Strain Field Characteristics of Coal-like Material under CO₂ Fracturing

Considering the problems related to hydrofracturing, CO₂ fracturing is a more effective way to enhance the permeability of coal seams. Conventional research has focused on the dynamic behavior of coal and rock under CO₂ fracturing. There is a lack of quantitative descriptions of the fracturing stages. Therefore, CO₂ fracturing experiments were carried out with coal-like material at several pressures. The maximum principal strain field was obtained by analyzing the surface images of the specimens. Nine characteristic parameters of typical grayscale and texture images were calculated from the strain field. The correlation between the maximum principal strain and the characteristic parameters was examined using gray relational analysis, and the effects of pressure on those parameters were investigated. We obtained some interesting results, for example, the standard deviation increased from 50 to 100, and kurtosis decreased from -1.0 to -1.5 with the fracturing pressure. The thresholds of the characteristic function F at different fracturing stages were determined, too. This research provides a basis for designing the drilling hole location and determining the millisecond delay of multihole fracturing, as well as for automatically dividing the fracturing stages with artificial intelligence based on the maximum principal strain field.