Current Affairs

The value of openhole resistivity andinduction logs as the primary source fordetermining hydrocarbon saturation is wellestablished. As reservoirs in the MiddleEast mature, it is necessary to monitor andreevaluate reservoir saturation behindsteel casing as a basis for decisions onenhancing recovery. The ability to measureresistivity behind steel casing has been amajor goal in the oil industry since the1930s. Until recently, pulsed neutrontechniques have been used, but these havea relatively shallow depth of investigation.

Paolo Ferraris explains how new, stableelectronics have made accurate, reliableand deeper cased-hole formationmeasurements a reality.

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Figure 4.1:The firstelectrical logwas recordedin an uncasedhole in 1927

F rom the early days of the oilindustry, petrophysicists have

sought to measure the saturation ofoil in reservoirs by simple electricalmeasurements. In their firstexperiments, pioneers Conrad andMarcel Schlumberger attempted tomeasure the effects of subsurface oilreservoirs on electrical current sentfrom and returned to surfaceelectrodes. Had this approach beensuccessful, the need for subsequentwireline logging techniques wouldhave been greatly reduced.

However, the Schlumbergerbrothers soon realized that it wouldbe necessary to lower electricallogging tools into wells to observereservoir saturation in detail. And soelectrical coring tools were born. Thefirst electrical log was recorded in anuncased or openhole well inPechelbronn, Alsace, France, in 1927(see Figure 4.1). The log recorded theextent to which the reservoir rocknear the wellbore resisted the passageof an electric current, with discretemeasurements at a number of depthsin the well. Since then, electricalsurveys in openhole have providedthe principal measurements used todistinguish hydrocarbon- from water-bearing zones.

While openhole resistivitymeasurements were to be invaluablein the search for oil, somefundamental drawbacks wereapparent from the earliest days. Inparticular, the effects of drilling mudon the measurements and of thedisplacement of reservoir fluids bydrilling-mud filtrate have taxed thedesigners of logging tools ever since.Many innovative ways have beendeveloped to make resistivitymeasurements and to focus themdeep into the formation whereinvasion has not had an effect on theoil saturation.

Two tool designs have emerged,working on entirely different physicalprinciples, but both with focusingcapabilities. Tools that pass currentdirectly from electrodes to theformation, such as laterolog tools,work best when the mud is highlyconductive and the formation ishighly resistive; for example,hydrocarbon reservoirs drilled withsalt-saturated mud. Induction toolsthat induce oscillating currents in the

formation are well suited tononconductive mud, such as oil-basedmud, and relatively conductiveformations. Both designs have finitedepths of investigation and benefitfrom shallow filtrate invasion.

In practice, it is rare for loggingconditions to enable directmeasurement of the uninvadedformation resistivity, which is normally evaluated by combining afew measurements acquiredsimultaneously at different depths ofinvestigation. More significantlimitations of the physics ofmeasurement of conventionalelectrical logging tools restrict them touse in openholes only. Despite theseproblems, openhole laterolog andinduction-resistivity measurementsremain the primary sources fordetermining hydrocarbon saturation.

CHFR – the challenge in cased holesCased-hole measurements offer a wayaround the effects of invasion andother saturation measurementproblems. Since the 1930s, cased-holeresistivity measurements have beensought to monitor both the initialhydrocarbon saturations around thewell just after running casing, and thechanging reservoir saturations at thewell as hydrocarbons are produced. Adeep-reading, cased-hole resistivitymeasurement would allow saturation tobe determined after drilling mudfiltrate had dissipated into theformation. Subsequent changes couldbe reliably attributed to the effects ofproduction on saturation.

As some Middle East oil fields reachmaturity, asset teams are seeking to

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improve recovery factors. Much of thelatest oilfield technology focuses onextending field life by identifying andproducing bypassed hydrocarbons. In other cases, field recovery isincreased through improved sweepefficiency resulting from secondaryand tertiary recovery schemes, such as water or gas injection. In thesecases, monitoring reservoirs in order totrack changes in saturation and themovement of fluid contacts isfundamental to success. Until recently,the only methods available to theindustry were based on pulsed-neutronmeasurements, which have significant,inherent limitations (see box). Onelimitation is the shallow depth ofinvestigation, with an order ofmagnitude of 1 ft or less, a distance atwhich filtrate may never fully dissipate.This permanently affects the readings,especially in damaged or low-permeability formations.

Today’s requirements for monitoringand reevaluating saturation haveresulted in an increased interest incased-hole resistivity that hascoincided with recent advances inelectronics. New, accurate and stabledownhole digital electronics have madeaccurate and reliable measurement offormation resistivity behind steel casinga reality. The new cased-hole formationresistivity technology incorporated inthe CHFR* cased hole formationresistivity tool is able to measureformation resistivity accurately behindsteel casing. The measurement is madevery much further away from theborehole than pulsed-neutronmeasurements, resulting in greatlyimproved saturation monitoring in thepresence of residual filtrate.

How it’s doneFor the past 60 years, many patentsrelated to the measurement of cased-hole formation resistivity have beengranted. Only those methods that passcurrent into the formation throughelectrodes, as in laterolog tools, haveemerged as feasible, and the CHFRtool is no different.

The central principle of both theCHFR and laterolog tools is anarrangement in which a measurecurrent flows into the formation. Theelectrical resistivity of the formation

Saturation determination in cased holes haspreviously only been possible using nuclearlogging tools. These use an electronicneutron source emitting high-energyneutrons that can pass through the casingand into the formation. Detectors in the toolreveal the slowing effects on the neutronsof the formation and the fluids contained inits pores, which can then be interpreted interms of hydrocarbon saturation.Alternatively, interactions of neutrons withcarbon and oxygen atoms in the formationgive characteristic detector responses thatyield hydrocarbon saturation.

The simplest and quickest nuclearlogging method is to measure sigma, aparameter that describes the ability of theformation and fluids to slow neutrons downto low energy. Porous formations saturatedwith salty formation water are moreefficient at slowing down neutrons andconsequently have a high value of sigma.The same formation filled withhydrocarbons has a much lower sigmavalue. Under these conditions, the dynamicrange of the measurement is sufficient toprovide quantitative saturation results.

Several collisions with chlorine atoms inthe formation water are necessary toreduce the energy to a thermal energy level,during which neutrons diffuse in a cloud upto 1-ft thick. Mud filtrate remaining fromthe drilling process may hide the trueformation saturation if invasion exceeds thesize of the neutron cloud and has notdissipated when the formation is logged.Experience shows that in undamagedformations with porosity above 20 p.u.sigma measurements reflect the truesaturation a few months after completingthe well. In lower porosities and in thepresence of formation damage there is noguarantee that the sigma measurement willever be valid, even after years of waiting.

Sigma logs are the preferred nuclearsaturation measurement, as they can berecorded at normal logging speeds.However, they require constant and knownformation water salinity for thecomputation of saturation. If this is not the case, for example because of theinjection of water of different salinity, anindependent method is required.

The carbon:oxygen ratio (C:O) method isthe only alternative neutron loggingtechnique. It relies on measuring theenergy of gamma rays that result whenhigh-energy neutrons interact with varioustypes of atoms. The resulting spectrum ofgamma ray energies is the sum of spectrafrom many elements, including carbon andoxygen. Analysis of the spectra yields theratio of carbon to oxygen recorded by twodetectors and in turn the calibratedhydrocarbon volumes.

Despite its advantages in unknownwater salinity, the C:O method has severaldisadvantages. Since the high-energyneutrons involved travel a shorter distancethan during the sigma measurement(interactions occur at higher energies, afterfewer collisions), it has a depth ofinvestigation in the order of 6 in.

The accuracy of the measurement is alsolow and requires a slow logging speed. Toincrease measurement accuracy C:O logsmust be recorded several times and stackedtogether. This means that total loggingtimes can be very long, up to 30 times thatof sigma measurements. However, even iflogging speed is not an issue, stacking morethan 10 passes is impractical, limiting theC:O technique in practice to formations withporosity above 15–20 p.u.

can be computed if the geometry ofthe current’s path is known.

To ensure the path of the measurecurrent is known, laterolog toolsemploy a second, variable, buckingcurrent, sent to guard electrodesabove and below the one emittingthe measure current. Together, themeasure and bucking currentsspread out in all directions from thetool. However, by adjusting themagnitude of the bucking current,the measure current can be forced to

flow radially outwards in a currentdisk of known shape and dimensionsinto the surrounding formation. Forexample, if changes in the formationresistivity cause the disk to thicken,by spreading upwards anddownwards, an increase in thebucking current will force it backinto the correct shape.

The tool’s geometrical factordescribes the geometry of themeasure current path. In the case ofthe laterolog, the geometrical factor is

Pulsed-neutron saturation logging

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well defined. Remote electrodesmonitor the drop in potential across apart of the current disk to completethe parameters needed to computeresistivity. The depth of investigationof the laterolog measurement is afunction of, among other things, theelectrode spacing.

Under appropriate conditions inopenhole, both laterolog and inductiontools produce currents that flow deep inthe formation. They do not work incased holes because of theoverwhelming difference in conductivitybetween the steel and the formation.The casing can be more than one billiontimes more conductive than theformation rock. Almost all of the currentemitted by either type of tool will flow inthe casing. Therefore, obtainingformation resistivity behind steel casingrequires a specially designed tool.

The CHFR tool is the first resistivitytool able to measure formationresistivity through casing using asimilar technique. However, thepresence of conductive casing has afundamental effect on both themeasure and bucking currents.

The CHFR measurement isessentially a laterolog measurement,with the casing acting as a giant guardelectrode that focuses the currentdeep into the surrounding formations.

A single source of low-frequency(typically 1 Hz) current is connected tothe casing downhole. The majority ofthe current flows up and down thehighly conductive steel. A smallfraction, the leakage current, flowsfrom the casing into surroundingconductive formations. This leakagecurrent is of the order of a fewmilliamperes per meter of casing, whilethe total current may be as high as 8 A.

The current source is at surfaceand current has to be conveyeddownhole using the logging cable.Several conductors are used inparallel to maximize the injectedcurrent. All the current, including themeasured leakage current, returns tothe surface. A remote return awayfrom the wellhead should maximizethe amount of leakage current,especially for shallow wells. Theeffects of the position of this current-return electrode were evaluatedduring CHFR field testing, usingphysical measurements andmathematical modeling.

Induction response,concentric formationsmeasured in parallel

Laterlog andCHFR response,

concentric formationsmeasured in series

Logging tool

Mud

Invaded zone

Virgin formation

Rt

Rt

Rxo

Rxo

Rm

Rm

Figure 4.2:Laterolog devicesmeasure boreholeand formationresistances inseries, whileinduction devicesmeasure resistancesin parallel

induces currents in the formationsurrounding the borehole. These ground-loop currents in turn induce a current in areceiver coil that is proportional to theformation conductivity. Induction tools canbe focused, for example to suppress theresponse from the mud column. In contrastto laterolog measurements, inductionresponse is made in parallel (Figure 4.2)and is suited to low-resistivity formations.

Both tools can be constructed with longelectrode spacing to substantially eliminatethe influence of mud and the invaded zoneon the rock volume being measured. Theresistivity of the virgin measurement canthen be read, more or less directly. Thisapproach must be traded off against theability to resolve thin beds. Array toolshave provided the means to obtainmeasurements with different depths ofinvestigation that can be combinedtogether using a suitable model to deriveuninvaded formation resistivity.

Openhole toolsPetrophysicists need to know the resistivityof the virgin formation as it was before thewell was drilled, and before the drillingfluid contaminated the near wellbore.

While laterolog tools benefit from beingfocused deep into the formation tominimize the effects of drilling-fluidinvasion on the measurement, they alsorequire the measure current to passthrough the mud and the invaded zonesbefore reaching the virgin formation. Sincethe measure current travels through themud, mudcake, filtrate-invaded zone andvirgin formation in series, laterologsbecome less accurate when the formationresistivity is low compared to the nearwellbore region. They will not work at all innonconductive mud.

Induction resistivity tools are designedfor situations where electrode contact isimpossible or unreliable, for example in thepresence of oil-based mud. Alternatingcurrent passed through a transmitter coil

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Conclusions were that armor(wellhead) return is preferable forwells deeper than 8000 ft, where thelogging interval is within 1000 ft of thecasing shoe, since there is nomeasurable increase in leakagecurrent when compared to using areturn electrode away from thewellhead. In addition, where theconductivity of the ground is low, forexample in desert conditions, a return

Casing voltagev

Current source

Current return

fish

Alternativecurrentreturn (cablearmor)

Figure 4.3: The effects of the position of thecurrent return electrode were evaluated duringfield testing. The conclusions were that armorreturn is preferable for wells deeper than8000 ft, where the logging interval is within1000 ft of the casing shoe

at the wellhead, or on the loggingcable itself, was found to be a bettersolution, as it is less susceptible topicking up spurious electrical noise.(see Figure 4.3).

The amount of leakage currentleaving the casing in the region of thetool is influenced by the formationresistivity. It is this current, and thevoltage that drives it, that must bemeasured to compute cased-holeformation resistivity. The magnitudeof the current is inferred bycomputing the currents flowingdownward in two adjacent, 2-ft thickintervals of casing, one above theother (Figure 4.4). The differencebetween these is the current that hasleaked into the formation.

Upward and downward currents inthe casing vary according to theposition of the tool and the formationresistivity. When the tool is near thesurface most of the current goes upthe casing. The downward currentdecreases and leakage into theformation increases when the tool isnearing the casing shoe. The voltagesassociated with these small currentsare very low. Advances in digitalelectronics now offer the downholecircuits necessary to measure thepotential differences of a fewnanovolts that result.

The corresponding casing voltageat the same point, measured withrespect to a remote ground referencepotential at surface, is also measured.This is the voltage that drives theleakage current to surface. Forpractical reasons, this is measuredindependently of the oscillatingcurrent measurements using a DCvoltage, first of one polarity, then ofthe opposite polarity.

Due to the high conductivity of thecasing, the voltage varies so littlealong the casing that thismeasurement needs to be made at arelatively low frequency (once every

40 ft of casing logged is adequate).The very small voltage changes alongthe length of the casing also meanthat the leakage current flowsperpendicularly away from the well. Itis, in effect, focused deep into theformation. Field testing found that theshape of the voltage variation alongthe casing followed a well-definedpattern that could be expressedmathematically as a function of casingproperties. This approach, theestimated voltage method, has theadvantage of being noise free, and isoften preferred to the physical voltagemeasurement, which is used mainlyfor quality control.

In principle, the CHFR measurementis similar to that made by a laterolog.Unlike the laterolog, in which themeasure current is sent separatelyand can be measured, the part of theCHFR leakage current to be measuredis a very small fraction of the totalcurrent leaving the tool and cannot bemeasured directly. As noted, it iscomputed as the difference betweentwo currents flowing in adjacentintervals of casing, but these currentsare also influenced by casingresistivity and must themselves beinferred. A second part of themeasurement, the calibration, orcasing step, is therefore needed todetermine the casing resistance.

Given a known geometrical factor,the voltage and current measurementsdescribed would allow the formationresistivity to be computed (see Figure4.5). The magnitude of the focusingeffect of the CHFR leakage current is a consequence of the casinggeometry and many other factors. The geometrical factor of the tool istherefore different in each well and an absolute resistivity cannot becomputed from CHFR data alone. Toovercome this, a gain must becomputed for each well, and appliedto all measurements made under the

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I

Top current-injectionelectrode

Casing

∆I and∆Rc

I+∆i ∆IV1

V2

Surfaceelectrode

0

500

1000

1500

Depth, m

2000

2500

3000

Leakage current

V0

Figure 4.4: Top injection measurement step (right-hand side). A low-frequency alternating current is injected into the casingfrom the upper centralizer (right), closing the circuit either to a return electrode at the surface or to the logging cable armor.When the current reaches the casing it splits, part of it flows directly back up to the surface and the remainder flows down thecasing. The ratio of these two components depends on the formation resistivity and the location of the injection point withrespect to both the surface and casing shoe. When the injection point is equidistant from both casing ends, the up:down split is50:50 for a 1-ohm-m formation resistivity. In most cases, the downward current is less than 50% of the total injected current forseveral reasons. Firstly, the fraction of the downward current is inversely proportional to formation resistivity. Secondly, at highformation resistivities, the amount of current leaking into the formation decreases. Both these factors result in the size of theleakage current decreasing with increasing formation resistivity, adding to the difficulties in measurement. A third factor is thatthe downward current is drastically reduced as the injection point approaches the casing shoe, however, the formation currentincreases in this region. This works in favor of CHFR measurement since most of the intervals to be logged are close to thecasing shoe. The green line shows the leakage current in a homogeneous formation with resistivity of 1 ohm-m for a 3000-mdeep well with 7-in. casing with current returns at the wellhead

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same conditions in that well. Thechanges in geometrical factor thatoccur due to variations in the currentdistribution near the casing shoe (or surface) are computed as afunction of casing size, thickness,properties and depth, and are used tocompensate for the nonlinearity of thegeometrical factor observed close tothe casing shoe.

CHFR gain is computed bycomparing the uncalibrated log withexisting resistivity logs in intervalsthat have not changed since logging.Clearly this introduces someambiguity into old wells. In new wells,openhole resistivity logs, followed byCHFR data recorded soon after thewell is cased, will provide accuratebaseline cased-hole logs of lastingvalue for monitoring changes insaturation over the life of the field.

The measurementThe tool consists of a cartridgeincorporating state-of-the-artelectronics to process the very smallmeasurement signals, and ahydraulically operated sondecomprising the centralizers andelectrodes. A third centralizer can beadded to improve centralization indeviated wells. The tool is 43 ft longwith a diameter of 333/8 -in. Thecentralizers are strong enough tosupport the tool in holes with up to a70° deviation.

The sonde has four sets of armscarrying electrodes in between thetwo centralizers used for currentinjection or current return. Since eachmeasurement requires only threeadjacent electrodes, the sonde designpermits two measurements to bemade 2 ft apart at each tool depth.

The centralizers that act as currentelectrodes are open at all times andare pressed firmly against the casingby strong springs. Electrical contactbetween casing and voltage electrodesis established by opening the hydraulicsonde at the appropriate depth.

During the measurement, the tool isstationary to minimize contact roadnoise and to allow the two steps ofthe measurement process to be madeat exactly the same depth. At eachdepth, measurements are repeatedmany times over a period that variesbetween two and three minutes,depending on the formationresistivity. Statistical techniques areused to reduce random variationsbetween measurements, so that moreaccurate readings require longermeasurements at each station. Goodelectrical contact with the casing ispreserved by leaving the tool’s armsopen while pulling it up the hole tothe next station, which is generally4 ft shallower.

The results of the measurementand calibration steps are combined toderive the formation current.Formation resistivity can then becalculated using the voltage withrespect to the surface and the tool’sgeometrical factor, similarly to otherlogging tools. Secondary results fromthe computation are the casingresistance and magnetic permeability.In addition, when the steel resistivityis known or assumed, casingthickness can be derived.

PerformanceCHFR field logs have shown that themeasurements are repeatable anddirectly comparable to formationresistivity recorded using conventionaltools, such as the Platform Express*integrated wireline logging tool, at thetime of drilling. Also, because of thephysics of measurement and the depthof investigation, CHFR is not, ingeneral, affected by borehole washoutsbehind casing (Figure 4.6).

In cased holes, the presence ofcement does not impede CHFRmeasurements. An electrode spacingthat focuses the measurement awayfrom the borehole eliminates theeffect of cement in the same way thatlaterolog measurements eliminate theinvaded-zone resistivity in similarsituations. Two-dimensional modelinghas shown that the effect of cementon CHFR is negligible for a conductivecement (Rt/Rcem >1), but must beconsidered when the ratio offormation resistivity to cementresistivity drops below one. Thickercements are worse than thin ones.Extensive testing has shown thatcorrection factors can be applied toproduce reliable results for cementswith known properties. However,where changes in cement resistivitymay have occurred over time, orwhere the cement quality is inquestion, the cement distribution and thickness can be evaluated usinga suitable tool, such as the CBT*Cement Bond Tool, the CET* CementEvaluation Tool or the USI*UltraSonic Imager, and some estimateof the effect of cement on resistivityreadings can be attempted.

Figure 4.5: Casing resistivity must be knownto compute the current leaking into theformation. It is measured by sending acurrent from the top centralizer electrode tothe bottom one through the casing. In thisconfiguration, there is negligible leakageand the casing resistance between thevoltage electrodes can be computed fromthe current flowing and the potentialdifference between the electrodes

Rt

Rc

I

Top current-injectionelectrode

Bottomcurrentelectrode

Casing

V1

V2∆Rc

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Abu Dhabi case studyThe CHFR service recently made asignificant contribution to theunderstanding of water saturationchanges in an onshore reservoir underwater injection in Abu Dhabi.

Prior to launching a full-scale water injection project, the Abu DhabiCompany for Onshore Oil Operations(ADCO) initiated a study to improveunderstanding of the movement of reservoir fluids and theirdisplacement mechanisms. Theydrilled a vertical observation well inwhich to monitor saturation changesduring injection in a deviated injectorwell 100 m away. The observation wellwas completed across two reservoirintervals with a standard 7-in. steelcasing. Injection would take place inthe lower of the two reservoirs, withperiodic saturation monitoringperformed across both reservoirs atthe observation well before andduring injection.

In the past, observation wellsdesigned for monitoring saturation byresistivity logging were completed withfiberglass casing. Nonconductivefiberglass does not interfere withinduction-log measurements, butpracticalities such as its fragility anddifficulties with cementing resulted inthe company reverting to steel casingin observation wells. In practice, theuse of fiberglass casing had to beabandoned because of the developmentof casing leaks over time and ADCOplanned to complete the newobservation well with a conventionalsteel casing.

The logging program includedpulsed-neutron logs. The addition of the newly introduced CHFRtechnology gave ADCO theopportunity to evaluate the tool’sability to overcome the drawbacks ofthe nuclear measurements.

The study’s main objectives were to

• integrate logging results withpressure data and core analysis

• establish a comprehensive andreliable understanding of thereservoir

• study saturation evolution with time and compare it with reservoirsimulation runs

• evaluate the efficiency of watersweep before application on a wider scale.

In the observation well, initial watersaturation was computed fromPlatform Express tool data. However,deep invasion caused by formationtightness and the use of nondamagingfluid affected the quality of thesaturation results. Low formationpermeability meant that dissipation ofthe filtrate was expected to beextremely slow with a high probabilitythat the annulus of filtrate around thewellbore would affect baseline pulsed-

neutron measurements. Furthermore,the accuracy of the C:O measurementsderived from the pulsed-neutron logswas compromised by the relatively lowporosity (>20 p.u.).

The CHFR tool had the potential toovercome many of these problems. Its much greater depth of investigation(in general from 7 to 32 ft, but above12 ft in the lower interval of this well)was expected to provide measurementsfrom beyond the invaded zone.

in.

Caliper

6 16

API

Gamma ray

0 150

in.

Platform Express shallow laterlog

0.2 2000

ohm-mDepth, ft

X400

X450

X500

X550

X600Washout

Platform Express deep laterlog

0.2 2000

ft3/ft3

Neutron porosity

0.6

sec/ft

Sonic slowness

140 40

0

ohm-m

CHFR resistivity

0.2 2000

ohm-m

Rxo

0.2 2000

Figure 4.6: In this Middle East well, at depth X600 ft, the caliper (track 1) indicates a washouthaving a borehole diameter of nearly 16 in. (the bit size plus 8 in.). In track 2, the CHFR resistivity(black dashed) overlays the laterologs (blue and red) and appears to be unaffected by the holewashout. In contrast, the porosity logs at the same depth presented in track 3 (blue = neutronporosity and green = sonic porosity) are significantly affected

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The outcome

Openhole evaluation of theobservation well included coringacross the reservoir of interest using alow-invasion technique. Theevaluation was performed initiallyusing a Platform Express tool with aHALS* Azimuthal Laterolog Sonde forresistivity. An MDT* ModularFormation Dynamics Tester tool wasused for pressure measurement andto collect two sets of samples abovethe oil–water contact using dualinflatable packers. The top samplingpoint yielded oil with some formationwater, while only filtrate was collectedat the lower sample depth.

Comparison of the ELAN*Elementary Log Analysis interpretationbased on the openhole logs and theMDT results revealed a saturationanomaly. The MDT data showed thefree water level in the observation wellto be at least 48 ft deeper than thatinitially estimated from the ELANinterpretation of openhole resistivities.It appeared that the HALS resistivities,despite their relatively deep level ofinvestigation, were affected by the deepinvasion, and that the ELAN saturationinterpretation could not be used safelyas a baseline for time-lapse logging.

In view of the main objective –monitoring water sweep efficiency –and the fact that logs were affectedby invasion, cased-hole logging wasseen as the key to providing accurateinformation. Having completed theopenhole evaluation, four CHFRsurveys to track saturation changeswere made over almost two years.

Cased-hole run 1The first cased-hole run was recordedfour months after the initial openholelogs. Both resistivity and pulsed-neutron logs were recorded, using theCHFR tool and RST* ReservoirSaturation Tool respectively in bothsigma and C:O modes. Logs wereacquired over both the upper andlower reservoir intervals.

The voltage gain required tonormalize the CHFR measurementswas chosen to make the CHFRresistivity equal the true formationresistivity (Rt) computed from the

openhole logs. To make this lesssubjective, the original openhole datawere reprocessed using an inversioncode to extract an improved referenceformation resistivity (Rt). The intervalused for gain matching was X920 toX020 ft, an interval unaffected byinvasion and excluding the high-resistivity peaks. Rt was then pickedfrom and used to normalize the CHFRresistivity. An acceptable matchbetween openhole and CHFRresistivity was obtained using thismethod in most of the loggedinterval. Figure 4.7 shows CHFRresistivity and Rt from PlatformExpress using a gain of 0.8 andestimated voltage processing.

After normalization, the CHFRresistivity was processed using theELAN program to compute saturation.The same parameters were used toprocess both the cased-hole andopenhole resistivities to ensurerepeatability over the duration of thetime-lapse surveys.

The equivalence of the openholePlatform Express and CHFRmeasurements is evident at resistivitiesof less than 100 ohm-m, the upper limit for CHFR measurements, with anaccuracy better than 10% for thenumber of cycles selected for thisacquisition (the upper limit of100 ohm-m could have been increasedby planning longer stations with extracycles, but was thought to beunnecessary). However, in oneinterval, the CHFR measurement wasapproximately four times higher. Thiscould not be explained by the effectsof cement behind casing, which evenin the worst conceivable case couldnot account for such a discrepancy, orby random errors as it was shown tobe repeatable (Figure 4.8).

The conclusion drawn from theCHFR base log (run 1) was that thisinterval of the lower reservoir hadseen considerable hydrocarbonreinvasion since the openhole logswere recorded. In other words,filtrate present during openholelogging had been replaced byhydrocarbon by the time the CHFRlog was recorded. Hydrocarbonresaturation was observed to a lesser extent in the upper reservoir.Figure 4.9 shows the ELAN-processed

volumetric analysis for the openholeand run 1 CHFR data, with significanthydrocarbon resaturation in the lowerinterval clear between X018 andX084 ft. This is consistent with theMDT pressure gradient measurementsthat identified the free water level ata depth of X088 ft.

Analysis of both the sigma and C:Opulsed-neutron RST logs recorded atthe same time as the run 1 CHFR,yielded water saturations that werehigher than the openhole logs, and verymuch higher than the CHFR-computedsaturation. Four months after drillingthe well, the RST data showed that thefiltrate near to the wellbore was farfrom having dissipated.

Cased-hole run 2A second set of monitoring runs wasmade four months later, eight monthsafter the openhole logging. Duringboth run 1 and this run, the currentreturn was made to a remote electrodeat surface. Since the casing geometry,and therefore the CHFR geometricalfactor, had not changed between runs,the same gain used in run 1 wasapplied to normalize the measurement.

No appreciable changes in the upperreservoir were seen on either thenuclear or cased-hole resistivity logs. Inthe lower reservoir, only very slight Sw

changes were evident. This indicatedthat hydrocarbon resaturation waspractically complete in the volumeinvestigated by CHFR, while the RSTresults showed only a minor increase inhydrocarbons near the well.

Figure 4.10 compares the CHFRand HALS depths of investigation,assuming Rxo = 0.5 ohm-m and Rt=10 ohm-m. When the well was loggedin openhole, invasion must have beendeeper than 50 in, blinding the HALSresponse. CHFR run 2, with thecombination of a depth ofinvestigation five times deeper thanthe HALS tool and hydrocarbonreinvasion, confirmed that astabilized baseline for time-lapseCHFR saturation monitoring hadbeen reached.

Water injection was started at2400 B/D into the deviated pilotinjection well, which was roughly330 ft from the monitoring well.

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Cased-hole run 3

Run 3 was recorded after six monthsof injection. For this run, the CHFRcurrent return was made to the cablearmor, instead of the fish, to reducespurious electrical noise that hadbeen observed in the earlier runs. The noise reduction that wasachieved significantly improvedaccuracy. Resistivity measurementsgave almost perfect repeatability overthe whole logging interval and themeasurement range. The voltage gainfactor was unchanged despite thechange in setup, confirming thatcurrent leakage and geometricalfactor were unaffected by the currentreturn selection.

A reservoir simulation performed inpreparation for the study hadpreviously modeled the effect of thewater injection. This predicted thatsaturation changes would be expectedat the observation well after ninemonths of injection. Further light wasthrown on the expected injectionprofile by a water-flow log recorded inthe injector well (Figure 4.11).Uniform injection over the intervalsuggests that an efficient sweep of thereservoir is possible, and that nopreferential injection into high-permeability streaks is occurring thatmight lead to premature breakthroughat the observation well.

The CHFR data confirmed that theinjector well was influencingsaturation at the observation well aspredicted. Figure 4.12 shows thedefinite change around the samedepth at which the water wasinjected, with a clear increase in Sw.As expected, some reduction in Sw

was noticed in the upper 25 ft of thelower reservoir, indicating that theinjection had swept hydrocarbonupward. In the upper reservoir therewas no appreciable change.

MD1 : 400,

ft

X900

X925

X950

X975

X000

X025

X050

X075

X100

X125

X875

ohm-m

RXOZ. SLB. PREPLUS. MCFL

0.5 500

ohm-m

HLLD. SLB. PREPLUS. HALS

0.5 500

ohm-m

CHFR run 1 main log

0.5 500

ohm-m

CHFR run 1 repeat

0.5 500

Figure 4.8: The repeatability of CHFR in the first cased-hole run of thestudy can be seen in track 1. This log differs from field data only by minordepth matching, the gain factor applied and removal of spikes due tocasing collars. Its repeatability is exceptionally good below 10 ohm-m,while some noise is present above 20 ohm-m. Detailed analysis showedthat this noise was showing only in the first of the two measurementssteps – the formation-dependent one – while the casing step was veryclean. The cause of the noise was not readily obvious, but electrodecontact problems were excluded, since the same electrodes were used forboth steps. Quality in later runs was improved by switching the currentreturn to the wellhead (cable armor)

Figure 4.7: Overlay of CHFR resistivityversus inverted Rt using a gain of 0.8and estimated voltage processing.Relative CHFR resistivities must benormalized with absolute openholeresistivities. HALS were reprocessedusing a 2D modeling code to extract abetter reference Rt. Since HALSprovides only two independent curves –high-resolution deep laterolog curve(HLLD) and high-resolution shallowlaterolog curve (HLLS) – only two out ofthe three unknowns: Rxo, Rt and Ri

(radius of invasion) could be invertedsimultaneously. Rxo was selected to beequal to Rxoz (pad microsphericalfocused log at 18-in. resolution)

MD 1:500, ft

CHFR run 1ohm-m0.2 2000

X900

X950

X000

X050

X100

X150

HALS RT 2Dohm-m0.2 2000

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run 1 was made. The effects of theinjection water are seen in runs 3 and4, tracks 2 and 3. First the appearanceof injection water is seen betweenruns 2 and run 3, then the increase inoil saturation that appeared in thelower zone between runs 3 and 4.

A corresponding log for sigma-derived volumes (Figure 4.13)includes the openhole reference log intrack 1, and differential changesbetween runs in the tracks to theright. The sigma logs showspractically no change after run 2,except possibly at the top zone of thelower reservoir where, despiteabsolute saturations remaining belowthose computed in openhole, thereappears to be a tendency towardshydrocarbon increases consistent withthose quantified by CHFR. Theproblem is that this effect is maskedby filtrate dissipation, which, in theabsence of any independent proof, isimpossible to discern from truehydrocarbon gain. The qualitativedecrease in Sw in both the intervalsX095 to X010 ft and X040 to X085 ftfurther supports the CHFRinterpretation. Comparison of RSTsigma and CHFR results may beconsidered representative of near andfar wellbore volumes.

MD1 : 400,

ft

X950

X000

X080 MDT, effectivefree water level

X050

X040 openhole ELAN,oil–water contact

X100

Moved hydroc.

Hydrocarbon (O)

Hydroc. CHFR R1

Filtrate/depleWater

PHIE. ELA. RESft3/ft30.4 0

PSW. ELA. RESUft3/ft30.4 0

PSXO. ELA. RESft3/ft30.4 0

PSW. ELA. RESUft3/ft30.4 0

PSW. CHFR R1ft3/ft30.4 0

PHIE. ELA. RESft3/ft30.4 0

Hydrocarbon OHFigure 4.9: ELAN-processedvolumetric analysisfor run 1 CHFR datashows hydrocarbonresaturation betweenX018 and X084 ftconsistent with MDTmeasurements thatidentify free waterlevel at X088 ft

Cased-hole run 4

Run 4 was recorded six months afterrun 3 with the same setup, andtherefore CHFR gain. As expected, theupper zone remained steady, with nochange in Sw. In the top section of thelower zone, the oil saturation was seento have increased since run 3.Contrary to expectations, oil saturationhad increased (Sw reduced) in theinterval where water sweep hadappeared six months earlier.

Figure 4.12 shows these tendenciesby displaying the differences betweenfluid volumes when comparing eachCHFR run with the previous one. Themarginal changes in hydrocarbonsseen between run 1 and run 2 (track 1) indicate that resaturationwas more or less complete when Figure 4.10: Comparison of CHFR and openhole laterolog depths of investigation. Comparing, for example,

the HLLD depth of investigation of 30 in. with the interpolated CHFR value for the same response in a 100-ft-thick bed, shows the CHFR depth of investigation to be almost five times greater (140 in.)

CHFR/HALS Depth of Investigation

100 150500 200Radius of invasion, in.

Radi

al re

spon

se, s

1.0

0.5

0

Bed thickness (CHFR)

HLLSHLLD

100ft (interpolated)50ft

200ft

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ConclusionsCareful analysis of MDT pressure datahad suggested an oil–water contactthat was much deeper than thatestimated from openhole formationevaluation. The presence of deepfiltrate invasion prevented anymeaningful quantitative cased-holesaturation monitoring based exclusivelyon shallow devices such as the RST.Time-lapse logging showed that thefiltrate annulus near the wellbore wastrapped for more than two years aftercompleting the well. But, CHFR datawere able to confirm the MDTinterpretation and provide a reliablebase reference run that allowedquantitative monitoring of Sw

saturation before and after startinginjection in the selected pilot well.

It is important to note that, like theRST sigma measurement, the use ofresistivity methods for monitoringsaturation changes relied on theinjection water having the same salinityas the formation water.

In the presence of mixed watersalinity, both the CHFR and RST sigmameasurements would have beenambiguous, leaving no other alternativeto RST C:O logging. In this case theconditions that negatively affectpulsed-neutron nuclear methodsactually helped the CHFR response.

Figure 4.11: The injection profile from thewater injection well shows uniform injection,suggesting that an efficient sweep of thereservoir is possible. (Lower rates are shownduring the water flow log , as injection waschoked down during this survey.)

Depth MD, ft

Dept

h TV

D, ft

Inje

ctio

n ra

te, B

/D

Total injection rate: 1342 B/D

11.3%

7.9%

13%4.9%

6.9%5.6%

3.6%

31.2%2.6%6.2%6.8%

InjectionprofileX040

X060

X080

X100

200

400

600

800

1000

1200

1400

1600Water Injector Well

Figure 4.12: Thedifference betweenoil and watervolumes computedfrom successiveCHFR runs. Track 1(left) shows that inthe four monthsbetween runs 1 and2 only a smallincrease inhydrocarbonsaturation occurreddue to resaturation.By the time run 3was recorded,injection water hadreached theobservation well. Thewater saturationbetween X015 andX085 ft reflects theadvance of theinjection sweep.Above X015 ft in thelower zone the oilsaturation increasedas the water floodadvanced. Tracks 2and 3 show thereduction in watersaturation thatoccurred betweenruns 3 and 4

MD1 : 400,

ft

X950

X000

X050

X100

PSW. CHFR run 1

ft3/ft30.4 0PSW. CHFR run 2

ft3/ft30.4 0

PHIE. ELA. RES

ft3/ft30.4 0

PSW. CHFR run 2

ft3/ft30.4 0PSW. CHFR run 3

ft3/ft30.4 0

PHIE. ELA. RES

ft3/ft30.4 0

PSW. CHFR run 3

ft3/ft30.4 0PSW. CHFR run 4

ft3/ft30.4 0

PHIE. ELA. RES

ft3/ft30.4 0

Filtrate/deple

Hydroc. CHFR R2

Hydroc. R1

Filtrate/deple

Hydroc. CHFR R3

Hydroc. R2

Filtrate/deple

Hydroc. CHFR R4

Hydroc. R3

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In fact, although the formation waterwas very saline and porosity neverexceeded 20 p.u., the resistivitymeasurements fell well within CHFR’sdesigned optimum range of 1 to100 ohm-m. CHFR resistivity remainedhigh enough not to be affected bycement (which needs only beconsidered at resistivities of 1 ohm-mand below) and, within the reservoir,resistivity never exceeded 100 ohm-m.

Overall, the message from theADCO cased-hole resistivity surveys isthat water injection is working veryefficiently and, possibly with othereffects such as gravity drainage, isproducing optimum hydrocarbondisplacement. Core thin-sectionanalysis confirmed the results that hadbeen established by MDT and CHFR.

During this series of logs,improvements in operatingtechniques were developed and thevalidity of the CHFR for monitoringsaturation changes over the duration of the pilot project wasdemonstrated. In these favorableconditions, CHFR proved to have the potential to provide the mostaccurate saturation monitoring.

The way forwardAlthough cased-hole formationevaluation is still in its infancy, and theCHFR provides only a relativeresistivity measurement, the industry islooking ahead to applications that willnot only allow deeper, morerepresentative measurement in existingwells, but will also lead to significanteconomies in future wells. The ABC*Analysis Behind Casing program islooking at alternative ways to evaluatereservoirs through casing in the future.

The success of this initiative couldlead to dramatic changes in wellevaluation practices. By allowing wellsto be cased immediately after drilling,the potential for borehole damagewould be minimized and drilling andcompletion costs reduced.

MD1:400,

ft

X950

X000

X050

X100

Moved hydroc.

Hydroc. (O)

Hydro. sigma R1

Filtrate/depleWater

PHIE. ELA. RES

ft3/ft30.4 0PSW. ELA. RESU

ft3/ft30.4 0

PSXO. ELA. RES

ft3/ft30.4 0

PSW. ELA. RESU

ft3/ft30.4 0PSW. SIGMA R1

ft3/ft30.4 0

PHIE. ELA. RES

ft3/ft30.4 0

PSW. sigma run 1

ft3/ft30.4 0PSW. sigma run 2

ft3/ft30.4 0

PHIE. ELA. RES

ft3/ft30.4 0

PSW. sigma run 2

ft3/ft30.4 0PSW. sigma run 3

ft3/ft30.4 0

PHIE. ELA. RES

ft3/ft30.4 0

PSW. sigma run 3

ft3/ft30.4 0PSW. sigma run 4

ft3/ft30.4 0

PHIE. ELA. RES

ft3/ft30.4 0

Filtrate/deple

Hydroc. OH

Hydro sigma R2

Hydroc. R1

Filtrate/deple

Hydro sigma R3

Hydroc. R2

Filtrate/deple

Hydro sigma R4

Hydroc. R3

Figure 4.13: Sigma-derived differential volumes show practically no change after run 2. Themeasurements are masked by filtrate dissipation that cannot be discerned from true hydrocarbongain. The qualitative decrease in Sw, in both the intervals X095 to X010 ft and X040 to X085 ft,further supports the CHFR interpretation

For example, drilling rigs could drilland case wells one after another,leaving formation evaluation andcompletion operations to less costlycompletion or workover rigs. Finally,there are environmental benefits inthis concept. Concerns over leavinglogging tools in the well would bealleviated to a great extent, becausethe risk of a stuck tool is much lowerwhen running in casing.