Simulation Study of N₂-Hydraulic Compound Fracturing Based on the Volumetric Opening Model

N₂-hydraulic compound fracturing (NHCF) is an innovative technology aimed at addressing coalbed methane development challenges in low-permeability, low-pressure coal reservoirs in China. However, limited research has been focused on the evolution of damage zones, pore pressure fields, and fluid pressure characteristics in this context. In this paper, we establish a finite element seepage equation based on the volumetric opening model and construct a finite element model for horizontal well stage fracturing. We used the physical and mechanical parameters specific to coal reservoirs in the Xinjing coal mine. Subsequently, we conducted numerical simulations of N₂ fracturing (NF), hydraulic fracturing (HF), and NHCF using ANSYS. The results indicate that the initiation-fracturing pressure of NHCF is lower than that of HF but higher than NF, but the steady-fracturing pressure is higher than HF and NF. Moreover, numerical simulation shows that under the same water injection volume, the total volumetric opening formed by NHCF is about 2 times that of HF, NF is the smallest, and the damage zone and pore pressure field caused by NHCF are the largest. Finally, when comparing the casing pressure curve of NHCF by field test with the fluid pressure curve of wellbore obtained from numerical simulation, we observe a strong correlation; the steady fracturing pressure of NF is about 13 MPa, which is basically consistent with the numerical simulation, and the steady- fracturing pressure of HF after NF is about 27 MPa, which is slightly lower than the 30 MPa in numerical simulation. This is because in the numerical simulation, the reservoir parameters after NF can be inherited to the subsequent HF, which cannot be done in the field test. This study presents a novel method for numerical fluid fracturing simulation, offering a fresh perspective on the subject.