Colloids and Surfaces A: Physicochemical and Engineering Aspects
Spatial characterization of heterogeneous nanopore surfaces from XCT scans of Niobrara shale
Graphical abstract
3D X-ray CT images of the surface for a complex nanopore channel defining surface areas and spatial distribution of carbonate and kerogen phases (voxel resolution 20 nm).
Introduction
Properties of multiphase fluids in nanoporous rocks differ from those of coarser-grained rocks with larger pores. Developing an understanding of heterogeneity and scale through natural and geo-architected materials is an important area of cross-cutting research. The multiphase fluid flow through natural and geo-architected nanopore media is fundamentally controlled by the geometry of the nanopore structure, pore wettability and interfacial tension [[1], [2], [3], [4], [5]]. Wettability is defined as the tendency of one fluid to adhere to a solid surface in the presence of other immiscible fluids and is determined, at the pore scale, by the local contact angle. The physics of liquid spreading on flat homogeneous surfaces is relatively well understood [6], however, the measurement of wetting on other than a flat surface remains in question. In this regard, multiphase fluid flow through the nanopore structure is further compromised when the pore surfaces are heterogeneous, having surface regions of different composition, polarity, and wetting characteristics.
Because X-ray CT imaging non-destructively measures the material characteristics of a sample at each volume element in three-dimensional space, multiphase systems can be quantified while determining structure and dimensions. For porous structures, the internal surface regions can be defined, and pore network connectivity can be determined.
The 2D sections in Fig. 1 show the complex geometric features of Niobrara shale using a multi-length scale CT (micro and nano) analysis. It is clear that nano-length scale resolution is required to characterize and analyze the complex structure of the Niobrara shale.
With the development of advanced X-ray nanoscanners such as the Xradia/Zeiss UltraXRM-L200 microscope, 3D pore network structures can be quantitatively described to define pore connectivity and permeability at the nanoscale. Further, with the appropriate software, spatial regions of defined phases can be distinguished at the pore surface. In this paper, the nanopore network structure of a Niobrara shale sample is defined in 3D at a resolution of 20 nm using X-ray UltraXRM-L200 tomography. The spatial extent of the mineral matter phases (carbonate and silicate minerals) and the hydrocarbon phase (kerogen) is defined. Such information is necessary for efforts to simulate fluid flow in such shale nanopore structures.
Section snippets
Sample preparation
Shale is one of the most abundant rocks found on the earth’s surface. Shale is of interest as an energy resource and consists of a mineral matter matrix and a hydrocarbon oil phase. The shale structures are further characterized by nanopore networks. In this regard, a Niobrara shale sample was selected for this study. The Niobrara shale consists of three phases, namely mineral matter, kerogen, and pores. The typical composition of Niobrara shale, by volume, is 78% mineral matter, 17% kerogen
Initial evaluation of CT images
In general, as shown in Fig. 3, shale consists of mineral phases and organic phases. The organic phase (kerogen) has nanopores which may contain low molecular weight hydrocarbons.
One of the key issues in shale gas exploration and production is the estimation of adsorbed gas. The amount of adsorbed gas depends upon the surface area of the organic pore network. Preliminary nano-scan results are shown in Fig. 4, including the original nanoCT image and the spatial distribution of nanopores for the
Preprocessing of original 3D nanoCT images
The processing of CT image data has limitations and problems regarding resolution and noise. A very basic limitation to the accuracy of the measurement taken by CT is the underlying statistical nature of the x-ray photon production, photon interaction with matter, and photon detection. Furthermore, noise is generated due to the mathematical artifacts of the reconstruction process. The challenge is how to improve image quality in order to increase the precision and accuracy of subsequent
Pore geometry and network analysis
Connectivity is an important concept when flow problems are considered. Fluid flow can occur between two points only when the pore space is connected. The three-dimensional interconnected pore structure is difficult to determine by stacking of two-dimensional thin slices. In addition, to characterize the three-dimensional interconnected pore structure, the term pore size is not easily defined. These open or interconnected pores cannot be simply regarded as discrete pseudo-particles [13].
The
Summary
As discussed in our previous paper [10], and used with permission from Elsevier, the multiphase fluid flow through natural and geo-architected nanopore structures is fundamentally controlled by the geometry of such nanopore structures, pore wettability and interfacial tension. Furthermore, the multiphase fluid flow through such nanopore structures is compromised when the pore surfaces are heterogeneous, having surface regions of different composition, polarity, and wetting characteristics. In
Declarations of interest
None.
Acknowledgements
This work was supported as part of the Multi-Scale Fluid-Solid Interactions in Architected and Natural Materials (MUSE) Project, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award #DE-SC0019285. The authors thank Ms. Dorrie Spurlock for her assistance with the preparation of this manuscript.
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