Effective evaluation of secondary hydraulic fracturing based on a modified criterion for new fracture creation

The effectiveness of secondary hydraulic fracturing is an important indicator for evaluating the stimulation performance of low-yield coalbed methane (CBM) wells. Following initial hydraulic fracturing, alterations occur in the in-situ stress and mechanical properties of the coal seam, rendering traditional criteria for fracture creation inadequate for accurately assessing the effectiveness of secondary hydraulic fracturing. Based on damage mechanics theory, this study identifies the key parameters governing new fracture creation after primary fracturing, and revises the post-fracturing in-situ stress and frictional resistance values. A modified criterion for new fracture creation in secondary hydraulic fracturing is thereby established, and the conditions under which new fractures can be created are determined. Using this criterion, the effectiveness of secondary hydraulic fracturing in CBM wells in the Shizhuang South Block is evaluated. The results indicate that after primary fracturing, both the horizontal principal stress difference and the tensile strength of the coal decrease. During secondary hydraulic fracturing, effective new fractures can be created in the previously fractured coal seam when the fracture initiation pressure is lower than the fracture propagation pressure; otherwise, effective new fracture creation is difficult. Gas production data from eight typical CBM wells subjected to secondary hydraulic fracturing validate the accuracy of the proposed criterion. The findings provide a theoretical basis for assessing the effectiveness of secondary fracturing in low-yield CBM wells.