Fluorescent Nanobeads: A New Generation of Easily Detectable Water Tracers
Authors: Agenet Nicolas , Moradi-Tehrani Navid , Tillement Olivier 
Published: May 2016

Abstract
Reservoir monitoring is an essential tool to optimize oil production. Among other techniques, water tracers are very useful to understand complex flow patterns that may arise between injection and production wells during waterflood operations. This paper proposes an innovative approach for developing new fluorescent tracers based on silica nanoparticles; it describes their synthesis and properties during coreflood experiments.
The new nanoparticles can be coded with various fluorophores, which can be easily detected using on-site equipment offering direct tracer quantification and even on-line monitoring capabilities.
Silica nanoparticles containing one or more organic dyes or lanthanide-based fluorophores were synthesized. The versatility of the synthesis procedure allows coding the nanoparticles with theoretically dozens of tagging combinations. The sol-gel synthesis technique yields 50-nm-diameter nanoparticles which are treated to tune their surface properties. The conservative tracer behavior of such nanoparticles is ensured by proper surface functionalization, and is demonstrated by coreflood experiments. This virtually large tracer library is combined to a unique time-delay fluorescence detection technique which allows simple on-site tracer quantification on multiple nanoparticle types with minimal sample preparation.

Introduction
InterWell Tracer Tests (IWTT) are routinely used in the petroleum industry (Du and Guan 2005). This technique allows gathering informations on well-to-well connections and subsurface flow paths. Aqueous tracers can be split in two main groups upon their function: passive tracers which travel at the same velocity as the injected water and partitioning tracers which are partly soluble in oil, leading to a delay in their breakthrough. Used along with passive tracers, partitioning tracers permit to evaluate the remaining oil saturation in the contacted zone (Tang 1991, Tang, 1992, Tang 1995, Wood 1990, Deeds 2000, Dwarakanath 1999, Jin 1995, Jin 1997, Mariner 1999). Tracers can be selected in the following families: naturally occurring tracers (isotopic or ionic composition footprints), radioactive isotopes, and chemical derivatives (Du and Guan 2005, Hutchins 1991). Each family possesses its own advantages and limitation: restricting regulation on radioactive tracers can make them difficult to implement on field in some areas; chemical tracers such as fluorobenzoic acids (FBAs) have become routinely used for field tracer campaigns as they can be detected with a low detection limit. However they require fine analyses which cannot be performed in standard labs. Moreover, in each family the number of available chemical tracers remains limited when repeated tracer campaigns are required on the same field over time.
This paper describes the design of nanoparticles as tracers and coreflood tests to assess their propagation in porous media. Rare-earth based organometallic complexes are known to possess a long fluorescence lifetime. A low detection limit of the nanoparticles can be achieved by incorporating such complexes. Taking advantage of the fluorescence lifetime differences, background fluorescence from organic oil contaminant of the sample is suppressed using time-resolved fluorescence detection. The nanobeads associate the transport behavior of nanoparticles in porous media with the fluorescence properties of rare-earth elements complexes to give access to easily detectable passive tracer.