Water tracers in oilfield applications: Guidelines

A key parameter in tracing tests is the selection of the molecules used as water tracers. Many previous tests have failed because of improper selection of these molecules. To address this issue, the first part of this paper provides guidelines, offering data and advice for choosing the best possible tracers for a tracing campaign. This part of the paper presents the different types of water tracers proposed and used in oilfield applications, from the first qualitative tracer study in the 1960s to tracer studies in the 2000s, with their respective advantages and drawbacks. The oil industry began to conduct interwell tracer tests with molecules already successfully used in hydrology. These compounds included radioactive species and stable isotopes, chemicals such as fluorescent dyes, and inorganic ions. Some of the early chemical tracers have been rejected because of issues with adsorption onto the rock. Radioactive species, with a low detection limit, a low reactivity, and a low presence in the environment, have been widely used. However, their use has become more restricted throughout the world in response to the radioactive hazards associated with their use. Therefore, new types of non-radioactive tracers were developed and tested in the 1990s. Currently, however, few chemical molecules possess characteristics matching the selection criteria of an effective water tracer (with regard to environmental and economic aspects, etc.). The most effective molecules currently used as water tracers are the fluorinated benzoic acids (FBA); these molecules can be detected with very low limits of detection (LOD) using analytical techniques such as gas chromatography or ultra-high-performance liquid chromatography coupled with mass spectrometers (GC/MS and UHPLC/MS-MS, respectively).The second part of the paper deals with the analytical aspect of a tracing test. An alternative technique to the fluorescence methods currently used for the naphthalene sulfonic acids (NSA) is proposed: UHPLC/MS-MS. With this original tool, FBA and NSA could be simultaneously detected in water samples in only one 5-min analysis. Other molecules, halogenated boronic acids, were also tested analytically for their potential application as tracers. However, these molecules were not retained because of their overly high LOD, requiring the injection of large quantities into oil reservoirs.

► State of the art of water tracers used in oilfield applications.
► Fluorinated Benzoic Acid (FBA) broadly used as passive water tracers.
► Direct determination of the FBA by a simple and fast analytical technique, UHPLC/MS-MS.
► Other tracers detectable with the same analytical tool and with low limits of detection.
► Naphthalene sulfonic acid (NSA) can also be detected with this technique.