Damage and permeability response modeling of coal reservoirs subjected to liquid nitrogen fracturing using a heterogeneous fully-coupled model

The liquid nitrogen (LN2) fracturing technology is a promising method to enhance coalbed methane (CBM) recovery. This study aimed to examine the impact of diverse factors on the fracturing of coal reservoirs using LN2. A thermo-hydro-mechanical-damage (THMD) coupling model that considers reservoir heterogeneity was developed. The effects of reservoir conditions on the damage characteristics and permeability enhancement of a coal reservoir fractured with LN2 were investigated. The results showed that the reservoir fracturing process with LN2 can be divided into three stages: the initiation stage, the breakdown stage, and the complete damage stage. Higher initial reservoir temperatures were found to be conducive to the development of damage cracks, increasing the permeability enhancement effect of reservoir fracturing with LN2. With an increase in the initial reservoir temperature from 25 degrees C to 95 degrees C, the final permeability increased by 13.81 %, and the initiation and breakdown pressures decreased by 21.35 % and 15.81 %, respectively. The permeability enhancement effect was optimal when the ratio of the maximum horizontal principal stress to vertical stress (H/V ratio) was 1.5. When the H/V ratio reaches 1.5, the final permeability also reaches a maximum (0.197 mD), and the breakdown pressure decreases slightly (20.98 MPa). The elastic modulus, thermal expansion coefficient, and thermal conductivity were key physical parameters affecting the damage and permeability of fracturing coal reservoirs with LN2. Our findings provide a theoretical basis for the efficient fracturing of coal reservoirs with LN2 from an engineering perspective.