Mechanisms and equation improvement for gas seepage in coal seams around boreholes subjected to high-pressure CO₂ foam fracturing

Objective The special low-porosity, low-permeability structures of high-gas-content coal seams restrict efficient gas drainage. To address this issue, this study accurately determined the seepage parameters of annularly fractured coals around boreholes subjected to CO₂ foam fracturing to enhance both the design accuracy of boreholes for gas pre-drainage from coal seams and gas drainage efficiency.

Methods Using an independently developed apparatus for gas seepage experiments on fractured coals around boreholes, this study investigated the coals with pores and fractures distributed in an annular pattern around boreholes from the perspective of varying initial pore structures. Specifically, this study analyzed the evolutionary characteristics of the permeability of fractured coals around boreholes by testing the permeability of coal specimens with varying initial pore structures. Based on the improved Ergun equation, this study derived the fundamental equation for gas seepage in fractured coal media and verified its rationality.

Results The results indicate that the annularly fractured coals around boreholes experienced three stages of rupture and deformations during their compaction. When the stress ranged from 1.2 kPa to 1.3 kPa, corresponding to the late deformation stage of particles, the quantity of fractured particles significantly decreased. The reason is that fine-grained particles produced before the late stage of particle deformation filled most of the pores. Under external loading, the coal particles underwent further compression and deformation. As a result, the particles were subjected to mutual compression, forming a dense whole. As the seepage velocity continuously increased, the flow was primarily governed by inertial force. By setting parameters in the quadratic term coefficient in the Ergun equation at a variable associated with the flow state, it can be found that the relationship between the a/b ratio of the Ergun equation and seepage velocity v roughly followed a negative exponential relationship. The improved Ergun equation overcame the limitations of the conventional seepage equation, enabling the effective prediction of the negative pressure loss during seepage in porous media under varying porosities, particle diameters, and flow velocities.

Conclusions For gas pre-drainage from coal seams, the permeability of annularly fractured coal around boreholes for gas pre-drainage can be calculated using the improved gas seepage equation for fractured coals. Accordingly, the effective fracturing radius can be determined accurately. This study provides an important theoretical basis for optimizing the arrangement of gas drainage boreholes. Meanwhile, it offers a novel approach to green, efficient gas drainage from widely distributed high-gas-content, low-permeability coal seams in China.