Development of alternative tracers for oil reservoirs

The availability of a row of alternative tracers is important in studies of a petroleum reservoir, whose domain is traversed by several simultaneous sources, either injected or natural (e.g.: aquifer intrusion). This calls for the injection of multiple tracers. Thus the research proposal contemplated the development (preparation or synthesis, measurement methodology, and laboratory tests) of tracers to be used in applications in which the sole injection of tritiated water is not enough. For inter-well tests the alternatives sought for included: thiocyanide tagged with either 35S or chemical tracers (fluorbenzoic acids, FBA). The synthesis route for K35SCN and the analytical procedure for FBA had already been developed at CDTN, and they had been used in fieldwork. However, both methodologies still required some improvement, in particular FBA required analytical refinement to achieve lower detection limits. Also 14C and complexes of rare earths as activable have been considered. Activable tracers can be a quite efficient option since they are exempt from radioactivity manipulation in the field and do not interfere with the measurement of other radiotracers. However, complexation of the rare earth salts is required to make sure that the tracer will remain in the water phase and will not suffer either partition to the organic phase or adsorption in the solid surfaces of the reservoir rock pore walls. 14C used as the S14CN- ion has the advantage over 35S (t1/2 = 87 d) of its longer half life. It had been dismissed for some time mainly because it has to be imported, as only high flux reactors can produce it. Importation of radioactive material is rather time consuming and hampered by administrative and legal requirements. Production based on local capabilities had been preferred. But, besides its longer half-life, 14C-tagged organics can apt tracers for the observation of NORM's generation processes inside oil reservoirs, as well as its migration characteristics once they have been disposed in controlled radioactive waste repositories. A batch of 10 mCi of Ba14CO4 has been procured by the IAEA to be used in synthesizing KS14CN by the same route as with the radiosulfur. Work has also been started in the synthesis and use of a family of organic compounds of the thio-dicarboxylic acid group has also been under consideration; these compounds could be used either as 35S labeled radiotracers or as chemical tracers (measured by chromatography), or even as complexants for the lanthanoid activable tracers. The natural radioactive gas 222Rn has been added to the list as a partition tracer that can be potentially useful for the quantification of the oil saturation. It also has been frequently mentioned in the literature as an able tracer for the quantification of NAPL and DNAPL. An adequate methodology for its measurement in water samples has to be developed and optimized.