Tracer applications can be found in almost any phase of oil field development. Interwell tracer technology is an important reservoir engineering tool for the secondary and tertiary recovery of oil. Interwell tracer testing is also used in geothermal reservoirs to gain a better understanding of reservoir geology and to optimize production and reinjection programmes. The main purpose of conducting interwell tracer tests in oil and geothermal reservoirs is to monitor, qualitatively and quantitatively, the injected fluid connections between injection and production wells and to map the flow field, reservoir heterogeneities and volumetric sweep (contacted volumes) between wells. Tracer is added into injection fluid via an injection well and observed in the surrounding production wells (Fig. 1). Tracer response is then used to describe the flow pattern and thereby gain a better understanding of the reservoir. This knowledge is important in optimizing oil recovery. Most of the information given by the tracer response curves cannot be obtained by means of other techniques.

Fluid flow in most reservoirs is anisotropic. The reservoir structures are usually layered and frequently contain significant heterogeneities leading to

FIG. 1. Principle of tracer injection method for interwell communications.

directional variations in the extent of flow. Hence, the effective fluid movement can be difficult to predict. This is where tracer technology plays an important role, assuming that the movement of the tracer reflects the movement of the injected fluid. Obviously, it is most important to assure that the properties of the tracer meet this requirement as closely as possible; there should be a minimum quantity of undesired loss or delay. The physical and geochemical conditions of the reservoir define the constraints. As a result, tracers found to work properly in one reservoir, may not work satisfactorily in another.

Apart from radioactive and chemical tracers, stable isotopes of the water molecule (2H and 18O) can be employed as effective tracing tools to identify the source (origin) of produced water, both in geothermal as well as oilfield applications. On the basis of stable isotope indices, the relative contribution of different sources of water towards produced water may be estimated. However, in the cases of geothermal reservoirs and high temperature oil reservoirs, the 18O content of injected water is likely to be modified due to 18O exchange between water and host rock. However, 2H is considered as conservative and can safely be used to estimate relative contributions.

A field radiotracer investigation consists, in brief, of the following main

steps:

(1) Design of tracer strategy, involving consultation with reservoir engineers

(2) Selection of applicable tracers

(3) Application to the relevant authorities based on a safety report

(4) Tracer mixture preparation, calibration and quality assurance

(5) Selection/design of tracer injection and sampling procedures

(6) Tracer transportation to injection site

(7) Implementation of radiation safety procedures at the injection site

(8) Tracer injection

(9) Radioactivity contamination survey

(10) Injection equipment decontamination and handling of radioactive waste

(11) Tracer sampling and sample transportation to analytical laboratory

(12) Tracer analysis

(13) Data evaluation and simulation

(14) Reporting of results

The IAEA Coordinated Research Project (CRP) on Validation of Tracers and Software for Interwell Investigations has developed, prepared, tested and validated several tracers, techniques and software packages. The main group activities of the CRP were:

• Laboratory intercomparison on analysis of the tritiated water (HTO) in field samples (brines);

• Intercomparison on evaluation of field data with a simple software package (Anduril);

• Laboratory intercomparison on analysis of mixtures of the two water tracers HTO and 14CH3OH;

• Application of the PORO streamline simulator on field data provided by different companies.

A short summary of the main achievements of the CRP is given below: Tracer preparation, quality assurance and analysis: Synthesis, preparation, analysis and quality control of several tracers both individually and in mixtures: HTO, SCN — (14C or 35S labelled), radiolabelled alcohols, [Co(CN)6]3- (radiolabelled), 125I — (131I ) and gold nanoparticles have been established or validated. Criteria for selecting the more adapted tracers have been investigated. Laboratory intercomparison analysis of mixtures of HTO and 14CH3OH was successfully done. Ions and stable isotopes in produced water have been used as indicators to support the interwell tracer test.

Experimental procedure for tracer tests: Intercomparison of the injection and sampling strategies has been done and rules have been proposed to carry out the tracer experiments. Tracer injection techniques, both bypassing and direct pumping into the well head, have been compared. Well head samplers and procedures for collecting water sample have been developed and tested in the field. Safety procedures have been established and implemented.

Interpretation and modelling: Several models for interwell tracer data interpretation have been tested and compared. Rules and advice have been established to select the more suitable model and/or software packages, depending on the field structure and configuration. The following models (software packages) have been studied: Brigham (home-made code), dispersion (Anduril), streamlines (PORO), chemical engineering (Disproof) and computational fluid dynamics (CFD)(Caste and CONSOL). New possible approaches of compartmental modelling have been suggested for fractured oil or geothermal reservoirs.

The publication represents a form of monograph dedicated to tracer methods as applied to interwell investigations in oil and geothermal reservoirs. It consists of three sections and four appendices.

The first section gives the background and arguments for the use of tracers and presents the general view on tracers and tracer techniques as applied in interwell investigations in oil and geothermal fields. The status of tracer technology worldwide is given as well.

Section two deals with technical steps in the practical application of interwell tracer technology, including planning of field tests (tracer selection, injection and sampling), field related operation and implementation, tracer measurement and data interpretation.

Section three covers new tracer development, including tracer quality control, behaviour of tracers in various environments, and analytical methods for tracer measurement. Finally, the CRP achievements are summarized in a short section.

The four appendices provide the following information:

• Appendix I is allocated to field case studies performed by the various institutions involved.

• Appendix II deals with laboratory intercomparison tests on analysis of HTO and HTO + 14CH3OH in mixtures as well as operation of the tracer interpretation software Anduril on practical cases common for all laboratories.

• Appendix III provides procedures and protocols for measuring tracers in produced water.

• Appendix IV describes two software packages produced and tested during the CRP period: Anduril software for simple data treatment and PORO software for more advanced streamline simulation.