Tracer technology for well experiments is one of very few applicable technologies for collection of unique dynamic data of reservoir flows. Two main tracer tests are commonly used for reservoir characterization: (i) the Single-Well Chemical Tracer Test (SWCTT) and (ii) the Inter-Well Tracer Test (IWTT) which includes Partioning Inter-Well Tracer Tests (PITT). SWCTT and PITT give access to the residual oil saturation (SOR) respectively in the near-well and interwell regions. Non-partitioning IWTT allows assessing qualitatively and quantitatively interwell flow connections, swept volumes, etc., resulting in an improved reservoir model.

We have previously introduced the concept for a new class of potential partitioning fluorescent tracers for SWCTT tests (IOR 2017 in Stavanger, Norway ; EAGE conference). The tracer is a complex compound by an organic chelate and a fluorescent metallic center. The results have shown that it was possible to functionalize the chelate with an ester function to theoretically increase the affinity of the final complex for the oil phase. However, the complexation of the new modified chelate with the metallic center was not solved.

In the present study, the complexation strategies and characterization tools for detecting and quantifying the tracer will be discussed. Especially, High-Performance Liquid Chromatography (HPLC) coupled with a Time-Resolved Fluorescence (TRF) detection allowed separating the different partitioning compounds and their passive form with a high specificity. A series of partitioning tests have been carried-out using synthetic production water and both synthetic oil and a crude oil from the Norwegian continental shelf.

Against the expectation, close to 100% of the ester tracer was found in the aqueous phase after contact with the oil. This result has been confirmed for two tracers with different ester chain length (ethylester and butylester). Liquid Chromatography coupled with Mass Spectroscopy (LC-MS) characterizations performed on the butylester form before and after contact with oil have confirmed the observations and results obtained by HPLC. Moreover, the LC-MS characterization provided a better understanding about the environment of the metallic ion, particularly on its degree of complexation which suggests that most of the final complex is negatively charge.

Given that the reaction of hydrolysis of the ester is dependent on temperature, pH and salinity the tracer could be relevant as a “probe” to obtain accurate data on those three parameters in-situ. The ester has in this case no partitionning behavior and any changes in the previously cited parameters will affect its kinetic of hydrolysis.


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