## 2 Establishment of an Evaluation Index System for CBM Development  
### Index System Construction  
Coal is both the source rock and reservoir rock of CBM, which has the dual role of hydrocarbon generation and gas migration [18; 19]. Therefore, the evaluation of a CBM reservoir involves not only resource characteristic parameters (e.g., reservoir thickness and gas content) but also production characteristic parameters (e.g., reservoir pressure gradient and permeability). According to the conditions of CBM reservoirs in China, referring to the evaluation index system of CBM blocks at home and abroad, and avoiding the superposition among various factors, eight resources and productivity characterization indices were selected as the main indices of block development. 1. Reservoir depth. Reservoir depth controls the economic value and economic efficiency of CBM reservoirs, and this factor affects whether a reservoir has commercial developmental value. It has been known that, as the reservoir depth increases until critical depth, the gas content of CBM increases [20]. However, the permeability of the reservoir decreases continuously [21], which has a negative effect on the seepage and drainage pressure of the CBM well. As a result, the reservoir depth is a cost-type index in reservoir evaluation;
2. Gas content. Gas content is a key index that simultaneously determines the level of CBM resources abundance and controls the CBM enrichment and production [22]. Along with the increasing of the gas content, the critical desorption pressure and the effective desorption area increased. Therefore, the shorter gas breakthrough time resulted in enhanced gas production per well [23; 24]. Obviously, the gas content serves as a profit-type index for reservoir evaluation;
3. Reservoir thickness. As the source rock and storage space of CBM, a thick enough reservoir thickness is a prerequisite for the enrichment, development, and production of CBM [25; 26]. Theoretically, the thicker the reservoir, the more CBM will flow into the wellbore. That is to say, the thick coal seam can provide continuous and sufficient gas source for gas wells and ensure a longer production cycle and higher gas production [27]. Consequently, reservoir thickness is regarded as a profit-type index in the evaluation model;
4. Reservoir pressure gradient. The reservoir pressure gradient is the main driving force of water flow [28]. In CBM wells, gas is produced mainly by water pressure transmission and the effective drainage radius for reservoir presents a depressurization funnel trend. The greater reservoir pressure gradient resulting in the stronger force of the water flow, the greater the distance of the water pressure transfer, which is more favorable for gas production [29]. For this reason, the reservoir pressure gradient is a profit-type index for reservoir evaluation;
5. Ratio of critical desorption pressure to reservoir pressure (hereinafter referred to as \"critical reservoir ratio\"). The critical reservoir ratio represents the dynamic features of CBM desorption, diffusion, and seepage to the bottom of the well. [PERSON] and [PERSON] found that with the increase of the critical reservoir ratio, the gas saturation in the reservoir increased. Then the stronger methane power field made gas evolution under the same declining pressure more easily [30; 31]. It was also found that a higher critical reservoir ratio shortened the time of CBM well drainage, which reduced the damage caused by effective stress on the permeability of reservoir during the drainage and depressurization stage, weakened the negative effect of fracturing fluid on critical desorption pressure of reservoir, and increased the aerodynamic force [3; 32]. The critical reservoir ratio is considered to be a profit-type index for reservoir evaluation;
6. Permeability. The reservoir permeability represents the development degree of pores and fractures in coal seam and influences the degree of the gas flow in effective pores of the coal seam [23; 33].