Thermal stability and interactions with sedimentary rocks under typical reservoir conditions of selected pyridines investigated as phase partitioning tracers

Pyridine, 2-hydroxy-6-methylpyridine, 3-hydroxypyridine, and 4-methoxypyridine are evaluated as potential phase-partitioning oil field tracers. Their stability is tested in a brine for 12 weeks at temperatures between 25 °C–150 °C, and at initial pH values of 5,5; 7,1; 8,0. Interactions with kaolinite clay, Berea sandstone and limestone are also evaluated. The main results are as follows: pyridine is stable up to 12 weeks at 150 °C, and not influenced by the rock substrates or pH. 2-hydroxy-6-methylpyridine becomes unstable at T ≥ 50 °C, is not affected by the rock substrates, and exihibts slower degradation kinetics at higher pH values. 3-hydroxypyridine is unstable at T ≥ 75 °C, sensitive to the presence of kaolinite in a combined effect with pH, and exihibts slower degradation kinetics at higher pH. 4-methoxypyridine degrades at T ≥ 75 °C, is characterised by a strong interaction with kaolinite, and is insensitive to pH.

The degradation of 4-methoxypyridine in the absence of kaolinite clay follows pseudo first-order kinetics. This compound could indicate the temperature in the swept volumes, and in conjunction with a fully conservative tracer indicate the presence of clays. Pyridine exhibits the required stability and lack of interaction with rock materials to be used as PITT tracer in oil reservoirs. However, it is present in oils and its concentration levels in production waters should be evaluated prior to its use.