Experimental Study on the Fracture Process Zones and Fracture Characteristics of Coal and Rocks in Coal Beds

In this work, the physical, mechanical, and fracturing properties of three different types of rock (shale, sandstone, and coal) are studied through three-point bending tests. The fracturing characteristics are further investigated by acoustic emission (AE) and digital image correlation (DIC). The modified fracture mechanics (MFM) theory, which considers the influence of fracture process zone (FPZ) length, is constructed to calculate the fracture toughness with greater accuracy than that obtained using standard linear elastic fracture mechanics (LEFM). The experimental results indicate that the coal shows more significant nonlinear fracture behaviour than sandstone and shale. For the geometry of the FPZ, coal has the largest FPZ area, and the FPZ length-to-width ratios for shale, sandstone, and coal are 0.86, 1.96, and 1.93, respectively. Furthermore, the FPZ length varies dramatically, while the FPZ width is almost stable among the three types of rock. Using the proposed MFM approach, the average fracture toughness values of shale, sandstone, and coal are 1.019, 0.419, and 0.058 MPa × m0.5, respectively, which are much lower than those calculated by the standard LEFM. For the average fracture energy, sandstone exhibits the largest value (236.55 J/m2), while coal exhibits the lowest value (42.99 J/m2). These newly observed fracture toughness and fracture energy differences among the three materials provide a theoretical basis for the implementation of hydraulic fracturing in the joint exploitation of shale gas, tight sandstone gas, and coalbed methane.