III.6.1. Analysis by chemical treatment of water sample for detection with liquid scintillation counter

There are several procedures for sample treatment, one of which is described below. The flow chart of this analytical procedure is shown in Fig. 93.

(1) The samples (2 L) are delivered in plastic bottles and are weighed to determine their volumes. A known quantity of inactive I — (5 mg) is added to act as a 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).

(2) The I — is then oxidized to IO — with KMnO4 and allowed to stand for about 20 min. At the same time, any sulphide present (S2-, HS- or H2S, which would form an Ag2S precipitate in competition with Agl) is oxidized to SO4-. A longer time might be used if organic matter is present or if there is a high sulphide concentration.

(3) The IO — (including the carrier) is then reduced back to I — by addition of an acid mixture (HNO3 and HF) followed by Na2SO3 solution. The HF is included to inhibit formation of silica (Si(OH)x), which would clog filters and interfere with the weight of the final precipitate. The SO4- is unaffected by this step, thus effectively removing sulphide interference. After standing, the solution is filtered to remove any traces of Si(OH)x which might have formed.

(4) 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.

(5) 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 and then immersing the vial in an ultrasonic bath to disperse the AgI into the cocktail. The AgI dissolves to form the silver complex Ag[SC(NH2)2]+. The paper is translucent and should be left in the vial (20 mL). The precipitates are dried and weighed.

(6) 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.

In order to qualify a water tracer on the basis of dynamic behaviour as passive (‘very good’) or near-passive (‘good’), the type of job the tracer is expected to fulfil needs to be carefully defined. If the job is to measure fluid communication exclusively, a near-passive tracer may work as well as a true passive tracer.

Non-charged tracer species: A true passive water tracer is one that mimics all movements and interactions that the water molecules undergo in the traced water volume. In practice, the only radioactive compound that fulfils this requirement is radioactive water (HTO). Movements can, for instance, be the free movement in and out of dead-end pores insensitive to columbic forces set up by negatively charged rock surfaces. Interactions can include exchange with connate water in the rock pores or exchange with crystal water molecules. Thus, it can sometimes be observed that HTO seems to lag behind the injection water breakthrough as measured, for instance, by salt balance (ionic logging) or that the HTO production profile is somewhat more skewed. In the literature this has been incorrectly interpreted to HTO instablility under reservoir conditions, and that it may be subject to isotope exchange reactions of tritium with hydrogen in neighbouring hydrogen-containing compounds, some of which are stationary.

Other non-charged radiotracers include, for instance, tritiated methanol (CH2TOH) and the other radiolabelled light alcohols. These will behave qualitatively similar to HTO with respect to the diffusive and convective parts, but differ as regards interactions.

Anionic tracers: Of electrically charged tracers, anions represent the more applicable ones. In laboratory experiments, however, ion exclusion is observed, i. e. negatively charged species tend to be repelled from the negatively charged rock surfaces. As a result, the tracers tend to flow in the middle of the fluid-conducting pores. They will not easily enter into dead-end pores or through narrow pore throats. This results in a somewhat smaller available pore volume for anions than for non-charged species. In laboratory experiments, the production profile differs in reproducible ways from that of HTO, but in full-sized field experiments this difference is not that obvious.

Anionic tracers are represented by S14CN-. A typical production profile is given in Fig. 3. This profile is compared with the production profile of the simultaneously injected HTO. The difference in shape is enhanced by subtracting the normalized HTO profile from the normalized S14CN- profile. The result is given in Fig. 4. It is evident from the curve that the breakthrough of HTO precedes that of S14CN — and that the tail of the HTO profile is more pronounced. This profile difference is qualitatively reproduced for all near-passive anionic water tracers and illustrates the phenomenon which has become known as anionic exclusion. The flow rate is increased, for instance, by a factor of 10 (i. e. to 200 cm/d), the first down-dip in the normalized difference curve disappears and the breakthrough of the anionic tracer precedes that of the reference tracer, HTO.

On the basis of such curves, retention factors may be derived from Eq. (4) and the production profiles found by such experiments.

1 + P = Vt/Vs (4)

where

P is the retention factor;

VT is the retention volume for the tracer candidate;

VS is the retention volume for the standard reference tracer.

The retention volume may be represented by the peak maximum value or the mass mid-point (first moment (p1)) for non-symmetric profiles. These values are best found by fitting the profile with an analytical function consisting of polynomials.

For monovalent anions, the retention factors are in the range 0.0 to -0.03, indicating that such tracers pass faster through the reservoir rock than the water itself (represented by HTO).

Some anionic tracers may show complex behaviour. Radioactive iodine (125I- and 131I) breaks through before water but has a substantially longer tail than HTO. Both reversible sorption and ion exclusion seem to play a role here.

Cationic tracers: These are in general not applicable. However, experiments have qualified 22Na+ as an applicable water tracer in saline (greater than sea water salinity) waters. In such waters, the non-radioactive 23Na+ will operate as a molecular carrier for the tracer molecule. The retention factor has been measured in the range of P « 0.07 at reservoir conditions in carbonate rock (chalk) [10]. Accordingly, the tracer is somewhat delayed by sorption on, and ion exchange with, the reservoir rock, but in a reversible fashion.

In the literature there is also a report on the successful use of 134Cs+ and 137Cs+ in a carbonate reservoir [11]. This tracer cannot, however, be generally used as it will adsorb strongly (and irreversibly under ordinary reservoir conditions) on clay-containing rocks. There is also the reported use of other cationic species such as 60Co3+ and other cobalt isotopes, but these compounds have never been produced back.

Experiments are normally conducted with a typical linear flow rate of 25 cm/d, which corresponds to the rate of the injected flow front in the reservoir.

The most frequently used testing method utilizes pulse injection into the core. The tracer candidate is always co-injected with a standard reference tracer. For water, this is normally HTO. The production profiles of the tracer candidate and the standard reference tracer may be directly compared. Examples of such profiles are given in Fig. 3. Figure 4 shows the difference curve of the normalized production profiles.

Another method that may be used is continuous injection of a constant concentration tracer solution. Figure 43 show results from such a test at ambient temperature where In3+ complexed with EDTA is compared with the behaviour of HTO. The curves show evidence of a slight reversible sorption of In-EDTA under the running conditions.

When a tracer candidate has also passed the dynamic tests in ‘good shape’, the next, and final, step is to perform a pilot test in an actual reservoir section. An

1. Electric heating elements

2. Thermally insulated column housing

3. Thermo elements

4. Air circulation pump

5. Balance

6. Column elution liquid

7. High-pressure pump

8. Piston cylinder

9. Six-port valve

10. Constant initial trac er mixture volume

11. Syringe for filing of tracer volume

12. Computer for logging

13. Differential pressure gauge

14. Collected liquid samples

15. Circulating ho t air

16. Pressure equalizer

17. Back-pressure regulator

18. Sample changer

19. Nitrogen reservoir for dome pressure

20. Chromatographic column with crushed reservoir material

FIG 42. Flow rig for dynamic tracer testing of water tracers in columns of crushed reservoir rock or reservoir-like material under simulated reservoir conditions. The rig contains a 200 cm long thermostatized chromatographic column.

acceptable tracer performance may finally be regarded as proven after several successful experiments under reservoir conditions.

Use of radioactive material has to be approved by a competent authority in the country concerned. One of the main principles governing the utilization of radioactive material is that it shall not pose danger to persons, neither those working with the materials nor the general public. In addition, the radiation doses

to which workers and the public are exposed should be within the limits set by laws and regulations.

Another principle is that use of radioactive material will be accepted only when the benefit is considerable, after having taken economic as well as health and environmental aspects into consideration.

A third principle is that when using radioactive material every effort should be made to keep the quantity of radioactivity to be used and the resulting radiation doses ALARA.

What is the work space?

PORO TracerSim allows use of a work space, that is, the field in which the wells and faults are inserted. Its definition is simple, only the coordinate limits of the area of interest need be entered in the text boxes.

While working, the work area may be modified as required.

Another way to modify the work space is by clicking the Setup menu and then on the calculation and graph parameters, or directly by clicking the following icon.

• Well flow rate (without +/-, the program set depends on whether the well is PRODUCER or INJECTOR);

• Position in X;

• Position in Y

It is not necessary to enter the X and Y positions with the keyboard, the mouse can be used anywhere in the field or work space and the values in the dialog box will change automatically. The coordinates of the mouse pointer in the position indicator can be followed.

Once writing values, click on the Accept button, and the element will be inserted. In the field or work space an image appears in the corresponding place that allows selecting whether the well is a PRODUCTOR or an INJECTOR:

How to delete a well?

Right-click the corresponding image of the well to be deleted and then the menu appears by which the selected well can be eliminated.

How to amend the wells inserted?

To modify a well already inserted, click on the image of the well. The same table which was used to insert the well appears. This enables the values to be corrected, and then press Accept.

How are the faults inserted, modified and removed?

Click on the button Faults.’A list of inserted faults is now shown in the field or work space. By clicking on the New Fault button the following box appears.

Faults are inserted in a manner similar to the wells by writing the positions of two points that define the line of the fault in the following box. To use the mouse instead just click on Position X in the text box of POINT 1 and then click on the position of the field or work space. The values for X and Y will be written automatically.

To insert POINT 2, click on the Position X text box of POINT 2, and then perform the same operation as for the first point. The values can be changed at any time, until the right position is found. In the field or work space, the following image appears:

To remove an already inserted fault, click on the corresponding fault in the list of faults and then on the Eliminate button in the same box.

To modify an already inserted fault, double-click on the corresponding fault in the list of faults, or select with a single click the fault, and click on the Modify button. A box appears similar to the one which appears when a new fault is being inserted. Only the values of the X and Y coordinates of both points defining the line of the fault should be modified. The mouse should be used in the same way as that for inserting a new fault.

(i) Direct method

In the oilfield, the simplest form of manual sampling is performed by using a plastic bottle. The bottle is connected to a bleed valve which is mounted on the production line. Further, the bottle should be connected to a gas treatment system (flaring, venting) that takes care of any associated gas. A schematic of the system is shown in Fig. 17. By careful opening of the valve, the liquids are bled off from the production pipeline into the bottle. A mixture of oil and water is normally collected by this method (Fig. 18).

(ii) Well head sampling method

Another somewhat more sophisticated manual sampling method involves the use of a phase separator that is connected to the well head (Fig. 19). The sampling procedure is as follows:

• The separator equipment is connected to the well head.

• The valve at the well head is opened and a mixture of oil and water flows through the main pipeline. A bypass is led into the separator.

• The oil phase is located in the upper part of the separator and is from there continuously transported back to the main pipeline.

• The water phase accumulates in the bottom of the separator.

• Water samples can be collected continuously from this equipment by opening the valve at the bottom of the separator.

I.4.1. Tritium tracer test in Lahendong geothermal field

The Lahendong geothermal field is located in North Sulawesi province at an elevation of 800-1100 m above mean sea level and is producing about 20 MW(e) of electricity annually.

Tritium tracer with an activity of 629 GBq (17 Ci) was injected into LHD-7 injection well in July 2006. Monitoring of tritium tracer has been done in several production wells surrounding LHD-7. The tracer monitoring in production wells LHD-8, LHD-11, LHD-12 and LHD-15 was done periodically every two weeks during the first three months after injection, and then periodically every month.

Tritium in water samples was analysed using a Packard-TR 1900 liquid scintillation counter. The direct counting method was used and samples were distilled before counting. A 10 mL sample of distilled water and 11 mL of Instagel cocktail (Ultima Gold) were added to the counting vial and homogenized by shaking. The counting time for each sample was 20 min and the counting rate was converted to tritium units.

I.4.2. Results and discussion

Table 15 shows the result of tritium tracer in Lahendong geothermal field, whereas Fig. 65 shows the plot of time versus tracer concentration curve.

111.1.7.1. Safety measures

Check that water hoses are available and connected to the supply. Check

that the operators have the required personal safety equipment and that the

injection site is roped off.

111.1.7.2. Injection procedure

(1) Starting with all outlets from the module plugged and all valves closed, then start the pump and bring it up to the injection line pressure. Open valves 1, B and 2.

(2) Open valve D slightly, close it and read the manometer pressures.

(3) Open valves E and F. Then open valve D slightly to admit pressure to the tube section between valves F and H. Close valve D and check for leakages.

(4) Break the wire seal on the tracer bottle and carefully open valve G while looking for possible leakage at the ends of the bottle. If leakage occurs, close valve G immediately and then close all other valves.

(5) If there are no leaks, open valve H and the tracer solution will be pressed into the injection pipe.

(6) Check by means of the gamma monitoring equipment that the tracer is transferred to the water injection line.

(7) After 30 min a sample should be collected from the injection water that has been diverted through the tracer bottle. The sample should be checked for tracer by means of the portable liquid scintillation counter.

(8) If the samples show that the tracer concentration in the water which has passed through the injection module is sufficiently low, the injection can be stopped and all valves closed. However, the injection should last for at least one hour before it is terminated.

(9) Disconnect the injection module and check for radioactivity at the outlet/inlet opening on the injection line.

(10) The injection module can now be decontaminated and prepared for another injection.

III.1.6. Decontamination

Personnel from the tracer group will clean all equipment and, if necessary, all areas that have been contaminated by radioactive material. Sweep tests will be performed in order to ensure successful decontamination. Equipment that cannot be decontaminated at the platform will be packed according to the rules and sent to the institute for further cleaning or storage.

The radioactive and non-radioactive tracer injections that are described in this report will not have environmental consequences.

When transport, storage and handling of the tracers are carried out according to laws, rules and regulations, the tracer injections can be performed in a safe manner. Under normal circumstances there will be practically no radiation exposure to the operators or to the general public.

When tracers are used to analyse industrial processes, it is common to express the tracer concentration in the system output in terms of elapsed time, and then to calculate the mean residence time, variance and other parameters related to the time response.

Although the time response is generally used in interwell studies it has some problems that reduce its usefulness. Effectively, alterations in the pattern, such as variations in the injection rate, which are very common in any oilfield, result in a biased response curve.

To avoid this inconvenience, a good alternative is to express the tracer concentration as a function of the cumulative injected or produced water volume, which is rate independent. Nevertheless, time representation is often preferred because in many cases the volumetric data are not available.

Figure 29 shows time (a) and volumetric (b) responses for the same well. It is obvious that they are nearly identical since the cumulative volume was calculated from the injection flow rate, which in this case was quite stable.

The volumetric response in a production well when an instantaneous tracer injection has been performed is a measure of the pore volume swept by the injected water.

The tracer breakthrough is sometimes used as an indicator of the swept pore volume, but the mean of the distribution is a better locator because it represents the average volume swept by the injected water and takes into account the shortest as well as the longest paths followed by the tracer. Then:

C(V) dV

о

The swept volume arriving at a given well is equal to the average volume swept multiplied by the fraction of the injected water that reaches this production

weH f).

Results from the various laboratories are summarized in Table 19. All datasets except the one from Argentina are internally relatively consistent and also relatively close to the nominal values.

Figure 81 compares the country results for each of the samples and Fig. 82 compares the results for each country with the nominal values of the tracer concentration in radar plots with a logarithmic concentration axis. In Fig. 82, a horizontal bar has been inserted showing a symmetric realistic 2ct error of ±5% around the nominal value. This value is derived mainly from a realistic assessment of the uncertainty in the accuracy of the secondary standard.

ю

-p-

FIG. 81. Bar plot of HTO laboratory tests for different participating countries compared with the nominal value of each sample (yellow bar). Width of bar represents uncertainty in nominal value (cont. on p. 127).

FIG 82. Radar plots of results for each country with the nominal values of the tracer concentration.

A few difficulties prevail:

• The high values reported from Argentina may possibly be explained by contamination of the samples. When the samples were received in Argentina, they were erroneously stored in a room with about 80 Ci of HTO for tracer injection. Samples were stored for about 14 d before being analysed.

• For higher HTO concentrations, China and Vietnam are consistently reporting values that are too low. This problem may be due to erroneous calibration at higher concentrations. If that is the case, the problem may be rectified by making a new calibration with new certified standards.

• Indonesia, although having reported the lowest background counting rate of all the laboratories, seems to have a problem with the low content samples of brine and produced formation water.

The round robin test conducted has been a useful exercise. The main conclusion from the test is that all laboratories seem to handle this kind of analysis in a satisfactory manner.