Fluid identification system comprising encapsulated DNA

A fluid identification system comprises a plurality of particles, each particle including at least one tracer material having an identifiable DNA, which is encapsulated by an encapsulation material wherein the at least one tracer material is complexed with a first polymer surrounded by the encapsulation material which comprises a second polymer. The particles preferably are adapted to retain the at least one tracer material in an encapsulated form after exposure of the particles to a temperature of at least 75 °C and/or a pressure of at least 1000 psi (6.9 x 106 N/m2). The system may be used as part of a method of identifying a fluid, by adding the particles to the fluid of a well, field, or reservoir of a hydrocarbon production unit, retrieving the fluid identification system from a sample, separating the tracer material, and analyzing the identifiable DNA. The particles may comprise at least one magnetic material to add retrieval. A library of a plurality of fluid identification systems is disclosed, and also a catalogue of the library.