# Energy Evolution Analysis of Coal Fracture Damage Process Based on Digital Image Processing
## 1 Introduction  
Coalbed methane (CBM) is often stored in coal rocks in an adsorbed state, and because of its extremely high calorific value and the fact that it produces almost no exhaust gas after combustion, it is a clean, unconventional energy source and a strategic national reserve resource [1; 2; 3; 4; 5; 6; 7; 8]. Coal rocks are the main reservoir of CBM, and the mechanical properties of coal rocks and the energy analysis during fracture damage have been the key scientific problems in oil and gas development, as well as an important research direction in CBM exploration and development. Coal rock is a mineral resource formed through a series of complex tectonic and geological processes, and is a key research target for national resource development. For the mechanical properties of coal rocks, scholars have done a lot of research work. It is found that the internal structural defects of coal are obvious, complex and changeable, and the dispersion is large, resulting in the low compressive strength of coal [9; 10; 11; 12; 13; 14; 15]. During the formation of coal under complex geological conditions, there are a large number of different meso-structures, such as mineral particles, holes and micro fractures. These meso-structures have a substantial effect on the mechanical properties and fracture damage processes of coal [16; 17; 18; 19]. [PERSON] et al. 2018 measured partial axial strains in coal rocks andextrapolated partial axial strains from the overall axial strains in general agreement with the test results [20]. [PERSON] et al. (2015) treated coal rock as equivalent homogenized and constructed stress state expressions and proposed a general compression shear damage criterion considering the bond strength between coal and soft rock [21]. The fracture damage process of coal, like other rocks, is an unstable phenomenon of energy storage and release. The use of energy to analyze the damage process of rocks is gaining more attention in the theoretical and engineering communities [22; 23; 24; 25]. Many scholars have conducted studies on the relationship between the energy released during the destruction of rocks and the damage, and fruitful results have been obtained [26; 27]. In terms of experimental analysis, [PERSON] found no direct correlation between the effect of size on energy dissipation density and compressive strength [28]. While great progress has been made in rock energy evolution, many scholars are also committed to the study of coal energy release in mining [29; 30; 31; 32]. As a complex natural heterogeneous material, coal is often filled with calcite veins under the action of geological structure. Complex calcite veins have crucial impacts on the mechanical properties, crack extension and energy evolution process of coal rocks. In the process of coal mining, understanding the mechanical properties, failure mode, energy input, accumulation, dissipation and release of coal is an important prerequisite for coal mine safety production. Many mine dynamic disasters are caused by the sudden release of a large amount of energy, such as common rock burst and mine earthquake [33; 34]. Many coalbed methane mining processes are also achieved by inducing energy dissipation when coal seams are damaged. For example, coal seam cutting, pressure relief and permeability enhancement pumping is through the conversion of coal seam energy into more fracture surface energy [35; 36]. Therefore, the research on the mechanical properties and fracture damage of calcite vein coal and rock has important theoretical guiding significance for the exploitation of coalbed methane. In this paper, the calcite-bearing vein coal rocks of Laofang and Enhong in Yunnan are studied, and the internal structure of coal rocks is characterized by digital image technology for spatial distribution. Numerical models of coal rocks with seven sets of calcite veins of different azimuths were established using RFPA2D-DIP, and uniaxial compression tests were conducted under diverse loading conditions.