The procedure for 125I analysis using gamma spectrometry and a

multichannel analyser is as follows:

(1) Filter the brine sample (1 L) by using 0.45 pm pore size filter paper.

(2) Adjust to pH6-7 by adding HCL or Na2CO3.

(3) Prepare an anion exchanger column with diameter 5 mm and 100 mm length (various resins are available commercially, e. g. Dowex 1, BioRad 1). Mesh size can be 80-100, and cross-linking x8.

(4) Percolate the solution through the column at a slow rate (a few millilitres per minute).

(5) Seal the column at both ends.

(6) Count the column in a detector set-up (preferably a well-type HPGE detector) connected to the multichannel analyser multichannel analyser by the gamma-gamma (photon-photon) coincidence method, where the sum coincidence between the X rays and between the X ray and the gamma ray are measured.

A photon spectrum from the use of the photon-photon (sum) coincidence method is shown in Fig. 94.

Appendix IV

For unambiguous, single phase tracing of water in secondary or tertiary recovery schemes, the following practical tracer selection criteria apply to passive tracers:

• Insignificant degradation, reservoir and production conditions (i. e. high stability against thermal, chemical, physical and microbial degradation and a suitable half-life for radiotracers);

• Insignificant phase partitioning;

• Insignificant sorption on reservoir minerals;

• Insignificant natural occurrence in involved fluids (low background);

• Detectable in very low concentrations in reservoir fluid samples;

• Toxicity and radiotoxicity at acceptable levels;

• Non-problematic preparations, handling and logistics;

• Adequate commercial availability of components for preparation of tracer mixture;

• Acceptable cost.

For geothermal tracers, all of the above criteria apply except for the second item. For the tracing of H2O movement, in many reservoirs it will be advantageous to apply simultaneously selective tracers for the condensed water phase, selective tracers for the vapour phase and tracers for both phases.

Tables 1 and 2 list some water radiotracers which have been conditionally qualified for water tracing in oilfield operations and in geothermal operations, respectively, conditionally because each one has certain limitations to be observed.

Tables 3 and 4 list examples of corresponding non-radioactive tracers used in oilfields and geothermal fields, respectively.

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a L/2 (>400 d) here defined as the time needed at the indicated temperature to reduce the concentration by thermal breakdown to half of its original value. b Pyrene tetrasulphonate.

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For tracers composed of single atoms, such as 125Г, 22Na+, etc., tests are simpler than for tracer compounds based on molecular complexes. This is exemplified here with the tracer compound cobalthexacyanide ([Co(CN)6]3-). This molecule may be labelled with 56Co, 57Co, 58Co, 60Co or 14C, and it may be used as the unlabelled complex. The latter requires a sensitive analytical method for Co, such as thermal neutron activation analysis. One single molecular carrier may then give rise to six different tracers. The total complex constant of this molecule (P6) is reported to be very high (1038-1064) [9]. This may wrongly be interpreted as indicating that the (CN)6 ligand molecule is very stable and that it will exist in this molecular form more or less regardless of the chemical environment. This argument has led to extensive and somewhat uncritical field use of radiolabelled versions of these molecules. In many cases, good results have been obtained, while in others the tracer has never been produced back. A thorough investigation of radiolabelled [Co(CN)6]3- has already been conducted and a few results of this study and some complementary experiments are discussed.

The 60Co labelled hexacyanide was purchased from one of the largest commercial producers of radiochemicals as a ready-to-inject solution. One of the quality control methods used is the electrophoresis technique. Batches purchased at different times showed different results, indicating a radiochemically impure product. A new synthesis was carried out using procedures provided by the company. Results from this experiment are shown in Fig. 44. The electrochromatogram shows a relatively broad distribution with two distinct peaks. This indicates that the 60Co label exists in, at least, two different anionic forms. These forms are not identified; they may have different stability and chemical behaviours. The compound was then synthesized by a modified method. The modified procedure produced chromatograms similar to that shown in Fig. 45.

Cobalthexacyanide from the commercial company was then subject to thermal stability and sorption investigations. There was fast sorption onto corroded steels already at ambient temperatures, but the sorption became even more pronounced at elevated temperatures. Liquid solutions, after heating to 120°C for 24 h, were again investigated by electrophoresis. The results are shown in Figs 46 and 47.

ID 2D ЗО 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

POSITION (mm)

A substantial fraction is non-charged and does not move away from the application point. On the positive potential side is a relatively low and broad, nearly constant, distribution indicating different 60Co labelled anionic forms. On the negative potential side is a substantial and broad distribution indicating various positively charged complexes where heating leads to breakdown of the

POSITION (mm)

hexacyanide complex into a range of different complexes with varying masses and charges.

The CN ligands may be exchanged to some degree with Cl-, OH — or even H2O to saturate the coordination number of 6. This leads to complexes of different charges and different chemical properties. The referred experimental investigations showed that the quality control on the radiochemical purity is very
important, and the purity of [Co(CN)6]3 should be ensured at the start of any field application. This tracer compound should not be used at reservoir temperatures >90°C.

In March 2002, the IAEA’s Board of Governors approved a Safety Requirements publication entitled Preparedness and Response for a Nuclear or Radiological Emergency [22]. This publication established the requirements for achieving an adequate level of preparedness and response for a nuclear or radiological emergency in any Member State. Amongst other things, the publication specifies requirements for emergencies involving a dangerous source. The Requirements define a dangerous source as one “that could, if not under control, give rise to exposure sufficient to cause severe deterministic effects”. The Requirements then go on to define a severe deterministic effect as one that “is fatal or life threatening or results in a permanent injury that decreases the quality of life”.

The operational definition of a dangerous source is known as the D value. The D value is that quantity of radioactive material, which, if uncontrolled, could result in the death of an exposed individual or a permanent injury that decreases that person’s quality of life.

For the purposes of determining D values, the exposure scenarios that were used fall into two groups: one for material that has not been dispersed and one for material that has been dispersed.

Different numerical values are provided for each of these groups:

• The D1 value is the activity of a radionuclide in a source that, if uncontrolled but not dispersed (i. e. it remains encapsulated), might result in an emergency that could reasonably be expected to cause severe deterministic health effects.

• The D2 value is the activity of a radionuclide in a source that if uncontrolled and dispersed might result in an emergency that could reasonably be expected to cause severe deterministic health effects.

• The D value is the lowest value of the Dj and D2 values for a radionuclide.

For pure beta emitters D1 values do not apply in this actual radiotracer application. For gamma tracers, both D1 and D2 values apply. The quantities (activity) of radionuclides typically used in interwell tracer examinations are evaluated against their recommended D values.

Click on the Setup menu and then in calculation and graph parameters, the following dialog box will appear.

This allows the line thicknesses of curves to be modified. As explained above, also from here, new limits for the work space can be set. Further, it allows modification of streamlines and colours of the daily fractional recovery curves.

For the latter, another practice option is available. By clicking on the coloured squares which are in the Wells button on the left of the window, this can be done in a more practical way.

IV. 2.6.1. Problem parameters

Here, the values of dispersivity and porosity must be entered. The injected mass of tracer and the time of injection must also be entered. The type of tracer to use can be either chemical or radioactive. There is another, more practical, way to amend the problem parameters, by clicking on the appropriate button.

made.

• Streamline calculation: Values required for defining the number of streamlines of the problem are the angle swept by them and the distance between two successive points on one line. It also requires the total number of points to be entered.

• Time answer: The range of times to be displayed in the daily fractional recovery curve is entered.

• Frequency set-up: These are values required by the program for the calculations. The method of calculation works in the frequency domain and these values are needed. By setting them, it is possible to find better solutions to the problem being considered.

How to calculate the daily fractional recovery curves?

Once all the elements (production wells, injection wells and faults) have been inserted, select a PRODUCTOR and an INJECTOR. A red square or circle around the element indicates that this element has been selected, as shown in the following image.

Note the difference between the production well (square) and the injection well (circle).

Verify that the parameters of the calculation are the correct ones and that the wells are not inserts on both sides of a fault (otherwise, incorrect values will be found on the other side of the fault).

Then click the Calculate and Graph button, to display a dialogue box that prompts whether to continue or not. Click Yes. The program will then begin to calculate the streamlines and the daily fractional recovery curves.

» PORO TracerSim 2 — [Ejemplo. poroJ

Files View Configuration Help

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t о Weis liCvv Daily fractional recovery curves Г" Concentratio

I Dady fractional recovery — Graph 1 Daily fractional

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Notice that two icons appear in the toolbar. One allows printing and the other copying the image displayed on the screen.

See the upper section of the frame.

Currently selected graph

Daily fractional recovery — Graph 1 Daily fractional recovery — Graph 2 Daily fractional recovery — Graph 3 Daily fr

Daily fractional recovery — Graph 1

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PORO TracerSim allows the creation of five different graphs. For example, if it is desired to calculate and show another daily fractional recovery curve of a different well, in another graph, then click on the Graph 2 button. Then return to the Wells button, and make the selection of the corresponding wells. Following a click on the Calculate and Graph button, a dialog box will be displayed that will give the option to continue or not. Click Yes. In this way, the program will begin to calculate the streamlines and the daily fractional recovery curves of the new well, and the result will be shown in Graph 2 (without erasing the other generated graphs).

To show a concentration graph, click in the Concentration output checkbox.

Concentration output

checkbox

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W [Concentration output Unit: І Кд/гпЗ

Graph 1 Daily fractional recovery ■ Graph 2 Daily fractional recovery

Note that PORO makes it possible to select and modify the unit of the output parameter. In this case, when the tracer is chemical, the unit is mass per unit volume. For a radioactive tracer, the unit will be in becquerel per unit volume.

Sampling is always a vital procedure for conducting interwell tracer tests and timely sampling is most important in ensuring that a test is successful. The automatic sampler shown in Fig. 20 is designed to be installed at the well head. It can automatically collect seven samples within a planned period, e. g. over one day, one week or two weeks. The automatic sampler consists of a separator (A), a control unit (B), a mini-pump (C), seven water sample containers (D1-D7) and electromagnetic valves (V1-V12) (see Fig. 20). In many cases, the electromagnetic valves should be substituted with pneumatic valves due to the probability of gas leakage and the danger of an explosion caused by the electronic circuits.

The functioning of the automatic sampler is as follows:

(1) The multiphase flow enters through valve V1 into the separator.

(2) The produced fluid will charge the separator A and the oil and water separate continuously by gravity.

(3) The separated ‘oil’ phase on top of the separator is pumped back to the bypass pipeline through valve V2 when keeping valve V3 (drainage) and valve V4 closed.

(4) At the end of a set collection time, ‘old’ water from previous samplings is removed from the manifold by drainage through valve V5.

(5) The fresh ‘water’ sample from the lower phase inside the separator is transferred to the sample container (D1) by gravity by opening valve V5.

(6) After sampling, the remaining water in the separator is pumped back to the bypass pipeline by opening valve V4.

(7) Sequences 2-6 is then repeated for the remaining sampling bottles.

(8) Valve V3 is used for pressure release and drainage when replacing the separator equipment. Oil samples can be taken manually through valve V3 as needed.

Dieng geothermal field is located in central Java. It has more than 15 production wells and 3 injection wells. It produces about 60 MW(e) of electricity annually. In August 2007, 5.6 Ci of 125I tracer was injected in the well HCE-29. The same injection method as tritium injection in Lahendong was applied to this field. The sampling and subsequent measurements were carried out for a period of 135 d after injection.

Analysis of 125I

Isotope 125I is a low energy gamma emitter (35 keV) with a half-life of 60 d. On the basis of its half-life, 125I is a suitable radiotracer for short to mid-term reinjection into a geothermal field. However, the low energy characteristic makes 125I difficult to detect directly in the field. Pretreatment of sample and use of a

sensitive detector, i. e. liquid scintillation counter, are required in order to analyse this tracer activity.

The procedure used to analyse 125I is as follows:

• The samples (2 L) are delivered in plastic bottles and are weighed to determine their volumes accurately. A known quantity of inactive iodide (5 mg) is added to act as carrier, as well as to ensure that the final precipitate is of sufficient mass (about 10 mg) to be reliably filtered and weighed. The samples are then filtered if inspection reveals any debris or cloudiness, and NaOH is added to make the samples slightly alkaline (~pH9).

• The iodide is then oxidized to iodate with KMnO4 and allowed to stand for about 20 min. At the same time, any sulphide present (which would form Ag2S precipitate in competition with AgI) is oxidized to sulphate. A longer standing time might be used if organic matter is present or if there is a high sulphide concentration.

• The iodate (including the carrier) is then reduced back to iodide by adding an acid mixture (HNO3 and HF) followed by Na2SO3 solution. The HF is included to inhibit formation of silica, which would clog filters and interfere with the weight of the final precipitate. Sulphate is unaffected by this step, thus effectively removing sulphide interference. After standing, the solution is filtered to remove any traces of silica which might have formed.

• An excess of AgNO3 solution is added soon after the filtration to form a precipitate of AgI. Because AgI is much less soluble than AgCl, it is precipitated preferentially despite the approximately thousand-fold excess of chloride ions. However, small quantities of AgCl (and AgBr) are formed. After standing in the dark, the precipitate is filtered through cellulose acetate paper under vacuum. The AgCl and AgBr are then removed by washing with ammonia. The precipitate, now pure AgI, is then passed quickly through a further oxidation-reduction cycle for purification purposes before being dried and weighed.

• The precipitate is then dissolved in the liquid scintillation cocktail. This is done by inserting the rolled filter paper into the cocktail vial, adding about 20 mg of acidified thiourea to complex the AgI, and then immersing the vial in an ultrasonic bath to disperse the AgI into the cocktail. The paper is translucent and should be left in the vial (20 mL). The precipitates are dried and weighed.

• The analytical yield is calculated by dividing the mass of iodide in the AgI precipitate by the quantity of iodide added plus that known from prior analysis to be in the sample, typically 0.1-0.2 mg/L.

Table 16 presents the sample counting rates obtained at different production wells. It shows that during 135 d of monitoring, the 125I tracer appears at wells HCE-7A, HCE-7B, HCE-7C, HCE-9B and HCE-28A. Figure 66 shows the tracer experimental response curve obtained at the production well HCE-28A.

III.2.1. General

HTO or 1H3HO is a commonly used radiotracer in many industrial applications, particularly in interwell communication studies (water flooding) during enhanced oil recovery operations in oilfields and various investigations in geothermal fields. Tritium is a radioactive isotope of hydrogen, which decays by emission of very low energy beta particles (Emax = 18.6 keV) and has a half-life of 12.3 y). As tritium emits a very low energy beta particle, it cannot be measured on-site or on-line. The samples are required to be taken to the laboratory for measurement by liquid scintillation counter. A liquid scintillator is added to the sample vial, which acts as a detector for beta particles. When beta particles interact with the scintillator, it emits scintillation photons, which are, in turn, detected by two PMTs placed around the sample vial. The unit of tritium commonly used in hydrology is the tritium unit. One tritium unit represents the ratio of tritium in common hydrogen atoms as: 1 tritium unit = 10-18 [3H]/[1H].

The unit of tritium measurement commonly used in industrial applications, including the oil industry, is becquerel per kilogram (Bq/kg) or becquerel per litre (Bq/L). One becquerel is equal to one disintegration per second. One tritium unit is equal to 0.11919 Bq/L.

Permeability is a property of a porous material and a measure of its capacity to transmit a fluid. Permeability is largely dependent on the size and shape of the pores in the substance and, in granular materials such as sedimentary rocks, on the size, shape and packing arrangement of the grains.

In general, permeability is evaluated in the laboratory by analysing samples taken from the oilfield, but the results obtained by this technique are subject to considerable uncertainty. Firstly, the core samples are small and limited to sectors around the wells. There is always a question about the representativeness of the sample for the reservoir. However, the use of interwell tracers allows average values of the permeability in the swept volume between wells to be derived.

On the basis of Darcy’s law and many simplifications, a simple formula for permeability evaluation can be developed:

к-тіг(->(7) (l3)

where

K is the permeability;

ф is the porosity;

Sw is the water saturation;

m is the viscosity;

r is the the radius of the production well;

d is the the distance between the injection well and the production well;

AP is the differential pressure between wells; t is the mean residence time.

Although this expression supposes a number of simplifications, it constitutes an acceptable approach from the experimental point of view. The main use is to derive comparative values related to the permeability of different layers in the same pattern or of several stratifications in a unique layer.

2.4.2. Interpretation

Interpretation of the response curves obtained in production wells is the final objective of an interwell study. The tracer method gives correct and comprehensive information about the reservoir’s hydrodynamic parameters, allowing the reservoir engineers to understand better the phenomena and probably to increase the recovery. Four levels of complexity are generally accepted:

(1) Qualitative: Important information can be obtained just by looking at the response curves or by means of simple calculations. Breakthrough and mean residence times, distribution of injected water, recovered tracer mass or activity and swept volume are among these parameters.

(2) Basic models and software: Decomposition of complex curves into simple ones easy to approximate by elemental functions, moment determination and evaluation of statistical parameters, simple calculations and the fitting of experimental data. Anduril software (developed by Argentina) fulfils these operations and is used for simple analysis of tracer response curves.

(3) Streamline models: The volume under study is divided into a quasi-two­dimensional grid in small cells. Assigning to each one certain properties (pressure, permeability, porosity), streamline pictures are generated by solving the pressure equations. By this method it is possible to fit the experimental data in order to obtain structural information from the reservoir.

(4) Reservoir simulators: Generally, these comprise commercial and expensive software with capabilities to simulate reservoir behaviour under different conditions. Some of them have a rather basic ‘tracer’ option to evaluate the application of water tracers.

Analysis of the response curves consists of several steps:

(1) The simpler interpretation is the qualitative one. Just by observing the curves, the following pattern characteristics can be derived: injection water arrival time (breakthrough); high permeability channels, barriers and fractures between both wells; communications between different layers; stratifications in the same layer and preferential flow directions in the reservoir. This interpretation level is completed by means of some simple calculations from the numerical response, firstly, the determination of the mean residence time. The cumulative response can be obtained by integration of the concentration versus time curve, assuming the production flow rate is known. From this new curve, the fraction of injection water reaching each producer is easily calculated. A standard spreadsheet is the best way to make all these calculations.

(2) A second level involves the use of basic mathematical models to fit simple response curves by means of theoretical expressions and to decompose complex responses in several simpler functions. In 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 patterns.

(3) Finally, it is possible to make use of complex mathematical models such as numerical simulators in order to achieve a more rigorous analysis. Such tracer simulators may be coupled to full field reservoir simulators where the current reservoir model is used as input (geology, stratification, etc.). This is especially useful when the well pattern is complex, the reservoir heavily faulted and there is a complex production strategy.

(4) Reservoir simulators with a tracer option are powerful tools for determining the parameters of systems under study, for planning infill well drilling and for future tracer examinations. Well-known reservoir simulators such as ECLIPSE and VIP both have relatively simple tracer options which may be used for passive water tracers, while it is probable that the simulators from Computer Modelling Group in Calgary, Canada, represented by STARS, have the most advanced tracer simulator included. This can also be used for reversibly sorbing and phase partitioning tracers.

11.2.1. Sample preparation

Samples were prepared by the Tracer Laboratory of the Centre for Applications of Nuclear Techniques in Industry (CANTI), Dalat, Vietnam, by adding various quantities of HTO and 14C-MeOH standard solutions into injection water (Table 20) to get the concentrations of HTO and 14C-MeOH in samples ranging from zero to a few hundreds of becquerels per litre and from zero to 74 Bq/L, respectively.

Samples were prepared in 10 sets. Each set contained 15 samples stored in polyethylene bottles. Seven sets were sent to seven laboratories on 28 January 2008. One set in a glass bottle was left at the Tracer Laboratory (CANTI) for analysis and the rest were stored in CANTI for further reference (preserved samples). Instructions for the analysis of HTO and 14C-MeOH in mixture in brine by distillation were prepared by CANTI and sent by email to participating laboratories on 19 February 2008.