Tracers have been used to measure fluid flow in reservoirs for several decades but little by little new applications have been proposed and tracers have been playing a more and more relevant role in oil industry.

As a general rule, natural production mechanisms, or primary production, contribute to extract from the reservoir about 25% of the original oil in place. This means that 75% of the existing oil remains in the pores and fissures of the rocks.

The production flow rate depends on the differential pressure between the permeable layer and the bottom of the well, the average permeability, the layer thickness and the oil viscosity.

The main natural production mechanisms are the expansion of the oil, water and gas and in certain cases the water influx from aquifers connected with the reservoir.

When primary oil production decreases in a field because of a reduction in the original pressure, water is usually injected to increase the oil production. Injected water in special wells (injection wells) forces the oil remaining in certain layers to emerge from other wells (production wells) surrounding the injector. This technique, commonly called secondary recovery, contributes to extract up to 50% of the original oil in place.

Although this technique was firstly used in old reservoirs in which oil production had decreased, it is today a common practise to begin the exploitation of new wells with fluid injection as a way to optimise oil recovery. For this reason, the name secondary recovery is being replaced by the more general term enhanced recovery.

The efficiency of the water flooding process is highly dependent on the rock and fluid characteristics. In general, it will be less efficient if heterogeneities are present in the reservoir, such as permeability barriers or high permeability channels that impede a good oil displacement by the injected water.

Tagging the injection water with a suitable nuclide and measuring samples taken from production wells make it possible to obtain the response curves (concentration of activity vs. time) which represent the dynamic flow behaviour of the pattern (injector plus producers) under study.

Most of the information given by the analysis of the response curves cannot be obtained by means of any of other technique.

On the other hand, nowadays is a common practise not only to drill wells at different angles but also to branch them in order to pass through several zones containing hydrocarbons along many kilometres.

Once oil production starts is very important to know which layers or which sectors of a unique layer contribute to the total production. This objective can be accomplished using some special tools capable of measuring the amount of gas or petrol flowing at specific locations but they have several disadvantages.

First of all in long horizontal wells it is almost impossible to lower logging tools to the desired depth in the borehole. Secondly, the production rate requires to be wholly or partially stopped in order to use such a tool and that leads to economic losses. Finally, the technique itself is rather expensive to use.

A new technology using tracers allows the evaluation of the oil flow during the first production of the well. For this purpose, a number of different tracers, in a chemical form that ensures their solubility only in oil crude, are located at specific points along the borehole. Samples are taken at surface over a short period of time during the production tests and then measured by gamma spectrometry to identify the tracers present in each sample and then to determine the areas contributing to oil production.

Small pellets containing a few amount of scandium-46, chromium-51, cobalt-60, silver-110 and/or antimonium-124 are commonly employed for this application. The tracers are attached to the explosive charges used by perforation guns and carried with the explosive pressure wave into the formation to tag different layers or sectors of a layer.

Due to the low tracer activity and the high dilution rate involved in the process, measurements must assure suitable statistic error and confidence interval. Low background, long measuring time and a good long time electronic stability are the basic requirements to accomplish this objective.

Hydraulic fracture is another operation widely used in oil industry to increase the hydrocarbon production by improving the rock permeability. The fracture begins with the injection of fluid and continues with the incorporation of several tons of sand in increasing concentrations in the selected layer.

The process ends when the fluid is extracted from the borehole and the high permeability sand takes its place improving the geological characteristics of the formation. Although the injection is performed at a certain depth and following a specific sequence, the fluid as well as the sand usually moved to random paths and the height and shape of the fracture seldom agree with the previous expectative.

Consequently, the determination of the fracture height and shape is a way of controlling the quality and accuracy of the operation. Radiotracers are an excellent and reliable tool to achieve this objective by tagging the sand with one or more radioisotopes in a suitable physical and chemical form and then comparing two gamma ray logging (run before and after the fracturing operation). There are no other tools able to determine the characteristics of an hydraulic fracture except theoretical calculations performed before the operation and based on a great deal of suppositions not always realistic. Scandium-46, antimonium-124 and iridium-192 as aluminium oxide are recommended to tag the sand.

Tracer is a detectable substance added to a chemical, biological, physical or industrial system by means of some injection or labelling technique in order to study and evaluate the characteristics and the dynamic behaviour of such a process.

Examples of tracers are solids in suspension, dyes, salts, alcohols and radioisotopes. The major advantage of radiotracers is that they can be detected without physical contact and therefore without affecting the system.

For some values of atomic and mass numbers the nuclei are stables, that is to say that they do not change as time goes by unless an external action takes place. When the ratio neutrons to protons differs from the value for stable state the nucleus trays to reach stability through the emission of alpha or beta particles. This mechanism is known as radioactive disintegration and it may additionally produce electromagnetic radiation originated in the nucleus (gamma radiation).

For a radioactive material, its disintegration velocity is proportional to its mass and is called activity, being the becquerel its measurement unit (1 Bq = 1 d/s). In practise is very common to use another unit, the curie (1 Ci = 3,7 x 10¹⁰ Bq = 37 GBq).

The time for a radioactive mass to be reduced at a half as a consequence of radioactive decay is the half-life of the radioisotope enclosed in this material.

Some nuclear reactions make it possible to produce a third hydrogen isotope known as tritium (³H) which has one proton and two neutrons in its atomic nucleus. Because of this additional neutron tritium nucleus becomes unstable and looks for stability emitting a low energy beta particle without emission of gamma radiation.

Since tritium nucleus emits low energy beta particles the only way to get a high measuring efficiency is using liquid scintillation technique. Tritiated water samples coming from oil wells have to be cleaned, filtered and fractioned before measuring. Later they are mixed with a commercial product called scintillation cocktail which has the ability of producing photons when beta particles interact with its atoms. A couple of electronic valves (photomultipliers) detects the photons and generate electronic pulses that are processed and counted by the electronic unit. As a result of that information on the beta energy and activity is provided by the system.

Tritium samples ready for measuring

The response is presented in "counts" per time unit (c.p.m.  = counts per minute) or disintegrations per time unit  (d.p.m. = disintegrations per minute) when counting efficiency is taken into account.

Most of the information given by the analysis of the response curves cannot be obtained by means of any of other technique.

Detailed analysis of the response curves obtained from interwell studies allows to:

All this information can be used to make operational water flooding decisions in order to increase oil production.

Interwell tracer tests give quantitative information on the fluid dynamics in a reservoir. Now, dynamic information from a reservoir may in addition be obtained by three other methods: Logging of production rates (profiles) of reservoir fluids, pressure testing and time-lapse seismic examinations (4D seismic). However, these methods and the tracer testing are complimentary, and cannot directly replace each other.

In this framework is especially convenient to use radioactive tracers and among them tritium whose main advantages are:

ability for tagging huge water volume using a small tracer mass (this property is common for any other radiotracer);

being a low energy beta emitter heavy shields are not necessary;

very high measuring efficiency and therefore detectable in very low concentrations;

ideal tracer behaviour .

Sampling operation is generally carried out by personnel belonging to the oil company. It is not a too complex task, especially when working with tritium due to the very low concentrations usually present in production water.

Ideally, one sample per day should be taken but only some of them sent to the analytical laboratory. When the tracer is detected in any sample, the previous ones can be measured in order to rebuild the response curve for detection of tracer break-through. However, in the case of having performed several simultaneous injections it is difficult to manage the number of samples involved and for that reason it is convenient to elaborate a sampling plan less ambitious. But all the same, it is always advisable to measure only some of the collected samples and return to previous samples when tracer appears.

The sampling plan should include a relatively high sampling rate during the first days after the injection and a somewhat more time-spaced sampling as time passes. The sampling rate should increase when break-through is detected in order to monitor more precisely that part of the tracer production curve where most abrupt changes take place. After having passed the maximum of the production profile, two samples a month should be enough and just one sample per month afterwards. The objective of a good sampling plan is to have enough information to get a good response curve with a minimum of samples to be taken.

The reason for a high sampling frequency right after injection is the possibility of some canalisation that makes the injection water flow very quickly to the production well. In such a case the tracer longitudinal dispersion will be very small and the response curve will be short with a sharp rise edge and high amplitude. All the information will be concentrated in a short time and many samples will be needed to recover it.

When counting a radioactive sample it is well known that the instrument reading is a measure of the sample activity plus the background activity. The latter must be subtracted in order to evaluate the actual net sample activity. The background activity is usually taken to be the activity measured by using the sample taken before the injection (blank sample). If, however, the tracer does not appear immediately (there is no canalisations), a more representative information on the background activity is obtained by measuring several of the samples and averaging the results taking into account that the more samples the lower is the background’s variation coefficient. The variation coefficient is the ratio between the standard deviation and the mean value.

At this point, it is convenient to remember that radioactive decay is an inherently random phenomenon that follows, strictly speaking, the binomial distribution. Nevertheless, Poisson distribution is an excellent approach taking into account some of the radioactive decay characteristics (the random event "disintegration" is repeated many times and the individual probability for an atom to disintegrate is very low). Furthermore, in the case that the number of events approaches infinity the binomial distribution and the Poisson distribution converge towards another statistic distribution called normal distribution or Gauss distribution which is continuous and symmetric around its mean value.

Consequently, two measurements may be said to belong to different populations when their measured mean values differ by at least five standard deviations. This criterion is also applied to determine whether a sample is active or not. That is to say that a sample has some radioactivity of its own when its count rate is five standard deviations greater that the background.

For each well the response curve is the activity concentration as a function of time. The original data is usually corrected by background subtraction and radioactive decay and probably filtered by means of some suitable mathematic algorithm.

The cumulative response plays also an important role when results are analysed. It is the recovered activity as a function of time and can be calculated from the concentration curve. From it the tracer distribution among the wells making part of the pattern is evaluated.

Analysis of the response curves can be focused from three different points of view or complexity levels.

The simpler interpretation is the qualitative one. Just observing the curves, the following pattern characteristics can be found out : injection water arriving time (breakthrough); high permeability channels, barriers and fractures between both wells; communications between different layers; stratifications in the same layer; preferential flow directions in the reservoir.

The next step in the analysis should be to make some simple calculations from the numerical response, first of all the determination of the mean residence time. Furthermore, the cumulative response can be got by integration of the concentration vs. time curve supposing the production flow rate to be known. From this new curve, the fraction of injection water reaching each producer is easily calculated.

Finally, it is possible to use mathematical models to fit simple response curves by means of theoretical expressions and to decompose complex response in several simpler functions. This way partial residence times as well as other parameters can be determined for each function.

Mathematical models also allow the evaluation of some important parameters such as permeability and make it possible to predict the behaviour of unknown patters.

NOLDOR S.R.L. has institutional and personal licenses for working with tritium and others radioisotopes granted by the Argentinean Nuclear Regulatory Authority (ARN).

Any practise performed by NOLDOR implying the use of radiotracers in oil fields or industrial plants or involving the environment are backed-up by a procedure especially developed for each particular case and approved by the customer.

NOLDOR has special containers for radioisotope transport according to requirements established by "Regulations for the Safe Transport of Radioactive Material" (International Atomic Energy Agency) and local regulations and that also agree with the specifications of other international organisations such as World Health Organisation (WHO) and Organisation for Economic Co-operation and Development (OECD).

Injection operation in which important tracer concentrations are manipulated is always carried out by specialists from NOLDOR following approved procedures and using different radioprotection equipment.

Production wells sampling is the only task involving tracer manipulation in charge of the oil field workers. However this operation is completely safe since the activity concentration in the samples is about one thousand lower than the annual intake limit (ALI) imposed by international regulations.

Chemical tracers are an alternative and a complement for radiotracers though, as a general rule, injection is usually more complicated due to the huge volumes involved in the operation. As a matter of fact, it is common to handle several thousand litres of a thiocyanate compound or of some alcohol in lieu to the small volume required when radioisotopes are used. Sample volume is also higher because of the lower detection sensibility.

NOLDOR S.R.L. strongly recommends the using of tritium as first tracer though also offers chemical tracers as a complement, preferentially ammonium thiocyanate.

As it was already exposed above, tracers can be used to detect canalisations and to evaluate their volume. If canalisations are really found, gel injection can be a good choice for reducing injectability in those high permeability layers.

Once gel injection has finished, tracers may be again used as a tool to control the efficiency of such an operation. If gels have adequately accomplished  their task of reducing the permeability in the "thief zones", then the breakthrough time of the tracers will be noticeable lower.

Several oil companies in Argentina have satisfactorily applied this excellent control tool.

Any interwell communication study starts with a feasibility analysis including the calculation of the required tracer activity. The customer has to supply some information on the pattern under study so as this evaluation can be carried out.

Afterwards a sampling plan is agreed between all the involved parts. A typical plan is shown in the downloadable file Information and sampling.pdf (62.5 KB) where the information needed for calculations also appears.

Before the tracer injection takes place, the most suitable technique for this operation should be chosen on the base of the operative conditions of the well. Probably, a special injection device will be needed usually very simple and cheap. In case of selective injection the layer or layers in which the tracer is going to be injected have to be isolated. This task is usually done by a wire-line company.

The following step is to inject the tracer. NOLDOR's specialists always take on this operation.

From this moment the oil field worker will be in charge of taken the samples following the plan implemented before. It is very important not to go away from the planned sampling sequence otherwise valuable information on the tracer behaviour could be lost. It is worth to underline again that samples coming from production wells are practically free of radioactivity (in this regard see the previous paragraph).

Samples are then send to NOLDOR laboratory for treatment a measurement. For tritium measuring it is recommended to take 250 cm³ water samples free of oil.

NOLDOR will issue a monthly report including all information concerning the measurements, some basic interpretation and recommendations about the sampling frequency. A the end of the study a final report will be brought out.

Publications issued by NOLDOR's engineers regarding interwell studies are listed bellow.

"Hacia una mayor eficiencia en la recuperación secundaria de petróleo".

H.R.Gómez, G.E. Maggio, G.B.Baró.

Petrotecnia - Año XXXVII Nº 2, 1996

"Aportes de los trazadores radiactivos e indicadores isotópicos a la industria petrolera".

H.R.Gómez, G.E. Maggio.

YPF - Boletín de Informaciones Petroleras - Año XII Nº 47, 1996.

"Radiotracer Applications in Oil Secondary and Tertiary Recoveries".

G.E. Maggio.

International Atomic Energy Agency. Report of the First Research Co-ordination Meeting on Radiotracer Technology for Engineering Unit Operation and Unit Processes Optimization, Vienna, 1998.

"Empleo de radiotrazadores en la evaluación hidrodinámica de un yacimiento de petróleo viscoso".

H. Najurieta (Bridas S.A.P.I.C.), G.E. Maggio.

IV Congreso de Exploración y Desarrollo de Hidrocarburos. Actas de Instituto Argentino del Petróleo y Gas, 1999.

"Guía de Procedimientos sobre Tecnología de Trazadores. Capítulo 7: Trazadores en Petróleo".

G.E. Maggio.

International Atomic Energy Agency, ARCAL XLIII Guidebook, 1999.

"Radiotracers Technology as Applied to Interwell Communications in Oil Fields".

Thor Bjornstad (IFE, Norway), G.E. Maggio.

Consultants' Meeting on Technical Report Series: Guidebook on radiotracer and sealed source applications in industry. International Atomic Energy Agency, 2001; Technical Report Series N° 423, 2004.

Teaching and tasks regarding interwell studies carried out by NOLDOR´s engineers are listed bellow.

Lectures in the "Regional Training Course on Radioactive Tracers in Oil Fields" organised by International Atomic Energy Agency (IAEA) and Cuyo National University, Mendoza, Argentina (March, 20 to 31, 2000). G.E. Maggio, H.R. Gómez, G.B. Baró.

"National Training Course on Tracer Techniques and Nucleonic Control Systems in Oil Industry" organised by International Atomic Energy Agency (Project ARCAL XLIII) and Research Institute in Geosciences, Mining and Chemistry, Bogotá, Colombia (may, 8 to 12, 2000) G.E. Maggio.

"Internal Workshop on Application of Nuclear Techniques in Oil Industry", Central University of Venezuela , in the framework of Technical Co-operation Project IAEA VEN/8/015 "Nuclear Techniques in Oil Fields". Caracas, Venezuela ( May, 14 - 18, 2001) G.E. Maggio.

Lectures in the "Regional Training Course on Tracer Applications in Oil Industry" organised by International Atomic Energy Agency (Project ARCAL LXI) and Comahue National University, Neuquén, Argentina (September, 10 - 19, 2001). G.E. Maggio, H.R. Gómez.

Lectures in the "Regional Training Course on Applications of Sealed Sources in the Petrochemical Industry" organised by International Atomic Energy Agency (Project ARCAL LXI) and Comahue National University, Neuquén, Argentina (March, 8 - 12, 2004). G.E. Maggio, H.R. Gómez.

Lectures in the "Radioisotope Applications for Troubleshooting and Optimizing Industrial Processes" training course organised by the International Atomic Energy Agency  (Project AFRA RAF8040 - 013) and the Centre for Energy Research and Training, Abuja, Nigeria (May 31 - June 4, 2010). G.E. Maggio.

Assessment tasks regarding interwell studies carried out by NOLDOR´s engineers are listed bellow.

Technical assessment to Ecuadorian Atomic Energy Commission in tracer applications in the oil industry, in the framework of Project ARCAL XLIII,  International Atomic Energy Agency. Quito, Ecuador (October, 13 - 17, 1998) G.E. Maggio.

Technical assessment to Vietnamese Nuclear Research Institute in the framework of Project RCA RAS/8/086/ 11-01, International Atomic Energy Agency. Dalat and Vung Tau, Vietnam. (November, 15 - 26, 1999). G.E. Maggio.

Technical mission in the framework of the Technical Co-operation Project VEN/8/015 "Nuclear Techniques in Oil Fields", International Atomic Energy Agency, in order to evaluate the capabilities of the Central University of Venezuela. Caracas, Venezuela. (January, 22 - 26, 2001) G.E. Maggio.

Technical mission in the framework of the Technical Co-operation Project VEN/8/015 "Nuclear Techniques in Oil Fields", International Atomic Energy Agency, in order to train engineers from PDVSA (Venezuelan National Oil Company), INTEVEP (Venezuelan Technical Institute) and Central University of Venezuela. Caracas, Venezuela. (July 21 - August 1, 2003) G.E. Maggio.

Participation as expert in the "Consultants' Meeting on Preparation of a Technical Report on Radiotracer Applications in Oil Fields". IFE, Kjeller, Norway, organised by the  International Atomic Energy Agency. (April, 7 - 11, 2003). G.E. Maggio.

 H Home                                                                                                                                                    ñ Back to top