2.1.3. !. Tracer classification

Reservoir tracers can be divided into two categories:

(1) Passive or conservative (or also, less precisely, termed ideal) tracers: The requirement is that the tracer shall passively follow the fluid phase or phase fraction into which it is injected without exhibiting any chemical or physical behaviour different from that of the traced component itself. In addition, the tracer must not perturb the behaviour of the traced phase in any way and neither must the fluid phase or its components perturb tracer behaviour. In petroleum reservoirs, passive (or in practice near-passive) tracers are used in studies of water flooding.

(2) Active (also, less precisely, termed non-ideal or reacting) tracers: The tracer behaves in a qualitatively predictable way and is used to measure a property of the system into which it is injected. The degree of active take-up is a quantitative measure of the property being determined.

Examples of active tracers include:

• Phase partition (with the potential to measure oil saturation in the water contact reservoir zone);

• Sorbtion onto rock, either reversibly or irreversibly (with the potential to measure ion exchange capacity of formation rock);

• Hydrolyzation (e. g. for measurement of water saturation, or temperature if the water saturation is known);

• Thermal degradation (to measure reservoir temperature away from wells);

• Microbial degradation (to measure microbial activity).

It is practical to divide the available interwell reservoir tracers into three types based on their different production mode, treatment and analytical methods:

(i) Stable isotope ratios;

(ii) Non-radioactive chemical species;

(iii) Radioactive atoms or molecules.

Zemel, in his book on Tracers in the Oil Field [9], argues that radioactive and non-radioactive chemical tracers are not necessarily different kinds of tracers, but that radioactive tracers are only radioactively tagged chemical tracers. He might as well have included tracers based on stable isotopic ratios in this argument by stating that isotopic ratio tracers are only chemical tracers labelled with a different stable isotope ratio. It is correct that the flooding properties and survivability in reservoirs are determined by the chemical properties of the tracer compound. It is not, however, generally valid that the same materials without a radioactive tag are also useful tracers. This all depends on the degree of their natural occurrence in reservoir fluids and on their detectability by non­radiochemical methods.

Tracers may also be classified as intrinsic or extrinsic:

Intrinsic tracers are molecules containing an isotope (radioactive or stable) of one of the molecules’ natural elements, which makes the labelled molecule particularly detectable by nuclear or mass spectrometric methods in systems where the dynamic characteristics and general behaviour of the non-labelled molecule are followed in a given medium. For example, in the case of water, there are three such labels: oxygen-18 (‘H218O) and deuterium (‘H2H16O) measured as isotopic ratios (18O/16O by S18O and 2H/1H by SD) by mass spectrometric techniques and tritium (1H 3H16O) measured by nuclear techniques (in practice, liquid scintillation counting). In this case, the water molecule is traced from the inside, in the confines of its nucleus. In this case, the water tracer will (in practice) follow all movements and reactions of the water itself.

Extrinsic tracers are made up of atoms or molecules supposedly to sharing the same dynamic characteristics and, in general, the same mass flow behaviour as the investigated medium. Falling into this category are all the substances that allow for tracing outside the molecular or ionic structure. For example, in case of water, 131I-, S14CN — and [60Co(CN)6]3- are examples of extrinsic tracers. They will not follow water in all its movements because of their charge and because they are basically a salt which will not, for instance, evaporate with the water.

Tracers may further be classified as artificial or natural:

Artificial tracers are generally defined as those tracers which are produced artificially and are deliberately introduced (injected) into the system under study. Most of the tracers employed in industrial applications, including geospherical tracing, are artificial tracers. They are further classified as radioactive and non-radioactive artificial tracers.

Natural tracers are those tracers that exist in nature, generated by nature itself. Such tracers are, for instance, the noble gas 222Rn that may be used to follow mass flow in extended open systems, isotopic ratios of hydrogen atoms (SD) to study, for instance, the movement of injected sea water in an oil reservoir, provided its SD value is sufficiently different from that of formation water, etc. Such tracers are mainly used to trace processes in environmental, geospherical, biological and agricultural studies and are not especially relevant for study of industrial processes in general.

The last class that will be mentioned here are activable tracers:

Activable tracers differ from other tracers only by the fact that they contain a chemical element with a special capacity (high activation cross-section) to be analysed in minute quantities by instrumental neutron activation analysis. As such, they are of special interest to the radiotracer specialists. These compounds are either organometallic covalently bound compounds or simply electrostatically bound chelate complexes. Their main advantage is that they do not pose any radiological hazards during operation and they have a practically infinite shelf

life in comparison to radiotracers. On the other hand, there is always a danger of contamination of the collected sample before activation.

In order to hinder sample contamination, all chemicals and mechanical components in contact with the liquid sample must be virtually free of activable element. These samples are activated with thermal or epithermal neutrons and sample measurements are carried out in a laboratory equipped with high resolution gamma spectrometers.

Metallo-organic compounds may also be analysed in trace quantities with other trace analytical techniques, for instance by inductively coupled plasma mass spectrometry. Such compounds may include other metals than those which are optimal for instrumental neutron activation analysis.

Different types of equipment are available. It is common to use cores of consolidated reservoir rock or reservoir-like rock (i. e. sandstones such as Clashack, Berea, Bentheimer, Felzer, etc., and carbonates such as chalk, limestone, etc.). Core dimensions normally range from d x l = 2.8 cm x 7.7 cm to d x l = 5.1 cm x 51.2 cm.

A flow rig constructed for smaller consolidated cores is shown in Fig. 40. This is constructed to operate under simulated reservoir conditions, i. e. temperatures up to 150°C and pressures up to ~450 bar. The core is mounted in a Viton or neoprene rubber hose and an overburden pressure of ~20 bar is exerted onto the rubber hose in the external chamber in order to prevent any leakages along the surface of the core.

Instead of the permanently mounted vertical core, a Hassler cell (illustrated in Fig. 41) may be used, which permits the choice of any angle of the core from horizontal to vertical. A method has also been developed to permit the use of unconsolidated material in this equipment.

Other equipment is based on the use of crushed rock material to fill chromatographic columns of varying dimensions.

Figure 42 illustrates a flow rig based on a 200 cm long chromatographic column with an internal diameter of 11 mm.

A quantity of radioactive material is characterized by the number of nuclear disintegrations or transformations that occur within a specified time interval. The parameter is termed the disintegration rate, D, and the unit for this quantity is the becquerel (Bq), which is equal to one nuclear transformation per second. Another unit that is still in use is the Curie, equal to 3.7 x 1010 disintegrations per second (dps). Following on, the:

• Activity concentration, Ac, has the dimension dps per unit mass (or unit volume) of a sample (i. e. Bq/g or Bq/mL).

• Specific activity, As, has the dimension of dps per unit mass of the inactive counterpart of the tracer atom or molecule. For HTO, the dimension is Bq/g H2O or Bq/mL H2O.

For HTO in pure water, Ac = As. For HTO in aqueous mixtures with other water soluble components, Ac ф As.

When ionizing radiation from a radioactive material passes through a mass of some other material, the radiation will interact with the atoms of the material in different ways. The net result is that energy will be absorbed in the material. The quantity of energy that is absorbed per unit mass is termed the absorbed dose and is measured in gray (Gy), whereby 1 Gy = 1 joule/kg. The old unit RAD is still in use, whereby 1 RAD = 0.01 Gy.

The biological effect of the radiation does not depend solely on the absorbed dose, but also on the quality factor of the radiation. For small dose rates of beta and gamma radiations, this quality factor is set to 1 (for alpha particles, neutrons, etc., the quality factor is >1). The absorbed dose multiplied by the quality factor renders the equivalent dose, which is measured in sievert (Sv). The old unit REM is still in some use: 1 REM = 0.01 Sv.

Humans are continuously receiving radiation doses from ambient (natural and artificial) sources. For most people these doses are in the range 3-5 mSv per year. Approximately 60-70% of this dose is caused by inhalation of radon (222Rn) from materials in the surroundings which contain uranium (decay of 238U). For comparison, 5 mSv corresponds to the dose that a person receives from the intake of 300 MBq of HTO. For members of the public, the International Commission on Radiation Protection has recommended the use of 1 mSv/y, averaged over 5 years, as a limit for radiation doses caused by application of radioactive material or other types of ionizing radiation.

A material emitting ionizing radiation will create an absorbed dose rate that is usually measured in Gy/h. The corresponding equivalent dose rate is measured in Sv/h or the smaller units pSv/h, or mSv/h.

Injection of a radioactive tracer into a hydrocarbon reservoir at high pressure entails handling of materials with properties that are unfamiliar to most people. As radioactive materials in quantities used for injections are potentially dangerous, it is important that they are treated properly. In an injection project, safety consideration should already have been taken into account when the tracer is selected and the quantities fixed. During transport and storage at the well head site, the safety aspect should be borne in mind and safety precautions should be taken according to international rules and regulations.

IV.2.5.1. Installation

Installation is an almost automatic process. In the CD there is an ‘auto run’ file that starts installation some seconds after having inserted the disk in the CD drive. However, in case this file does not work properly, installation can be started by clicking in the Windows start menu at the ‘run’ option. Then browse for CD drive and the ‘setup. exe’ file. Accepting will force the installation sequence to start.

The set-up software has been written in Spanish but most steps are performed without human participation. First of all, eight auxiliary files are copied from the CD to the hard disk in order to prepare installation. Once files have been copied, the ‘welcome’ window appears, recommending closure of any open application.

Instalacion de PORO TracerSim 2.0 (english)

Instalacion de PORO TracerSim 2.0 (english)

After accepting, the installation windows open. To install PORO TracerSim 2.1 in the default folder, click on the icon, otherwise click in the Cambiar directorio button.

• The first step is to select a program group for adding a shortcut for PORO TracerSim.

• The second step is the installation itself. An advance bar is shown.

• Finally, a new window appears indicating that installation was completed successfully.

To remove PORO TracerSim 2.1, open the control panel and select Add or Remove program options and follow the instructions.

Geothermal fluid sampling for tracer analysis of isotopes such as tritium and 125I — (131I) in both vapour and liquid systems is done through a condensation process of the fluid, as shown in Fig. 15.

Steam pipe to turbin

Condenser

Master valve

Condenser sampling

valve sampling

FIG. 15. Schematic diagram of a condenser used for sampling in geothermal wells in Indonesia.

The sampling procedure is as follows:

• The condensation of the geothermal fluid is achieved using the condenser apparatus shown in Fig. 15.

• The condenser is connected to the well head. Cool water flows into the condenser countercurrent to the geothermal fluid which is transported in the spiral tube.

• The valve at the well head is opened to allow hot fluid to flow into the condenser.

• Water from the vapour or hot liquid is condensed to ambient temperature. This water is collected in a plastic bottle of 1 L or 2 L capacity.

Sampling in the Philippines (at the Energy Development Corporation) makes use of a Weber separator, where steam and water are separated during sampling. In the Philippine geothermal fields, the Weber separator is connected along two phase lines (shown in Fig. 16) and samples are collected separately for water and steam condensate.

The experience gained through the work carried out in oilfield 1 strengthened the capabilities of the tracer group at PINSTECH and resulted in better cooperation and enhanced acceptance of the technology by the end user. This provided an opportunity to extend radiotracer applications to another oilfield. Oilfield 2 is situated about 20 km away from oilfield 1. There are four production wells and two injection wells in this field. The pattern of wells is shown in Fig. 62. The distance between injection wells and production wells varies from 1125 to 3975 m and the depth of the wells varies from 4068 to 4267 m below mean sea level.

Objectives

The objectives of this study were to:

• Determine the breakthrough time between the injection and production wells;

• Assess the contribution of injected water in the production wells;

• Determine relative contribution of injected water and formation water to individual production wells;

• Investigate the presence of channels (if any) between the injection and production wells;

• Determine the mean residence time of floodwater in the reservoir;

• Determine the percentage radiotracer recovery and the swept volume by floodwater.

Tritium (as HTO) was injected in well 1 through a bypass loop and production wells 3, 4, 5 and 6 were monitored for tracer response. Tracer breakthrough was recorded in well 3 in 17 d. However, no tracer breakthrough was observed in production wells 4, 5 and 6 up to June 2008. Stable isotopes of water (2H and 18O) were also utilized to identify different water sources and their relative contributions to produced water. The data are shown in Figs 63 and 64.

The work related to oilfield 2 is in progress and the results obtained to date are as follows:

• The breakthrough time of well 3 is 17 d.

• The early breakthrough in well 3 indicates the channelling effect between injection well and production well 3.

• Mean residence time of tracer with respect to injection well 1 and production well 3 is 127 d.

• The volumetric response of tracer has determined that mean produced water volume from well 3 is 52 000 m3, which was achieved in 136 d after radiotracer injection and 301 d since water injection was started. These figures are in good agreement with a mean residence time of 121 d.

• The volumetric response of tracer has determined that mean injected water volume from injection well 1 is 200 490 m3, which was achieved in 131 d after radiotracer injection and 301 d since water injection was started. These figures are again in good agreement with a mean residence time of 127 d.

•
The maximum and mean velocities of injected water between injector well 1 and producer well 3 are 80.9 m/d and 110.8 m/d, respectively.

• About 53% of injected tracer has been recovered through producer well 3 within 427 d of radiotracer injection (up to 15 May 2008).

• Considering a mean produced water volume of 52 000 m3 and 53% recovery of radiotracer from well 3, the mean swept volume is determined as 27 560 m3. This is the average volume of reservoir swept by injected water which was produced by well 3.

• No radiotracer breakthrough was detected in wells 4, 5A and 6 up to 15 May 2008.

• Relative contributions of injected water and formation water to production well 3 are 77% and 23%, respectively.

I.3.3. Conclusions

The tracer test carried out in both oilfields provided excellent data, which can be used to validate modelling software. The highlighting point of these tracer tests was that the conjunctive use of the stable isotopes of water (2H and 18O) along with radiotracer provided very useful supplementary information, giving more credibility to tracer technology as applied to interwell communication studies. Therefore, stable isotopes can be successfully applied for interwell communication studies where there is reasonable difference in stable isotope indices of injection and formation waters. Further, stable isotopes are unique tools to identify different sources of groundwater.

A typical check list for testing the injection equipment shown in Fig.7 prior to injection is: [2] [3] 3

(4) The relief valve is set to 30 MPa (350 bar). Check that it does not relieve at working pressure.

(5) Open valves D and B. Check for leakages.

(6) Remove drain plug and open valve C slightly to check for flow through the module.

(7) Close valve C and replace drain plug.

(8) Close valve D and open valve 2. The two manometers on the module will now read injection line pressure.

(9) After having passed through the steps 1-8 successfully, close all valves. The module is now ready for installation of the tracer bottle.

(10) Bring the tracer bottle to the site.

(11) Check that the wire seal between the valves on the bottle is not broken.

(12) Remove the plugs from the two valve outlets.

(13) Connect the bottle to the injection module.

(14) Starting with all valves closed, open valves 1 and E and check for leakages.

(15) Close valve E and open valves D and F. Check for leakages.

(16) Close valves 1, D, F and check that all other valves are closed.

(17) Having passed the above steps successfully, the tracer injection module is ready for injection of tracer.

The model is based on a number of assumptions, mainly: (i) flow in parallel non-communicating aerially homogeneous layers, (ii) constant water saturation, Sw, and (iii) regular and balanced patterns. These assumptions are acceptable in cases where they represent the situation of an ideal reservoir. By comparing the experimental tracer records with those of the Abbaszadeh-Dehghani and Brigham

2.4.1. Response curves

A good sampling programme and the measurement of these samples with adequate detectors (high efficiency, low background and low statistical error) is the way to obtain good response curves which form the basis for further interpretation.

2.4.1.1. Tim e response

The time response is the graphic representation of the concentration of activity (after background subtraction and decay correction) as a function of time. A preprocessing of the experimental data can also be used in order to smooth the response.

From this curve, the cumulative response (recovered activity versus time) is derived by a simple numerical integration. The application of complex integration methods is not justified because of statistical dispersion in the original data and variations in the pattern parameters.

The example illustrated in Fig. 28 was taken from an actual field exercise and shows both instantaneous and cumulative response curves to HTO injection.

Concerning the cumulative response, the following expression gives the activity recovered up to an instant, ti:

Aif) _ f g(t) C (t) dt

0

where

A(t) is the total tracer recovery up to ti (kg or Bq);

g(t) is the production water flow rate as a function of time (m3/d);

C(t) is the tracer concentration as a function of time (kg/m3 or Bq/m3); ti is the elapsed time after the injection (d).

Information about the production flow rate is usually available in the oil company. Among the information obtained from the time response, tracer breakthrough is the first to be obtained. It is the time interval during which the tracer concentration exceeds the general background level of the samples.

The mean residence time is another important parameter. Its definition is identical to the one used in process studies, i. e. the ratio between the volume (V) involved in this process and the flow rate that feeds it (Q).

(9)

I tC(t) dt

t =^z (10)

I C (t) dt

Jo

The final time is the time in which the response reaches the general background level of the sample. However, in oilfield experiments it is very common to stop sampling before this point. Thus, the final time is evaluated from the extrapolated response curve. For extrapolation purposes the exponential function gives the best fit for the tail of the experimental curve.

Knowing the distance between injection and production wells it is easy to calculate the maximum, mean and minimum water velocities from the breakthrough, mean residence time and final time respectively.

The tracer recovery in each well is determined from the extrapolation of the cumulative response for time approaching infinity on the basis of the exponential approximation of the concentration curve. The fraction of injected tracer recovered in each well in the pattern (f) equals the fraction of the injected water that arrives at this well:

where Аш is the extrapolated tracer activity recovered in the well at time infinite and A is the injected activity.

The total tracer recovered in all the wells belonging to a given pattern should be identical to thequantity of tracer injected in order to obtain a perfect mass balance. However, tracer recovery is seldom higher than 80% and it can be as low as 20% for tritium, which is supposed to be an ideal tracer for water. There are three reasons for this behavior. Firstly, the tracer molecules continue moving towards second line wells and not all of them emerge from the wells immediately surrounding the injector, secondly the injected water pushes the oil to production wells and replaces it in the rock pores and finally, a fraction of the tracer mainly in the tail of the response curve suffers dilution that causes the concentration to fall under the detection limit. Sampling second line wells is a good idea in order to improve the mass balance and to gain additional information about the pattern under study.

Table 18 lists the experimental parameters used in different laboratories.

TABLE 17. SALINE WATER COMPOSITIONS

Series 3: Artificial Gullfaks formation water, Series 4: Tordis produced water

salts dissolved in distilled tap water (samples have been in contact with live oil)

Indonesia, Hydrology and Geothermic Laboratory Center for the Application of Isotope and Radiation Technology, National Nuclear Energy Agency

Pakistan, Radioisotope Hydrology Laboratory, Pakistan Institute of Nuclear Science and Technology