Development and application of an improved constitutive model for acidizing coal around fractured boreholes.

The fracture radius surrounding the borehole is a key parameter for assessing the effectiveness of acidizing and fracturing in coal seams. To determine this radius, a self-developed physical experimental platform for CO foam acidizing and fracturing in low-permeability coal seams was designed. Fracturing fluid erosion tests were carried out on raw coal samples, and the stress-strain relationship of the acidified coal around the borehole was analyzed. The effect of acidization on the intrinsic constitutive parameters of the surrounding coal was clarified, and an improved constitutive model for the coal body around the acidized fracturing borehole was established. It was found that increasing the erosion time of the fracturing fluid leads to reorganization of the coal body's physical structure, an increase in free volume, and enhanced compressibility and brittleness. Additionally, increasing the erosion period intensifies the chemical reactions within the minerals inside the pore fissures of the coal body, leading to more intense chemical reactions and significant brittle damage in the saturated samples. The improved damage constitutive model can accurately characterize the stress-deformation behavior of low-permeability coal seams under acid fracturing. Using this model, the effective fracturing radius of the 226 comprehensive working face was determined to be 5.2 m, and the "three-flower hole" borehole arrangement was proposed accordingly. This study provides theoretical guidance and significance for the borehole arrangement in acid fracturing of low-permeability coal seams.