Smart Tracers for Geothermal Reservoir Assessment

Abstract

We have developed a method of manufacturing and testing ?smart? encapsulated tracers that release at specified temperatures. These encapsulated tracers can be used to simultaneously map both flowfield distributions and flowpath-specific temperature within a reservoir. Here we report on the methods of fabrication and initial tests of durability, obtaining release as a function of temperature, and flow through replicated rock fractures. The tracer capsules were created as double emulsions, using microfluidic techniques, with 4 wt% fluorescein solution inside shells formed from commercially available materials. The smart tracer capsules were subjected to several geothermal temperatures in internally pressurized, externally heated vessels for durations of 2 to 16 hours. Fluorescein contents in the surrounding fluid were measured afterwards with UV-Vis to quantify encapsulated tracer release. Capsules of one shell material displayed significant fluorescein release at temperatures as low as 80 °C, while capsules made from another shell material showed tracer release over a much higher and narrower temperature range (160 ? 200 °C). The fluorescein tracer itself also underwent substantial degradation over times > 2 hours at 200 °C. To validate the transport of these encapsulated tracers, natural rock fracture surfaces were duplicated via 3-D printing techniques to form a transparent flow cell. Smart tracers were pumped through the flow cells at a variety of fluid velocities to verify that they can be easily transported into fracture systems. These initial tests demonstrate the ability to manufacture encapsulated tracers capable of tracer release at temperatures above a specified threshold, and rugged enough to survive pumping and transport through a fracture.