Thermo-Sensitive Nanogels for Targeted Tracer Release in Push and PullOperations

摘要

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 wellshut-down.In the present paper,we propose using thermo-sensitive nanogel carriers for targeted releaseof tracers for SWCTTs.This approach has the potential to significantly reduce the time and amount ofchemicals required by a SWCTT.The targeted tracer release method was inspired by previously developed drug delivery applications usingstimuli-sensitive nano-capsules.Nanoparticles loaded with medical cargo were synthesised to target specificsites.The release of the active principles would then be triggered by an in-situ(temperature,pH)or ex-situ(magnetic field,light)stimuli.For the application to tracer studies(with focus on SWCTT),a poly-N-isopropylacrylamide(pNIPAm)based nanogel was synthesized and tested due to its thermo-sensitivenature.PNIPAm molecule has a unique memory effect with respect to temperature.This effect is exploredas a mechanism to both load and release the tracers for a SWCTT.PNIPAm nanogels or hydrogels is a highly hydrophilic,cross-linked polymeric network.When thetemperature of the solution is increased above the lower critical solution temperature(LCST)of PNIPAmmolecule,the capsules exhibit a reversible collapse effect,which is capitalised in the release of the tracermolecules.The hydrodynamic diameter of capsules were measured using Dynamic Light Scattering(DLS)and were found to be 195 ± 1.4 nm at 25 °C and 73 ± 1.45 nm at 45 °C respectively.The nanogels exhibita reduction in volume to 8 times when the temperature is increased from 25 °C to 45 °C.This change involume acts as a lock-in mechanism once the tracer is loaded and open-up to release loaded tracers.In-orderto study the encapsulation and release of tracer compounds,passive and partitioning tracers were loadedinto the structures.The capsules showed a significant tracer loading efficiency.For studying the release rateand mechanism,increase in temperature was used to trigger the release of tracers.The developments in the SWCTT have been slow since it was originally introduced in 1973,eventhough this test is one of the most used tracer tests in the oil industry.In the present work,we show hownanotechnology can be used to reduce the large amounts of chemicals and time required by SWCTTs.Concepts and results about the synthesis of the nano-carriers,loading and releasing of the tracers arepresented and discussed.