111.1.3.1. Exposure in a normal situation

For pure beta radioactive tracers used in a normal situation, there will be no exposure to radiation during transport and injection because the soft beta radiation will not penetrate the walls of the bottles and equipment in which the tracers are kept before injection.

For gamma radioactive tracers, appropriate shielding has to be applied to reduce the dose to the personnel involved to below an acceptable limit.

111.1.3.2. Exposure due to a transport accident

For pure beta radioactive tracers, the tracer is shipped in a sealed 95 mL Monel bottle surrounded by water absorbent material and placed in a steel container. This container is again placed in a steel barrel surrounded by shock absorbing Ethafoam.

For gamma radioactive tracers, the volume of the tracer solution will be similar to the beta emitting tracers, i. e. 50-75 mL, and the packaging and containment are the same except for a higher level of shielding surrounding the inner steel container in order to give an acceptable transportation index (see necessary shielding thicknesses above).

If an accident occurs during the transport of the tracers, the radioactive material is supposed to be retained in its containment. Even in a major accident it is very unlikely that radioactive material will be dispersed. The situation will be dealt with in a normal way according to national and international laws and rules.

There are two scenarios that could result in the release of tracer:

(1) Very small leakages of tracer containing liquid at tube connections, etc.

(2) Breakage or destruction of injection equipment due to some accident

onboard the platform.

(a) Scenario 1

As a general rule, injection of radioactive tracers should always be performed by at least two persons with the necessary technical skills and safety competences. One person is implementing the injection process while the other is handling monitoring instruments and safety precaution equipment.

In case a leakage occurs, valve B (Fig. 7) connecting the tracer injection equipment to the water injection pipe on the platform will be closed. This will stop the leakages and the released fluid will be removed and treated as contaminated material by operators carrying personal protective equipment. It is supposed that this type of tracer dispersion will not cause any significant exposure to radiation or intake of radioactive material by the operators.

(b) Scenario 2

If a serious accident (e. g. an explosion) occurs in the vicinity of the injection site during the few seconds when the tracer is being flushed from the tracer bottle to the water injection pipe, causing destruction of the injection equipment, there could probably be a release of tracer.

It could also be envisaged that a valve or pipeline failure on the high pressure side would cause extensive leakage in the form of an unidirectional ejection of tracer-containing fluid.

The action to be taken, if possible, in this case will be to stop the flow through the injection cylinder by closing valve B. Then the injection equipment and the site will be flushed with large quantities of water to disperse the tracer into the sea (for offshore installations). As the tracer will be diluted with water very rapidly in this case, it is believed that there will be no significant intake of tracer by the operators. If the injection operator, who wears the mandatory protective water repellent clothing, has been splashed by tracer-containing fluid, the nearby support person immediately starts decontamination of the operator according to established procedure, including dousing of the operator with water followed by removal of the protective clothing. The support person also operates

monitoring equipment to ensure that the operator is clean before they leave the injection site.

In the case of onshore operations, injection equipment should be mounted in a trough in order to collect any spillage from the injection equipment itself. In addition, the nearby surface area should be covered with a plastic sheet with absorbent tissue paper on top to collect any spillage and facilitate site decontamination after a spillage. If soil contamination occurs, the area should be evacuated for a time long enough that the remaining activity in the soil has either evaporated (in the case of volatile tritiated liquids) or disappeared deeper into the ground (in the case of non-volatile beta and gamma emitting tracers). Surface monitoring of gamma radiation and/or vapour samples taken by radiation safety workers and analysed with respect to the content of volatile tritium labelled tracers will decide when the area is opened for general and normal work.

IV. 2.8.1. Interpretation of experimental field data

Some field tracer experimental data has been analysed using the PORO simulator. Some of them are described below as case studies. In all cases, the first step in the simulation was carried out under the following conditions:

• Residual oil saturation (Sw = 1 — (Sor — Swi));

• Total layer thickness watered (h = hmax);

• Reported porosity;

• Nominal water flow rates;

• 100% tracer recovery;

• No faults;

• No anisotropy;

• Dispersivity equal to 10% of the distance between wells.

Case application 1: Carmopolis oilfield (Brazil)

The pattern is illustrated in Fig. 107. The layer thickness is 10 m, the porosity 17%, the water saturation 63%, the injection flow rate 83 m3/d and the injected activity 55.5 GBq.

In the first step of the simulation of well CP-1091, using the parameters reported by the oil company, it was observed that the simulated tracer breakthrough time was double the experimental value.

Additionally, the measured cumulative tracer recovery in the well was 55.33% (extrapolated to 60%) instead of the simulated value (43.80%). It is believed that this difference is due to the fact that the streamlines were too ‘open’ because only the water flow rates in the wells were taken into account in

calculating their paths. Introducing modifications to confine the streamlines simulation became more realistic (Fig.108).

Thickness, water saturation and porosity values used in the second step of the simulation were provided by the company. The dispersivity value was in

FIG 109. Well 1 pattern (Pindori oilfield, Pakistan).

agreement with the criteria requiring that it must be equivalent to 10% of the distance between wells.

Case application 2: Pindori oilfield (Pakistan)

The pattern and the reservoir parameters are illustrated in Fig. 109. Up to day 150 after HTO injection, the only well in which tracer had been detected was well 3. On day 60, the injection conditions were modified, introducing strong perturbations in the tracer record. For that reason, only its non-perturbed portion was considered for this study.

Figure 110 shows both the experimental and the simulated tracer responses under the mentioned conditions.

The PORO simulator allowed an acceptable fit to the experimental records to be obtained by employing the reported injection water flow rate and the same water flow rates for all the producer wells. A layer thickness of only 0.35 m was used.

However, the simulation predicts a quick tracer breakthrough in well 4, which is not in agreement with actual tracer behaviour. Several reasons may be responsible for the lack of tracer detection in this well, such as different location of the fault, anisotropy and low water flow rate to the well. Additional information will be necessary for selecting the right scenario for this case.

Gamma tracers are commonly measured using either solid scintillation detectors or semiconductor detectors.

Solid scintillation detectors: These are of different types, but the most generally applicable is the detector based on a single crystal of sodium iodide doped with traces of thallium, the so-called NaI(Tl) detector. The crystal is optically coupled to a PMT. Interaction of a gamma photon with the scintillation crystal results in the emission of light, which is detected by the PMT.

The light output is proportional to the gamma energy. The electronic system associated with the PMT analyses the pulses according to pulse amplitude (energy) and stores the results in a multichannel analyser. Thus, energy and intensity are recorded, and the result is the gamma energy spectrum of the radiation source.

The NaI(Tl) detector has a high intrinsic efficiency but limited energy resolution. The scintillation crystals are provided in different sizes. The efficiency for high gamma energies increases with detector volume.

Common counting equipment has cylindrical crystal sizes of 50 mm x 50 mm to 125 mm x 125 mm (height x diameter): the larger the crystal, the higher the price. The detectors can be made quite rugged and are suitable in field instrumentation.

Semiconductor detectors: Today, these are mainly based on high purity germanium crystals, so-called HPGe detectors, where a semiconductor junction is created by suitable elemental dopants on the crystal surface. A gamma ray interacting with the detector will result in an excitation of electrons from the valence band to the conduction band in the crystal, and a small electrical pulse is created in a high voltage field. The pulse height is proportional to the gamma energy. The pulses are sorted and stored in a multichannel analyser.

The intrinsic efficiency of semiconductor detectors has, for many years, been lower than that of NaI(Tl) detectors. At present, it is, however, possible to purchase detectors with efficiencies >100% relative to that of a 75 mm x 75 mm NaI(Tl) detector, but prices are very high. The main advantage of an HPGe detector is, however, its excellent energy resolution. This property may be indispensable for the analysis of complex radiation sources. HPGe detectors need cooling to the temperature of liquid N2 during operation and are not generally practicable as field instrumentation.

In general, gamma tracer detection requires little sample preparation except for the extreme low energy emitters (i. e.125I). There are several ways to reduce the minimum detectable concentration in gamma detection:

• Increase the intrinsic detector efficiency: This is a matter of cost.

• Increase counting sample volume (constant activity concentration in the sample leads to higher total activity in the sample): There is a practical limit to the sample size.

• Optimize the counting geometry by shaping the counting sample: For a given radionuclide, a selected detection set-up and a certain sample volume, there is an optimum shape of the sample volumes. For practical reasons these are most often cylindrical shapes.

• Enrich the tracer from a large to a smaller sample volume (increased total activity for a better sample counting geometry): This requires sample treatment either by liquid evaporation or by chemical separation. Sample treatment time and cost increase.

• Reduce the background level by effective detector shielding: This is most often done by passive shielding with lead walls (5-10 cm thickness) around the detector and sample.

A typical counting set-up for a NaI(Tl) detector is shown in Fig. 22. A 1000 mL Marinelli sample container, 75 mm x 75 mm NaI(Tl) detector, Pb shield (5-10 cm), a Sn (or Cd) screen to filter Pb X rays generated by the sample activity in the Pb shield, Cu filter screen to filter away Sn (or Cd) X rays generated by the Pb X rays in the Sn (or Cd) screen.

• With NaI(Tl) detector based analytical equipment, detection limits of <0.2 Bq/L can be obtained using Marinelli beakers and reasonable counting times for common radionuclides such as 22Na, 60Co and 125I.

• For HPGe detectors, the corresponding detection limits are <0.1 Bq/L.

I.5.3.I. Tracer recovery

Among the ten wells monitored for a year, three wells (W2R3D, 214 and 202) yielded positive results for tritium tracer. These same wells also yielded positive returns with NDS. These returns confirmed the communication between the injector well 1R8D and the nearby production wells.

Figure 68 shows the results of the NDS tracer test. Among the wells in Tongonan-1 nearest to 1R8D, wells 2R3D and 2R4D showed NDS breakthrough starting about 19 d after injection. Well 214 manifested breakthrough 40 d after injection, while breakthrough in well 202 occurred much later, at 131 d.

202

FIG 68. Plot showing the breakthrough of wells from NDS tracer injected into well 1R8D.

The wells which showed NDS breakthrough also showed positive returns with tritium; among the wells monitored, these are the only wells which gave tritium breakthrough. Tritium in well 2R3D appeared 28 d after injection; while it appeared in wells 214 and 202, 61 D and 189 d after injection, respectively. The vapour rich wells (i. e. 101, 105 and 109D) did not show a positive manifestation of tritium, even after six months of monitoring, thus analysis in these wells was terminated after December 2006 (Fig. 69).

Tritium and NDS returns in wells 2R3D and 2R4D exhibit sharp breakthroughs at the start and gradually tapered off over time. This is opposed to the relatively semi-broad nature of the graphs for wells 214 and 202. Figure 70 shows the processed curve for well 2R3D for both tritium and NDS tracers. The plots show that there are two pulses of tritium and NDS that entered the well.

Well 214 showed two pulses of HTO breakthrough, while only one pulse was detected for NDS (Fig. 71).

Reduction of HTO and NDS curves in well 202 showed only one pulse for both (Fig. 72). Further, the recovery yield for both NDS and tritium were different. Using the Anduril software, the tracer recovery for tritium was 0.4% for well 2R3D, while wells W214 and W202 both gave yields of 0.1%. NDS recovery, on the other hand, was 1.3% for well 2R3D, 0.2% for well 214 and 0.1% for well 202. On the basis of these results, NDS recovery was higher by almost 100% for wells 2R3D and 214, while recovery was the same for well 202, the well furthest from the injector.

It is possible to enrich HTO in water by electrolysis of the water to produce hydrogen and oxygen gases. In this process, water containing the lighter hydrogen atoms 1H and 2H are preferentially removed from the water, leaving an electrolyte which is increasingly enriched in HTO. Starting with a volume of 250 mL, it is possible to reach an enrichment factor of 25-35, depending on the equipment and procedure used. However, in all cases where HTO is added artificially as a tracer to water, there is not much gain in this enrichment technique because the natural HTO content is increased, as well as the background. Thus, the quality of the analysis is not improved even though the number of counts increases. Sample enrichment is indispensable when analysing natural samples that have low contents of tritium, however.

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 an efficient volumetric sweep and thereby a good oil displacement by the injected water.

Natural production mechanisms, or primary production, contribute to extraction from the reservoir of 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 to 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 termed secondary recovery, contributes to the extraction of 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 nowadays a common practice to begin the exploitation of new wells with fluid injection as a way to optimize oil recovery. For this reason, the name secondary recovery is being replaced by the more general term enhanced oil recovery.

For oil reservoirs, interwell tracer data are important in order to optimize the production strategy (injection balance) in the reservoir and thereby maximize the oil recovery. In geothermal reservoirs, interwell tracer tests are used to improve the understanding of reservoir geology and to optimize production and re-injection programmes and thereby enthalpy production from the reservoir. During the last 10-15 years there has been substantial progress on tracer technology development. This has resulted in improved basic knowledge and new technology.

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

— Detection of high permeability channels, barriers and fractures;

— Detection of communications between layers;

— Evaluation of the fraction of the injection water reaching each production

well;

— Determination of residence time distributions;

— Indication of different stratifications in the same layer;

— Determination of preferential flow directions in the reservoir;

— Determination of swept volume of the reservoir.

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

Tracer technology is a powerful tool for tracing the movement of the injected fluid through the oil reservoir, monitoring reservoir performance, investigating unexpected anomalies in flow and verifying suspected geological barriers or flow channels. Generally, the injected fluid is labelled with tracer (radioactive or non-radioactive) and the produced fluid from the well(s) of interest is sampled and analysed to determine the tracer response curve. The analysis of tracer response curves can provide important information about the character of the reservoir and makes it possible to optimize the injection regime and improve production strategy.

Such information can be used to evaluate flood performance, optimize the balance between injection and production rates, help make decisions on infill drilling and enhanced oil recovery programmes and improve the accuracy of the reservoir model.

In industrialized countries, tracers have been used to measure fluid flow in reservoirs for several decades [1-4]. A summary of theIR earlier use (before 1990) from the perspective of tracer behavior is provided by Bjornstad [5]. There are some success stories and some reports on experiments, which have largely failed. The reason for failures is mainly due to insufficient knowledge of tracer behaviour under changing reservoir conditions.

Knowledge of tracer behaviour is gained through dedicated laboratory investigations, through the above-mentioned oil field experience, groundwater movement investigations, atmospheric tracing experiments and also, to a significant degree, through the work carried out on the migration of radioactive species in soil for the purpose of evaluating radioactive waste repository sites.

Although the integrated knowledge from these areas is substantial, the information obtained is not always consistent. Results from one area of investigation cannot readily be transferred to new fields because of both scaling problems and changing experimental conditions. During the last 20 years there have been substantial programmes on tracer technology development in a few R&D laboratories in Europe and North America. This has resulted in new basic knowledge and new technology.

During the past few years, a number of ‘traditional’ radioactive and non­radioactive water tracers have been re-examined along the lines described above, and the search for new possible tracer compounds is ongoing. Ultra-low detection limits are required. Among the non-radioactive compounds, the fluorinated aromatic acids have attracted special attention because of their success in tracing groundwater flow. A comprehensive quantity of information has been generated with respect to their thermal stability and reservoir flow behaviour in dynamic laboratory experiments under simulated reservoir conditions. Some of the compounds passed through the quality checks in good shape. Others show instability or other unwanted properties which excludes them from use in reservoirs, at least under certain specific reservoir conditions. The compounds with sufficiently ‘good marks’ from laboratory experiments were extensively tested in full field experiments in the early 1990s.

A selection of the non-radioactive polyfluorinated benzoic acids were established as industry standards for tracing water flow in oil reservoirs more than 10 years ago and details have been published in open literature [6]. Currently, the continued development has resulted in new families of non­radioactive tracers qualified for oil reservoir water tracing. However, the identity of these compounds is not revealed in the open literature. Individual compounds have certain limitations on their use and are not generally applicable. It is important to know these limitations in detail in order to apply them correctly.

Some information can be found in Refs [7] and [8], but most of the data remain unpublished as private confidential research reports.

3.1. TRACER STABILITY AND INTEGRITY

The quality of a new tracer candidate is examined by subjecting it to a test sequence where stability and associated properties are examined. Another sequence where detectability is examined and developed will only be represented here by an example where a method for analysis of radiolabelled [Co(CN)6]3- and SCN — in the same sample is employed.

3.1.1. Thermal degradation

The thermal stability of tracers is typically tested in batch experiments where solution aliquots of the actual tracer candidate are heat sealed in individual glass cylinders and exposed to different temperatures for different time periods. HTO is always added in a known quantity to act as a standard reference tracer. The experiments may be carried out under aerobic or anaerobic conditions. Water of different quality may be used, ranging from distilled deoxygenized water to seawater and various formation waters.

Samples are analysed with respect to the remaining original tracer concentration as a function of time at the different temperatures. The analysis is carried out by liquid scintillation counting or gamma spectrometry if the radionuclide permits.

If RTo is the volume specific count rate (cps/mL) of the tracer in the original vials before the start of the experiment (t = 0) at temperature T, and RTt is the volume specific count rate of the tracer after time t, the surviving fraction, Y, is found by the simple expression:

One example of a Y-plot is shown in Fig. 34. For an ideal water tracer under test conditions, Y should stay at ~100%. In the example given, a measurable degradation of S14CN — at 120°C in seawater occurs over time.

" MATRIX: Seawater

110 и——————- .——————- .————

Contact time at 120°C (days)

11 Seawater

The results from the analysis of 14C-MeOH are not consistent with nominal values in most cases (Table 23). Some of the participants reported that they could not find any 14C-MeOH in any of the samples while others reported a low concentration of 14C-MeOH in comparison with the nominal value for some samples. Some laboratories did not apply the enrichment process (distillation) to measure low concentrations of 14C-MeOH while others attempted to distill samples but still found zero or low concentrations.

The results reported from Vietnam are acceptable. Measurements were carried out soon after receipt of the samples, on 21 January 2008. Upon receipt of various comments from participating laboratories regarding the difficulties of measuring 14C-MeOH in the samples, CANTI’s tracer laboratory was requested to measure a stored reference set of samples (29 June 2008). The new analysis showed the same difficulty or problem previously observed and reported by the other laboratories. There was close to zero activity of 14C in the stored samples (Table 24).

Bq/L о S Й

іґ> и I І 5 о

-о

3-

FIG. 84. Radar plot representation of the datasets.

The moment analysis method was originally developed for closed reactor vessels [25, 26], but has been applied to the more general conditions of open boundaries [27] for characterization of fractured media under continuous tracer reinjection [28, 29] and for estimation of flow geometry [30, 31]. Certain restrictions are inherent in the calculation, for example, steady state conditions and conservative tracer behaviour are assumed a priori. Nevertheless, the method has a rigorous mathematical basis and has been extensively validated analytically and experimentally.

The governing equations used in moment analysis are based on knowledge of the residence time distribution of the tracer, defined as:

where C is the collected sample tracer concentration (ppm, ppb, Bq/L, etc.), and Qinj and Minj are the injection water flow rate (m3/d) and the injected tracer quantity (kg, g, Bq, etc.), respectively.

If the tracer experimental curve is not fully recorded (which is very frequently experienced because of limited the sample collection and analysis), it is important to employ some criteria for extrapolating.

The parameters of the experimental residence time distribution curve are calculated by the moment method. The nth moment of a residence time distribution curve is defined as:

Jtn ■ E(t) • dt

tn = (37)

J E (t) ■ dt

0

The zero moment (equivalent to the fractional cumulative recovery of the tracer) is:

■ dt (38)

The first moment (equivalent to the residence mean time) is:

J t ■ E(t) ■ dt

t* = ——————- (39)

J E (t) ■ dt

0

IV. 2.1.2. Pore volume determination

Pore volume determination is based on the knowledge of the zero and the first moments. It is calculated from:

The calculated pore volume represents only the watered pore volume. The complementary volume occupied by the oil cannot be reached by a passive tracer.

IV. 2.1.3. Calculating flow geometry

It has been proposed that the flow and the storage (pore volume) geometry of the formation can be estimated directly from a tracer test [30, 31]. The cumulative flow capacity at any streamline ‘i’ of a formation (F) is the sum of the contribution of each streamline that has a velocity greater than the ‘i’ and is normalized by the ensemble properties. Darcy’s law gives:

X j

}=i J N kA

X Xj

J =1 J

The cumulative storage capacity of these streamlines (Фг) is simply the sum of their individual pore volumes:

XVp

F, = j——————————————————————————————— (42)

j=1

These can be estimated from a tracer test, where Фг — is the incremental first moment calculated at the time t and normalized by the true first moment:

0

The cumulative flow capacity is simply the cumulative tracer recovery at time t normalized by the complete recovery:

(44)

Flow and storage capacities are most often plotted in a (F Ф)-р1оі The shape of a (FФ)^Ы is useful as a diagnostic tool indicating what fraction of the pore volume contributes to what fraction of the fluid flow. The (F Ф)-p1ots are widely used in oil reservoir engineering. Figure 102 illustrates the (F Ф)^Ы showing experimental values compared with the case of uniform flow (parallel equidistant streamlines).

Radiotracers for interwell purposes are often purchased from a commercial company as an aqueous solution, sometimes as a dry salt. Whenever the radiotracer can be purchased in dry form this is preferred because it enhances the shelf life of the tracer due to reduced autoradiolysis.

The radiotracers are provided in suitable transport containers which depend on characteristics of the radioisotopes. In most of cases, the supplied radiotracers

are ready for injection, and transported to injection sites according to transportation regulations [21].

In the case of exclusively beta emitting tracers, for instance HTO and S14CN-, it is advantageous to add a small quantity of a short lived gamma emitter in the form of a water tracer, for instance 131I-, for easy monitoring of the injection process during field operation.

Figure 5 shows an example of container for transportation and flow through injection of beta emitting tracers. It has a volume of 100 mL and is rated to a pressure of 350 bar.

The container in its plastic and lead support is mounted inside a transportation drum lined with shock absorbant material, as shown in Fig. 6. The transportation drum is labelled with the correct radioactivity transportation index and information about the type and quantity of the radionuclide according to the national regulations.