*2.1. Sample Preparation*

In this work, two fresh, overmature marine shale samples were collected from the Upper Ordovician Wufeng Formation of Well TY1 and Lower Cambrian Niutitang Formation of Well RY2, northwest of Guizhou Province, respectively (Figure 2). Information regarding the composition and maturity of selected samples is listed in Table 1. The raw shale samples were cut into cubes with a side length of 1 cm for the FE-SEM observation and MICP test. Then, the cubes were carefully hand-crushed and sieved into three particle size subsamples: 20–35 mesh (size A), 35–80 mesh (size B), and 80–200 mesh (size C). The shale samples used for analysis were well preserved, and with no sign of oxidation or weathering. Each subsample was dried in a vacuum oven at 60 ◦C for more than 48 h (until mass constancy) to remove the initial moisture content and subsequently analyzed via SANS, low-pressure N2 and CO2 physisorption, water vapor adsorption, and MICP measurements to determine various parameters of pore characteristics.


mixed-layer mineral; 7 %S = percentage of smectite in mixed-layer mineral.

**Table 1.** Basic properties of shale samples used in this work.

54

**Figure 2.** Location map of sampling wells in southern China (modified from [10]).

## *2.2. SANS Experiment*

SANS was performed at the Suanni SANS instrument at the China Mianyang Research Reactor using three sample-to-detector distances (10 m, 4 m, and 1 m) and two neutron wavelengths λ = 5.3 Å (4 m, 1 m) and λ = 8 Å (10 m). The scattering vector (Q) range of this test was 0.0039–0.3 Å−1, which corresponds to the pore diameters (D) from 128 nm to 1.7 nm, according to an approximate relation D = 5/Q [30]. Shale samples with different particle sizes were placed into Hellma cells with a 1 mm path length for the SANS measurement. The raw scattering data were corrected for scattering from the background and space between sample particles by acid-washed quartz sand with the same mesh and empty-cell [21]. The corrected SANS data were analyzed using the polydisperse size-distribution model (PDSM) in IRENA macros of the IGOR Pro software, which assumes that the pores are in a spherical shape and have a random size distribution [31]. Additional background information on the application of the SANS technique for pore characterization of shales can be found in a review article of Sun [3].

#### *2.3. Low-Pressure N2 and CO2 Physisorption*

Shale samples with different particle sizes were analyzed via low-pressure N2 and CO2 physisorption on a Quantachrome Autosorb-iQ apparatus after the SANS tests. The samples were degassed at 105 ◦C for 12 h to remove any adsorbed moisture and volatile matter. The relative pressures (*P*/*P*0, where *P*0 is the vapor pressure of the adsorbing gas, and *P* is the actual gas pressure) of the N2 and CO2 adsorption ranged from 0.0009 to 0.995 and 0.0006 to 0.03, respectively. The surface area and pore size distribution (PSD) of the samples were calculated from N2 adsorption data. N2-based and CO2-based adsorption data were interpreted using the density functional theory (DFT) [32].

#### *2.4. Water Vapor Adsorption Experiment*

Water vapor adsorption (WVA) tests were carried out on the shale samples, which completed low-pressure gas physisorption using the dynamic vapor sorption (DVS) method at 25 ◦C. The DVS apparatus (Quantachrome Aquadyne) accurately measures the mass change (resolution of 0.1 μg ± 1%) of shale samples from 2% to 95% RH to obtain the water vapor ad-/desorption isotherm. The RH usually corresponds to the ratio of the pressure of water vapor ( *P*) to the pressure of saturated vapor ( *P*0). Based on the Kelvin equation, the relation between RH and pore radius (*rp*) of the water-filled capillary can be described [33]:

$$\ln\left(\frac{1}{RH}\right) = \frac{2\gamma V\_m \cos\theta / RT}{r\_p} \tag{1}$$

where γ is the surface tension, *Vm* is the molar volume of the water vapor, θ is the contact angle, *R* is the universal gas constant, and *T* is the temperature. In addition to the water-filled in the pores by capillary condensation, the water adsorbed on the pore surfaces also occupies part of the pore volume. Before capillary condensation, it is assumed that the pore surface of the shale is covered by multiple layers of water vapor with the same interfacial forces to form a water film of a specified thickness. When van der Waals force is the main controlling factor of water film thickness, the thickness (*t*) of the water-adsorbed layer on the pore surface could be calculated using Hasley's equation [17]:

$$t = 0.354 \left[ -5 / \ln \left( \frac{P}{P\_0} \right) \right]^{\frac{1}{3}} \tag{2}$$

Since the bound water in clay minerals is not removed during sample treatment, only water film and capillary water are considered when calculating the PSD of the shale sample by WVA isotherm. The mass change could then be converted to the pore volume. The actual pore radius (*r*) measured by WVA could be expressed as follows:

$$r = r\_P + t \tag{3}$$

## *2.5. MICP Measurement*

MICP measurements were conducted on intact cube samples and crushed samples (size A, size B, and size C) using a porosimeter (Autopore IV 9520, Micromeritics) located at the China University of Geosciences (Wuhan) at pressures up to 60,000 psia (~413 MPa). In addition, to determine the influence of the space between the particles on the intrusion curves, acid-washed quartz sand with di fferent particle sizes was used as a reference for MICP. The porosities and distribution of pore-throat sizes ranging from 3 nm to 36 μm were calculated from the mercury intrusion data.

## *2.6. FE-SEM Imaging*

The intact TY1-20 shale sample was first cut into a 10 mm × 10 mm × 5 mm slice, and then ion-milled on a 10 mm × 10 mm surface using an argon-ion-beam polisher (LEICA EM XTP) to obtain a smooth surface for FE-SEM observation. After imaging, the sample was carefully hand-crushed into particles of approximately 0.5 mm in diameter (~35 mesh). Then, the crushed subsamples were further observed using FE-SEM to identify the influence of crushing on the shale sample.

## **3. Experimental Results**

#### *3.1. Characteristics of SANS Results*

Figure 3a,b display the neutron scattering curves of the two shale samples with di fferent particle sizes. As shown in Figure 3a,b, the scattering intensity of the shale samples with size C in the low Q region is higher than that of shale samples with size A. The neutron SLD values of the shale samples were calculated by averaging the SLD value of each component in the shale, recorded in Table 2. The detailed calculation method can be found in our previous study [3,20]. Shale is treated as a pseudo-two-phase system of pores and solids when the pore structure parameters are determined from SANS tests [34]. Table 2 shows the results of the porosity and specific surface area (SSA) derived from SANS. For the two shale samples, with decreasing particle size, the PDSM porosity increased

(Table 2). Because the porosity measured by SANS represents the total porosity (open and closed pores), the increase in porosity with the decrease in particle size is primarily due to the artificially increased pore and fracture space induced during the sample crushing. However, the SSA of the shale samples derived from SANS did not show the same trend as porosity.

The relationship between the scattering vector (*Q*) and pore radius (*R*) can be transformed by the empirical equation *R* = 2.5/*Q* [30]. The PSD of the samples with different particle sizes are illustrated in Figure 3c,d for comparison. For the TY1-20 sample (Figure 3c), the pore volumes of both size B and size C were significantly higher than those of size A within the pore size range tested by SANS. For the RY2-18 sample (Figure 3d), as the particle size decreased, the pore volume of the pores with a diameter greater than 20 nm increased considerably. In addition, the pore volumes of size B and size C within the pore size range of 2~5 nm were also significantly higher than that of size A.

**Figure 3.** (**<sup>a</sup>**,**b**) Small-angle neutron scattering (SANS) profiles and (**<sup>c</sup>**,**d**) pore size distribution (PSD) for the TY1-20 and RY2-18 samples with three different particle sizes.


**Table 2.** Pore parameters of samples from SANS and gas (N2 and CO2) adsorption.

1 The SLDs (×1010 cm−2) of organic phase used in the samples (TY1-20 = 3.3, RY2-18 = 3.7); 2 PDSM = Polydisperse sphere model; 3 SSA = Specific surface area; 4 N2 Porosity is calculated

by N2 pore volume and bulk density of shale cubes obtained from mercury injection capillary pressure (MICP).

#### *3.2. Low-Pressure N2 and CO2 Physisorption*

Figure 4a,b show the N2 adsorption–desorption isotherms of the two shale samples for the three different particle sizes. All samples display distinct hysteresis loops, and the hysteresis influence of desorption is mostly due to the pore morphology (ink-bottle shape). A comparison of the adsorption branches shows that the shale samples with the minimum particle size exhibit the maximum adsorption capacity at the maximum equivalent pressure. The observation of the desorption branches demonstrates that the desorption rate increases as the particle size decreases from size A to size C. In other words, the decrease in sample particle size shortens the distance required for desorption, thus enhancing the pore connectivity and gas transport capacity.

**Figure 4.** (**<sup>a</sup>**,**b**) Low-pressure N2 adsorption/desorption isotherms and (**<sup>c</sup>**,**d**) PSD for the TY1-20 and RY2-18 samples with three different particle sizes.

The N2 pore volume and surface area of the two samples are listed in Table 2. For sample TY1-20, the N2 pore volume increased with decreasing particle size, and the increased pore volume is predominantly concentrated on the range of pore sizes larger than 10 nm (Figure 4c). Similarly, the BET SSA of sample TY1-20 does not show the same trend as the pore volume changes with particle size. For sample RY2-18, the N2 pore volume and SSA increased with the particle size reduction from size A to size B, but then decreased for size C. Similar to sample TY1-20, the pore volume with a pore size larger than 10 nm increased with the decrease in particle size (Figure 4d). However, sample RY2-18

at particle size C exhibited a significant reduction in pore volume within the pore size range of less than 10 nm.

The CO2 adsorption capacity of the two samples decreased slightly with a decrease in particle size (Figure 5a,b). Determined by CO2 adsorption, the pore volume and SSA of the shale samples were consistent (Table 2). The consistency of the CO2 PSD curves of shale samples with different particle sizes (Figure 5c,d) indicates that the effect of particle size reduction on the micropore volume of the overmature shale is limited.

**Figure 5.** (**<sup>a</sup>**,**b**) CO2 adsorption isotherms and (**<sup>c</sup>**,**d**) PSD for the TY1-20 and RY2-18 samples with three different particle sizes.

## *3.3. WVA Analysis*

The water vapor adsorption–desorption isotherms of the shale samples are presented in Figure 6a,b. As the particle size decreased, the total water adsorption of the shale samples under 95% RH increased continuously (Table 3). When the RH is higher than 70%, the WVA curves of samples with different particle sizes are the most distinct. According to Equations (1)–(3), the results of the distribution relationship between pore size diameter and water incremental intrusion are shown in Figure 6c,d. Within the pore size range, less than 6.2 nm (corresponding to approximately 70% RH), the water absorption capacity of the samples with different particle sizes did not differ significantly. However, with pore sizes larger than 6.2 nm, the water vapor uptake increased dramatically with decreasing particle size. In general, the adsorption of water vapor in shale occurs in three stages: monomolecular-layer coverage, multimolecular-layer adsorption, and capillary condensation with an increase in humidity [18,35]. Therefore, the influence of particle size on the WVA of the overmature shale is primarily reflected in the stage of capillary condensation.

Moreover, pronounced hysteresis loops were observed in all the water vapor adsorption/desorption isotherms (Figure 6a,b). Based on the IUPAC classification, the hysteresis loops of sample TY1-20 and sample RY2-18 can be classified as type H3 and type H2, indicating slit-like and ink-bottle shape pore networks, respectively [36]. In this work, the Areal Hysteresis Index (AHI) was used to quantitatively describe the characteristics of the hysteresis loop. The AHI is expressed as follows [37]:

$$AHI = \frac{A\_{d\varepsilon} - A\_{ad}}{A\_{ad}} \times 100\% \tag{4}$$

where *Aad* and *Ade* are the areas under the adsorption and desorption isotherms, respectively. The AHI values of sample RY2-18 were significantly higher than those of sample TY1-20 (Table 3). In addition, the values of AHI tended to increase with the decreasing particle size.

**Figure 6.** (**<sup>a</sup>**,**b**) Water vapor adsorption/desorption isotherms and (**<sup>c</sup>**,**d**) PSD for the TY1-20 and RY2-18 samples with three different particle sizes.


**Table 3.** Parameters obtained from water vapor adsorption (WVA) and MICP analyses.

RH = relative humidity; 2 AHI = areal hysteresis index; 3 APtS = average pore-throat size.

## *3.4. MICP Analysis*

1

The cumulative mercury intrusion curves and pore-throat size distribution curves for all sample sizes of TY1-20 and RY2-18 are illustrated in Figure 7. As shown in Figure 7, as the sample size decreased, the cumulative mercury intrusion volume increased. The cumulative intrusions of sample TY1-20 from the cube to size B increased even at the maximum pressure (413 MPa), indicating that mercury will continue to enter the pore space if the pressure is increased. However, for TY1-20 with size C, the cumulative intrusion would not increase when the pressure was higher than 4000 psia. For sample RY2-18, the cumulative intrusion volume of all the samples except the cube will be constant after a certain pressure.

The pore-throat size distribution curves (Figure 7) indicate that the accessible pore volume connected with the pore throat less than 10 nm in the cube samples will be greatly reduced in the particle samples. For the samples with particle size C, the pore network space connected with a pore throat of less than 100 nm almost disappeared. Contrarily, the connected pore volume of the samples with pore-throat diameters larger than 100 nm dramatically increased with the decrease in sample size. Furthermore, the MICP results of acid-washed quartz sand with di fferent particle sizes demonstrated that the influence between particles was mostly manifested on the micron scale (Figure 8).

Table 3 summarizes the results of the pore structure parameters corrected by the acid-washed quartz sand using MICP data and conformance volume calculations [16,38] for the di fferent sample sizes of TY1-20 and RY2-18 samples. The results demonstrate that porosity is strongly related to the sample size, which increased with decreasing sample size. With the decrease in the sample size, the pore throat with a small diameter gradually disappeared, which led to an increase in the average pore-throat size (APtS) and a decrease in the total pore area. The particle density of the same samples with di fferent sizes also exhibited slight di fferences, indicating that there will inevitably be subtle di fferences in composition in the sorting process.

**Figure 7.** (**<sup>a</sup>**,**b**) MICP intrusion curves and (**<sup>c</sup>**,**d**) pore volume distribution vs. pore-throat diameters for the TY1-20 and RY2-18 samples with four different sample sizes.

**Figure 8.** Pore volume distribution vs. pore-throat diameters of quartz samples with three different particle sizes.

#### *3.5. Observation of FE-SEM*

Various mineral components and organic matter can be clearly observed under FE-SEM on the smooth surface of shale obtained by argon-ion-beam polishing (Figure 9a). The crushed subsamples were then fixed with epoxy resin and further observed using FE-SEM, revealing that the smooth surface became rough (Figure 9b). Artificial microcracks at the nanometer scale formed by the fracture of rigid minerals could be observed on the rough surface of crushed subsamples (Figure 9c). In addition, in the process of particle size reduction, the structure of the clay minerals was prone to collapse, leading to the generation of smaller fragments and new artificial pore space (Figure 9c).

**Figure 9.** Field emission-scanning electron microscopy (FE-SEM) images of the TY1-20 sample. (**a**) Smooth surface of shale obtained by argon-ion-beam polishing; (**b**) Crushed subsamples were then fixed with epoxy resin; (**c**) Artificial pores and microcracks; (**d**) Artificial microfractures perpendicular to the bedding direction; (**e**) Marks of the crushing process on the smooth surface; (**f**) An enlargement of the crush mark in the rectangle area in Figure 9e.

Among the artificial microcracks, artificial microfractures perpendicular to the bedding direction are more conducive to enhancing pore connectivity. As illustrated in Figure 9d, the hydrocarbon fluids in the original pores could flow out through artificial microfractures, whereas the small molecular fluid in fluid-invasion porosimetry can enter the original pores through the artificial microfractures. The marks of the crushing process on the smooth surface by argon-ion-beam polishing could be

observed in the local area of the particle sample (Figure 9e). By magnifying the observation of the crush area (Figure 9f), it was found that the artificial pore-fracture is manifested in three forms: (1) the separation of rigid minerals (quartz, calcite, etc.) and plastic components (organic matter and clay) to form artificial pore-fractures; (2) the fragmentation of mineral grains to form artificial fractures; and (3) the spalling of mineral grains, such as pyrite, to form artificial moldic pores.
