TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The synthetic crystal is in the form of a mineral sulfate, a mineral carbonate, a mineral halogenide, mineral silicate, a mineral sulfide, mineral molybdate, mineral tungstate (wolframate) a mineral phosphate or a mineral oxide or hydroxide. The encapsulating compound: has a hardness of 2 or greater on Mohs scale, a density of greater than 2g/cm3, and no phase transitions within the pressure-temperature field of (1-1000 bar and 20-500 degrees C), preferably 50-350 bar and 70-250 degrees C; is low solubility mineral salts and/or and mixtures; comprises salts that are normally present in formation water such as sulfates, carbonates and phosphates, preferably calcium, magnesium, barium, strontium, lead, silver, or zinc (including barium sulfate, and/or calcium carbonate); and is at least one compound having at least one cation comprising calcium, barium, strontium, lead, silver, magnesium, manganese, iron, titanium, zinc and aluminum and an anion comprising sulfates, halogenides, silicates, molybdates, tungstates, phosphates, carbonates, sulfides, hydroxides and oxides. The encapsulating compound is at least one barium, strontium salt and/or mixed barium/strontium salt (including barium sulfate and strontium sulfate, (barium, strontium)sulfate or (barium, strontium)carbonate). The tracer is a water tracer, an oil tracer and/or gas tracer. The crystals are consolidated into rods or bars.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Composition: The two controlled-release compositions comprise different tracers. The controlled-release composition is used in a clean-out valve or tracer-release valve. Each of the two or more controlled-release compositions: is placed at a different location within the reservoir, such as in a different zone and/or different level; and has the same encapsulating compound but a different tracer. Preferred Components: The oil-field chemical is at least one chemical that is added in order to improve performance of the production in a petroleum reservoir or reduce unwanted reactions or precipitations. The chemical include scale inhibitors, corrosion inhibitors, wax inhibitors, hydrate inhibitors, surfactants, antifoaming agents, biocides, demulsifiers, hydrogen sulfide or oxygen scavengers, asphaltene inhibitors, or nanoparticles. Preferred Method: The method further comprises relating the absence, presence and/or concentration of the tracer or each tracer to the flow of fluid within the reservoir. Two different controlled-release compositions are applied to the reservoir. The method of producing the release composition comprises contacting a first and second salt in a gel crystallization zone containing the tracer, where the crystallization zone is placed between: two semi-permeable membranes, which permit passage of the first and second salts but prevent or impede passage of the tracer; and a first reactant zone containing the first salt and a second reactant zone containing the second salt, and each of the first and second reactant zones is in the form of a gel. The presence of the tracer in the produced fluids is taken to indicate a leakage from the plug. EXAMPLE - No suitable example given.