Thermo-Sensitive Nanogels for Targeted Tracer Release in Push and Pull Operations

Abstract The single-well chemical tracer test (SWCTT) is widely used in the determination of residual oil saturation (SOR) in the near-well region. SWCTTs typically require large amounts of chemicals and some days of well shut-down. In the present paper, we propose using thermo-sensitive nanogel carriers for targeted release of tracers for SWCTTs. This approach has the potential to significantly reduce the time and amount of chemicals required by a SWCTT. The targeted tracer release method was inspired by previously developed drug delivery applications using stimuli-sensitive nano-capsules. Nanoparticles loaded with medical cargo were synthesised to target specific sites. The release of the active principles would then be triggered by an in-situ (temperature, pH) or ex-situ (magnetic field, light) stimuli. A novative approach to address the current limitation of classical SWCTT is based on the biomedical background. It consists in using a poly-N-isopropylacrylamide (pNIPAm) based nanogel, known for its thermo-sensitive nature, to ensure the in-situ delivery of tracer molecules. This effect is explored as a mechanism to both load and release the tracers for a SWCTT. PNIPAm nanogels or hydrogels are highly hydrophilic, cross-linked polymeric networks. When the temperature of the solution is increased above the lower critical solution temperature (LCST) of PNIPAm molecule, the capsules exhibit a reversible collapse effect, causing the release of the tracer molecules. The hydrodynamic diameters of capsules were measured using Dynamic Light Scattering (DLS) and were found to be 195 nm at 25 °C and 73 nm at 45 °C. The nanogels exhibit a reduction in volume to 8 times when the temperature is increased from 25 °C to 45 °C. This change in volume acts as a lock-in mechanism once the tracer is loaded and open-up to release loaded tracers. The study of the encapsulation and release of tracer compounds was achieved using passive and partitioning tracers loaded into the structures. The capsules showed a significant tracer loading efficiency. For studying the release rate and mechanism, increase in temperature was used to trigger the release of tracers. Although the SWCTTs are the most used tracer tests by the oil industry, their development have been relatively slow since it was originally introduced in 1973 (Deans 1971). The present study aims at presenting a novel approach on how nanotechnology can be used to reduce the large amounts of chemicals and time required by classical SWCTTs. Concepts and results about the synthesis of the nano-carriers, loading and releasing of the tracers are presented and discussed.