Development of a Nanofluidic Chip Representative of a Shale Sample

Abstract:

Fluidic models are used to replicate and improve understanding of fluid flow in rocks. In this study, a shale-mimicking fluidic model is developed by characterizing a shale sample, converting that information into a 2D representation, excising this pattern into a substrate while preserving its nano-scale features, and sealing the etched pore structure without altering nano-scale components. Imaging, helium pycnometry, nitrogen sorption, mercury injection, and total organic carbon (TOC) measurements are collected on a Bakken shale sample. Using Focused-Ion-Beam (FIB) Scanning Electron Microscopy, a 3D image of the sample is collected with a voxel size of 2.44 nm² by 5 nm. Pore volume and organic content are calculated using image segmentation and confirmed via helium pycnometry, mercury injection, and TOC measures. Pore and throat sizes used in the model result from 3D image analysis and pore size distributions calculated from sorption data. These findings are used to create a quasi 2D representation of the sample with interconnected pore space. To create a nanofluidic chip, one consideration is the smallest size that can be etched onto a substrate. This depends on etching technique, equipment capabilities, user experience, and substrate itself. To that end, channels were etched using FIB under various settings onto substrates. For Pyrex, the smallest feature created was about 50 nm; however, characterization revealed porosity features as small as 5 nm. We are exploring the use of e-beam lithography techniques to create more scalar representations. Successful bonding is as a major challenge. We have had success bonding Pyrex to Pyrex using a combination of pressure and temperature to create microfluidic chips – chips with smallest features of a few micrometers. The main concern is that pressure and temperature treatment will likely result in significant degradation of smaller pattern features in nanofluidic chips.