Compact multimeasurement logging-while-drilling tool provides essentialinformation to solve a petrophysicalpuzzle.

AUTHORSMicheal Stadnyk, Derek Harrold, andMarty Haynes, Talisman Energy (UK)

Limited; Katy Heidenreich, Meretta Qleibo,and Ian Tribe, Schlumberger

The Auk field, located in theCentral North Sea, comprises oilbearing aeolian sandstones of

the Rotliegend formation. In 2009,Talisman Energy (UK) Limited plannedto drill three new horizontal develop-ment wells into the Auk North field,which was originally drilled over 30years ago. The well design consisted oflong 81⁄2-in. production intervals ofbetween 5,000 ft (1,525 m) and 6,500 ft (1,980 m). The first two wells havebeen drilled and were evaluated usingthe Schlumberger EcoScope multifunc-tion logging-while-drilling (LWD) tool.

One of the issues in the petrophysicalevaluation of the Auk field is the neces-sity to discriminate between the duneslipface facies and dune windripplefacies, since the latter is non-productive.Talisman was interested in evaluatingthe sigma and spectroscopy measure-ments available from the LWD tool todetermine if they could be used to dif-ferentiate between the two facies.

Integrated formation evaluation in single collarThe EcoScope service combines stan-dard LWD measurements of formationdensity, propagation resistivity, gammaray and compensated neutron responsewith the previously wireline-only meas-urements of elemental capture spec-troscopy and thermal neutron decaytime (Sigma), all in a single 26-ft (7.9-

m), 6.75-in. diameter collar. The addi-tional measurements are possible dueto a major change in the technologyused for measurement of formationneutron response. The traditionalchemical neutron source has beenreplaced with an electronic neutrongenerator. In addition to the increasedflexibility of measurements, the use of a Pulsed Neutron Generator (PNG)reduces risk by minimizing the require-ment for chemical sources in the well.The multifunction LWD tool still con-tains one chemical source, which isrequired for the measurement of for-mation density; however, a new tech-nique for measuring a sourcelessformation density using the neutrongenerator is under development. Byusing this measurement, known as neu-tron gamma density (NGD), it will bepossible in the future to run EcoScopefor formation evaluation without put-

ting a chemical source in the hole. While drilling the lateral section in

the first well, an unusual amount ofsevere wear was encountered on all ofthe bottomhole assembly (BHA) compo-nents, including wear on the EcoScopestabilizer that effected the bulk densitymeasurement. The data processing cen-ter was able to use the tool memory datato compute the NGD density, an inde-pendent formation density that was notaffected by the stabilizer wear.

Analysis of this data compared withthe conventional density in the first rungave sufficient confidence in the newdensity porosity measurement to evalu-ate the reservoir, saving the time andcost of a dedicated logging run.

Sourceless real-time density measurementPrior to drilling of the second well,several changes were made to the BHA

‘Sourceless’ LWD improves horizontal analysis

The Auk field sits on the western margin of the Central Graben, in Block 30/16 of theUK North Sea under 240 to 270 ft (73 to 82 m) of water. It is about 169 miles (270km) east southeast of Aberdeen. (Image courtesy of Schlumberger)

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to minimize the tool wear. To monitorthe effectiveness of these adjustmentsand to give confidence in the real-timedensity measurement, techniciansdecided to compute downhole andtransmit in real time the sourcelessNGD in addition to the traditionalgamma-gamma density.

By monitoring these two densitycurves, Talisman confirmed their con-fidence in using the sourceless NGDdata to evaluate the first well and wereable to establish while drilling thatdata quality in the reservoir sectionwas good. A comparison between thetwo measurements shows excellentagreement between the sourcelessNGD and traditional density. In thisfirst instance of employing the realtime NGD measurement in the NorthSea, it was proven that sourceless den-sity can be delivered reliably for use inreal time formation evaluation.

Talisman established that they couldrely on the NGD density for futureapplications where it was not appropri-ate to use a chemical source in the tool.

New data for a mature fieldIn the case of the Auk North field devel-opment project, the field evaluationfiles were populated mainly with histori-cal vertical well log data. By acquiringadvanced data in two new horizontalwells, the quality of reservoir evaluationcan take an important step forward.The newly acquired data was used toproduce correlations that allowed thereservoir model to be upgraded.Talisman’s evaluation of the EcoScopemeasurements acquired in the first twowells indicated that variations in thesigma response could be used to con-firm the classifications of dune slipfacefacies and dune windripple facies, assist-ing determination of reservoir quality.Furthermore, Orion II high speedtelemetry provided very high qualitydensity images to be transmitted in realtime for improved identification offaults and dune bedding dips.

Future developmentsThe ability to provide consistentnuclear measurements-while-drilling(MWD), exclusive of chemical sources,with no compromise on quality, histori-cal comparability or workflow compati-bility will be valuable for our industry.Replacing the chemical neutron sourcewith the PNG results in an 85% reduc-tion in immediate surface exposure risk and a nearly 100% reduction inlong-term contamination risk down-hole. Further removal of the chemicalgamma source will eliminate the HSErisks associated with handling ofradioactive material on surface andprovide for zero downhole radiationcontamination risks. EcoScope empow-ers users to obtain the critical densityand porosity measurements, in theform and quality they are used to,while taking advantage of technicalinnovations that makes the rig site saferand poses less environmental risk.

The Orion II telemetry platformenables more of the valuable downholeinformation to be available in real timethan previously. Improved data densityat high rates of penetration with quality

drilling and formation evaluationparameters will have significant impacton the user’s operations and time sensi-tive decisions. Decisions based onimproved geomechanical and petro-physical evaluation will help the opera-tor to drill successfully in marginalmechanical environments and betterplace wells in the reservoir for improvedproduction. A more frequent updatefor downhole parameters will allow thedriller to better control the surfaceparameters to ensure optimum penetra-tion rates and safe treatment of the bitand drilling assembly for longer drillingintervals.

The EcoScope service combines standardLWD measurements with previously wire-line-only measurements of elemental cap-ture spectroscopy and thermal neutrondecay time (Sigma) all in a single 26-ft(7.9m), 6.75-in. diameter collar. (Imagecourtesy of Schlumberger)

Azimuthal density images from theEcoScope tool from well #2 (left) helpidentify and quantify faults and dune dipsessential to reservoir characterization. Onthe right, the gamma-gamma densitycurve (red) is seen to track the sourcelessNGD curve (black) validating its use forformation evaluation. (Logs courtesyTalisman Energy (UK) Limited)