The efficiency of the water flooding process is highly dependent on the rock and fluid characteristics. In general, it will be less efficient if heterogeneities are present in the reservoir, such as permeability barriers or high permeability channels that impede an efficient volumetric sweep and thereby a good oil displacement by the injected water.

Natural production mechanisms, or primary production, contribute to extraction from the reservoir of about 25% of the original oil in place. This means that 75% of the existing oil remains in the pores and fissures of the rocks. The production flow rate depends on the differential pressure between the permeable layer and the bottom of the well, the average permeability, the layer thickness and the oil viscosity. The main natural production mechanisms are the expansion of the oil, water and gas and, in certain cases, the water influx from aquifers connected to the reservoir.

When primary oil production decreases in a field because of a reduction in the original pressure, water is usually injected to increase the oil production. Injected water in special wells (injection wells) forces the oil remaining in certain layers to emerge from other wells (production wells) surrounding the injector. This technique, commonly termed secondary recovery, contributes to the extraction of up to 50% of the original oil in place. Although this technique was firstly used in old reservoirs in which oil production had decreased, it is nowadays a common practice to begin the exploitation of new wells with fluid injection as a way to optimize oil recovery. For this reason, the name secondary recovery is being replaced by the more general term enhanced oil recovery.

For oil reservoirs, interwell tracer data are important in order to optimize the production strategy (injection balance) in the reservoir and thereby maximize the oil recovery. In geothermal reservoirs, interwell tracer tests are used to improve the understanding of reservoir geology and to optimize production and re-injection programmes and thereby enthalpy production from the reservoir. During the last 10-15 years there has been substantial progress on tracer technology development. This has resulted in improved basic knowledge and new technology.

Detailed analysis of the response curves obtained from interwell studies allows the following:

— Detection of high permeability channels, barriers and fractures;

— Detection of communications between layers;

— Evaluation of the fraction of the injection water reaching each production

well;

— Determination of residence time distributions;

— Indication of different stratifications in the same layer;

— Determination of preferential flow directions in the reservoir;

— Determination of swept volume of the reservoir.

All this information can be used to make operational water flooding decisions in order to increase oil production.

Tracer technology is a powerful tool for tracing the movement of the injected fluid through the oil reservoir, monitoring reservoir performance, investigating unexpected anomalies in flow and verifying suspected geological barriers or flow channels. Generally, the injected fluid is labelled with tracer (radioactive or non-radioactive) and the produced fluid from the well(s) of interest is sampled and analysed to determine the tracer response curve. The analysis of tracer response curves can provide important information about the character of the reservoir and makes it possible to optimize the injection regime and improve production strategy.

Such information can be used to evaluate flood performance, optimize the balance between injection and production rates, help make decisions on infill drilling and enhanced oil recovery programmes and improve the accuracy of the reservoir model.

In industrialized countries, tracers have been used to measure fluid flow in reservoirs for several decades [1-4]. A summary of theIR earlier use (before 1990) from the perspective of tracer behavior is provided by Bjornstad [5]. There are some success stories and some reports on experiments, which have largely failed. The reason for failures is mainly due to insufficient knowledge of tracer behaviour under changing reservoir conditions.

Knowledge of tracer behaviour is gained through dedicated laboratory investigations, through the above-mentioned oil field experience, groundwater movement investigations, atmospheric tracing experiments and also, to a significant degree, through the work carried out on the migration of radioactive species in soil for the purpose of evaluating radioactive waste repository sites.

Although the integrated knowledge from these areas is substantial, the information obtained is not always consistent. Results from one area of investigation cannot readily be transferred to new fields because of both scaling problems and changing experimental conditions. During the last 20 years there have been substantial programmes on tracer technology development in a few R&D laboratories in Europe and North America. This has resulted in new basic knowledge and new technology.

During the past few years, a number of ‘traditional’ radioactive and non­radioactive water tracers have been re-examined along the lines described above, and the search for new possible tracer compounds is ongoing. Ultra-low detection limits are required. Among the non-radioactive compounds, the fluorinated aromatic acids have attracted special attention because of their success in tracing groundwater flow. A comprehensive quantity of information has been generated with respect to their thermal stability and reservoir flow behaviour in dynamic laboratory experiments under simulated reservoir conditions. Some of the compounds passed through the quality checks in good shape. Others show instability or other unwanted properties which excludes them from use in reservoirs, at least under certain specific reservoir conditions. The compounds with sufficiently ‘good marks’ from laboratory experiments were extensively tested in full field experiments in the early 1990s.

A selection of the non-radioactive polyfluorinated benzoic acids were established as industry standards for tracing water flow in oil reservoirs more than 10 years ago and details have been published in open literature [6]. Currently, the continued development has resulted in new families of non­radioactive tracers qualified for oil reservoir water tracing. However, the identity of these compounds is not revealed in the open literature. Individual compounds have certain limitations on their use and are not generally applicable. It is important to know these limitations in detail in order to apply them correctly.

Some information can be found in Refs [7] and [8], but most of the data remain unpublished as private confidential research reports.