Nonlinear tracers and subsurface sensing with chemical waves

We demonstrate that autocatalytic reaction schemes generate and propagate nonlinear spatio-temporal chemical waves, which interact with, are excited by, and retain memory of, detailed characteristics of the porous media through which they traverse. The use of chemical waves as nonlinear tracers in the subsurface is proposed for advanced characterization, sensing, and non-traditional computing for subsurface engineering. Arguably the most famous and oft-studied examples are the Belousov-Zhabotinsky-type reaction systems involving metal-catalyzed oxidation of an organic substrate by bromate. We investigate chemical wave interactions of the bromate-ferroin-1,4-cyclohexanedione system with microfluidic experiments and computer simulation, demonstrating on-off switching, simple logic gates, and excitability to fracture and pore texture. Contrasting with typical "linear" tracers, chemical waves can propagate faster than conservative tracers, even against an advective gradient. Using a generalized "oregonator" type dynamics, numerical experiments of field-scale push-pull or cross well injection show that chemical waves act as nonlinear tracers that can report on form or connectivity of fracture networks, presence or absence of certain organic constituents, and trace amounts of contaminants. Simpler autocatalytic reaction schemes, such as pH oscillators, could possibly play a role in sensing stimulated fracture networks in granites, siliceous shales, or other rocks with low short term pH-buffering capacity. Inasmuch as the chemical kinetics are tunable, engineered chemical waves can alternatively overcome temporal and spatial limitations of other subsurface sensing mechanisms, e.g., increasing resolutions below that of seismic imaging, or overcoming attenuation of electronic/RF sensors.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. SAND2018-12720A.