**1. Introduction**

With the commercial development of shale gas in North America and China, the pore characteristics of shale reservoirs have been extensively studied [1–3]. Pore characteristics not only control the hydrocarbon gas storage capacity [4,5], but also have an important influence on the gas flow mechanism and producibility [6,7], which can provide basic information for the evaluation of shale reservoirs and

shale gas accumulation mechanisms. Therefore, a series of quantitative and visual techniques are used to characterize the pore structure of shales and the characterization scales are summarized in Figure 1.

**Figure 1.** Methods used to investigate pore characteristics in shale (modified from [3]).

Among the above methods, fluid intrusion analytical methods are the most widely applied to characterize the pore structure of shale [8–10]. However, for each test, there is no uniform standard as to whether the test sample size should be an intact or crushed sample, as well as the particle size of the crushed sample. Chen et al. [11] carried out gas (N2 and CO2) physisorption measurements on New Albany Shales with different particle sizes (4 mesh, 20 mesh, and 60 mesh), and the results indicated that the mesopore volume increased with a decrease in particle size, whereas the micropore volume changed irregularly. Subsequently, Wei et al. [12] and Han et al. [13] evaluated the effect of particle size (5–250 mesh) on the change in pore structure through gas (N2 and CO2) adsorption experiments for Longmaxi Shale samples. Their conclusions were consistent with those of Chen et al. [11], who found that the decrease in particle size primarily affected the pores larger than 10 nm, and suggested that 60–140 mesh is the most suitable particle size for gas adsorption tests. In addition, Mastalerz et al. [14] suggested that gas adsorption tests on low-maturity (Ro~0.57%) and high-maturity (Ro~1.30%) shale samples with smaller particle size (200 mesh) could eliminate the equilibration problems and attain accurate results. However, Hazra et al. [15] proposed that shale particle sizes that were too fine would lead to destruction or alteration of the mesopore structures. Similarly, the results of the mercury injection capillary pressure (MICP) analysis for Barnett and Haynesville shale samples demonstrate that permeability and accessible porosity increase with decreasing sample particle size, indicating that the shale matrix has a higher connectivity on a small scale [16]. Moreover, the water vapor adsorption results for shale show that the total adsorption at 95% relative humidity (RH) is smaller in larger particle size samples, which is associated with fewer accessible pores [17,18]. Nevertheless, there is still a lack of a systematic analysis explaining the above results in the various fluid intrusion experiments.

In addition to fluid intrusion techniques, the nondestructive small-angle neutron scattering (SANS) technique has been used to evaluate the pore characteristics of shale reservoirs in recent years [19–21]. The main advantage of SANS in characterizing pore structure compared with fluid intrusion methods is that it contains information on closed pores (inaccessible to fluids) [3]. Thin sections of thickness

from 0.15 mm to 0.5 mm are commonly used in the SANS test for shale for neutron transmission and avoidance of multiple scattering [22,23]. In addition, shale grain samples can also be used for the SANS test to avoid the anisotropy of the SANS images for wafer samples [24–26]. However, few studies have been performed on the e ffect of particle size on pore structure using SANS. Previous studies using fluid intrusion techniques considered the enhancement of the pore connectivity and ignored the artificial pores and fractures generated in the process of particle size reduction [14,16,27]. Therefore, the revelation of pore structure changes in the shale samples with di fferent particle sizes by SANS can compensate for the deficiencies of previous studies. Moreover, the results of the crushed shale pressure-decay test for di fferent particle sizes show that the helium permeability decreases with decreasing particle size [28]. However, the adsorption capacity of methane increased with decreasing particle size [29]. The reasons for the above phenomena can be explained by the mechanism of pore structure changes during the process of shale particle size reduction.

This study aims to reveal the influence of particle size on the pore characteristics of overmature organic-rich shales. Two shale samples were prepared as 1 cm cubes and particles of 20–35 mesh, 35–80 mesh, and 80–200 mesh. With a combination of SANS, low-pressure gas (N2, CO2, and H2O) physisorption, and MICP, pore structure changes in shale with di fferent sample sizes were first analyzed quantitatively. Then, the grinding positions of the shale were observed and characterized using field emission-scanning electron microscopy (FE-SEM). Finally, the e ffect of the pulverization process on the original pore characteristics of the shale was revealed. Thus, this study attempts to provide a reasonable suggestion on the size of shale samples that should be selected for the characterization techniques of di fferent principles.

#### **2. Materials and Methods**
