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The use of chemical tracers to water injection processes

applied on Romanian reservoirs

Rajan Patel

Rajan.pbt13@gmail.com

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Outline

• Introduction

• Types of tracer

• Types of tracer test

• Partition coefficient ( Kd or Ki )

• Lab tests

• Field tests

• Conclusion

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Introduction

• Tracers are the chemical compounds that are used to track the fluids.

• Few applications of tracer :

To find the residual oil saturation.

Reservoir heterogeneity.

Fracture job evaluation.

Acidization evaluation.

Gelling (blocking) job evaluation.

Water channelling determination.

Finding sealing or non-sealing characteristic of fault.

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• Tracer test is mainly used for the residual oil saturation and the reservoir

heterogeneity or connectivity between injector and producer.

• Residual oil saturation can be measured using coring, logging, and with tracers.

• Core gives the information of in the inches.

• While logging gives the information in range of 4 ft.

• While Single well tracer test gives information in range of 4 to 12 ft.

Fig.1 Illustration of test zone sizes.

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Quality of tracer needed

• Nontoxic and not affecting the fluid.

• Travel at the speed of the injected water.

• Not to be present in reservoir water.

• Reservoir fluid and bacteria should not decompose it.

• Not to be adsorbed on rock.

• Detectable at very low concentration.

• Injected and produced at secured conditions.

• Cheap and already available.

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Types of tracer

1. Halogens (Chlorides, Bromides, Iodides)• Not environment hazardous.

• Contained by most reservoir waters.

• Sometimes res. Water contains quite big quantity of halogens so implicitly of test is

expensive.

2. Thiocyanates• Not present in reservoir waters.

• Avoiding the disposal of high concentrations on the soil or other places from where it

might reach the underground waters.

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3. Ammonium Nitrate• Used by agriculture as a fertilizer, to enhance and maintain soil fertility.

• Not environment hazardous.

• Disposal of high concentrations in places from where it might reach the underground

waters must be avoided.

• Solid ammonium nitrate must be carefully handled because under certain conditions

it may become explosive.

4. Alcohols• Low molecular weight ( Methanol or Ethanol).

• Water soluble but not Oil soluble.

• Bio degradable and Environment friendly.

• Mostly not present in reservoir water but might be there after stimulation jobs.

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5. Dye Tracers• Fluorescein and B Rhodamin.

• Detected using spectrofluorimeter.

• Advantage : Detectable at very low concentration

• Disadvantage : Adsorbed on the rock surface

• They can be use in fractured reservoir in which the connectivity between producer

and injector is very good.

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Types of tracer tests

• Inter well tracer test (IWTT)

Two well tracer test (TWTT)

• Single well tracer test (SWTT)

• The IWTT has been initially only applied in water injection and EOR processes,

later on SWTT and TWTT are implemented to find residual oil saturation around

single well or between two wells.

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IWTT (inter well tracer test)

• IWTT is the basic test first started to applied in the fields.

• In this test we used to find the reservoir heterogeneity.

• One of the disadvantage of this

test is its time requirement.

• Other disadvantage is the

control on the test.

Fig.2 Illustration of the IWTT.

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TWTT (two well tracer test)

• Two tracers with different distribution coefficients to water and to oil are used.

• One tracer is only soluble in water and other one is soluble in both water and oil.

• Both are injected by injector and monitored at producer

• Based on the time difference of the arrival of the two tracers, can find the residual oil

saturation along the tracer’s path between the two wells.

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Fig.3 Illustration of TWTT and its outcome.

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SWTT (single well tracer test)

• Alkyl esters of fatty acids and alcohols are used as tracer.

• Ester selection depends on the reservoir characteristics.

• Mostly used ester is ethyl acetate.

• Tracer injected into the reservoir in certain predetermine quantity to certain distance

and then shut in to allow ester to react with reservoir water.

• This reaction produce ethylic alcohol (new tracer) and acetic acid (Hydrolysed- Ester)

new tracer in reservoir.

• Some part of tracer is non hydrolysed.

• Well is started to flow and both tracers are produced and detected along with water.

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• Because of the difference in water and in oil distribution coefficients, they will reach to

the well bore at different time.

• Based on this time difference we can calculate our oil saturation around the well.

Fig.4 Illustration of SWTT.

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Fig.5 Outcome of the SWTT.

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Kd (Partition Coefficient)

• Oil and Water tracers are characterised based on this.

• This is unique for each tracers for unique reservoir conditions.

• Ratio of the two immiscible phases tracer concentration.

• Dependent upon : Temperature (partially), Salinity of water, Type of oil.

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Fig.6 Arrival of different Kd tracers, with 28.1 Sor

Fig.7 Kd with different oil type and water salinity

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Laboratory Teats

• Why???? Selecting the tracer and setting the methods to detect them.

Under reservoir conditions to determine moment of tracer arrival and its

adsorption on the rock and partition coefficient.

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Tracer selection and Detection methods

• Tracers used for the lab tests are

Thiocyanates.

Ammonium Nitrate.

Methanol.

Sodium Chloride (Nacl).

Methyl Tertiary Butyl Ether (MTBE).

N – Butanol.

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Detection Methods

• In beginning of tracer detection we use colorimetric methods.

• This method is successfully replaced by the chromatography method.

• The nitrate ions, thiocyanates, halogens (Chloride, Bromide, Iodide), and acetate

ions are presently identified by the ion-chromatography.

• In this method detected up to tens of ppb (parts per billion).

• Finding the sensitive detection method has our permanent objective, because it is the

only way we can decrease the tracer quantity used for tests.

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Flow trough core tests performing

• The FRT Chandler (Formation Response Tester) was used for dynamic testing on

sandstone and limestone core.

• Solution mixture contains 1000ppm of each tracer.

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Fig.8 Tracer’s arrival in flow through core test in carbonate.

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TracerMaximum concentration, ppm

Recovery, %

Thiocyanate 342 99.00

Nitrate 291 98.70Bromide 274 97.80Iodide 264 95.40

Hydrazine 186 67.90

Rock type Sandstone Carbonate

Concentration of injected methanol, ppm

100 100

Adsorption, mg/100g rock 2.87 4.36

Methanol recovery degree, %

88.27 82.6

Table.1 Tracer recovery degree.

Table.2 Adsorption of methanol.

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Field Tracer Tests

• All 4 tests are applied in different fields of Romania.

• From last 13 years counted 50 IWTT with chemical tracers on 23 reservoirs and 2

TWTT.

1. Identifying Injected Water Channelling• Test applied on Tazlau reservoir.

• Rock type is mainly Kliwa siliceous sandstone.

• The chemical tracer Methanol was injected from I-1 Tazlau.

• Methanol is very soon identified in producer well P1-1 Tazlau.

• Approximately around 40,000 ppm after 245 hours.

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• Other 6 observation wells are P2-1, P6-1, P9-1, P10-1, P11-1, P12-1.

• Tracers arrived at very less concentration in this wells.

• Based on this the 4 block A,B,C,D are separated by non-sealing faults.

Fig.9 Area of Tazlau reservoir Fig.10 Tracer arrivals in P1-1, P2-1, P11-1

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2. Set the preferential flow paths of injected fluid• Test applied in Turnu Nord reservoir.

• Rock type is mainly sandstone, sand and marl limestone, with marl and shale

intercalations.

• Chemical tracer used was Thiocyanate.

• Test was performed in injector-I.

• Here in figure shown the tracer concentration map based on the arrival of the tracer.

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• From the figure we can say that the mainly flow of the tracer is towards the southern direction.

Fig.11 Evidence of preferential water flow direction

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• Test was performed in injection well I-2

of Moinesti reservoir.

• Here the shown fault is separating 2

blocks.

• Ammonium nitrate was used as tracer.

• Approx. after 7 days from injection in

wells P1-2 and P2-2 situated 58m and

103m from injector was first identified

in concentration up to 10ppm.

• Tracer was identified in wells P3-2, P4-

2, P5-5 and P6-2 after 8, 11, 13, and

17 days respectively.

3. Checked faults sealing characteristic

Fig.12 Area of Moinesti reservoir.

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• The concentration of the ammonium nitrate was in between 3 to 10ppm.

• Arrival and concentration observed in the wells differs based on the distance.

• As the tracer was produced from the well P3-2 we can say that the fault has non

sealing characteristics.

Fig.13 Tracer arrivals in wells P3-2, P2-2, P1-2.

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• The application of gelling systems is

one of the method used to reduce

water in the production wells.

• This method works for both producer

and injector, when the injected water

channels towards the one of the

producer.

• The efficiency of such treatments can

be assessed by performing a tracer

test before and after the blocking

treatment.

• Injector I3 Strambu and producer P-3.

4. Evaluated the efficiency of water shut-off treatments.

Tracer : Methanol

Fig.14 Difference between tracer arrivals before and after the job.

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Conclusion

• The tracer test provides information on the injected fluids flow through reservoir

related to different recovery process, and residual oil saturation.

• The way of reduction in the tracer quantity is only by the use of the most sensitive

detection method which is done in the laboratory testing.

• The displacement tests are performed on the cores in reservoir conditions.

• Up to present day, Romania performed 50 IWTT with chemical tracers on 23

reservoirs and 2 TWTT.

• Most frequently used tracers were Thiocyanate, Methanol, and Nitrate.

• In recent advancement tracers are using in unconventional oil sand reservoirs.

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References

• The use of chemical tracer to water injection processes applied on Romanian

reservoirs, by M. Zecheru and N. Goran in EDP Sciences 2013.• Assessing EOR potential from partition tracer data, by Olaf Husbey, Sven K. Hartvig,

Kjersti Jevanord, Qyvind Dugstad presented in SPE 11/03/2015.

• Evaluation and comparison of available tracer methods for determining residual oil

saturation and developing an innovative single well tracer test technique: dual salinity

tracer, by Rasoul Khaledialidusti, and Jon Kleppe presented in IPTC 12/12/2014.

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Thank You!