Development of new oil/water partitioning tracers for the determination of residual oil saturation in the inter-well region of water-flooded reservoirs

Most of the hydrocarbon-rich large unexplored basins are located in remote and/or highly environmentally sensitive regions. As more and more oilfields reach maturity at the end of the secondary stage of recovery, while on average more than 50% of the original reserves of hydrocarbons are left in place, the future of oil production on the Norwegian Continental shelf (NCS) will increasingly rely on IOR projects to face the demand. A partitioning inter-well tracer test (PITT) is a type of tracer test that measures the water contactable saturation of immobile oil (SOR) in the inter-well region of water flooded reservoirs. Knowledge about SOR in the swept volumes between injector/producer pairs is a key parameter for the design and evaluation of IOR projects. The PITT explores the lag in production experienced by an oil/water partitioning tracer relatively to a passive water tracer which directly correlates to SOR. This principle was introduced to the industry in 1971 and relied on the use of molecules successfully applied in hydrology and/or labelled with radioactive nuclides for easier detection. The conditions encountered in oil reservoirs, particularly in deep oil formations, are significantly harsher than in the near surface subsoil. Thus, several unsuccessful inter-well tracer tests resulted from a poor selection of the tracer compounds used at that time because of insufficient knowledge about their behaviour under typical reservoir conditions. Much work has been done to improve the original concept of the PITT regard its deployment and interpretation. However, little effort has been put in place to establish a systematic procedure for selecting, testing and describing the dynamic behaviour of the substances used as oil/water partitioning tracers. Thus, this thesis aims to present a methodology for selection and testing of new PITT tracer candidates, with the results and findings of its application to a selected group of molecules. The methodology presented here starts by describing the base requirements for selection of new oil/water partitioning tracer candidates. Additionally, guidelines for testing and qualification are presented. There are several steps in the qualification procedure. These can be divided into static stability experiments, development of analytical methods for laboratory samples, development of analytical methods for identification and quantification of the stable molecules in real produced waters, characterisation of the oil/water partition coefficient (K) of the molecules, and dynamic flooding experiments using cores of consolidated sedimentary rock. Following this method, step by step, 16 molecules from 4 different chemical “families” were selected and tested for qualification as new oil/water partitioning tracers. The static stability experiments evaluated the thermal stability of the PITT tracer candidates, in the absence and presence of typical reservoir rock materials, different pH conditions, and elevated salinity up to 150 0C for 12 weeks contact time. Ultra-performance liquid chromatography (UPLC) coupled with ultra-violet detection (UV) and high-resolution mass spectrometry (HRMS), and gas chromatography (GC) coupled with flame ionisation detection (FID) were the techniques used to follow the concentration of the PITT tracer candidates along the 12 weeks of experiment. UPLC-HRMS was used to try to obtain relevant information to describe the observed phenomena. The static stability experiments proved that only 5 of the 16 tested compounds were stable for 12 weeks up to 150 ℃. Two additional compounds were stable for the same period up to 125 ℃. This is sufficient to allow for their use in most oilfields, and they were thus included as possible inter-well PITT tracers. These experiments also revealed dramatic interactions between some of the studied molecules and clay minerals of undefined nature, as well as thermally driven degradation of the candidates that is well described by a pseudo-first order kinetic model. The latter two findings open the possibility of using tracers to retrieve information about temperature and geochemistry/mineralogy in the inter-well region, though the latter requires further development. A method based on sequential direct immersion (DI) headspace (HS) solid-phase microextraction (SPME) proved effective as analytical sample pre-treatment followed by GC-MS/MS for analysis of the PITT tracer candidate concentrations in real production waters. The DI-HS-SPME-GC-MS/MS method allows for quantification of the stable molecules investigated in low ng/L concentrations and introduces significant improvements in comparison to the commonly used methodologies for analysis of tracers in produced waters: it requires just 5 mL of sample and eliminates the need for use of organic solvents in the laboratory. Furthermore, sample processing times are significantly reduced as the cleaning/concentration step becomes much faster. This is of utmost importance for a PITT, as several hundreds of samples are analysed in these examinations. The characterisation of the K-values of the stable PITT tracer candidates revealed that they will likely vary along the volume swept between injector/producer pairs. K is influenced primarily by the ionic strength of the aqueous phase and composition of the hydrocarbon phase, and to a smaller extent by temperature T. The influence of temperature can, however, be very relevant: The K-value is used in the calculation of SOR together with the times of arrival of the different tracers using the same landmark of their respective production curves (i.e., the theoretically most correct is the first moments of the curves). It is likely that the temperature varies between injector and producer well-pairs. The variation of the K-value as function of T needs to be accounted for to determine accurate SOR values. Variations of the ionic strength will lead to even bigger variations of the K-value independently of the valency of the ions present in the aqueous phase. The experiments performed also confirmed the constant and reversible equilibrium distribution of the oil/water tracer candidates between hydrocarbon and aqueous phases, as well as their suitability for use on most oilfields of the NCS. The flooding experiments were performed on sandstone and chalk cores prepared both to pure water saturation and to residual oil saturation, SOR. Residence time distribution analysis (RTD) was used to interpret the production curves. These experiments proved that the PITT tracer candidates behave as water tracers in the absence of hydrocarbons, with no significant difference encountered between their production curves and the ones obtained from the reference water tracer (tritiated water). SOR was determined for two different mass recovery landmarks in each of the experiments for all partitioning tracers using the K-values previously determined in the experiments for characterisation of the K-values. Good agreement between all SOR values measured by the tracer candidates and the values determined by a balance to the oil used to prepare the cores was encountered. This is also valid when SOR measured by the tracers is compared to the value obtained by the differences in water flooded pore volumes measured by tritiated water. The methodology presented and applied in this thesis produced 7 new oil/water partitioning tracers ready to be used in oil fields with low probability of failure. The findings and observations presented here can be incorporated into reservoir models to obtain more accurate data from PITTs, and consequently better reservoir description. Furthermore, the reinjection of produced waters will lead to contaminations of the inter-well region with tracers used there. Thus, the present study offers guidelines and methods for the development of new tracers. The oil industry, service companies, and other researchers working with tracer technology will be the primary beneficiaries of this study, that will hopefully contribute to disseminate the use of PITTs by the industry. This technology has a large potential to contribute to a future efficient and profitable oil production