Fracture growth and acoustic emission response in natural coal-rock blocks with different stress, fracturing medium and injection rates

Abstract

Parameters optimization during fracturing to form fracture networks is important to improve the permeability of reservoirs. In this paper, fracture growth in natural coal-rock blocks under different stresses with different fracturing medium and injection rates was studied, and the injection pressure evolution and acoustic emission dynamic response were further analyzed. The results indicate that the stress state, fracturing medium and injection flow rate significantly affected the fracture propagation behavior. When the stress difference was greater than or equal to 7 MPa ($\Delta \sigma \ge 7\text{MPa}$), the fracture could penetrate the coal-rock interface. In the same stress state (${\sigma }_v=12MPa,{\sigma }_H=8MPa,{\sigma }_v=5MPa$), the fracture geometry and injection pressure evolution significantly differed when using different fracturing media (SC–CO₂ and H₂O). When supercritical CO₂ (SC–CO₂) was used as the fracturing medium, secondary fractures were created with small residual fracture widths. When H₂O fracturing was adopted, single and straight fractures were obtained with a large residual fracture width, penetrating the coal-rock interface straightly. There existed significant differences in the critical stress difference, injection pressure evolution and the acoustic emission dynamic response between natural and artificial coal-rock block fracture during interface penetration. The results could provide important references for fracturing parameter optimization in the efficient exploitation of coalbed methane.