Analysis of crack propagation and hydraulic fracturing behavior of coral reef limestone

Understanding the hydraulic fracturing (HF) characteristics of coral reef limestone (CRL) is of great significance for improving the mining efficiency of seabed energy (such as gas and oil) and ensuring the stability of rock masses in marine underground engineering. To investigate the crack evolution mechanism of CRL under hydraulic coupling, numerical simulations of HF on CRL are carried out using particle flow code (PFC). Firstly, a numerical model method based on two-dimensional particle flow code (PFC2D) is proposed to establish the random pore distribution model of CRL, and its effectiveness is verified through indoor experiments. Then, based on the random pore distribution method (RPDM), a numerical model of HF is created, and a calculation formula for breakdown pressure during HF of CRL is established. The breakdown pressure obtained by these two methods is relatively consistent. Finally, the influence mechanism of porosity and confining stress on the hydraulic behavior of CRL is studied. Results indicate that the propagation direction of hydraulic fracture is related to porosity and confining stress. The interactions between pores and hydraulic fractures primarily include penetration, deflection, and obstruction. The presence of pores hinders the transmission of pore pressure, reducing the seepage capacity. With increasing porosity, CRL is more likely to develop macroscopic fractures, leading to fluctuations in water injection pressure. The fluctuations are related to the number of pores involved in crack propagation, pore volume, number of propagation paths, and path length. The breakdown pressure of CRL is affected by the stress on hole walls and confining stress. A higher breakdown pressure on hole walls indicates a greater stability of the surrounding rock under high hydraulic pressures. As for the initiation stress, it is influenced by the confining stress. As the confining stress increases, the breakdown pressure on hole walls increases. For non-uniform confining stress conditions, the breakdown pressure can be determined by the minimum confining stress.