The Reservoir Rock GeoBioCell: A Microfluidic Flowcell Developed for Controlled Experiments on Subsurface Microbe-Water-Rock Interactions

A better understanding of subsurface microbe-water-rock interaction in the Earth’s outer crust is of critical importance because it strongly influences the basic petro-physical properties of sedimentary rock. Over the past decade, miniaturized microfluidic flowcell prototypes of subsurface reservoir systems, usually called micromodels and named GeoBioCells herein, have been used to replace traditional column experiments. However, the inert pore structure in these micromodels does not contain the biogeochemical grain surface heterogeneities in actual subsurface rock reservoirs. In this study, we developed a next-generation microfluidic experimental test bed, herein called the Reservoir Rock GeoBioCell (RRGBC), in which an actual piece of subsurface reservoir rock is mounted within a microfluidic flowcell for experimentation.

Siliciclastic sandstones core samples of an oil-bearing subsurface reservoir were obtained for construction of the RRGBC. Custom petrographic rock sections (0.5 mm thick) were prepared from these core samples impregnated with Super Glue adhesive. Acetone was then used to remove the Super Glue and physically separate thin sections from the glass slides. A PDMS mold (~3-4 mm thick) was prepared to hold the thin section between a microfluidic inlet and outlet channels. The thin section in PDMS mold was covered with a PDMS-coated glass coverslip to help provide a pressure seal for core thin section (Figure 1, left-top).

Multi-photon laser confocal microscopy of the RRGBC showed pore connectivity to an imaging depth of ~400 µm within the thin section. The geochemical reactive sites were characterized using Raman Backscattering Microscopy, confirming the presence of reactive quartz. A fluorescent tracer test was conducted to identify micro-flow paths and solute breakthrough within the thin section (Figure 1). A multiphase flow experiment was performed to trap residual light oil in the thin section. A mixed-culture of oil-degrading biofilm was developed to degrade the entrapped oil, and an oxygen sensitive fluorophore was used to observe regions of microbial activity. The RRGBC developed in this study is useful for quantitatively testing and monitoring the physical, chemical and biological factors that affect subsurface carbonate diagenesis and other biogeochemical reactions.