**4. Discussion**

#### *4.1. E*ff*ect of Particle Size on Pore Structure Characteristics*

The results of the SANS show that the total porosities (including both open and closed pores) increased with the decrease in particle size (Table 2). This characteristic indicates that the increased porosity is due to the artificial pores or fractures created by the pulverization process. Previous studies [11,15] have shown that shear and compression forces in the crushing process induce the formation of new fractures and smaller fragments in the shale, generating new porosity, which is consistent with the results of our FE-EM observations. In addition, during the crushing process, the shale samples commonly fracture along the inherent weak parts, which will also lead to the disappearance of some microfractures [28,39]. Based on the analysis of the above results, a hypothetical model of the effect of the crushing process on the pore structure in shale was established (Figure 10). As illustrated in Figure 10, when an intact shale sample is crushed into particles, the pore connectivity in the shale is significantly enhanced. For a particle, artificial fractures or pores may form on the surface or interior of the particle. Meanwhile, some isolated pore networks may also be exposed to the particle surface or connected by new fractures.

The results of the MICP data indicate that MICP porosity increases with the decrease in sample size (Table 3). For the MICP test, shale can be divided into three constituents: accessible pores, inaccessible pores, and solid matrix [40,41]. The primary pores in the overmature shale have been greatly reduced after a long period of compaction and cementation [42,43]. In addition, the closure of minerals to the organic matter pore system results in low overall pore connectivity in the overmature shale [10,44,45]. Therefore, with the increase in sample size, the pore connectivity of shale decreases further, which results in a lower cumulative mercury intrusion, consistent with the reduction in accessible porosity. Moreover, as can be seen from the schematic representation of particle size reduction (Figure 10c,d), the disappearance of some pore throats results in a lower driving pressure for mercury entering the pores. When pore throats less than 3 nm (corresponding to a maximum pressure of 60,000 psia) are damaged, the pores that are inaccessible to mercury become accessible, thus increasing the accessible porosity. Gas injection porosimetry experiments from Sun et al. [46] also indicated that the crushing process decreases the required total diffusion time and minimum gas injection pressure. The results of MICP show that the destruction of the smaller pore throat is the primary cause of the increase in APtS and the decrease in total pore area with an increase in the crushing level.

The studies of Davudov et al. [47] and Hu et al. [7] on Barnett shale show that permeability and diffusivity increase as the sample size decreases. The permeability measured by MICP in shale is a strongly correlated function of accessible porosity and APtS [47]. Therefore, the increase in these two parameters is bound to increase shale permeability. However, the permeability values obtained from the helium shale matrix permeameter (Core Laboratories SMP 200) in the Woodford Shale decrease with a decrease in the sample particle size [28]. Compared with the permeability measured by MICP, the permeability of the shale matrix measured by the helium pressure-decay method is primarily at the nano-Darcy scale [28], which is inconsistent with the micro-Darcy scale permeability reported by Davudov and Moghanloo [16]. This also indicates that the shale matrix permeability is controlled by pore-throat distribution. With the reduction in particle size, the disappearance of a smaller pore throat will cause some transport pores to become dead-end pores, which will lead to a loss of seepage capacity and a decrease in matrix permeability.

**Figure 10.** Model of an intact shale and a particle shale (**<sup>a</sup>**,**<sup>c</sup>**) before and (**b**,**d**) after crushing (the gray areas denote the shale solid matrix, and the black areas denote pores or fractures space).

The effect of the particle size on the pore structure parameters obtained by low-pressure gas (N2, CO2, and H2O) physisorption are compared with those generated by MICP and SANS (Tables 2 and 3). The pore structure characterization in the shales using the gas adsorption techniques range from micropores to macropores with the increase in pressure, which is opposite to the order of Hg entering the pores in MICP [48,49]. For N2 adsorption, the connectivity of the pore network, especially for sizes larger than 10 nm, is significantly enhanced (Figure 4), which is consistent with previous studies [11,13]. However, according to the results of sample RY2-18 (Figure 4), the particle sizes of 80~200 mesh will cause some small pores to be damaged. A similar phenomenon of pore destruction was observed in the results of CO2 adsorption, but the overall effect on the pore system parameters was not significant. For WVA, an increasing trend was found for the total adsorption at 95% RH with a decreasing particle size, which could be dependent on the enhanced connectivity of the hydrophilic pore network [50]. The pore structure changes associated with clay minerals observed by FE-SEM may be responsible for the increased water adsorption sites. Similarly, previous studies also found that the methane uptake rate and excess sorption capacity of shale increased with a decrease in particle size [29,51]. This also indicates that the physical disruption of shale fabric will improve the pore connectivity and, thus, more methane adsorption sites. In addition, the increased artificial space can improve the swelling ability of shale after methane adsorption [52,53].

Moreover, evidence for the weaker hysteresis effect of N2 in the pores associated with smaller particle sizes (Figure 4) is related to the increased pore connectivity due to shorter flow paths. However, the hysteresis loops of water vapor sorption did not tighten with a decreasing particle size (Figure 6). In contrast, AHI increased with decreasing particle size, suggesting that wettability, in addition to pore structure, also caused fluid retention in the pores [35,36,50]. The closed porosities of various shales calculated by comparing the results of N2 adsorption and SANS have been reported in some studies [20,54,55]. However, in this work, the results of SANS were found to contain information on artificial pores rather than those of gas adsorption. Therefore, for the same particle size, the closed porosity measured by comparing the results of N2 adsorption and SANS is generally overestimated.

#### *4.2. Implications for Sample Size Selection of Di*ff*erent Methods*

The size of the sample has a grea<sup>t</sup> influence on the determination of the petrophysical parameters of shale using different measurement methods. The heterogeneity of shale is characterized by rich microstructural features (microfractures and laminae), complex components (organic matter and minerals), and multi-scale pore structure (from nanometers to micrometers) [56–58]. As shown in Figure 11, for the same intact shale sample, sampling at different locations for testing may produce different results. In addition, based on the analysis of Section 4.1, the difference in the results of particle size variation obtained by several methods is primarily related to the different theoretical bases.

For the neutron scattering technique, the most significant advantage is the nondestructive determination of the total porosity, including open and closed pores. Gu et al. [59] studied Marcellus shale with different sampling directions (parallel and perpendicular to the lamination) and found that the scattering pattern of the parallel layered samples is isotropic, while that of the vertically layered samples is anisotropic. The study of pore anisotropy in oriented shale wafers using SANS provides new insights for shale gas storage, migration, and preservation [23,60,61]. The SANS test on particle shale samples results in isotropic scattering patterns due to the reduced effects of bedding and microfractures. For the same shale sample, the scattering intensity for particles is higher overall in the low *Q* range than in the intact wafer [22]. The results of this work also indicate that the downsizing of particles will increase the SANS porosity due to the creation of more artificial pore space. Therefore, it is imperative to select an intact shale wafer in the SANS test to limit the analytical error caused by artificial space on the shale surface. Shale wafers with a thickness of 0.15~0.5 mm seem to be optimal for performing SANS analysis, considering the scattering time and multiple scattering of the shale.

The MICP measurement provides multi-scale (nm-μm-scale) information regarding the pore throats of shale [62–64]. Compared with the intact shale sample, the real pore throat distribution characteristics could not be provided with the destruction of small pore throat with particle reduction (Figure 7). Meanwhile, the artificially increased pore space is also reflected in the results of MICP. According to Yu et al. [27], the MICP results for 20~35 mesh particle shale samples can be corrected by fractal theory. However, the study only eliminated the error of mercury intrusion between particles without mentioning the effect of pore-throat damage. In addition, the smaller the particle is, the larger the modified exterior surface will be, causing the edge-accessible pores in shale to constitute a high percentage of the interior volume, which cannot truly reflect the pore-throat distribution of the shale [7,65]. As shown in Figure 11, the selection of representative positions at the centimeter scale can reflect the pore characteristics of shale more comprehensively. Therefore, the cubic samples (~1 cm3) with polished surfaces reduced the effect of cutting pockmarks and pore-throat damage and were more suitable for the MICP test. Moreover, previous studies have confirmed that MICP with cubes from overmature shale has minimal compression effects [40,66].

Many studies have demonstrated that more pores can be accessible to the adsorption gas (e.g., N2, CO2, and H2O) as the particle size of pulverized shale decreased [11,14,50]. Meanwhile, samples with smaller particle sizes can shorten the equilibration time and increase the desorption rate [41]. Therefore, Mastalerz et al. [14] suggested that a 200 mesh size of shale is more suitable for N2 and CO2 analysis to determine the total pore volume. However, the N2 and CO2 adsorption results in this

work indicate that an extremely fine particle size (80–200 mesh) may result in the destruction of finer mesopores (i.e., less than 10 nm) and micropores, which is consistent with the study of Hazra et al. [15]. Gas adsorption experiments typically measure pore size ranges of less than 100 nm and rarely involve artificial pores and fractures on the shale surface during the grinding process [12,13]. Considering the equilibration time and original pore connectivity characteristics of shale, 35~80 mesh particle samples provide good quality and reliable data for characterizing the PSD and calculating the pore volume of the overmature shale. For WVA of overmature shale, it is recommended to use consistent particle sizes for comparison with other gas adsorption results.

**Figure 11.** Sedimentary structure characteristics of the TY1-20 sample.

Due to the intrinsic heterogeneity of shale, the information to be included should be selected when characterizing the pore characteristics of organic-rich shales. Pore structure characterization is a significant part of a petrophysical evaluation of shale reservoirs and for assessing shale gas productivity. However, even for samples with the same particle size, discrepant results will be produced owing to the different principles and error sources of each test method. Therefore, future research on pore characteristics will need a multidisciplinary approach to obtain a more comprehensive, larger scale, and more reliable results.
