Transport Simulations on Scanning Transmission Electron Microscope Images of Nanoporous Shale

Abstract

Digital rock physics is an often-mentioned approach to better understand and model transport processes occurring in tight nanoporous media including the organic and inorganic matrix of shale. Workflows integrating nanometer-scale image data and pore-scale simulations are relatively undeveloped, however. In this paper, a workflow is demonstrated progressing from sample acquisition and preparation, to image acquisition by Scanning Transmission Electron Microscopy (STEM) tomography, to volumetric reconstruction to pore-space discretization to numerical simulation of pore-scale transport. Key aspects of the workflow include (i) STEM tomography in high angle annular dark field (HAADF) mode to image three-dimensional pore networks in µm-sized samples with nanometer resolution and (ii) lattice Boltzmann method (LBM) simulations to describe gas flow in slip, transitional, and Knudsen diffusion regimes. It is shown that STEM tomography with nanoscale resolution yields excellent representation of the size and connectivity of organic nanopore networks. In turn, pore-scale simulation on such networks contributes to understanding of transport and storage properties of nanoporous shale. Interestingly, flow occurs primarily along pore networks with pore dimensions on the order of tens of nanometers. Smaller pores do not form percolating pathways in the sample volume imaged. Apparent gas permeability in the range of 10−19 to 10−16 m2 is computed.

Chemical and Reactive Transport Processes Associated with Hydraulic Fracturing of Unconventional Oil/Gas Shales

“…The 3D reconstructions of pores teach that nanopores less than 20 nm dominate in terms of number , consistent with the observations of Curtis et al (2012) but do not form percolating networks at the scale of observation. Roughly one-half of the porosity is unconnected and this unconnected porosity lies mainly in the smaller pore sizes.…”

“…Interestingly, the inferences of conductive pore sizes from pulse-decay tests 112 are consistent with measurements from imaging of nanopore networks 44,140 These drastically different methods support the assertions that pulse-decay tests give insight into the pore sizes through which gas is flowing and that the Darcy−Klinkenberg description of single-phase transport is appropriate to describe transport in many shale samples.…”

“…Recently, Frouté and Kovscek (2020) , used STEM to image a representative volume of organic matrix material (3.5 μm × 3.5 μm × 100 nm) from a Barnett shale sample at a spatial resolution of about 1.24 nm 3 . Measurements span pore sizes between about 5 and 80 nm.…”

Scalable Simulation of Pressure Gradient-Driven Transport of Rarefied Gases in Complex Permeable Media Using Lattice Boltzmann Method

“…Technologies, such as scanning electron microscopy (SEM) [24] and scanning transmission electron microscopy (STEM) have enabled characterization of the fabric and pore structure of shales and tight formations in detail [25]. Nanoscale characterization techniques elucidate geometric details of the pore network structure of shales and tight formations.…”

“…BB boundary treatment may be combined with either SR or MDR to capture the slip velocity at the walls in simulations of gas flow in complex microchannels. An example is a combination of BB and MDR that was used to simulate flow of methane in a three-dimensional pore network, which was developed using nanometer-scale STEM images of a shale sample [25]. These simulations enable examination of transport property of the characterized sample by computing the apparent gas permeability [29].…”