Nanometer‐scale characterization of microscopic pores in shale kerogen by image analysis and pore‐scale modeling

Abstract

[1] Nanometer-scale scanning electron microscopy was applied in visualizing the microscopic pores within shale kerogen. Geometrical information of all individual pores was extracted by image analysis. Image segmentation and separation showed that most of the intrakerogen pores are discrete and isolated from each other, having relatively spherical morphology. These isolated intrakerogen pores result in huge challenges in gas production, because they are not effectively connected to natural and hydraulic fractures. Statistical results showed that nanopores, which have diameters smaller than 100 nm, make up 92.7% of the total pore number, while they make up only 4.5% of the total pore volume. Intrakerogen porosity and specific surface area are 29.9% and 14.0 m 2 /g, respectively. Accurate visualization and measurement of intrakerogen pores are critical for evaluation of gas storage and optimization of hydraulic fracturing. By lattice Boltzmann simulations, permeabilities and tortuosities were simulated in the three principal directions. Long tails were observed in breakthrough curves, resulting from diffusion of solute particles from low-flow-velocity pores to larger conduits at late times. The long-tailing phenomena at the pore scale are qualitatively consistent with those observed in real productions. Understanding the porescale transport processes between microscopic pores within kerogen and large fracture systems is of great importance in predicting hydrocarbon production. Upscaling methods are needed to investigate largerscale processes and properties in shale reservoirs.

Methane Adsorption Rate and Diffusion Characteristics in Marine Shale Samples from Yangtze Platform, South China

“…The pore structures of shale are highly heterogeneous, with the pore size mainly ranging from nanometer to micrometer-sized [5,6]. Previous research has reported that nanometer-sized pores, which are predominantly associated with organic matter and clay minerals [6][7][8], could provide larger surface areas and higher adsorption energy for gas molecule adsorption [9][10][11][12], while micrometer-sized pores-which are generally associated with inorganic minerals [6]-are more likely to connect with induced fractures for gas flow and transport [13,14]. For gas desorption, once external conditions, such as temperature and pressure changes, for example, and artificial fractures disturb the equilibrium, the free gas molecules in fractures and macropore networks flow from high-pressure zones to low-pressure zones, and then gas that adsorbed onto the surface of organic matter and clay minerals starts to desorb, inducing a concentration gradient between the bulk particles and their surface.…”

Parameter Determination Using 3D FIB-SEM Images for Development of Effective Model of Shale Gas Flow in Nanoscale Pore Clusters

“…Every pore cluster can be identified and labelled using the bwlabeln function in MATLAB ® . The pore clusters may be connected by hydraulic fractures (Chen et al 2013). An understanding of shale gas transport through pore clusters is thus critical to the evaluation of shale gas recovery.…”

“…The 3D binary images are obtained by FIB-SEM imaging or reconstruction of a 2D SEM image (Chen et al 2013(Chen et al , 2015Ma et al 2014;Tahmasebi et al 2015), and the PNs can be extracted from the 3D images or artificially generated using constraints. Mehmani et al (2013) used local shrink and adjustment of the pore networks extracted from sandstone to approximate the micro-and nano-pore networks in shale and investigated the effect of the ratio of the nano-pores to the micro-pores on the apparent permeability using Javadpour (2009)'s flow flux formula.…”

“…He found that the spatial configuration of the nano-pores significantly affected the gas permeability. Chen et al (2013) used the lattice Boltzmann method (LBM) and FIB-SEM images of a shale sample to simulate Darcy flow and obtain the breakthrough curves. The non-Darcy flow was neglected because the Knudsen number was considered to be sufficiently small.…”

A kinetic Monte Carlo approach to study fluid transport in pore networks

“…3,4 The term shale play is used to describe a geographic area that contains an organicrich fine-grained sedimentary rock and exhibits low production rates. 4,5 Shale rocks are mainly composed of kerogen, quartz, clay, carbonates, and pyrite. Secondary components such as uranium, iron, vanadium, nickel, and molybdenum can be found in the shale matrix.…”