Quantitative Analysis of Nanopore Structural Characteristics of Lower Paleozoic Shale, Chongqing (Southwestern China): Combining FIB-SEM and NMR Cryoporometry

Abstract

Characterizing nanopore structure is one of the most important factors in understanding gas storage and transport in shale reservoirs, but remains a significant challenge. In this work, we combine the benefits of Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) and Nuclear Magnetic Resonance Cryoporometry (NMR-C) to characterize the nanopore structure of lower Paleozoic shales from Chongqing, southwestern China. Mineral composition is qualified through X-ray Energy Dispersive Spectroscopy (EDS) and Field-Emission Scanning Electron Microscopy (FE-SEM). Voids in 2D micrographs are classified into types of meso- and/or microfractures, inter particle pores (InterP pores), intraparticle pores (IntraP pores), and pores in organic matter (OM pores). An Otsu thresholding algorithm and an edge detection algorithm (in Avizo) are used to segment OM, InterP, and IntraP pores and to establish 3D pore structure for pore-network modeling (PNM). The pore size distribution (PSD) is measured via PNM and compared to the PSD recovered from NMR-C. Results indicate that the small OM pores have favorable potential for storing adsorbed gas because of their apparent interconnectivity and higher porosity; conversely, InterP and IntraP pores are mostly isolated and with low porosity. The peaks of the PSD, recovered from PNM, are in the ranges 10–20 nm and 60–120 nm and are in good agreement with the peaks of the PSD recovered from NMR-C. This suggests that the NMR signals in these discontinuous ranges are due to OM, InterP, and IntraP pores. This indirectly demonstrates that the probe liquid in the NMR-C experiments (i.e., water) may indeed enter the nanoscale pores in the shale during centrifugal saturation. Comparison of the PSD from PNM and NMR-C also shows that NMR-C has a higher sensitivity and accuracy in detecting nanopores. Combining FIB-SEM and NMR-C is a promising technique in detecting and characterizing the nanoporosity of shales.

Mechanical Property Measurements and Fracture Propagation Analysis of Longmaxi Shale by Micro-CT Uniaxial Compression

“…Specific surface area (the surface area per bulk unit volume) and tortuosity are important macroscopic pore structure parameters of porous material [48][49][50], which can reflect structural properties of the cracks in shale [51]. In this study, the tortuosity is defined as the ratio of the length of the path and the distance between its ends along z-axis [52] (Figure 14). Specific surface area (the surface area per bulk unit volume) and tortuosity are important macroscopic pore structure parameters of porous material [48][49][50], which can reflect structural properties of the cracks in shale [51].…”

“…Specific surface area (the surface area per bulk unit volume) and tortuosity are important macroscopic pore structure parameters of porous material [48][49][50], which can reflect structural properties of the cracks in shale [51]. In this study, the tortuosity is defined as the ratio of the length of the path and the distance between its ends along z-axis [52] (Figure 14). Although SSA and tortuosity are positively correlated at different load stages, they display different patterns in different specimens (Figure 13b).…”

Probing Nanomechanical Properties of a Shale with Nanoindentation: Heterogeneity and the Effect of Water–Shale Interactions

“…The mechanical parameters of rocks, such as the elastic modulus, hardness, and fracture toughness, have a great influence on the geometry of the fracture network, impacting the proppant embedment and permeability of the stimulated formations . In the past, the most common ways to evaluate the mechanical properties of shale and the degree of the water–shale interaction are the conventional mechanical tests, including uniaxial and triaxial compressive strength tests, shear tests, and ultrasonic measurements. − These meso- or macroscopic laboratory tests can provide only the average value of the all-solid phase, which fail to consider the microstructures’ heterogeneity and the mineralogical compositions. − In fact, the length scale involved in the production of shale gas can span up to 13 orders of magnitude . The mechanical characterization of shale on a macroscale is not enough for engineering applications.…”

Probing Multiscale Structure of Mineral and Nanoporous Kerogen Phase in Organic-Rich Source Rocks: Quantitative Comparison of Small-Angle X-ray and Neutron Scattering

“…As it is also well-known, this material has low permeability (in the range of nanodarcy), low porosity, and high retention capacity through a physisorption mechanism, , which may vary depending on the rock micropore content. Currently, scientific interest on source rock transport and storage properties led to an investigation of rock porosity. ,− Studies are focusing on the improvement of nanoporous media understanding through the assessment of total porosity, ,− open versus closed pore content, , pore size distribution, ,,,,− ,,,− pore connectivity, ,,,, and porous network tortuosity. , Accordingly, several analytical methods are employed such as gas adsorption/desorption, ,− , mercury injection capillary pressure, ,,,,, NMR relaxometry, − NMR cryoporosimetry, multifrequency and multidimensional NMR, and small-angle neutron scattering (SANS). ,,,,,,,,, …”