Through-tubing,behind-casing formation resistivity

CHFR-Slim

Applications■ Location of bypassed

hydrocarbons

■ Reevaluation of existing fields

■ Monitoring of reservoir saturation

■ Monitoring of gas-oil-watercontacts

■ Primary formation evaluationin cased holes

Benefits■ Rig time savings from more

efficient operations

■ Identification of unsweptreserves

■ Optimized sweep efficiency for improved production

■ Better input for decisions onrecompletions and sidetrackplacement

■ Maximized total hydrocarbonrecovery

■ Savings in time and cost byeliminating need to pull tubing

Features■ Deep-reading resistivity

measurement behind casing

■ Through-tubing capabilities

■ Redundant electrode contactsto ensure data quality in perfo-rated or corroded intervals

■ Quality measurements, evenwith small amounts of scaleand corrosion

■ Ability to measure throughmost cements without envi-ronmental corrections

■ Measurement independent ofwellbore fluids

■ Dynamic measurement rangefor evaluation of low-porosity,low-salinity formations

■ Accurate saturation measure-ment in low-permeability for-mations with deep mud-filtrateinvasion

Formation resistivity measurement technology for cased holesThe Schlumberger CHFR-Slim* tool, oneof the ABC* Analysis Behind Casingtechnologies, provides deep-readingresistivity measurements from behindsteel casing. Its enhanced hardware andmeasurement principles improve theoperational efficiency of cased hole resis-tivity measurements, and its small diame-ter allows it to be run through tubing.

The abilities to detect and evaluatebypassed hydrocarbons and to trackfluid movement in the reservoir are fundamental to improving productionand increasing reserves. Measuringresistivity behind casing has been recog-nized as a necessary component of thesecapabilities since the first openhole resis-tivity log was recorded in 1927; however,advanced electronics and electrodedesign are only now making these chal-lenging measurements possible.

Making the measurementThe CHFR-Slim tool introduces a cur-rent that travels up and down the casingbefore returning to the surface electrode.This current follows a path similar to thecurrent from openhole laterolog tools,as shown in the figure to the right.

While most of the current remains inthe casing, a very small portion escapesinto the formation. Electrodes on thetool measure the voltage difference cre-ated by this leaked current, and that dif-ference is proportional to the formationconductivity.

Typical formation resistivity is about1 billion times that of steel casing. Duringthe resistivity measurement, the currentescaping to the formation causes a volt-age drop in the casing segment. Because

casing has a resistance of a few tens ofmicrohms and the leaked current is typically on the order of several mil-liamperes, the potential difference meas-ured by these tools is in the nanovoltrange (one-billionth of a volt).

The measurements are performedwhile the tool is stationary.

To obtain the contact between thetool and the casing that is essential tothe CHFR-Slim measurements, the smallelectrodes on the sonde are designed toscrape through small amounts of casingscale and corrosion to establish goodelectrical contact. The tools use fourlevels of voltage electrodes in sets ofthree with 120° spacing. This 12-electrodeconfiguration allows faster operation

The CHFR-Slim tool introduces current to thecasing so that extremely small amounts escapeinto the formation. The voltage drop caused by this phenomenon, expressed in nanovolts(10 –9 V), is measured by tool electrodes andused to determine formation resistivity.

and provides redundant measurementsto help ensure data quality in perforatedor corroded intervals. For quality control,the electrode-casing contact is measuredat each station. Scale that has formedin amounts that prevent adequate elec-trical contact can be removed using theSchlumberger Scale Blaster* engineeredapproach to scale removal.

Low-resistivity (1- to 5-ohm-m range)cements that are typically found in oilwells do not impact cased hole resistiv-ity measurements, but measurementsmade behind cements having unusuallyhigh resistivity require environmentalcorrections.

Because its electrodes make directcontact with the casing, the tool is ableto work in all wellbore fluids. Thus thetool is not limited to conductive bore-hole fluids and can operate in wells withoil, oil-base muds, or gas in the casing.

A range of saturation measurementsThe depth of investigation of the resis-tivity measurement is between 7 and 32 ft (2 and 10 m), which is more thanan order of magnitude deeper thanthat of pulsed-neutron saturation measurements.

The dynamic range of measurementsalso allows evaluations in reservoirswith low porosity and formation salinity.Resistivity and nuclear measurementscan be combined for an enhanced satu-ration evaluation that is equivalent to an openhole interpretation.

Case study: Faster logging with equivalent results (Indonesia)The CHFR-Slim tool uses provenSchlumberger technology and makesan average station time in just 1 min. The result is an effective logging speedof 240 ft/hr.

Measurements made with the toolmatch openhole resistivity data quality,as shown below by the logs from a wellin Indonesia. The 240-ft/hr logging speedallowed the job to be completedovernight.

The CHFR-Slim tool’s fast loggingspeed is the direct result of the newmeasurement technique. First, whilecurrent in the casing is flowing past the

electrodes, a small amount of current is introduced opposite the casing current. This action reduces the sensi-tivity of the measurement to the resis-tance of the casing. Second, the casingresistance measurement (or calibrationstep) is made during the formationresistivity measurement, but at a differ-ent frequency.

The combined effect of these improve-ments is the elimination of a separatecasing resistance measurement. Stationmeasurement time is reduced by half,and logging speed is doubled. Measure-ment quality is the same for both tools.

X400

Casing Segment Resistance

(ohm)0 0.0001

Openhole Induction Deep

(ohm-m)0.2 2,000

Openhole Induction Medium

(ohm-m)0.2 2,000

Openhole Gamma Ray

(gAPI)0 150

CHFR-Slim Resistivity

(ohm-m)0.2 2,000

CHFR-Slim Casing-StepInjection Impedance

(ohm)

Depth(ft)

0 1

These CHFR-Slim measurements, which were acquired at 240 ft/hr and saved the operator daylight rigtime, closely match the openhole resistivity log of this well.

Case study: Extending field economic life (Bakersfield)The CHFR* tool provides an excellentmeans of identifying bypassed hydro-carbons before the decision is made toabandon a well or field. This informa-tion can have a significant impact on field economics.

In Bakersfield, California, a well in a field with a water-injection programwas abandoned when the water ratereached uneconomic levels at 1,600 B/D.Years later the well was reevaluatedusing the CHFR tool.

As shown on the right, the logs con-firmed that the interval below X720 fthad watered out. They also showedhydrocarbons in the interval betweenX680 and X700 ft.

A plug was set at X710 ft, and theinterval between X680 and X697 ft wasperforated. Following the workover pro-cedure, the well produced oil at 300 B/D.

Two other wells in the field were alsologged with the CHFR tool with similarsuccess. The decision to reevaluate thethree wells proved very profitable forthe operator because of the commer-cially significant volume of oil produced.

SpontaneousPotential

Gamma Ray

Openhole Deep Resistivity

Cased Hole Resistivity (CHFR)

2

–80

0

Saturation Ratio

3

2

2

Openhole Shallow Resistivity

Cased Hole Resistivity < Openhole Resistivity

Depletion

Original oil-watercontact

Oil-watercontactafter field is swept

20(mV)

200(gAPI)

(ohm-m) 200

(ohm-m) 200

(ohm-m) 200 0

X800

X700

Oldperfs

Newperfs

Casing Segment Resistance(ohm)0 0.0001

Gamma Ray

(gAPI)0 150

Cased Hole Resistivity (CHFR)

(ohm-m)0.2 200

Openhole Induction Deep

(ohm-m)0.2 200Depth

(ft)

This log from a well in a water-injection project demonstrates the value of CHFR technology in identifyingbypassed pay. Abandoned earlier, the well produced 300 BOPD after it was logged and perforated again.

This CHFR log shows an unlikely case in which the oil/water contact had moved down 12 ft. In most wellsthe water level moves up as the oil is produced. In this well, the waterflood swept oil to the well, indicatingthat the amount of oil was increasing. When the zone with swept oil was perforated, it produced 2,050 BOPDwith no water.

Case study: Reservoir monitoring (UAE)To monitor fluid movement and injectionperformance in a large Middle Easterncarbonate oil reservoir, CHFR tools wererun in time-lapse mode across a reser-voir being waterflooded with seawater.

A CHFR-Slim tool was run back toback with a CHFR-Plus* tool. Repeata-bility was excellent between the twotools, and their measurements comparedfavorably with those from a previousCHFR-Plus run, as well as with theoriginal openhole resistivity log data.

In this example CHFR-Slim data areused for time-lapse monitoring, demon-strating the excellent repeatability thatis critical for detecting subtle move-ments of reservoir fluids.

Case study: Logging for primary evaluation (Mexico)The CHFR-Slim tool can provide pri-mary formation evaluation data in newwells for which there are no openholeresistivity data.

In the Mexico log displayed belowright, well control issues prevented theacquisition of openhole log data acrossthe primary reservoir. The operatorneeded to identify sandstone intervalsthat could be perforated to maximizegas production while minimizing waterproduction.

Traditional cased hole neutron-gammaray logs alone are inadequate to clearlyidentify gas-bearing sands. After thecasing was set, the CHFR-Slim tool was run with other ABC* AnalysisBehind Casing services, including ECS*Elemental Capture Spectroscopy, CHFP*Cased Hole Formation Porosity, andCHFD* Cased Hole Formation Densityservices. The combination of resistivity,lithology, porosity, and density infor-mation enabled clear identification of the sands.

X480

X470

X460

X450

X440

X430

X420

X510

X500

X490

X410

X390

X400

SpectroLith* Data

1 0(lbf/lbf)

Clay

Quartz-Feldspar-Mica

Carbonate

Pyrite

0 20(ohm-m)

Cased Hole Rt (CHFR-Slim)Depth(m)

Cased Hole Density

1.65 2.65(g/cm3)

Cased Hole Neutron Porosity

60 0(p.u.)

Density-Neutron Crossover

Gamma Ray

0 100(gAPI)

Openhole RtOpenhole Bit Size

6 0.2 2,000(ohm-m)

Openhole Rxo

0.2 2,000

2,000

(ohm-m)

0.2 (ohm-m)

Cased Hole Rt (CHFR—Run 4)

0.2 2,000(ohm-m)

Cased Hole Rt (CHFR—Run 5)

0.2 2,000(ohm-m)

Cased Hole Rt (CHFR-Slim—Run 5)

16(in.)

Openhole Caliper

6 16(in.)

Gamma Ray

0 15(gAPI)

CCL

–19 1

Openhole Thermal Neutron Porosity

50 0(p.u.)

X800

X750

Depth(ft)

This log shows the repeatability of back-to-back CHFR-Slim (blue dots) and CHFR-Plus (red dots) meas-urements and those from an earlier CHFR-Plus run (white dots). Data from all three runs compared favor-ably with the original openhole resistivity log (brown line).

When openhole log data were unavailable, the CHFR-Slim tool was run with gamma ray, ECS, CHFP, andCHFD tools to identify gas-bearing sands.

Accuracy and precisionThe accuracy of CHFR-Slim measure-ments depends on the thickness andresistivity of the cement. For typical oil-field cements with resistivities of a fewohm-meters, the accuracy of the meas-urement prior to a cement correction is around 10% at 1 ohm-m and improvesto better than 3% above 10 ohm-m.Applying environmental corrections tothe log data can improve the accuracyof the measurement if the cementparameters are well known.

The precision of the CHFR-Slim meas-urements depends on the signal-to-noiseratio of the voltage measurements. Atlow resistivities (high conductivities), theprecision of the measurement is betterthan ±1%. However, at high resistivitiesand in large casing sizes, the precisionmay be closer to ±7% (at 100 ohm-m).The precision can be improved by increasing the station time.

7-in. casing, 0.75-in. cement thickness

–10

0

10

20

30

0 100Resistivity (ohm-m)

Relative erroron Rt (%)

101

Cement resistivity0.1 ohm-m1 ohm-m2 ohm-m5 ohm-m

For a given casing and cement thickness, the accuracy of the CHFR-Slim measurements improves withincreasing formation resistivity.

0 1 10 100 1,000

Resistivity (ohm-m)

0

2

3

1

4

Relativeprecision (%)

7-in. casing, 1-min station7-in. casing, 2-min station41⁄2-in. casing, 1-min station41⁄2-in. casing, 2-min station

The relative precision of the CHFR-Slim measurements depends on the formation resistivity and casingsize; it can be improved by increasing the measurement time.

CHFR-Slim Tool Specifications

Diameter (in. [mm]) 2.125 [54]Length (ft [m]) 37 [11.3]Weight (lbm [kg]) 253 [115] Max. temperature (°F [°C]) 300 [150]Max. pressure (psi [kPa]) 15,000 [103,425]Casing OD range (in. [mm]) 2.875–7 [73–178]Max. well deviation HorizontalStationary measurement (min) ~1Equivalent logging speed (ft/hr [m/hr]) 240 [73.15] Resistivity range (ohm-m) 1–100 (nominal)Vertical resolution (ft [m]) 4 [1.2]Depth of investigation (ft [m]) 7–32 [2–10]Accuracy 3–10%Precision 1–7%

04-FE-216 ©Schlumberger

January 2005 *Mark of Schlumberger

Produced by Marketing Communications, Houston

www.slb.com/oilfield