Evolution of the breakdown pressure and fracture induced by water-/liquid CO2-based indirect fracturing

Abstract

Indirect fracturing represents an innovative approach for the extraction of coalbed methane (CBM) from broken-soft coal seams, with the primary objective being the penetration of fractures and the generation of a complex fracture network with high conductivity within the coal. To investigate the impact of borehole-interface distances on breakdown pressure and fracture characteristics, as well as the suitability of liquid CO₂ (L-CO₂) for CBM extraction via indirect fracturing, a series of fracturing experiments utilizing water and L-CO₂ on coal-rock blocks were conducted. The findings indicate that reducing the borehole-interface distance can lower the breakdown pressure and acoustic emission (AE) energy, while enhancing the complexity and roughness of fractures in coal; however, it may also impede fracture penetration. In comparison to water-based fracturing, L-CO₂-based fracturing significantly decreases the breakdown pressure and AE energy, resulting in a greater number of rougher fractures in coal. Utilizing fracture mechanics theory and the physicochemical properties of fluids, the mechanisms by which borehole-interface distance and L-CO₂ influence breakdown pressure and fracture characteristics were thoroughly analyzed. This study offers theoretical insights for CBM extraction employing water- and L-CO₂-based indirect fracturing techniques.