504 m and for the different stresspath K = 0.5, 0, 0.5, and 1. Figure 7 shows the same for the threeshear velocities. Because the nonlinear model was calibrated near areference stress, we analyze only perturbations within 6 MPa of thevertical stress of the considered depth (Donald et al., 2013).For comparison of the plots, we show the results from theclassical empirical Eberhardt-Phillips et al. (1989) model that

For a visual comparison of stress-sensitivity effects, we arbitrarily shifted V PEP89 and VSEP89at the reference vertical stress (Donaldet al., 2013).We make several observations (Donald et al., 2013): The compressional velocities are sensitive mainly to thestress in the polarization-propagation direction (e.g., V P33,V P11, V P22 depend, respectively, mainly on SV, SH, and Sh )with a slight dependence to stresses in orthogonal directions, as expected from nonlinear elasticity. The shear velocities are sensitive mainly to the stresses inboth the propagation and polarization directions (e.g., VS31to SV and SH, VS32 to SV and Sh, and VS12 to SH and Sh ), asexpected from nonlinear elasticity. The three compressional and shear velocities vary greatlywith vertical stress depending on the stress path becausethey depend on the three principal stress magnitudes. The empirical model cannot capture differences caused bystress path because it relies on only one stress. The stress sensitivities are significantly stronger than theempirical VPEP 89 and VSEP 89 for all considered stress paths (K= 0.5, 0, 0.5, and 1). The model is calibrated for in situ conditions for three independent stresses and orthorhombic elastic media (nine independent constants), whereas the empirical model had to becalibrated artificially to the in situ conditions.

Figure 6. Variations of vertical (VP33, green) and horizontal (VP11 and

VP22, red and blue) compressional velocities as a function of smallperturbations of the effective vertical stress for the different stresspaths K = 0.5, 0, 0.5, and 1 for the Malaysia case study at a depthof 2504 m. The empirical model VPEP89 for 26% porosity and 5% clayvolume also is reported as a function of effective vertical stress and isshifted arbitrarily for each reference velocity.

Figure 7. Variations of vertical (VS31 and VS32, green and red) andhorizontal (VS12, blue) shear velocities as a function of small perturbations of the effective vertical stress for the different stress paths K= 0.5, 0, 0.5, and 1 for the Malaysia case study at a depth of 2504m. The empirical model VS EP89 for 26% porosity and 5% clay volumealso is reported as a function

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