| The function, current status and development trends of worldwide oilfield inter-well waterflood tracer technique are reviewed. Discussions are made on the potential, feasibility and future real scale in-situ application of fluorinated benzoic acids (FBAs) as waterflood tracer for middle and high temperature reservoirs. Some physical, chemical and biological properties of FBAs are supplied for the concern of their candidature as waterflood tracers. 2-FBA, the most difficult FBA to detect is chose for research in this work. Ultra-sensitive analysis method of 2-FBA in oilfield brine is developed based on solid-phase extraction (SPE) in combination with gas chromatography- mass spectrometry (GC-MS) technique. The method includes pretreatment of sample, the derivation before column and instrumental GC-MS analysis.SPE technique is introduced and the influencing factors for separation and enrichment of 2-fluorobenzoic acid in trace concentration in oilfield brine by SPE are discussed. Operating parameters of SPE are studied and optimized including choice of SPE column model, acidity of medium, speed of sample loading, elution reagent and eluting speed. The extraction and enrichment of 2-FBA in brine are made by established SPE method that can treat 250ml sample in 60 minutes with a high recovery level of 2-FBA over 95%.Three routes of derivation for 2-FBA are compared and the PFB-ester method is selected to be the radical route of the sample treatment. To optimize the derivation procedure, the temperature and time of the derivation are studied. To avoid obtaining a large number of degradation products from the derivation agent, the derivation procedure was performed using solid K2CO3 as catalyst. The 4-methyl -fluorobenzoic (4-TMFBA) acid is used as the internal standard to modify the whole derivation procedure. The suitable concentration of derivatising agent and internal standard are studied also in the work. The recovery efficiency was over 95% and no complex interferences were produced with this PFB-ester method.GC-MS was used to measure the derivative of 2-FBA. Procedures of GC-MS measurement are studied and optimized. Fragments and their abundance of derivatives of 2-FBA and 4-TMFBA by PFB-ester derivisation are discussed and examined. A number of experiments for the optimization of instrument operating parameters were performed, including the mode of sample injection, time of injection, the programming of oven temperature, the voltage of detector, the acquisition mode of data, etc. The range of calibration curve in the measurement by GC-MS was estimated from 5 to 2000 ng/ml (R2 >0.9998) with internal standard calibration. The relative error was lower than 15% at 2-FBA concentration lower than 20ng/ml when a 1μ1 aqueous sample was analyzed. However, the relative error is down to 3% when 2-FBA concentration is over 50ng/ml. The measurement procedure of GC-MS takes 10 minutes per sample.The expanded uncertainty of the method is assessed and expressed. A figure model is established for evaluating the uncertainty. According to the rule of JJF1059-1999, thepartial uncertainties of every steps in the whole analysis procedure are calculated and the expanded uncertainty of a once measurement result is synthesized ultimately.Based on the studies of above, a method of ultra-trace measurement of 2-FBA in oilfield brine is established using SPE enrichment & separation solid-phase in combination with GC-MS. The lowest detective limit of 2-FBA is 4×10-9g/L when the 250ml sample was analyzed. The expanded uncertainty of a once measurement for 400ng/ml is 50ng/ml,that means the relative uncertainty is over 14%.The method described here is rapid, accurate, and reliable for ultra-trace determination of 2-FBA in oilfield brine. It can be used in analysis of 2-FBA in other samples such as environmental, biological and drug samples. The work makes also the base for analysis of other FBAs in oilfield brine when used as waterflood tracers in future.
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