PROCEEDINGS, Twenty-Seventh Workshop on Geothermal Reservoir EngineeringStanford University, Stanford, California, January 28-30, 2002SGP-TR-171

TRACER LEAK OFF TESTS AS MEANS OF CHECKINGWELL INTEGRITY. APPLICATION TO PARIS BASIN

GEOTHERMAL PRODUCTION WELLS

Pierre UNGEMACH, Anne Véronique VENTRE and Sébastien NICOLAON

Géoproduction Consultants (GPC)Paris Nord II – Lot 109- 14, rue de la Perdrix

BP 50030 – 95946 ROISSY CDG Cedex – Francee-mail : office@geoproduction.fr

ABSTRACT

Exploitation of geothermal district heating systems,located most often in sensitive, densely populated,urban environments require thorough monitoring ofwell integrity.

The latter is commonly controlled via casing caliperlogs and packer leak off tests, provided productionequipment, such as submersible pump sets anddownhole chemical injection lines, have beenpreviously removed.

Tracers, or fresh water, either injected downhole orsqueezed from surface, production equipment in hole,may prove a feasible and cheaper alternative inassessing a reliable damage diagnosis.

The present paper reviews the figures of merit ofcandidate tracers, radioactive isotopes (131I, 82Br,99mTc), chemical (NaI, Li2CO3), fluorescent(Rhodamine WT), fresh water, and fieldimplementation protocols, based on selected ParisBasin case studies.

Field achievements led to the selection of combined,short duration, squeeze of Lithium carbonate/freshwater slugs regarded as the most rewarding, routine,and cots effective procedure.

BACKGROUND AND SCOPE

Surveillance of geothermal district heating wellsrequires thorough monitoring of casing/tubingintegrities, particularly in the Paris suburban areasheated by thirty four doublets (i.e. 68 production andinjection wells) operated in a sensitive, denselypopulated, urban environment.

Several wells have undergone severe casing damageowing to a thermally hostile corrosive fluid and loosecementing, the latter noticed most often in the uppercased sections, leading to casing piercing and leaks

whenever exposure to active aquifers vis-à-vis or/andvia channeling occur.

Evidence of such damage can be provided byproduction monitoring and subsequent depletion ofdischarge rates and wellhead pressures. However, atthis stage, the diagnosis may prove ambiguous as itcalls for several damage source mechanisms,reservoir plugging and casing scaling among others.Decline in static well head pressures is another, apriori reliable, means for identifying casing leaks,after due substraction of pressure interferences fromall possible interacting wells, in deed a delicateexercise. As a result, from these precursory showsremains the problem of assessing whether or notthere is a casing piercing/leak and, if such is the case,to locate it precisely.

Direct assessment by casing leak off tests is the mostpopular method, extensively practiced in the oil andgas and geothermal industry. It requires to kill thewell and remove the in hole production equipment(production pump, tubing, pressure control anddownhole chemical injection lines) prior to runningthe pipe string (or coiled tubing)/straddle packerassembly, thus mobilizing a workover facility, eithera servicing rig or a coiled tubing unit. It leadshowever to high operative costs, which add tosignificant exploitation losses, bearing in mind thatrig move in/move out and completion of packerpressurization tests result in most instances in a caone week interruption of production.

Therefore tracer testing has appealed to geothermaloperators as it offer a means of expecting a soundleak diagnosis in less than two calendar days,production equipment in hole, thus at reasonablecosts.

The rationale behind the method consists of (i)injecting/squeezing, from production well head or viaa resident downhole chemical injection line, a giventracer (either radioactive or chemical, fresh watereven) volume, and (ii) monitoring the volumes

recovered at surface in terms of restituted tracer ratioand resident times, indicative of the leak magnitudeand location.

The present paper reviews the tracer selectionproblematics, field implementation of tracer injectionprotocols and relevant interpretation of tracerrestitution sequences.

Along this line it should be readily emphasized that,in the Paris Basin, all tested wells are over pressured(i.e. eruptive) and equipped with submersible, eitherelectrically (ESP's) or hydraulically, turbine (HTP's)driven pump sets and, last but not least, residentdownhole chemical inhibition lines (of the auxiliaryinjection, type AIT, coiled tubing) (1).

The case studies presented and discussed in theforthcoming sections address the following wellconfigurations and tracer injection procedures:• case 1 : downhole injection, via the resident AIT,

of a radioactive isotope, on a ESP equipped well.• case 2 : injection in the turbine energizing tubing

(HTP equipped well) of a combined fluorescentchemical tracer sequence.

• case 3 : squeeze from surface, on a HTPequipped well, of a combined freshwater/fluorescent tracer sequence.

• case 4 : squeeze from surface (ESP equippedwell) of a combined freshwater/chemical tracersequence.

Case history 3 enabled to identify and quantify acasing leak.

Case histories 1, 2 and 4 concluded to casing/tubingintegrities, further confirmed, respective to cases 1and 4, by similar protocols implemented five and twoyears later.

Accuracy of the tracer flow back curves and relatedrestitution ratios is discussed in (2).

TRACER SELECTION

A variety of candidate tracers can be contemplated,namely :Fluorescent agents. Rhodamine WT is preferred asit is chemically neutral, i.e. non adsorbed by exposedpump, casing metal and AIT thermoplastic surfaces.Tracer restitution is easily monitored by means ofstandard calorimetric recording devices. It's use ishowever more or less qualitative as compared tochemical and radioactive tracer monitoring/recordingtechniques, regarded as more rigorous. Fluorescenttracers are best used as a quick look method enablingto appraise tracer restitution sequences (duration,resident times) prior to chemical/radioactive tracers.

Chemical tracers. Iodine is the most popular inhydrogeological applications. Lithium is anothercandidate. Limitations in use depend on the Iodineand Lithium contents in geothermal waters whichimply that injected concentrations stand one order ofmagnitude higher than nascent (geothermalbackground noise) concentrations, to allow nonambiguous and reliable interpretation of tracerresponse.

Radioactive tracers. Three candidates have beenconsidered, Iodine 131I isotope, Brome 82Br isotopeand Technetium 99mTc radioisotope. Their selection isguided by (i) their lifetimes (periods), which stand at8 days (131I), 1,5 day (82Br) and 6 hours (99mTc)respectively, (ii) their supply and availability (forinstance 82Br was no longer available at the time theexperiments were carried out), and (iii)environmental regulations and authorizations in force; in France utilization of 131I, owing to its fixation onhuman thyroid, is subjected to (re)injection of thetraced fluid in deep seated reservoir formations, afterformal approval by an ad hoc commission.Monitoring of tracer response is achieved via aradioactive impulse counter.

Fresh water. It is the most simple and, by all means,the cheapest "tracer". It can be reliably utilized aswater slugs, in conjunction with chemical tracers,provided there exists a strong physico-chemicalcontrast, (salinity, temperature, pH) with thegeothermal water which happens to be the case in theParis Basin (presence of a hot, mineralized,geothermal brine). Water conductivity is therefore therecorded parameter.

TRACER INJECTION PROCEDURES

Depending on the depth of the investigated leakeither downhole or surface injection will beimplemented.

A deep seated target would be best evidenced byinjecting the tracer downhole via the AIT lineaccording to the design shown in fig. 1A. When thewell is equipped with an ESP, the master valve isshut and the traced fluid produced in self flowingmode, through the annulus, (see fig. 1B), to avoid anytracer trapping whatsoever in the ESP and productiontubing. For a non eruptive well, production would beachieved via the ESP in which case only the casedsections exposed to tracer circulation, i.e. belowpump intake, are investigated.

In the absence of a downhole control (or chemicalinjection) line the tracer can be squeezed from thesurface wing valve as depicted in fig. 2, a procedurealso eligible to leaks located at shallow depths.

master valve

heat plant

production columnsuspension spool

chemical injection linesuspension spool

casing head

ground level

pumping chamber

ESP

production casing

down hole chemicalinjection line

wing

A. TRACER INJECTION SEQUENCE

master valve

heat plant

production columnsuspension spool

chemical injection linesuspension spool

casing head

ground level

pumping chamber

ESP

production casing

down hole chemicalinjection line

tracerflow back

B. TRACER FLOW BACK (SELF FLOWING WELL)

Figure 1. Tracer injection by means of a resident down hole injection line and restitution (self flowing production)trough 2" casing head wing valve

Injected tracer quantities must allow to access thedepth of the leak and can therefore be as large as thecased well volume. However, whenever the damageis located close to the casing shoe the interpretationmay be biased by tracer losses in the underlyinggeothermal reservoir, which requires special careduring the squeezing stage.

The presence of a HTP instead of an ESP somewhatcomplicates the exercise. Here the pump is driven bya hydraulic turbine fed by a high pressure chargepump and fluid (driving and reservoir producedgeothermal water) produced by the annulus. Asdescribed in fig. 3 the tracer is injected in the surfacepart of the energizing loop, getting therefore dilutedby the reservoir fluid after passing through theturbine. The energizing loop must therefore becirculated long enough to ensure the whole of thetracer has been evacuated to the heat plant andinjection well. In such circumstances preliminaryfluorescent tracer tests are recommended in order todefine the proper circulation time.

master valve

heat plant

production columnsuspension spool

chemical injection linesuspension spool

casing head

ground level

pumping chamber

ESP

production casing

down holeinjection line

lateral valve

presumed leakdepth

fortracer

injection/flow back

Figure 2. Tracer squeeze from surface in productionwell

ground

heat plant

feed pumptracerinjection

energetizationsuspension

own hole injectionsuspension

casing

energetizatiocolumn

HTP

inflatable

energetizatioloop

pumping

production

tracer

tracer

tracer dilutedwith reservoir

Figure 3. Tracer injection in a turbopump producedgeothermal well

TEST INTERPRETATION

The general chemical tracer caseThe experimental tracer concentration vs. time plotshown in fig. 4 is quite illustrative of the ideal tracerrestitution curve

Figure 4: Experimental tracer restitution curve

The recovered (restituted) tracer mass, corrected frombackground noise is calculated by integrating theexperimental response over the duration of the test.Hence the restituted mass Mg(g) is given by :

(1) [ ] [ ]∫∫ −=−=+∞ Ct

t

ppR dttQctcdttQctcm0

)()()()( 0

0

0

with :c(t) : concentration of the tracer (g/l),c0 = initial tracer concentration in formation fluid(g/l),Qp(t) : instant restitution flowrate (l/h),t0 : time restitution starts (h),tc : time restitution ends (h).

Restitution rate R (%) is given by :

(2)0

R

m

mR =

with :m0 : initial injected mass (g).

The restitution rate must be corrected frommeasurement (flowrate and concentrationdeterminations), initial mass and integrating errorsrespectively.

Integration of [concentration x flowrate] vs. time iscalculated from concentrations (ci) measured on the nsamples and flowrates (Qi) monitored during theexperiment at times ti.

An easy way to handle error determination consistsof integrating the above formula by two differentmethods (maximizing and minimizing methods)discussed in Appendix.

The specific radioactive tracer casePart (Qc) of the total flow production (Qp) is divertedto a measuring cell, which counts the total number ofradioactive impulses (Nc) during the whole restitutiontime (tc - t0). Background noise (NBN) from the totalimpulses number must then be substracted from thetotal impulses number. This last figure is correctedfrom cell volume (Vc) down to the total measuredvolume of fluid circulated through the cell [Qc(tc-t0)],and the corresponding total number of impulses inthe produced geothermal fluid is adjusted to theflowrate ratio (Qp/Qc). The number of impulses (N2)corresponding to the total extracted geothermal fluidis therefore :

(3) ( )c

p

0cc

cBNc2 Q

Q

)tt(Q

VNNN

−−=

This injected tracer needs also to sampled with analiquot sample (a) taken before injection in order toaccess the number of impulses generated by theinjected tracer mass. The number of impulses (Na)generated by the aliquot is then corrected bysubtracting the geothermal background noise (NBN)because geothermal water is used for aliquot dilutionprior to measurements. This last figure is corrected

up to the total volume (A) of injected tracer and theradioactive decay of the injected tracer during thetime between injection in the well (t0) andmeasurement of the aliquot (t), gives the initialnumber of impulses, (N1) of the injected tracer.

(4) ( )

−−=

−

a

aAeNNN T

tt693.0

BNa1

0

Restitution rate is therefore :

(5)1

2

N

NR =

with :QP = production flowrate,Qc = cell sampling flowrate,Vc = cell volume,N1 = number of impulses generated by the tracerinjected in the well,

N2 = number of impulses generated by the tracerproduced by the well,Nc = number of impulses generated by the fluidcirculated through the measuring cell,NBN = number of impulses generated by thegeothermal fluid (background noise),Na = Number of impulses generated by the aliquot,T = half time of radioactive decay, (radioactiveperiod)a = aliquot volume,A = dilution volume of tracer,t0 = time measuring of the extracted fluid started,t = time measuring of the aliquot started,tc = time experiment in the measuring cell ended

CASE STUDIES

A series of tests have been conducted on typical ParisBasin geothermal production wells whose featuresare highlighted in table 1.

Case 1. Radioactive Tracer Test

The clean up, via casing jetting, of the localgeothermal district heating well in 1992 evidenced apiercing of the pumping chamber. After unsuccessfulattempts to seal a casing patch over the damagedcasing interval, well integrity was reestablishedthanks to a remedial cement squeeze. A tracer testwas carried out a year later to check the integrity ofthe cement squeeze.

Iodine isotope 131I and Technetium radioisotope99mTc were selected as a result of their availability,easy operation and, last but not least, the existence ofan injection well solving the 131I disposal problem.Tests were carried out from 23 to 25 June, 1993, withthe assistance of the French Atomic Energy Agency(CEA), Grenoble DAMRI Laboratory (3)

Tracers were injected downhole via the resident AITaimed at permanent injection of corrosion inhibitorsinto the geothermal source reservoir.

Field set up is shown in fig. 5. Two counting cellswere used. A first dynamic cell to deliver a quicklook (fairly inaccurate) of the tracerresidence/restitution time. An accurate determinationof the tracer restitution rate is given by the secondcounting cell proper. The sampling and counting loopis depicted in fig. 6.

Case no. Year completed Pump type Tracer type Objective Diagnosis

1 1993 ESP RAT Casing leak

No leakConfirmed byDuplicate test(CT+FWS) 2001

2 1995 HTP CT Energizingtubing leak

No leak

3 1997 HTP CT+FW Casing leak Pumpingchamber leak

4 1998 ESP CT+FW Casing leakNo leakConfirmed byDuplicate test 2001

Table 1. Test features

Abbr. ESP : Electrosubmersible pump RAT : Radioactive tracer FW : Fresh waterHTP : Hydraulic turbine pump CT: Chemical tracer

Test n° 2. 131I restitution curve is shown in fig. 4

Figure 5. Field set up

Three testing sequences were conducted whoseresults are summarized in table 2. Test n°2 131Irestitution curve is shown in fig. 4. The first test wasdesigned to adjust the relevant testing parameterssuch as injection duration (tc-t0), geothermal (self)production flowrate (Qp) and sampling flowrate (Qc).This preliminary tests evidenced that:- the geothermal self production rate (Qp) was of

piston type i.e. non dispersive (see fig. 4), whichreduces the early estimated monitoring durationby a factor of 3 to 4, and

- the sampling flow rate (Qc) in the counting cellcould, as a result, be increased by 3 or 4 times,thus limiting the risk of getting the measuringvalves plugged by particulate suspensions in thegeothermal fluid. This also contributes tomaintaining a constant sampling flowrate whichwas not the case in the first test.

dynamicmeasurement

cell

countingcell

samplingflowrate

measurement

heat plantflowmeter

by-pass

production well injection well

heat

injection

Qp

Qc

Qp-Qc

Figure 6. Experimental monitoring and measurement loop

Test number Symbol 1 2 3Tracer used 131I 131I 99mTcRadioactive period (hours) T 192 192 6Quantity injected 74 MBq 74 MBq 740 MBqDilution volume of tracer (l) A 1 1 1Injection type Down hole Down hole Down holeInjection rate (l/h) 45 45 45Aliquot (l) a 0.7/1000 1/1000 2/1000Cell volume (l) Vc 293,2 293,2 293,2

Well characteristicsSelf flowrate (l/mn) QP 465 465 465Well head pressure (bar) 6.9-7.0 6.9-7.0 6.9-7.0Sampling parametersSampling time duration (mn) tc 344 294 154Sampling flowrate (l/mn) Qc ≈ 0,400 1,00 1.2Total extracted volume (l) Qctc 137.6 293.2 184.8Radioactivity measurementsGeothermal background noise (impulses) NBN 9 873 in 30 mn 9 873 in 30 mn 18 000 in 30 mnOutput signal (impulses) Nc 45 369 in 30 mn 71 419 in 30 mn 61 541 in 20 mnAliquot signal (impulses) Na 63 050 in 30 mn 95 225 in 30 mn 73 525 in 3 mnTime duration (mn) t-t0 717 52 55

Restitution rate R 111 % 111 % 11,6 %

Table 2. Case 1 tracer tests results.

Discussion

The first result is mainly depending on the samplingflowrate (Qc) accuracy. This sampling flowrate wasinitially set at 0.430 l/mn, but decreased quitesignificantly during the experiment, down to less than0.400 l/mn. However, its average value could beestimated at 0.400-0.410 l/mn, resulting in arestitution ratio ranging from 105 (Qc=0.410 l/mn) to111 % (Qc=0.400 l/mn).

The second result is more accurate thanks to theexperimental parameters' adjustment achieved afterthe first experiment (see above). Hence the errorcalculation can be estimated as follows:- number of impulses accuracy : 0.5 %,- volume measurements accuracy : 1 %,- sampling flowrate (after degassing of the

geothermal water) accuracy : 3 %.

These parameters lead to a global error on therestitution rate of 9 %, which ranges therefore, saybetween 102 and 120 %. The error on the producedflowrate (heat plant flowmeter) can therefore beestimated to at least 2 %, since no additionalradioactive impulses can be generated from theproduced flow. A better estimate of the error on theproduced flowrate would be 3-4 %, which matchesthe usual electromagnetic flowmeter precision.

The second experiment, realized in optimumconditions (steady flowrates) shows undoubtedly thatno leak is to be expected in the pumping chamber(neither elsewhere in the well).The third experiment exhibits a very low restitutionrate (12 %) with same errors, i.e. ranging between 3and 9 %, despite reliable experimental conditions(steady flowrates). Explanation lies in the fact thatthe tracer used (99mTc) is obviously interacting withand trapped (adsorption) by the geothermal mediumand exposed materials. 99 mTc is therefore readilydiscarded for tracer leak off tests on Paris Basingeothermal wells.

Case 2. Chemical Tracer Test. HTP EquippedWell

Doublet monitoring showed in 1995 a 1.5 bar drop inwell head static pressure, not induced by surroundingwell interferences, thus suggesting a possible casingleak located between the HTP packer and surface.In order to investigate this damage an ad-hoc test wasdesigned to trace the water circulating between thecharge (feed) pump and the hydraulic turbine.Testing was undertaken in July 1995 with theassistance of CEA/DAMRI (4).

(131I) was discarded because of the risks induced bysmall leaks noticed on the surface loop.

As a consequence a combination of fluorescent(Rhodamine WT) and chemical (Sodium Iodure NaI)was selected instead, as there are of routine use inhydrogeological tracing experiments and do notinteract neither with the geothermal fluid nor with theexposed equipment.

Tracers were injected through the HTP energizingcircuit at charge pump intake (see fig.3) and thegeothermal discharge rate set at a fairly high level (ca155 m3/h) to create a strong pressure contrast (highgradient) with the receiving aquifer and reliablyassess the leak, if ever, and its magnitude. As thecirculation transit time through the energizing circuitis short (1 to 2 min) for the aforementioned dischargerate, duration of the tracer injection sequence must belong enough (several hours) to capture the wholeplateau of the restitution curve and allow a reliableintegration of the signal and derivation of therestitution ratio.

Since a fraction (ca 125 m3/h) of the total producedflow (ca 280 m3/h) is recirculated through theturbopomp the net geothermal production standing atca 155 m3/h, circulation and sampling times ought tobe designed so as to allow the whole of the injectedtracer to leave the energizing loop (see fig. 3).

One hundred liters of a tracer solution (114 g ofRhodamine WT and 224 g of NaI) were injected at30 l/h upstream from charge pump intake (see fig. 3).Because of a leak, a second injection (32.5 l at anaverage 25 l/h rate) was completed after rinsing thetracer preparation tank. Rhodamine WTconcentration (Fluorescent units) measured on site,enabled to determine the tail of the tracer restitutioncurve.

The Rhodamine response (instant measurementscorrected from background noise) is depicted infig.7A. The curve, somewhat chaotic, reflects twoinjection pump stops at 95 and 135 min respectively.Oscillations following injection resuming relate tothe restart of the pump stepwise. The peak noticed at180 min is caused by the reinjection of the surfaceleak, and the secondary peaks correspond to the tankrinsing cycles.

Fig. 7B illustrates the evaluation of Rhodamineconcentrations measured, in fluorescent units (FU),on samples. Rhodamine contents are derivedstraightforwardly as FUs volumes. Whenever neededthe Rhodamine mass can be calculated from thefluorimeter calibration curve (see fig. 8).

Figure 7A. Rhodamine restitution curve (instantmeasurements

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

0 50 100 150 200 250 300

duration (mn)

Flu

ores

cent

unit

xl

Figure 7B. Rhodamine samplings results.

1

10

100

1000

10000

100000

1,00E-10 1,00E-09 1,00E-08 1,00E-07 1,00E-06

concentration (l/l)

fluor

esce

ntun

it

Figure 8. Fluorimeter calibration curve

Iodine measurements are biased by nascent Iodinedissolved in the geothermal fluid. This is exemplifiedin fig. 9A which clearly shows that amount ofinjected Iodine were unable to defeat the residentbackground noise. Nevertheless, thanks toadjustments to the Rhodamine restitution curve, thebackground noise could be estimated at 4200 ppb +200 pbb thus leading to the corrected restitutioncurve plotted in fig. 9B. whose integration deliversthe tracer restitution ratio.

0

1000

2000

3000

4000

5000

6000

0 50 100 150 200 250

duration (mn)

conc

entr

atio

n(p

pb)

Figure 9A. total Iodine (non corrected) concentrationin geothermal production flow.

0

200

400

600

800

1000

1200

0 50 100 150 200 250

duration (mn)

conc

entr

atio

n(p

pb)

Figure 9B. Iodine concentration corrected frombackground noise.

Discussion

Results relating to Rhodamine WT and NaI aredisplayed in table 3.

It is quite clear that, owing to the strong Iodinebackground noise from the geothermal fluid, only canthe Rhodamine WT corrected curve allow a fairlyreliable assessment of restitution ratios which, in thepresent case, allow to discard the existence of anypumping chamber leak whatsoever. This conclusioncould be confirmed later, when static well headpressure recovered its previous value.

It became also pretty obvious that, unlesssignificantly higher Iodine quantities be injected tocounter background noise, a substitute neutralcandidate tracer should be selected.

TracerRhodamine WTAbsolute figures Precision Na I- Precision

Injected mass m0 2.8 109 FUxl 1.3 % 229.6 g 1.3 %

Restitution function integrationminimummaximum

1.18 106 FU x mn1.39 106 FU x mn

4 %4 %

47.5 10 –3 g/l x mn120.0 10 –3 g/l x mn

5 %5 %

Geothermal flowrate (m3/h) 154 (*) 4 % 154 (*) 4 %

Restituted mass M minimum 1.18 106 x 154 103 /60 =3.03 109 FUxl

8 % 47.5 10 –3 x 154 103 /60 =121.9 g

9 %

Restituted mass M maximum 1.39 106 x 154 103 /60 =3.57 109 FUxl

8 % 120.0 10 –3 x 154 103 /60 =308.0 g

9 %

Global restitution ratio R (%) 99 - 136 42.8 – 144.4Table 3. Case 2 tracer test results.

Case 3. Fresh Water/Fluorescein Tracer Test

Since severe production losses had been noticed onthe geothermal, district heating, production well a testwas designed to assess whether it could be caused bya casing piercing likely to be located at the HTPpacker anchoring depth (101 m).

The simple test design (5) consisted of pumping firsta ca 62 m3 volume of freshwater the tail of which istraced with 60 g of Fluorescein. This volumecorresponds to a net cased hole depth of ca 825 m.

The self flowing sequence which followed evidenceda flowed back volume of fresh water estimated at ca5 m3, based on water conductivity recording.Furthermore no return of Fluorescein was observed.

Therefore a leak depth comprised between 80 and115 m could be inferred.

A more accurate spotting of the leak depth wasimplemented reconducting a similar protocol. 10 m3

of fresh water were placed into the wellbore, fillingthe upper 165 m.

The flowback sequence, whose results are illustratedin fig. 10 led to a non ambiguous diagnosis. Only5m3 of fresh water (out of the former 10 m3 injected)were recovered, allowing to locate the leak at a depthof 100 m, as initially suspected (see fig. 10).

These convincing and simple experiments weredefinitively useful guidelines in designing future,combined chemically traced/fresh water, leak offtests.

Water conductivity (mS/cm)

13"3/8 casing(80,6 l/m)

Energising tubing(30,5 l/m)

Conductivity monitoring Well pumping chamber geometry

Flow

edba

ckco

ntro

l(m

3 )

Dep

th(m

)

Inflatablepacker

HTP

Figure 10. Case study 3. Water conductivitymonitoring and volume/depth assessments(source CFG)

Case 4. Squeeze From Surface Of A CombinedChemical Tracer/Fresh Water Sequence

The test was designed in order to investigate whethera casing piercing identified, on a damaged well, at adepth of ca 460 m, was leaking or not (6).In so doing, and based on experience acquired onearly tracer experiments, the following rationale wasadopted. Chose a chemical tracer quasi inert withrespect to the geothermal fluid and pumped in

alternance with fresh water slugs at volumes equal tothe well capacity above the casing piercing.Lithium carbonate (Li2 CO3) proved the bestcandidate as (i) nascent Li concentrations information waters are low (< 2 mg/l), thus minimizingbackground noise, and (ii) laboratory determinationon samples are easier and faster to perform (atomicabsorption) than is the case with anions such Iodine I.

Taking advantage of the sharp resistivity contrastwith the geothermal fluid, a hot saline brine, freshwater offers, via continuous monitoring of electricalconductivity, an additional means for assessingrestitution ratios and, eventually, casing leaks.Furthermore integration of fresh water volumesaddresses flowrates.

The injection sequence (via the 2"wing valve) intothe annulus is illustrated in fig. 11. It starts with thepumping of one unit volume (ca 15 m3) of, freshwater diluted, Li2 CO3, followed by the injection ofan equal volume of fresh water. The well is then selfproduced via the annulus and 2" wing valve torecover the fresh and traced water slugs (each 15m3

in volume) and ultimately underlying geothermalwater.

Field set up is described in fig. 12 (injectionproduction phases).

Injection and (self-flowing) production rates arecontrolled by a flowmeter gauge and volumetricmeasurements on the 1.5 m3 tanks. Both injection andproduction rates were set at 15 m3/h the latter bymeans of a throttle valve.

Sampling intervals varied from 2.5 to 5 min as shownin fig. 14, which displays the conductivity, Li+

concentration, flowrate and temperature vs. timeplots.

Fig. 14 evidences an almost ideal piston type Lithiumresponse, with minimum dispersion at the sharpwater/fresh water interface (hardly 10 minutes)enabling reliable material balance calculations.Temperature transients, as already suspected in casestudy 3, cannot, owing to heat conduction, beexploited for any volume calculation whatsoever.

Hence masses and volumes of fresh and Li tracedwater, based on water conductivity an Li+

concentration, led to reliable assessments of relevantrestitution ratios, summarized in table 4.

From the measuring accuracy close to 100 % meanrestitution ratio is derived thus conclusive as to theabsence of detectable casing leaks, a diagnosisconfirmed two years later thanks to a replicate test.

ground level

shut down wellgeothermal water

tracer added

fresh water injection

fresh water injection

self flowing wellproduction of fresh water

production of tracer addedfresh water

production of geothermal fluid

1

2

3

4

5

6

Figure 11. Case 3. Injection/flow back sequence.

heat plant valvepressure

flowmeterGPC AIT spool

casing head

10"3/4 casing

production column

electro submersible pump(-284 m)

(-300 m)

(-460 m)presumed leak

9"5/8 casing

down hole chemicalinjection line (AIT)

master valvegauge

ground level

measuringcontainers

1.5 m3

B. PRODUCTION PHASE

heat plant valvepressure

triplex pump

10 m3 container

flowmeter

containers (1.5 m3)

pump

GPC AIT spool

casing head

10"3/4 casing

production column

electro submersible pump(-284 m)

(-300 m)

(-460 m)presumed leak

9"5/8 casing

down hole chemicalinjection line (AIT)

master valve

measuring

gauge

round level

A. INJECTION PHASE

Figure 12. Experimental set up. Injection and flow back sequences.

Figure 13A: Field set up. Triplex pump with small tanks(1.5m3) and mixing tank (10 m3).

Figure 13B. Well head with injection/production via 2"wing valve

Tracer Conductivity Precision Lithium PrecisionInjected mass (m0) or volume (V0) 30 m3 2 % 375 g 0.8 %Geothermal flowrate 15 m3/h 2 % 15 m3/h 2 %Tracer concentration/measurement inrestituted water

0.2 % 1.3 %

Restitution function integrationminimum 29.375 m3 360.98 gmaximum 30.000 m3 380.83 g

Global restitution ratio R (%) 93.72 – 104.2 92.16 – 105.65Table 4. Case 3 tracer test results.

0

10000

20000

30000

40000

50000

60000

70000

0:00:00 1:00:00 2:00:00 3:00:00

Duration (hour)

Conductivity(µS/cm)

0

10

20

30

40

50

60

70

flowrate (m3/h)[Li+] (mg/l)

Temperature (°C)

Conductivity [Li+] (mg/l) Flowrate Temperature

Figure 14. Case 3 restitution results.

CONCLUSIONS

Tracer tests carried out on Paris Basin geothermaldistrict heating wells aimed at identifying casingleaks production equipment in hole. These tests wereconclusive. Tracer testing proved a viable alternativeto conventional packer leak off tests in assessingcasing integrity, owing to easier and faster fieldimplementation and cheaper operative costs.

This stated, tracer choice remains the key segment intest design.

Radioactive tracers secure the best accuracy thanks tocontinuous radioactive impulse-counting thusavoiding any integration whatsoever of the tracerrestitution curve. However tracer selection ingeothermal applications was limited to twocandidates, 131I and 82Br respectively, both subjected

to a number of limitations. 82Br is not easily suppliedand 131I requires special care in handling and disposaldue to its human contamination risks. Use ofradioactive tracers requires elsewhere dueauthorization by a competent Authority and theassistance of a specialized, authorized, laboratorywhich add to delays and costs.

Chemical tracers must be compatible in the sense nochemical nor physical interaction with the formationfluid and exposed equipment materials occurs whichwould definitely bias signal processing. Along thisline a high signal to noise ratio is mandatory i.e. theselected element must avoid conflicting with thesame, nascent, element in the formation water forobvious background noise considerations and also tokeep the injected tracer amounts to a minimum. Thisaspect could be evidenced in case study n° 2 with theIodine tracer.

In this respect Lithium, injected as, fresh waterdiluted, Li2CO3 proved the best candidate so far.Reliability of chemical tracing in assessing, via theintegration of the restitution curve, tracer materialbalance and leak occurrence, is strongly dependanton the tracer sampling rate during well flowback and,last but not least, measurement accuracy.Adequate sampling and equipment calibrationachieve an overall 5 % accuracy of the materialbalance exercise.

Neither should fresh water be overlooked as long asthere exists a sharp resistivity contrast with thegeothermal fluid, which was the case here as itaddressed a hot saline brine. Furthermore continuousresistivity recording enables to by pass theconstraining integration procedure inherent tochemical tracers.

Case study n° 4 led to the design of an optimumtesting protocol combining sequential injection of,chemically traced and fresh, water slugs, thusallowing reliable, redundant, interpretation.It has been shown (case study n° 3) that chemicaltracing could match the depth of the leak damage. Itcan address also the calculation of leaking rates, andbe extended ultimately to multiple leakconfigurations via relevant inversion methods. (7)

REFERENCES

(1) UNGEMACH P. : "Chemical Treatment of LowTemperature Geofluids", International Courses onGeothermal District Heating Schemes, Cesme,Turkey 1997, pp. 10-1 – 10-14.

(2) VENTRE A.V. : "Integration Methods forProcessing Tracer Restitution Curves", GPC OpenFile Report, Oct. 2000.

(3) CALMELS P. and GETTO D. : "Contrôle del'Etanchéité du Tobago dun Puts Géothermique"report DTA/DAMRI/SAR/RAP/93.18/PC/CR, July1993.

(4) CALMELS P. and FRANCOIS O. : "Traçage surle Puits Producteur du Doublet Géothermique d'Orlyle Nouvelet" report SAT/RAP/95.22/PC/CR, July1995.(5) CHERADAME J-M. : Opération de Traçage duPuits en Production de Bonneuil sur Marne. Rapportd'intervention 98 CFG 04 du 18/02/98, CFG, Orleans,France

(6) GEOPRODUCTION CONSULTANTS (GPC).Essai de Traçage par Injection de Traceur Chimiqueet d'Eau Douce. Puits de Production Géothermique deCréteil Mont-Mesly. Rapport GPC 98266d25, 3 sept.1998.

(7) UNGEMACH P. More About Tracer Leak OffTests. Determination of Leaking Magnitude andDepth. An Approach to the Multiple Leak Case:Paper in preparation.