25 / 2023-05-14 16:18:45

Study on damage evolution law of coal pore structure under multi-pass pulse loads

Pulsating loading; Hydraulic fracturing; Microstructure characteristics; Evolution of pores and fractures; Coal structure strength

Pulsating hydraulic loads can achieve better fracture propagation effects under the same equivalent pressure, which is of significant importance for improving the permeability of coal seams. This study is based on CT three-dimensional reconstruction technology and combined with a self-built pulsating water injection experimental device, aiming to explore the laws of coal fracture propagation and evolution under different pulsating peak pressures from a micro level. The results show that: The pore structure in coal is highly irregular and concentrated in the middle and lower parts, and the primary pore structure determines the trend of coal damage. The ratio of pores with equivalent cubic side lengths of 20-90 μm in G1N1, G1N2 and G1N3 is nearly 90%, and reaches 65% in G1N4. However, the pore space is still mainly composed of a small number of large pores with equivalent cubic side lengths greater than 200 μm. High-density minerals will reduce the porosity of coal, but coal damage is more likely to occur here. The pulsating hydraulic load can tilt the arrangement of pores and small fractures towards the direction of water injection, and the greater the pulsating peak pressure, the better the fracture expansion effect. The pores with diameters of 100-500 μm are the main contributing part of the fracture section formation, and the sign that the coal body is propagated and penetrated is that the large pores with diameters larger than 500 μm become the main body of the pore structure. The evolution and distribution  characteristics of surface porosity and fractal dimension in the S1 to S4 stages are different under different loading conditions. Among them, the distribution under G1N2 is the most regular, and the fractal dimension of pores all have the "small and large bimodal distribution". The occurrence stage, number, and distribution range of this distribution are also different, which is caused by the different pulsating peak pressure. The simulation results of uniaxial compression show that the pulsating hydraulic load will greatly weaken the structural strength of the coal body. The damage first occurs at the fracture, and the stress value of the fracture section connecting the coal body is the highest.