Mapping 3D Surface Charge Distribution in Nanoporous Shales

Shales have recently emerged as leading producers of oil and natural gas and as potential targets for geologic carbon sequestration. There is thus an increased need for enhanced prediction and manipulation of fluid behavior in these reservoirs. Shale porosity is dominated by micron to nanometer size pores that exist within clay and organic-rich matrices. Characterizing pore connectivity and surface charge within these sub-micron pore networks is critical to improving models of multiphase flow in shale reservoirs. This study presents an approach for mapping sub-micron surface charge distribution through a combination of imaging techniques. Scanning electron microscopy (SEM) and focused ion beam (FIB) milling are coupled with atomic force microscopy (AFM) to evaluate shale pore network structure and surface charge in three dimensions at sub-micron scales. This is accomplished through use of Kelvin probe force microscopy (KPFM) via the AFM that is capable of describing surface potential distribution at the scale of 10's of nanometers. FIB milling is used to dissect a series of small segments of shale for KPFM analysis and subsequent reconstruction of 3D surface charge distribution. SEM-FIB analysis also provides 3D mineral composition data through backscatter electron imaging and energy dispersive spectroscopy, which can be used to inform reactive transport models. Combining these techniques to characterize shale pore space and establish its accessibility to fluid flow helps predict the behavior of shale reservoirs when contacted with reactive fluids like hydraulic fracturing fluid or CO₂-saturated brine. The added knowledge of pore wall surface potential distribution may help to inform the use of charged surfactants for tuning reservoir wettability. Improving cross-scale characterization of nanoporous materials will assist in validating models for multiphase reactive fluid transport in shales.