**1. Introduction**

Shale gas has received grea<sup>t</sup> attention from governments all over the world, especially after the successful shale gas revolution in North America. As a type of clean energy with abundant reserves, shale gas is believed to be one of the most promising replacements for conventional energy in the future. According to the Energy Information Administration (EIA) (2016), shale gas production is expected to drive world natural gas production growth in the coming decades and will account for approximately 30% of world natural gas production by 2040 (Figure 1a). The United States and China are predicted to be the two largest shale gas producers in the world by the end of the forecast period, with shale gas production making up 70% and 40%, respectively, of each country's total natural gas production (Figure 1b).

**Figure 1.** Current and predicted situations of shale gas resources in different countries or regions (from EIA). (**a**) Natural gas production by type. (**b**) Shale gas production in different countries. (**c**) Natural gas consumption in different regions. (**d**) Natural gas supply by type in China.

Taking China as an example, due to the relatively high economic growth and increasing attention to environmental protection, natural gas consumption is expected to increase from 19 Bcf/d in 2015 to 57 Bcf/d in 2040 (Figure 1c), accounting for a quarter of all global natural gas consumption growth between 2015 and 2040 (EIA, 2017). Driven by the development of shale gas resources, China's domestic natural gas supply will grow from 13 Bcf/d in 2016 to 39 Bcf/d by 2040, with shale gas production increasing from 0.7 Bcf/d in 2016 to 10 Bcf/d by 2030 and 19 Bcf/d by 2040. As we can see from Figure 1d, shale gas is expected to increase the fastest and will account for more than 30% of the total natural gas supply in China by 2040.

As one of the unconventional energy resources, shale gas reservoirs have uniqueness and complexity in terms of the gas storage type, transporting mechanism, and reservoir development mode, which makes the commercial production of shale plays a very challenging task for petroleum engineers. The United States, Canada, and China are the only three countries that produced commercial volumes of natural gas from shale formations by 2015. Although horizontal well drilling and hydraulic fracturing have been applied to produce shale gas in Australia and Russia, no commercial gas volumes were obtained from low-permeability shale formations. Currently, the commercial development of shale gas resources in North America and China mainly benefits from advanced engineering technology. The theoretical understanding of shale gas storage capacity, gas transporting mechanism in nanopores or micropores, and pore structure characterization is still not clear, being far behind engineering practice [1].

In this review, our attention will mainly be paid to three aspects: (1) petrological, organic geochemical characteristics and micropore structures of shale formations; (2) different types of adsorption models as well as their principles and application range, including Gibbs excess sorption, supercritical adsorption phenomenon, and adsorption/absorption models; (3) different methods of calculating gas physical properties, such as virtual saturated vapor pressure, adsorbed gas density, free gas density, free gas viscosity, etc. Different models on each subject will be compared and evaluated based on their physical meaning, reliability, accuracy, and applicability, which are significant for accurate numerical simulation and enhancing hydrocarbon recovery in shale gas reservoirs.

#### **2. An Overview of Pore Structures and Gas Types in Shale Formations**

The complex micropore structure of shale plays is determined by its special accumulation processes. An overall and deep understanding of pore structures and gas storage types is key to proper reservoir assessment and precise numerical simulation.

The pore structure of shale formations can be detected by the observation description method and the physical test method [2,3]. The observation description method adopts radiation techniques, mainly referring to the means of optical microscopy, scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), and small angle X-ray scattering (SAXS) to directly describe pore geometrical shapes, connectivity, and pore filling of shale formations [4,5]. The physical test method mainly refers to fluid penetration experiments, which utilize tests of fluid mass, volume, and pressure to obtain the pore sizes volumes indirectly, including mercury injection capillary pressure (MICP), helium (He) porosity, liquid N2 adsorption, low-temperature CO2 adsorption, etc. [6–9]. The resolution of the di fferent methods is shown in Figure 2.

**Figure 2.** The resolution of di fferent methods to characterize micropore structures [2–4].

In this section, we summarize the petrological and organic-geochemical characteristics of shale gas reservoirs in Sichuan basin in China compared to the shale gas development in North America. The chosen samples are marine shale of the Lower Silurian Longmaxi Formation. The porosity and permeability of the main shale gas reservoirs in North America and China are collected and tested, respectively, based on which the storage space types and pore size distributions are analyzed. Finally, di fferent kinds of shale gas as well as their modeling methods are identified and compared.

#### *2.1. Petrological and Geochemical Characteristics*

In this part, our previous work related to shale gas reservoir characterization and assessment is introduced. Samples of the Longmaxi Formation in south Sichuan basin are collected as experimental objects.

The highly mature Longmaxi marine shale is one of the most important candidates for the commercial development of shale gas resources in China [10]. The total organic carbon (TOC) content ranges from 0.4% to 18.4%, with the organic matter (OM) mainly composed of type I and II1 kerogen [10,11]. Its vitrinite reflectance (R0) values range from 1.8% to 4.2% [11]. The Longmaxi Shale is found to be porous and permeable [11,12], with porosity ranging from 1.2% to 10.8% and permeability ranging from 0.25 μD to 1.737 mD. Other geological and petrophysical characteristics of the Longmaxi Shale Formation can be found in previous publications [13,14].
