High-resolution structural characterization of shale nano-porosity
Keywords: Energy Resources Engineering.

Characterization of the microscopic details of the fabric of shales is important to understand transport and reaction processes. Insight into key shale properties, including mineral composition, organic content, wettability, porosity, pore geometry and permeability, is paramount to understand displacement and trapping mechanisms within shale. Today, there remains a significant lack of fundamental understanding of the multiple physical processes occurring in shale systems, especially at the micro- to nano-scale. Current characterization methods often fall short of accurately measuring properties at the relevant scale, down to the nanometer. With exceptional sub-nanometer resolution, electron microscopy techniques alone offer fine enough insight into the pore network to resolve shale nanostructures. In this work we combine complementary imaging techniques with nanoscale resolution such as Focused Ion Beam - Scanning Electron Microscopy (FIB-SEM), and Scanning Transmission Electron Microscopy (STEM) to characterize the microstructure, texture, and mineralogy of shale specimens of different origins to better understand fundamental mechanisms at the pore scale. With exceptional sub-nanometer spatial resolution, Scanning Transmission Electron Microscopy (STEM) tomography provides compelling opportunities for shale imaging. STEM involves rotation of a thin sample through an angular range, acquisition of two-dimensional images at angles throughout the range, and tomographic reconstruction of a three-dimensional volume. We quantitatively evaluated the capabilities of Electron Tomography for shale applications, by performing tomographic reconstructions of a numerical phantom, comparing storage and transport metrics, and evaluating the challenges imposed by the acquisition and processing modalities. We found that petrophysical properties typically used to characterize shales at the macroscale, such as porosity or permeability, are often ill-suited to evaluate STEM tomography outcomes, and proposed a misclassification index to assess pore network prediction performance. The missing wedge (a limitation in rotational angle range), experimental noise, and the reconstruction algorithm each contribute to pixel misclassifications, with the missing wedge being the main experimental bottleneck. A sensitive balance between angular range, angular increments, and electron dosage is required to achieve sufficient spatial resolution and signal-to-noise ratio (SNR), while preserving the finest features from electron damage. Our work revealed that, for fields of view of roughly 3.5 μm by 3.5 μm and pixel widths of 1.24 nm, describing porous structures below 10 nm in diameter remains challenging, but the remarkable nanometer spatial resolution offers a clear opportunity for shale applications. Next, we used complementary FIB-SEM and STEM imaging techniques to provide high-resolution insight into the pore networks of a Barnett sample, to understand better how nanopore networks contribute to transport and storage properties. The correlation of high-resolution datasets provides quantitative measurements of fundamental storage and transport properties at the relevant pore scales. Different pore types can be identified (intra-OM, inter-OM, mineral pores, fractures), and classified in terms of pore geometries (size, shape, orientation), and their control over petrophysical properties (volume storage, connectivity). STEM tomography shows that a large number of nanopore networks exist both within the organic matter and in between organic matter and particles. Observations in FIB-SEM indicate that, although nanoscale networks dominate in number and have an important volumetric contribution, most of them remain isolated from connected microfractures in the regions sampled from the Barnett plug. Macroscopic transport in the FIB-SEM volumes probed occurs through the fracture network. Combining 3D STEM tomography with numerical simulation methods, we demonstrated a digital rock workflow to study methane transport through nanoporous shale. The workflow includes sample preparation, image acquisition by STEM tomography, volumetric reconstruction, pore-space discretization, and numerical simulation of pore-scale transport. The lattice Boltzmann method (LBM) provides a tool to transform spatial data into information relevant to transport of gases and liquids. We selected a sub-volume to create a computational mesh suitable for simulation, comprised of roughly 1 million voxels (sub-volume: 79 x 82 x 194 nm, voxel size: 1.24 x 1.24 x 1.5 nm). LBM simulations, conducted by our collaborators at the University of Wyoming, offered an insight into the pressure distribution and velocity profiles through the distinct pore channels, for the slip flow and transitional flow regimes that dominate gas transport in shale reservoirs. Identifying sedimentary facies, their characteristics and distribution, is a common method to tackle shale heterogeneity and is key to identifying high resource intervals (i.e. sweet spots), optimizing well placement (i.e. landing points) and forecasting production. We identified six microfacies representative of a Vaca Muerta sample, and characterized the two most porous microfacies through FIB-SEM and STEM imaging to understand the characteristics and roles of its connected pores. In both microfacies, the larger nanopores play a key role in connecting the abundant organic nanopores to meso- and macro-scale flow paths. Finally, abundant large macropores and fractures are key to connecting the mesopore pore networks in the nearby microfacies to macropore flowpaths. Importantly, magnification in SEM or FIB-SEM is insufficient to characterize fully the pore system, and 3D STEM tomography is needed to probe connectivity and identify important roles played by the different pore types. Given the impact of nano-scale processes on hydrocarbon production, high-resolution imaging techniques are necessary to characterize fully the formation. In particular, we showed that most of the porosity exists below 100 nm for both microfacies, and nanopores can be as small as a few nanometers, even though the pore system is only connected above 15-20 nm. Fluid-rock interactions can result in important porosity and permeability changes. In shale formations, reactivity with aqueous fluids plays an important role, especially during hydraulic fracturing and during Enhanced Oil Recovery (EOR). Our first study of fluid-rock interactions focused on describing microstructural shale alterations occurring as a result of alkaline waterflooding (a common chemical EOR method). The effect of pH on shale geochemistry has been extensively documented, especially for inorganic rock components at low pH. In this work, we bring attention to the role that both the mineral matrix and the organic matter play during the reactive alkaline process. Characterization using FIB-SEM and XRD showed that mineral dissolution contributed to the creation of secondary porosity, with the composition of the brine post-reaction showing that quartz and clays undergo dissolution. Additionally, organic-rich samples showed an important increase in inter-organic porosity, likely due to the abundant organic acids within kerogen reacting with the alkali stimulation fluid. We showed that some of the main controls on fabric strength and pore alteration are abundance, morphology and distribution of organic matter. Finally, we also highlighted the dissolution process of iron-rich minerals, including framboidal pyrite, as another important source of secondary porosity. We also investigated the effects of the reaction between acid Hydraulic Fracturing Fluids (HFF) and shale on mineralogy and pore structure. Using FIB-SEM microscopy, we provided direct microscopic observation of mineral and structural alterations occurring at the pore/matrix scale at various stages of acidic HFF-shale reaction (t0, 2h, 7d). FIB-SEM cross-sections showed a progressive increase in porosity, which μCT data puts at 5.6% after 7d of reaction. Quantifiable mechanisms include the dissolution of carbonates and the oxidation of pyrite. FIB-SEM data, complemented by EDS mapping and XANES spectroscopy, sheds light on the types of chemical reactions occurring, the evolution of mineral compositions and the qualitative evolution of porosity over time