111.1. PREPARATION FOR RADIATION SAFETY OF RADIOTRACER TESTS

111.1.1. Introduction and general comments

Tracer techniques have wide application in petroleum exploration and production. For several decades, well-to-well tests have been used to study the movement of injected fluids through reservoirs. When properly carried out, such investigations can render information on the reservoir structure and fluid flow behaviour that could not have been obtained by other means. HTO is regarded as the standard reference tracer for water. With regard to safety, HTO is the most favourable radioactive water tracer generally available. However, there are other applicable radiolabelled water tracers, some of which are labelled with pure beta emitters such as 14C and 35S and some with both beta and gamma emitters such as 58Co, 60Co and 131I, while others emit only gamma radiation (electron capture decay) such as 57Co and 125I.

This appendix attempts to cover both the more general parts of the safety consideration as well as aspects that are nuclide specific for the individual radionuclide. In order to avoid an overly complicated treatment, tritium is considered to represent the beta radiolabels and 60Co to represent the gamma emitting radiolabels.

Radioactive tracer injection can be carried out in a safe manner with little impact on the environment. A typical safety report has to be prepared for the approval of the radiation protection authority and for the acceptance of end user management. The report contains:

• The design of the tracer experiment (calculation of tracer quantities, description of practical procedures for tracer preparation, transport and injection, sampling procedures, etc.);

• The description of safety precautions during handling and treatment of HTO during the whole operation.

To perform tracer injection in a safe manner, the injection equipment should have a sound design and meet pressure and temperature requirements. It should be properly tested before injection and operated by well-trained personnel. If these requirements are satisfied, an injection of radioactive and non-radioactive tracers can be carried out with very little risk.

Experience has shown that typically 370-3700 GBq (10-100 Ci) of HTO is injected into an injection well for interwell investigation in oilfields. A typical quantity for a 14C or 35S labelled tracer is 3.7-37 GBq (0.1-1 Ci) and the same for a gamma emitting tracer. A common design for an injection device for beta emitting tracers is presented in Fig. 7. This equipment can be operated in a bypass mode to the main injection line or by application of a high pressure pump. The pneumatically operated Maximator pump, which can be seen in Fig. 12, can deliver up to 10 L/min at a working pressure of 200 bar; lower at higher pressures. The pump is normally run at a rate of 5 L/min. The available pneumatic pressure for operation of the pump should be at 7-10 bar.

The PORO software is based on principles similar to those reported by Abbaszadeh-Dehghani and Brigham [35, 36], but includes no regular and unbalanced injection patterns, anisotropic permeabilities and faults effects. The strategy is based on the computation of the streamlines and the numerical evaluation of the convection diffusion equations on each stream tube. In this process, the time is converted to frequency, employing Fourier transforms. All the numerical computations are made using FORTRAN.

It is considered that, in a mature secondary recovery project, only water is flowing and that the stationary state has been reached. Therefore, if there is a horizontal, homogeneous and non-isotropic layer, then the Darcy velocity components are [39]:

where

where

is the well index; is the number of wells; are the spatial coordinates;

Equation (60) was obtained from the non-isotropic version of the Laplace equation:

where X and Y are the spatial coordinates related to the principal axis.

When sealing faults are present, additional ‘image wells’ are included in Eq. (60) for confining the flux. The streamlines are computed from the Eq. (60), by solving:

(61)

where Ф is the porosity. The boundary conditions at the beginning of each streamline are: and, at the end of each streamline:

rwp=Vc^^WP^+c^-rWP)7, *=i,2,-5nsi (63)

In Eq. (62),

is the starting angle of the streamline (i), Nsl is the number of streamlines, and xwI and ywI are the spatial coordinates of the injection well. In Eq. (63), rwP is the radius of the production well, and xwP and ywP are its spatial coordinates.

For illustrating the results, Fig. 103 shows the obtained streamlines of a five spot pattern for an isotropic, balanced case (above), non-isotropic balanced case (half) and sealing fault case (under).

IV2.3.1. Tracer transport

On each streamline, the tracer transport problem can be considered as one dimensional. Hence, a new spatial coordinate, s, (along each streamline) must be defined, satisfying:

with the boundary condition: si(0) = 0.

As in the Abbaszadeh-Dehghani and Brigham model, the convection diffusion equation governs the tracer transport along each streamline: where aL is the longitudinal dispersivity.

By considering that C(s, m) is the Fourier transform of C(s, t):

(66)

and taking into account the fundamental property:

(67)

it is possible to write the Eq. 67 as:

Converting the spatial variable, s, in a discrete form:

Ct = C(tAs, w) ~ C(s, w)

dC(5, t) 1 л 1 1 1 (3C’ __ — — C(s + ^t)-—C(^і) ~ —C,+1 -~C, — I I (70)
and the second spatial derivative:
(71)
a*) (d? C+2 ~~Ds[ C+1+lb C J-«s) (£C+1 — iC> — iwf Ct — 0
(72)
and:
C (2v(s)a + v(s)Ds) ■ )+ — av(s) ■ Ci+2 ‘ av(s) + v(s)Ds — iwfDs 2 The input boundary condition (in the Fourier domain) is: C(0 , f( , V2Coe-mT/2 . wT C „ ) C(0,w) — f (w) — 0 sin— = C — f (w) w-jn 2 where C0 is the pulse height and T its lifetime. At the output, a ‘flow’ condition was imposed:
(73)
(74)
dC(L, w) 1 — kC( T 1 C 1 C 1 — kC ds — a C(L, w)~ DsCn Ds Cn-1 — a Cn-1
(75)
where N is the greatest value taken by the index, i.

a

By solving these equations, it is possible to obtain the C(L, ra) for each streamline. After this, by composing the individual responses and returning to the time domain, the program obtains the complete tracer response.

To illustrate the final result, Fig.104 shows the tracer records (expressed as daily fractional recovery) of a five spot pattern (for an isotropic balanced case). It can be seen how the layer thickness controls the breakthrough, the peak position and the broadness of the tracer records.

Dispersivity controls the breakthrough, the broadness and, to a slight extent, the peak position of the tracer records. The effect of anisotropy (for wells along the direction of Kmax) is opposite to that of the dispersivity. While increased dispersivity results in greater peak width, increased anisotropy reduces the peak width. Figure 105 illustrates the tracer records for an isotropic case and a non-isotropic case.

FIG 105. Daily fractional recovery of a five spot pattern (for a non-isotropic balanced case). Influence of Kma/Kmin-

Additionally, the presence of a sealing fault enhances the injection water support in the producers located in the same fault block as the injector, especially in the wells along the fault (e. g. P-1 in Fig. 106).

The effect on the daily tracer recovery is similar to that caused by anisotropy (on well P-1), giving earlier breakthrough and peak position, but reducing the peak width. Finally, sometimes the lack of tracer (or the scarce production of tracer) in a producer, may be the consequence of actions outside the involved injectors. As a consequence, the tracer daily fractional production in well P-1 is strongly reduced (Fig. 106).

Detailed procedures for operations before, during and after injection should be described in the technical safety report. Some of the important points to be considered for radiation safety at an injection site involve:

• Informing well site personnel about the nature of the work to be done. In the case of radiotracers, explanations should be given in some detail, especially with regard to contamination/decontamination, radiation risks, doses etc.

• Preparing the injection site, setting up the equipment and connecting to the injection pipeline. In the case of injection against high pressure (several hundred bars), all connection points have to be leak tested before starting to pump the radiotracer solution. The injection area has to be cordoned off and signs displayed in order to deter unauthorized persons from entering.

• Ensuring that all operational personnel wear suitable protective clothing and personal dosimeters.

• Preparing monitoring and decontamination equipment to handle potential spills and leakages according to the written procedures.

• Flushing pure injection water through the injection apparatus and connected equipment for an extended period (~1 h) to clean out any traces of the radiotracer before disconnecting injection equipment.

• In the case of any leakages, performing the planned and prepared-for decontamination operation and enclosing any contaminated parts in suitable plastic bags for transportation back to radiotracer company’s premises for further decontamination.

• As a final check of the injection site before leaving, carrying out sweep tests for on-site analysis with relevant nuclear detection equipment.

• Finally, collecting urine samples from all personnel present during injection for subsequent laboratory analysis for any ingested or inhaled activity.

In the pattern shown in Fig. 55, well CP-804 was injected with 55.5 GBq of

HTO.

The response curves are shown in Fig. 56 where the tracer concentrations are represented in a relative way as tracer daily fractional recovery (TDFR) according to Eq. (27).

This way of expressing concentration is used in several other examples.

Days after injection

Figure 57 shows that the tracer is moving mainly in the direction of well CP-1091.

Before the injection of tracer takes place, the complete injection equipment will have been pressure tested and checked for proper functioning. The tracer solution will be delivered in a closed Monel bottle that will be connected directly to the injection equipment before the valves on the bottle are opened. Thus, the radioactive material will not be exposed to the environment or to the personnel.

Tracer injection will be carried out either by connecting the injection equipment as a bypass to the main injection tubing or by pumping water from a 200 L container through the injection module and into the injection line using a high pressure pump. The pumps are pneumatically and not electrically driven in order to reduce the risk of sparks igniting any gas leakage from the nearby petroleum operations. The injection of the water tracer is expected to last 1.5 h. The main injection (99.9% of the tracer) takes place in a few minutes and the remaining time is used to clean out traces of radioactivity from the injection system.

Decontamination of the equipment and the site (if needed in case of accidents) will be carried out by personnel from the tracer company. In order to check water samples for radioactivity, a portable liquid scintillation counter will be available at the well head site.

Four models and/or software packages have been tested, the Brigham model, PORO software (Streamlines approach), and the simple diffusion model using two different CFD software packages (Comsol and Castem).

PORO software was used successfully to simulate the experimental data. Figure 119 shows the good agreement obtained for wells 1 and 2. These simulations permitted the estimation of the porosity of the sand bed as 40%. The dispersivity was estimated to be 6 mm, this value being consistent with the characteristic of the sand bed.

Figure 120 shows the good agreement between experimental data and CFD simulations using the two CFD codes (Castem CEA made, ‘finite element toolbox’ and Comsol multipurpose finite element package, Comsol group, Sweden). However, it should be pointed out that the CFD code will be time consuming and more difficult to use for actual complex oil field simulations.

Finally, Fig. 121 shows comparison between the PORO and Brigham models, with several dispersivities (from 1 to 100 m) for a five spot configuration. If the tendencies of the two models are similar, the dispersions of the curves are different due to the original hypothesis of the two models.

FIG 122. Theoretical example of a ‘compartmental model’ in the case of a fracture surrounded by porous media zones with different characteristics.

For oil samples of types A and B in Fig. 24, the simplest pretreatment procedure is to remove any visible oil layer by pipette and to filter the water through a lipophobic filter. Transfer a measured volume of water, for instance 1000 mL, to a Marinelli beaker and count with a NaI(Tl) detector, as shown in Fig. 24. A regular cylindrical beaker may also be used, but the counting efficiency is somewhat lower. Owing to complex background radiation, an HPGe detector may be used if desired.

Upconcentration of f50Co(CN)^3- before gamma spectrometry

• After pretreatment to remove oil, the water is percolated through an anion exchange resin in order to concentrate the tracer molecule in a small volume in the resin.

• The column can be mounted directly onto a gamma detector (NaI(Tl) or HPGe) for gamma spectroscopy measurement.

• To prepare a sample for liquid scintillation counting the [Co(CN)6]3- on the column may be eluted with a suitable elution liquid into a small volume (a few millilitres) and mixed with a liquid scintillation cocktail. Thus, the counting efficiency is considerably improved. However, it requires that other beta emitters are not present in the sample.

A gamma spectra of 60Co accumulated with an HPGe detector with high energy resolution is compared with a gamma spectrum accumulated with a NaI(Tl) scintillation detector in Fig. 25.

The [60Co(CN)6]3- ion may also be detected with a liquid scintillation counter of high efficiency. It is the beta radiation and the Compton electrons which are registered. Liquid scintillation count detection is sensitive to quenching effects, as illustrated in Fig. 26 where a sample of 60Co activity is counted in a quench-free scintillation mixture and in a mixture where 8 mL of water is added. Water acts as a quenching agent in this case, resulting in a decrease in counting efficiency.

More details on the analysis of [60Co(CN)6]3- tracer is given in the analysis protocol in Appendix III.

(iv) Analysis of 14C or 35S labelled SCN-

Both 14C and 35S are beta emitters with similar beta energies (156 and 168 keV, respectively). Thus, they cannot be analysed simultaneously in the same counting sample. Therefore, use of these two compounds simultaneously in the same reservoir section should be avoided. If, however, both tracers are needed, it is, in principle, possible to analyse them in the same sample by special sample treatment and separation technique. The SCN- ion may be broken down to leave S in one type of molecule (e. g. SO4- by oxidation) and C in another (e. g. CN — or CO2). These may be isolated separately and counting samples prepared separately for each of them. Although the process given deals exclusively with the separation, enrichment and analysis of only one of them, the basic procedure for the two is identical.

Analysis of radiolabelled SCN — in produced waters from oilfields has been described previously in the literature. Two methods are outlined, one based on liquid-liquid extraction and the other on anion exchange separation of the tracer

ions from produced water samples. A detection limit <0.01 Bq/L was obtained by using low background liquid scintillation counting equipment (Quantulus 1220). A third method proposed by the China Institute of Atomic Energy is given in detail. It is based on solvent extraction with tributyl phosphate of a metallic thiocyanate complex after the purification process of oil removal followed by filtration as described previously.

Details of the procedure are given in the analysis protocol of radiolabelled SCN — in Appendix III, but the general steps include:

(1) Removal of particles and oil droplets by filtration through 0.45 pm filter paper;

(2) Addition of ZnCl2, KSCN carrier and HCl to form Zn(SCN)2 in a clear solution;

(3) Extraction of the electrically neutral Zn(SCN)2 into tributylphosphate;

(4) Conduct of phase separation by gravity segregation or centrifugation;

(5) Removal of sample from an aliquot of the tributylphosphate phase and mix with liquid scintillation cocktail in a liquid scintillation counting vial;

(6) Detection of the activity by liquid scintillation counting.

(v) Analysis [60Co(CN)6]3 in presence of radiolabelled SCN

The general procedure is as follows. Purify the water by removing oil as described previously. Isolate and enrich the two tracers from the bulk water volume, either sequentially or simultaneously. Two possible methods are solvent extraction and ion exchange. After the first step involving extraction/stripping or feed/elution operations, prepare samples for radioactivity detection. Ion exchange is less labour intensive than solvent extraction.

A separation procedure has recently been reported that is described in the analytical protocol for [60Co(CN)3]2- in presence of 35SCN- or S14CN — in Appendix III, but the general steps include:

• Removal of particles and oil droplets by filtration through 0.45 pm filter paper.

• Preparation of an anion exchange column of the resin Dowex 2 x 8.

• Percolation of the purified sample solution through the column. The [60Co(CN)3]2- will bind to the resin while 35SCN- or S14CN — will not.

• Removal of any remaining radiolabelled SCN — on the column by elution with a small volume of low concentration NaClO4 solution, which is added to the raffinate.

• Concentration and purification of 35SCN — or S14CN — in the raffinate by the procedure described for radiolabelled SCN — in Appendix III.

• Evaluation of column directly by gamma spectroscopy. In the cases where liquid scintillation counting is desirable, radiolabelled [Co(CN)3]2- may be stripped from the column by, for instance, a small volume of NH4NO3 solution at medium-high concentration and mixed directly with a scintillation cocktail.

• Detection of activity by liquid scintillation counting.

The necessity for performing a separation operation is underlined in Fig. 27, where a spectrum of 60Co is compared with a spectrum of 14C. The spectra overlap substantially. The liquid scintillation counting equipment may be run in so-called dual label mode, but the sensitivity becomes lower and the uncertainty in the results greater when spectra overlap to this extent.

As indicated in Fig. 77, three radioactive tracers were used in the test, HTO, 14C tagged KSCN and 60Co tagged K3[Co(CN)6]. In the test, 20 mCi of 14C tagged KSCN was injected into well 20-2 on 10 July 2005, and 10 Ci of HTO, together with 25 mCi of 60Co tagged K3[Co(CN)6], was injected into well 21-3 on 11 July 2005.

1.6.2. Tracer responses

All tracers were detected in corresponding production wells:

• The 14C from well 20-2 was found on 30 January 2006 at well 12-3, 204 d after injection.

• The HTO from well 21-3 was found on 30 November 2005 at well 13-4, 141 d after injection, and was found on 23 January 2006 at well 13, 195 d after injection.

• The 60Co from well 21-3 was found on 05 January 2006 at well 13-4, 177 d after injection.

Tracer injection and directional tracer movements are shown in Fig. 78.

PORO software simulation was used for tracer data treatment. Tracer response curves are shown in Figs 79 and 80.

141 192 222 243 266 286 304 324 342 364 398 454 524 573 674
Days after Tracer Injection

Appendix II

• pH meter, balance (1/10 000), 0.45 pm filter paper, funnel, flask (1000 mL), pipette, 5 mm x 100 mm glass column, spectral gamma analyser or gamma counter (well-type detector is recommended).

• No. 717 anion exchange resin (Beijing Analytical Reagent Factory), HCl (A. R.), Na2CO3 (A. R.).

111.4.2. Analytical procedure

The analytical procedure is shown in Fig. 92.

111.4.3. Recommendations

It is recommended that a reference solution (sample with known tracer concentration in the brine of the target reservoir) should be prepared and applied in an actual scale test in order to obtain very reliable results through use of the above analytical procedures. The concentration of the tracer in the reference solution should be at a similar level to that in the samples collected from the field.

In order to perform a proper planning process, a variety of detailed reservoir and well information should be considered. These are listed below.

Parameters to be provided by the oil company:

• Well pattern (e. g. inverted five-spot, line drive, irregular pattern, etc., map of the lateral distribution is preferable);

• Well types (vertical, horizontal, undulating, complex, etc.);

• Distance from injector well(s) i to producer well(s) p (rip (m));

• Whether it is possible to inject different tracer in each perforated zone or producing layer (j) and height of each layer (hj (m));

• Height of the combined pay zones (Zhj (m));

• Permeability (or water relative permeability) in each layer, j (kj);

• Average porosity of reservoir rock (Ф);

• Reservoir pressure (for gas tracers) (p (bar));

• Reservoir temperature (T (K));

• Estimated water saturation (Sw);

• Estimated oil saturation (So);

• Salinity (main salt components and their concentration (g/mL));

• Reservoir water pH;

• Special gas composition (e. g. H2S) (to plan analytical strategy/method);

• Special oil composition (API);

• Estimated or calculated water cut (for planning sampling strategy);

• Injection water rate (m3/d).

Parameter to be provided by the analytical laboratory:

• Lower limit of detection for each type of tracer t (LDt) and associated required sample volume.