1. n. [Drilling Fluids]

A chemical or isotopic marker that is uniformly distributed in the continuous phase of a drilling, coring, drill-in or completion fluid and used to later identify the filtrate in cores or in fluids sampled from permeable strata. A tracer must become a part of the filtrate, remaining in true solution and moving with the filtrate into permeable zones. It must not be a component in the strata that is expected to migrate, be adsorbed on clays, or degraded. It should be measurable in trace amounts and safe to handle.

Examples of filtrate tracers include

• Radioactively tagged compounds (isotopes of elements). Tritium, a weakly-emitting radioisotope of hydrogen, can be a safe and effective tracer in both oil and water (as T₂O) muds. It is measured by scintillation counts.
• Bromide or iodide compounds are practical to use because they do not occur naturally in most muds or reservoirs. They are detectable in small amounts by electron-capture gas chromatography.
• Fatty acids (or their derivatives) normally present in an oil-mud emulsifier can serve as oil-filtrate tracers and are analyzed by gas chromatography.
• Nitrate (NO₃^-) anion, added as sodium, potassium or calcium nitrate, is one of the earliest tracers used. It is limited by being difficult to analyze and lost by degradation.

See: bland coring fluid,  completion fluid,  continuous phase,  core,  coring fluid,  drill-in fluid,  dynamic filtration,  low-pressure, low-temperature filtration test,  mud tracer,  static filtration