Abstract
Binary mixture (CO2/N2) displacement can effectively enhance coalbed methane recovery. A small gas injection amount in low-permeability coal seams leads to poor recovery enhancement efficiency (REE) by gas displacement. Constructing pressure-relief chambers inside the targeted coal seam can effectively improve the gas injection amount, yet the correlation between heterogeneous coal failure and the recovery enhancement by binary gas mixture remains unclear. Artificial parameters that influence recovery enhancement, including the diameter of pressure-relief chambers, drilling spacing, CO2 concentration, gas injection pressure and initial gas pressure, can be controlled or optimized in field practice. However, these parameters are not independent of but correlated with each other. This study aims to enhance gas recovery in deep mining coal seams. To this aim, a model of multi-component gas flow and geo-mechanics was established to model the gas displacement process in destressed coal seams. Next, advantages of the model considering coal failure and heterogeneity were revealed by comparing effective breakthrough radius and gas recovery amount, respectively. Subsequently, the evolution law of multi-physics was analyzed and the conclusions are as follows: (1) the model considering heterogeneous coal failure can well exhibit the storage and migration of multi-component gas; (2) the gas displacement influencing region can be divided into strong, weak and pianissimo displacement zones according to the regions that binary mixture acts on. Moreover, the pre-breakthrough REE and the post-breakthrough REE were proposed based on the characteristics of different gas injection stages. The respective and coupling effects of the artificial parameters on REE were further investigated by response surface methodology. The results reveal that the above five parameters (pressure-relief chamber diameter, drilling spacing, CO2 concentration, gas injection pressure and initial gas pressure) are dominant factors controlling the post-breakthrough REE, yet the latter three parameters have a notable impact on the pre-breakthrough REE. In addition, the pre-breakthrough REE and the post-breakthrough REE can hardly be improved at the same time and it is necessary to improve the REEs according to different gas injection stages. Finally, an injection-extraction alternating and multi-level optimization technology was proposed to further enhance gas displacement in low-permeability coal seams. The research results provide a theoretical reference for engineering practice.
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