Mário Silva1,2,3 , Tor Bjørnstad3 Results and Discussion Solid-Phase Microextraction as Sample Preparation Technique in Tracer Technology Mário Silva1,2,3 , Tor Bjørnstad3 1National IOR Centre of Norway 2University of Stavanger, Department of Petroleum Engineering 3IFE – Institute for Energy Technology, Tracer Department Introduction Although rare in the oil industry, there is significant evidence [1] that Partitioning Inter-Well Tracer Tests (PITT) are an effective tool to assess residual oil saturation (Sor) in the swept volumes of the inter-well region. This is highly important information for efficient reservoir management. The PITT is based on the time delay experienced by a partitioning tracer compared to a passive tracer when transported in an oil reservoir. The time delay is caused by the equilibrium guided distribution of the partitioning tracer between the oil phase and the flowing phase [1]. In a field in Sor conditions, where the oil flow rates are negligible compared to the water flow rates, the injection of a selection of partitioning tracers, combined with frequent sample analysis of produced fluids will allow Sor determination from the tracers production curves [1,2]. A PITT tracer needs to be thermally, chemically, and biologically stable under reservoir conditions, show no or negligible adsorption to reservoir rock, be unique in the reservoir environment, environmentally acceptable, and have high analytical sensitivity. Therefore analytical techniques play a key role in tracer technology. Lower quantification limits provide more accurate production curves, and allow reducing the amount of injected tracer on a field test. Solid-Phase Microextraction (SPME) is a solvent free sample preparation/concentration technique, based on the adsorption of target analytes into a thin extraction phase that can be inserted directly into the chromatograph injection port. Here, desorption occurs thermally (GC) or by dissolution into the eluent phase (HPLC). SPME can be directly immersed in the sample (DI) or in its headspace (HS), and the analytes will adsorb to it until equilibrium is reached. Nowadays, SPME is widely used in environmental sciences, food and beverage characterization, and in bioanalytical and clinical applications, in the identification and quantification of trace amounts of a wide range of compounds [3-5]. To the best of our knowledge, SPME has never been used as sample preparation technique in oil reservoir tracer technology. This study focused on the development of a test method based on SPME coupled with GC-FID for the analysis of 1,2-hexanediol and 1,6-hexanediol, two of fifteen PITT tracer candidates under testing. Analytical conditions were optimized, and samples from PITT tracer candidates stability experiments were analyzed. References [1]. Viig, S.O.; Juilla, H.; Renouf, P.; Kleven, R.; Krognes, B.; Dugstad, Ø.; Huseby, O.K.; (2013), "Application of a New Class of Chemical Tracers to Measure Oil Saturation in Partitioning Interwell Tracer Tests", SPE 164059, presented at the “SPE International Symposium on Oilfield Chemistry”, Woodlands, Texas, 8-10 April 2013. [2]. Cooke, C.E. Jr. 1971. "Method of determining fluid saturations in reservoirs". US patent No. 3,590,923. [3]. Érica A. Souza-Silva, Ruifen Jiang, Angel Rodríguez-Lafuente, Emanuela Gionfriddo, Janusz Pawliszyn, "A critical review of the state of the art of solid-phase microextraction of complex matrices I. Environmental analysis". Trends in Analytical Chemistry 71 (2015) 224–235. [4]. Érica A. Souza-Silva, Emanuela Gionfriddo, Janusz Pawliszyn, "A critical review of the state of the art of solid-phase microextraction of complex matrices II. Food analysis". Trends in Analytical Chemistry 71 (2015) 236–248. [5]. Érica A. Souza-Silva, Nathaly Reyes-Garcés, German A. Gómez-Ríos, Ezel Boyacı, Barbara Bojko, Janusz Pawliszyn, "A critical review of the state of the art of solid-phase microextraction of complex matrices III. Bioanalytical and clinical applications". Trends in Analytical Chemistry 71 (2015) 249–264. Materials and Methods Investigated Conditions: Type of SPME fibre (PA, PDMS-DVB, CAR-PDMS-DVB) Direct immersion (DI) or headspace extraction (HS) Time of extraction (5 – 30 minutes) Temperature of extraction (20 – 100 °C) Matrix salinity (Gulfaks formation water – saturation with NaCl) (salting out effect) Optimal Conditions: CAR-PDMS-DVB fibre HS extraction 15 minutes of extraction 70 °C Matrix saturated with NaCl Analysis was carried out with a GC-FID Varian 3800 equipped with a 30 m SGE HT5 capillary column, splitless injection at 250 °C, helium at 1 mL/min as carrier and detector at 300 °C. Calibration curves for 1,2-Hexanediol and 1,6-Hexanediol were build up to 10.000 ppb. Figure 1. Types of tested SPME fibres: (A) PA; (B) PDMS-DVB; (C) CAR-PDMS-DVB The developed method was used to determine the concentration of 1,2-Hexanediol and 1,6-Hexanediol in samples from the stability experiments, where a mixture of 15 PITT tracer candidates with 10 ppm concentration was prepared in Gulfaks formation water. 2 mL of solution were sealed in glass vials with three types of rock material (Kaolinite, Berea sandstone and limestone) and without any rock. The vials were put in thermal cabinets, with gentile agitation, at temperatures ranging between 50 °C and 150 °C. Tracer concentration is monitored along time. Figure 2. General representation of the entire process: (A) Sealed vials setup in a thermal cabinet; (B) Samples retrieved for analysis prior to preparation; (C) One sample ready for analysis of 1,2-Hexanediol and 1,6-Hexanediol through the developed HS-SPME-GC-FID test method; (D) Extraction of the analytes from a sample with the CAR-PDMS-DVB fibre in the SPME system. Results and Discussion LoQ = 250 ppb LoQ = 210 ppb The example chromatogram in figure 3 shows that the developed test method identifies 7 other PITT tracer candidates in addition to the 2 targeted ones. No significant matrix effect is evident. A linear calibration up to 10.000 ppb was obtained with LoQ of 250 and 210 ppb for 1,2-hexanediol and 1,6-hexanediol respectively. The results are generally encouraging, and so further investigations about use of SPME coupled with GC-MS and its application to real field samples are ongoing. Figure 3. Example chromatogram from a stability test sample with peak identification and quantification limits (LoQ) for 1,2-Hexanediol and 1,6-Hexanediol Each point in figure 4 is the average result of three replicas analysed with the developed test method. Variation coefficients varied from a minimum of 3,8% to a maximum of 8,2%. 1,2-Hexanediol and 1,6-Hexanediol show excellent thermal stability after one week. No degradation is observed in every temperature tested. Figure 4. Preliminary thermal stability data for 1,2-Hexanediol and 1,6-Hexanediol after 1 week at 5 temperatures Acknowledgements The authors acknowledge the Research Council of Norway and the industry partners: ConocoPhillips Skandinavia AS, BP Norge AS, Det Norske Oljeselskap AS, Eni Norge AS, Maersk Oil Norway AS, DONG Energy A/S, Denmark, Statoil Petroleum AS, ENGIE E&P NORGE AS, Lundin Norway AS, Halliburton AS, Schlumberger Norge AS, Wintershall Norge AS of The National IOR Centre of Norway for support. Conclusions A test method for the determination of 1,2-Hexanediol and 1,6-Hexanediol by HS-SPME-GC-FID was developed. The two compounds are part of a group of 15 PITT tracer candidates currently under study. They were successfully analysed with the developed method in laboratory samples from the stability experiments containing a mixture of all 15 candidates with 10.000 ppb of initial concentration. SPME appears as a promising technique for sample preparation/concentration in tracer technology and its use will be investigated further.