Improving the accuracy of Rt

HRLA High-ResolutionLaterolog Array Tool

Focused array technologyThe HRLA* High-Resolution Laterolog Arraytool attacks the difficult task of resolving trueformation resistivity (Rt ) in thinly bedded anddeeply invaded formations by providing fiveindependent, actively focused, depth- and resolution-matched measurements. Thesemeasurements, together with a 2D earthmodel and inversion scheme, simultaneouslyaccount for borehole, shoulder-bed and inva-sion effects, yielding a more accurate, morerobust Rt —critical for identifying and estimat-ing reserves. The array spacing is optimized to obtain the maximum amount of informationabout the invasion profile, thereby improvingresistivity estimations. The unique feature ofbridleless operations improves wellsite effi-ciency, and the absence of the surface current return and the tool’s through-wireddesign offer unprecedented combinability.

Wellsite benefits—clear answers for quick decisionsOptimal array focusing, enhanced by thesymmetric HRLA tool design, ensures all signals are measured at exactly the sametime and same tool position. This focusinghelps avoid horns and oscillations producedby irregular tool motion and ensures themeasurements are depth aligned. The tooldelivers an array of five resistivities, eachwith increasing depth of investigation. Thisresistivity profile, in conjunction with theimproved quality of the HRLA measurements,provides the necessary elements to affect a robust 1D inversion at the wellsite. Thisinversion assumes radial resistivity variationand an infinitely thick bed to produce a trueformation resistivity, Rt , an invaded zone resis-tivity, Rxo , and a diameter of invasion, di .

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Distance from center of tool (in.)

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Applications

� Resistivity determination in conductive mud systems

� Thin-bed evaluation� Invasion characterization

for permeability indication� Water saturation

determination � Identification of fluid

contacts

Benefits and Features

� Bridleless operations saverig time and improve loggingefficiency.

� All currents return to toolbody, eliminating Groningenand other voltage referenceeffects.

� Data measured from com-mon central electrode usingmultiple frequencies givesimultaneous measurementsthat are naturally resolution-matched and depth-aligned.

� Arrays are actively focusedusing both software andhardware, reducing shoulder-bed sensitivityand enhancing thin-bed definition.

� Five resistivity measure-ments, each with increasingdepth of investigation, give a clear indication of invasion.

� Tool operates without auxil-iary current, enabling combi-nation with other formationevaluation tools that requirespecial power, such as theFMI* Fullbore FormationMicroImager tool.

� Advanced 2D processingimproves Rt accuracy and enhances reserves estimates.

Mode 0 is used to measure mud resistivity. Modes 1 through 5 are used to measure formation resistivities at differentdepths from the borehole. The red lines are the measure currents, and the white lines are the focusing currents.

Depth of investigationThe HRLA tool operates using six differentmodes. The shallowest mode, Mode 0, isprimarily sensitive to the borehole and is usedto estimate mud resistivity, Rm. The array spac-ing is designed to supply the most informationpossible about the invasion profile. The plot atleft compares the Platform Express* resistivitymeasurements from the HALS High-ResolutionAzimuthal Laterolog sonde and the resistivitiesfrom the HRLA tool. The borehole-correctedHALS deep resistivity (HLLD) compares wellwith the Mode-5 response from the HRLA arraymeasurement, while the HALS shallow resistiv-ity (HLLS) is intermediate between the Mode-2and Mode-3 HRLA responses. The additionalHRLA resistivity measurements identify resistivity changes resulting from invasion.

Reduced shoulder-bed effectsHRLA wellsite data, significantly less affected by shoulder beds than traditional laterologmeasurements, allow quick, reliable interpre-tations. Active focusing and multifrequencyoperation, together with the symmetric tooldesign, reduce the sensitivity to the shoulder-bed effect that often complicates dual laterologinterpretations. Since hardware focusing issubject to physical limitations, the HRLA toolalso employs software focusing, which usesmathematical superposition of signals toensure the focusing conditions are respectedand rectifies any voltage imbalances.

No reference effectsThe HRLA tool design eliminates voltage refer-ence effects because all the current returns to the body of the tool, permitting unambiguousformation evaluation. Traditional laterologmeasurements use a current return at surfacethat requires the logging cable be electricallyisolated from the tool by use of a long insulatingbridle. A shift in the deep-resistivity measure-ment, called the Groningen effect, arises whenhigh-resistivity formation layers force currentsreturning to the surface electrode into the bore-hole. An artificially high formation resistivityresults, and therefore incorrect saturation estimates (log at left). Long tool strings anddrillpipe-conveyed logging have a similareffect. Since the HRLA tool design returns thecurrent to the tool body, a bridle is no longerrequired, and voltage reference effects areeliminated. This design feature has the addedbenefit of reduced rig-up time, anywhere from15 to 45 minutes per job, as well as improvedcombinability and reliability.

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Invasion radius (in.)

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Apparentresistivity(ohm-m)

RLA1(Mode 1)

HLLDRLA5(Mode 5)

RLA4(Mode 4)

RLA3(Mode 3)

RLA2(Mode 2) HLLS

Rt = 10 ohm-mRxo = 1 ohm-mdh = 8 in.Rm = 0.1 ohm-m

Plot of HRLA and HALS resistivities versus depth of mud filtrate invasion. RLA5 from the HRLA tool is comparable to HLLD from the HALS tool.

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mMD Groningen Separation

MSFL (Logarithmic scale)

(ohm-m)0.2

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Array Resistivity - RLA2 LLS

LLD

(ohm-m)2 200

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Array Resistivity - RLA1

Array Resistivity - RLA4

Array Resistivity - RLA5

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Bit Size (BS) LLGArray Resistivity - RLA3

Comparison of HRLA and DLL* Dual Laterolog resistivity field data showing improved resolution and the absence of Groningen effect on the HRLA response. Microresistivity is presented in the left track to confirm the high-resolution HRLA data.

Clear invasion profileIn thin beds, where deeper measurementstend to degrade in both depth of investigationand vertical resolution, the HRLA service pro-vides an array of measurements that presenta clear invasion profile. The shallow readingsimprove the radial sensitivity to resistivitychange, which results in greater curve sepa-ration in an invaded zone. With no referenceeffects and only minimal shoulder-bed effects,the five resistivity measurements reduceambiguity and improve interpretations.

The wellsite example shown below com-pares 1D Rt and di from HRLA and traditional

dual laterolog data. The laterolog curves areout of sequence as a result of shoulder-bedeffect. Because the input data are inconsis-tent with the 1D formation model, Rt is setequal to the deep reading of the laterolog and the di is forced to bit size, indicating no formation invasion. The additional infor-mation from the HRLA resistivity data allowsa realistic estimate of di, which in turn allowsinvasion corrections to be applied to find amore accurate Rt. For the peak around XX00ft, the Rt is 45% higher, resulting in a 16%overall increase in reserves estimates overthe interval.

Improved combinabilityThe HRLA tool is compatible and combinablewith a host of Schlumberger logging tools,giving improved wellsite efficiency. The combi-nation of the AIT* Array Induction Imager andHRLA tools yields complementary data forquantification of complex resistivity environ-ments. The new HRLA tool design eliminatesthe need for auxiliary power, allowing combi-nations with services such as the FMI andCMR* Combinable Magnetic Resonance tools,which require special power. In addition, use of the HRLA real-time invasion analysis stream-lines formation pressure sampling operations.

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1D Rt Increase

HALS Invasion

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MCFL Microresistivity MCFL Microresistivity

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HRLA Array Resistivity 5 HALS Shallow Resistivity 1D Rt from HALS

HRLA Array Resistivity 4 HALS Deep Resistivity

HRLA Array Resistivity 3

HRLA Array Resistivity 2

HRLA Array Resistivity 1

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Comparison of HRLA and HALS data showing reduced shoulder-bed effect and improved characterization of invasion, which led to a better Rt after the 1D inversion. The real-time 1D inversion facilitates quick decisions.

Advanced 2D inversion—improving the accuracy of Rt

More accurate representation of the forma-tion and borehole environment means moreaccurate Rt estimates, especially in thinlybedded formations. The HRLA tool not onlyprovides a coherent array of measurements,but the improved quality of these measure-ments and the additional information aboutthe invaded zone allow advanced 2D inver-sion processing. The 2D formation modelsimultaneously accounts for all 2D effects,including those from the wellbore (caves)and from vertical (shoulder-bed) and radial(invasion) resistivity variations. The 2D inver-sion process begins with the information-rich

raw HRLA data. First, layers are definedthrough inflection-point segmentation, and a “first guess” is made for the initial forma-tion parameters: Rt, Rxo and di. The programthen computes the tool response in the theoretical formation and compares it to the actual response. The formation parame-ters are updated, and after successive iter-ations an acceptable match is found. Thismathematical inversion technique, usedwith a 2D formation model, yields a moreaccurate Rt and therefore a more accuratesaturation estimate.

On the log above, the 2D inverted Rt andRxo are shown in track 3 with the raw HRLAcurves. The red shading indicates normal

invasion (Rxo < Rt); the green indicatesreversed invasion (Rxo > Rt). In track 4, the 2D inverted resistivities Rt (red) and Rxo(green) are compared with the 1D invertedformation resistivity Rt (magenta) and the Rxo (black) from the Platform Express MCFLMicroCylindrically Focused Tool. The 2Dinversion shows a significant increase in Rt obtained in thin beds—such as thosebetween XX30 and XX70 ft—over the 1Dinversion results. A good match between the 2D inversion-derived Rxo and the oneindependently obtained from the MCFLmeasurement adds confidence to the inversion results.

Formation model used for the 1D inversion. The only variable is theamount of invasion.

Formation model used for the 2D inversion. This model allows forvariations in hole size, zone thickness and the amount of invasion.

HRLA Array Resistivity 3

Rt from 2D Inversion

Rxo from 2D Inversion

Rt from 1D Inversion

MCFL Microresistivity

(ohm-m)(ohm-m)

HRLA Array Resistivity 2

HRLA Array Resistivity 1

HRLA Array Resistivity 4

Rxo from 2D Inversion

HRLA Array Resistivity 5

Rt from 2D Inversion

DifferentialCaliper

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Comparison of raw log, field 1D inversion and 2D inversion. Note the many thin zones across this 100-ft interval.

HRLA Tool Specifications

Physical CharacteristicsTool diameter 35⁄8 in.Tool length 24.1 ftTool weight 394 lbm

Environmental LimitationsMaximum temperature 300ºF [150ºC]Maximum pressure 15,000 psiShocks and vibrations 2000 shocks of 250 g for 2 ms, LWD qualification standardsMinimum borehole diameter ≥ 5 in. (43⁄4 in. under certain borehole conditions)Maximum borehole diameter ≤ 16 in. (≤ 12 in. preferred)Maximum dogleg severity 50°/100 ft in 8.5-in. borehole

Measurement Performance (8-in. borehole)Resistivity range (Rm = 1) 0.2–100,000 ohm-mResistivity range (Rm = 0.02) 0.2–20,000 ohm-mDepth of investigation 50 in. (median response at 10/1 Rt /Rxo contrast)Vertical resolution 12 in. Sampling rate 2 in. at 3600 ft/hr logging speed

www.slb.com/oilfield 06-FE-211 October 2006*Mark of SchlumbergerCopyright © 2006 Schlumberger. All rights reserved.Produced by Schlumberger Marketing Communications