Numerical study on enhancing coalbed methane recovery using stress relief by multiple cavities

Conventional horizontal well direct fracturing technology is inadequate for deep coalbed methane (DCBM) surface extraction. The extraction of DCBM presents a technical obstacle that impedes the advancement of China's CBM industry. This study explores the potential of a novel approach involving horizontal wells with multi-cavities to enhance DCBM extraction. A three-dimensional numerical tool, based on the elastic-plastic damage model and material point method (MPM), was developed to examine permeability enhancement and CBM desorption induced by cavities. Systematic engineering simulations were conducted to validate the effectiveness of DCBM extraction. The findings are as follows: 1) The three-dimensional numerical tool accurately simulates large deformation, large displacement, and inner boundary self-contact issues during cavity-induced collapse in deep coal. 2) A single cavity contributes an effective stress-relief volume 23.95 times greater than the cavity volume; permeability increases by 5–800 times in the plastic zones and by 1–5 times in the elastic zones over the initial permeability, and a total CBM desorption of 197.22 m³ is achieved. 3) The DCBM production volume enhanced by horizontal wells with multi-cavities is 24.98 times greater than hydraulic fracturing, with an average production exceeding 10,000 m³ /d and remaining above 6000 m³ /d after one year of extraction. 4) Optimal performance of horizontal wells with multi-cavities can be achieved by reducing the cavity angle and spacing while increasing the cavity width and length. Based on these results, implementing compound fracturing in horizontal wells with multi-cavities is recommended. This study provides a numerical tool and new insights for enhancing DCBM extraction.